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Understanding Your Sailboat Propellers

• By Michael Cilenti
• Updated: July 6, 2015

Out of sight and out of mind, sailboat propellers are often an afterthought for cruising sailors. Ironically, because manufacturers have developed a variety of efficient sailboat propellers designed to maximize thrust under power while minimizing drag under sail, selecting the right prop for a sailboat can be even more complex than it is for a power vessel. In order to make an informed decision and choose the right propeller for your needs, it’s important to understand the advantages and disadvantages of all the options available.

Propeller Geometry

Before delving into specific types of sailboat propellers, in order to grasp how all the design elements work together, it’s ­important to understand the basics of propeller geometry. All propellers — from sailboats to power cruisers, outboards to aircraft carriers — share the following characteristics.

To start, the hub is the central part of the propeller, to which its blades are attached either as a single casting or mechanically; it, in turn, is connected to the propeller shaft in traditional drive trains, or to the saildrive , which has become more and more prevalent in contemporary cruising boats. Regarding those blades, the leading edge is just that: the edge that leads into the water when turning ahead. (It’s also the edge closest to the forward part of the propeller hub.) The trailing edge , conversely, is the opposite edge of the blade (the one closest to you when standing on the aft side of the prop).

The root is the inboard side of the blade and forms the transition where the blade connects to the hub, while the tip is the outboard edge of the propeller blade. The blade face is also known as the pressure side, and is the side that faces aft. The blade back is also known as the suction side, and is the side that faces forward.

The majority of propellers are right-handed ; viewed from astern, they rotate clockwise when moving forward, and have a slight tendency to swing the stern to starboard. Since the propeller is designed to be most efficient when going ahead (the direction one travels the vast majority of the time), this side force is minimal. When going astern, however, the opposite is true, and the stern will “walk” to port, which is usually the more pronounced of the two side forces and can therefore be a distinct advantage, or disadvantage, when maneuvering in close quarters. Prop walk is dependent on several factors, including hull shape, but also the propeller itself. An inefficient propeller will introduce more prop walk than an efficient one.

Diameter and pitch are two of the most basic dimensions used to describe a propeller. If you were to attach a pencil to the tip of one of the blades and ­rotate the propeller one full turn, tracing a circle, the distance across this circle would be the diameter of the propeller. Next, if you were to rotate the propeller one full revolution in a solid medium that does not slip (think of a screw in wood), the pitch would be the forward distance that the propeller would travel in that single revolution. With modestly sized yacht propellers, diameter and pitch are routinely measured in inches (as opposed to feet or meters for ship propellers). So a 12-by-20 prop has a 12-inch diameter and a 20-inch pitch.

Given the definition of pitch, you might wonder why you wouldn’t choose a propeller with higher pitch in order to gain additional power. But propellers must be sized in relation to the rest of the propulsion system, and must match the power curve of the engine, as well as the rpm range afforded by the reduction gear. As such, a propeller with ­excessive pitch may not actually generate additional thrust, since it would overload the engine and prevent it from reaching the designed rpm, which can reduce fuel efficiency and increase wear to critical engine components. ­Additionally, an overloaded propeller is susceptible to cavitation , which happens when a prop rotates and creates a partial vacuum. Water rushes in to fill that vacuum, creating tiny bubbles; when they collapse, they basically pound the prop. While cavitation will occur in small amounts in very localized areas on almost every propeller, excessive cavitation will result in a loss of thrust, excessive noise, and vibration, and can be violent enough to actually pit or erode the propeller.

Many blades also have rake and skew . Rake is the forward or aft slant of the blades when viewed from the side. Propellers are generally given some degree of aft rake to help increase efficiency while decreasing vibration. In fact, many propellers on outboard engines exhibit a significant amount of aft rake. A propeller blade is skewed if it is asymmetrical with respect to a straight radial line drawn outward from the hub. Skew helps reduce pressure fluctuation and therefore vibration and noise.

As an example, think of those classic World War II films, when the submariners would huddle together, waiting for the next depth-charge attack while listening to the pulsating sound of the destroyer’s propellers as it passed overhead. That distinct pulsation was caused by the difference in pressure as each propeller blade passed close to the ship’s hull. Even if the propeller blades were equidistant from the hull on all sides (think of a propeller in a tunnel or nozzle), the pressure difference between the propeller tip closest to the surface and the tip that is deepest can be substantial enough, especially in larger propellers, to cause similar pulsation. The prop on a modern submarine is an example of a highly skewed propeller; the sickle-shaped blades are designed to reduce noise and vibration to an absolute minimum, even at the expense of propulsive efficiency.

The number of blades can also play a significant role in the propeller’s performance. As with skew, at least in theory, a greater number of blades will generally reduce vibration, since for a given rpm there is less time between each blade passing a given fixed point. That said, if a propeller is well designed and dynamically balanced, a two-bladed prop will not vibrate more than a three- or four-bladed prop. However, in instances where a shaft is slightly out of alignment, a three-blade prop could help decrease shaft vibrations.

Along these lines, if a propeller had an infinite number of blades — if it were a solid disc — there would be no vibration. Additionally, a propeller with more blades will generally have a larger expanded area than one with fewer blades, although that is not necessarily always the case. Expanded area is essentially the total area of the blades. For a given horsepower, a larger expanded area will result in lower thrust loading, or conversely, a propeller with a larger expanded area can absorb more power prior to experiencing significant cavitation. At the end of the day, it’s a delicate balance between optimum blade area and the potential for excessive diameter. Wide diameter causes excessive tip speed, which results in cavitation.

Propeller Options

There are several propeller types available for auxiliary sailboat applications. To begin, let’s look at some basic but often confused definitions for different types of propellers.

Blades may have constant or variable pitch. A propeller with constant pitch has blades whose pitch, or angle of attack, remains the same from the root to the tip. Conversely, a variable-pitch propeller has blades that appear twisted; that is, the blades will have greater pitch at the root and less pitch at the tip.

While constant-pitch propellers are easier and therefore cheaper to manufacture, variable-pitch blades are more efficient. Consider each blade’s motion as the propeller spins. At a given rpm, the part of the blade near the root is moving much slower than the part near the tip. If this seems confusing, think about the change in circumference as we move outboard on each blade; a point closer to the hub has less distance to travel to complete a full circle than does the tip of the blade. Since they both must complete the revolution at the same time — they are, after all, two points on the same blade — the point near the tip must move faster than the point near the root. Now, we also know that the faster the blade moves, the more thrust it will produce. So, in order to maintain constant thrust along the entire blade, the pitch is reduced as we move farther out toward the tip.

Propellers may also have either fixed pitch or controllable pitch . Fixed props are available in a wide range of diameters and pitches. All fixed props have twisted blades, but as the name suggests, they can’t be adjusted. Feathering props, alternatively, have flat blades with no twist. This makes the blade area relatively inefficient, but that drawback is offset by the ability to adjust the blades’ pitch.

A controllable-pitch propeller also has the ability to change the pitch of the prop blades, by rotating each blade at the root, but these props are generally used on superyachts, not your normal cruising boat. On these larger vessels and ships, controllable-pitch propellers are used to keep the engine operating in the most efficient rpm band by varying the pitch of the propeller to change its thrust rather than the speed of the engine.

Additionally, many controllable-pitch propellers also have reversible pitch: In order to provide astern thrust, the pitch of the propeller blades is reversed while the propeller continues to spin in the same direction. For larger ships, this has some distinct advantages, as it eliminates the need to stop the propeller before turning it in the other direction. This simplifies the machinery required and dramatically reduces the time before reverse thrust is produced, therefore increasing maneuverability. For sailing vessels, in addition to the advantages previously described, controllable-pitch propellers can often “feather” the blades to minimize drag while sailing.

Controllable-pitch props can also be adjusted to maximize the power and efficiency of your particular propulsion system and boat, something that fixed-pitch propellers cannot do. This ability to fine-tune the propeller — coupled with advanced blade designs in controllable-pitch props — may promote a notable increase in power, speed and efficiency. It’s important to note, however, that compared to fixed-pitch models, these controllable-pitch benefits do come at increased cost. Cost aside, the complexity of the system also may make it more prone to issues than a fixed-pitch propeller, and more difficult to find parts and complete repairs when voyaging in remote regions.

Of course, many sailors are familiar with folding propellers, which are fixed-pitch models designed to fold back in order to minimize drag while under sail. Like fixed props, these come in a wide variety of diameters and pitches, have twisted blades, and aren’t adjustable. Originally, folding props were generally two-bladed, but three-bladed props are now widely used and very popular, and four-bladed models are also available. Folding props are designed so the centrifugal force of the spinning shaft will cause the blades to unfold when the engine is engaged. Folding propellers are less complicated than their controllable-pitch cousins, but compared to standard fixed props, they still offer reduced drag under sail. When folded, they are also less prone to being fouled by seaweed or other debris than a feathered, controllable-pitch propeller.

In days gone by, many folding propellers got bad raps for several reasons. They wouldn’t open immediately at low rpm, particularly in reverse, or they required a revved engine to provide the sufficient centrifugal force to open them. Conversely, at low speeds while under sail, the flow of water was sometimes insufficient to keep the blades folded, particularly if the prop blades were aligned top to bottom (where gravity could open the bottom blade) rather than side to side.

When we raced sailboats when I was younger, we used to dive down and put a rubber band around the folded prop once we got out to the racecourse. Or we’d make a mark on the propeller shaft with paint or a permanent marker when the boat was on the hard; we could then position the shaft so the blades were aligned side to side once we secured the engine. However, those issues have largely been eliminated with today’s modern, highly engineered folding props. These days, the blades in most folding props are geared and synchronized, and very efficient in all conditions, whether under sail or motoring. Their reliability is underscored by the fact that they’re now standard equipment on new boats from Beneteau, Dehler, Gunboat, Hanse, J-Boats, Jeanneau, Morris and many other manufacturers.

The variety of propellers available today provides a wide range of options for sailors to choose from. The advantages and disadvantages of each propeller type must be weighed against your sailing and motoring style. When investigating a new prop, shop wisely and ask lots of questions of the specific manufacturers; it’s important to get personalized recommendations for your boat and your plans. Understand the differences to ensure you find the best fit for your needs.

Propeller Resources:

The following is a list of propeller manufacturers and their websites.

• Autoprop Automatic Variable-Pitch Propellers
• Flexofold Folding Propellers
• Gori Folding Propellers
• J Prop Feathering Propellers
• Kiwiprop Feathering Propellers
• Max-Prop Feathering Propellers
• Martec, Autostream and Slipstream Folding and Feathering Propellers
• Variprop and Variprofile Feathering Propellers
• Volvo Penta Folding Propellers

A graduate of the U.S. Coast Guard Academy, where he raced inshore on the varsity sailing squad and offshore to Bermuda and Halifax, Lt. Cdr. Michael Cilenti is currently second-in-command of the Coast Guard cutter Waesche out of Alameda, California .

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Sailboat Propellers

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Uncompromising, Impartial Solutions

King propulsion offers uncompromising and impartial solutions to all your marine propulsion needs.  our unique portfolio of sailboat propellers ensure that we only match the customer with the propeller they need.  folding and feathering propellers have unique characteristics that do not apply to every boat or sailor in them.  we strive to make this connection for you and give you the best technical advice possible. our team are all highly qualified and respected naval architects with countless years of experience in all types and sizes of propulsion systems from sailboats to warships., in addition king propulsion also supports and distributes sigmadrive, and exceptional drive train solution for coupling propeller shafts to engines quietly and efficiently., us distributor for autoprop feathering propellers, varifold folding propellers & sigmadrive, based in usa.

King Propulsion is a US owned and operated company based in Virginia Beach, VA supporting the USA, Canada and the Caribbean.

Tech Support

King Propulsion is staffed by Professional Naval Architects who can meet all of your propulsion needs. From accurately sizing a propeller to shafting and alignment issues, we have you covered.

King Propulsion is a technical consultancy with substantial experience in propeller analysis and propeller design.  King Propulsion represents Bruntons Propellers, a 150 year old propeller company based in the UK.

King Propulsion is an exclusive Bruntons dealer selling the full line of Autoprop propellers, Varifold propellers and SigmaDrives.  King Propulsion also provides full after sales support and propeller refurbishment.

Choosing a Sailboat Propeller

In the market for a new sailboat propeller ow about a folding sailboat propeller a feathering sailboat propeller what about autoprop, they are. supposed to be good or was it varifold, maxprop or gori choosing between them can be an impossible task and understanding the basis of each manufacturers claims and counter claims even harder. at king propulsion we are professional engineers first, technical sales second and we pride ourselves on the fact that we offer all flavors of propeller to suit every boat and every sailor. we don’t need to fiddle facts on propellers, we use science to help you choose what is right for you., if you look at the market you can distill the sailboat propeller types into 3 broad categories, the fixed pitch propeller, the folding propeller and the feathering propeller, each suits a type of sailing, a type of boat and a type of sailor. there is no one propeller to fit all. as any engineer will tell you, design is a compromise to achieve your objectives. so let us look at some of the compromises all the companies don’t want you to know about. the descriptions below are features that transcend the designs to give you some idea of the choices and options available., remember at king propulsion we sell all 3 types of sailboat propeller, so it is all about the right choice for you, propeller drag, the reason sailboat propellers exist in the first place is to reduce the appendage drag of the sailboat and sail faster. each family of designs achieves this in a different way  and compromises on very different parts of the puzzle to get there. for serious racers, this category is the only consideration.  for the regular sailor, it is a choice, low drag, backing performance, motor sailing as you can’t have everything in this category the folders and the feathering propellers fight for the title, autoprop is a bronze medal winner and fixed pitch propellers just add plain old drag., cruising speed at 2500 rpm, not only is the hull shape and the size of the main engine a factor in speed through the water, but propeller type play a big part of this as well.  fixed pitch propellers are the benchmark but autoprop with its self pitching mechanism gets more speed.  the folder and the feathering options are sub optimal here., backing and maneuvering ability, backing down can be one of the most fraught times for a sailboat owner. for some race boats it is not a consideration, for day sailing and backing into tight marina spots it is a requirement., fuel consumption, the wind is of course free, but catching the wind is expensive, especially if you have to motor long island sound or the icw to get to wind. all of the sailboat propellers impact fuel burn in different ways with autoprop the clear winner and the feathering propeller requiring more fuel for the same voyage. the values below are in us gallons per hour., initial cost of the propeller, sailboat propellers are an aspirational product as soon as you move away from the stock fixed pitch propeller. the cost increases as the complexity of the propeller goes up. the folding propellers are the least expensive of the sailboat propellers; the feathering propellers typically the most expensive and autoprop is slightly behind the feathering propellers. these prices are based on 2024 18″ diameter shaft drive propellers., so what type of sailer are you, what type of boat do you have, find out how a sailboat propeller can improve your sailing performance and heighten the sailing experience. simply fill out the form below with basic details about your boat and we will work with you to size and select a propeller that will be a perfect fit for you and the boat, please note, the above data is drawn from actual sea trials, magazine publications and academic research performed by king propulsion and bruntons propellers on their line of fixed, folding and feathering propellers..

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Our products are in use around the world on a vast array of vessels. here are some of the comments we have had back for our propellers, couplings and service..

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Stainless Steel Inboard Boat Propeller Shafts

The "best" shaft material depends on a number of different variables. These include whether the boat will be exclusively used in fresh water or be operated in brackish or salt waters. If intended solely for fresh water use, corrosion problems should not be a major issue. As such, the propeller shaft will, most likely, be less expensive. However, salt water can cause a range of corrosion problems, making the task of selecting an appropriate shaft material more crucial.

Basically, shaft materials are typically made of a variety of stainless steels or copper bearing materials. The copper bearing metals commonly used for propeller shafting are Monel alloys, Naval brass, and a variety of bronzes. Naval brass has a content of approximately 60 percent copper, while bronze must have a copper content of 90 percent or more; this makes it more superior.

Stainless Steel Shaft

Typically, the stainless steels used to make propeller shafts include types 630, 316, 304, and 303. All of them are strong, particularly the 630 variety. When used in salt water, stainless steel shaft is vulnerable to pitting which often results in crevice corrosion. When it comes to crevice corrosion, types 303 and 304 are widely viewed as the worst.

Type 316 is a lot less vulnerable and as such, it is much better suited for use in salt water. At this point in technology, the 630 type is perhaps right up there with the K-500 Monel competing for the top spot as the "best shaft material." Type 630 is priced higher than the other kinds of stainless steel materials. It is also stronger and is less susceptible to corrosion.

Stainless steel shaft has particular requirements for usage and installation that should be followed for the duration of its lifespan. For example, the sacrificial galvanic anodes are supposed to be positioned on or adjacent to the propeller shafts in brackish or salt waters, when the hull stays in the water. If the shafts are outfitted with anode collars, these could cause the shafts to become out of balance.

In addition, these types of anodes are more susceptible to erosion than those not located on the shaft. Careful attention should be paid to locating these anode collars so the flow of water to the propeller will not be restricted. For boats that have an elongated shaft tube that traps water beside the stainless steel propeller shaft, there must be a way to foster a positive water flow circulation to escape the corrosive impact of stagnant water when it makes contact with the stainless steel. All underwater equipment and electrical equipment must be grounded properly to a negative ground plate underwater. On trailerable boats, these issues are not a major concern.

Propeller Depot's expert machine shop is able to meet all of your inboard prop  stainless shafting needs.

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Propeller Shaft Size Calculator

Start » Propeller Shaft Size Calculator

This free propeller shaft size calculator helps you determine the proper propeller shaft diameter for your boat. The calculator determines a safety factor (design coefficient) based on shaft diameter , max engine RPM , shaft horsepower , gear ratio , and torsion strength of the shaft material used. The calculator utilizes the following industry accepted formula endorsed by shaft manufacturers, classification societies, and industry governing bodies:

SF = Safety Factor (Design Coefficient)

D = Shaft Diameter (this unit is squared [^3] in formula

St = Shaft Strength, Torsional Yield Shear (psi) Note : see note below

N = Max Shaft RPM

321,000 = Formula Constant

HP = Engine Horsepower

Interpreting Shaft Safety Factor

This calculator computes an objective safety factor (design coefficient) based on user provided data. So what is a propeller shaft safety factor?

Think of a propeller shaft safety factor as a protection against risk. The higher the safety factor, the more protected your boat is from a catastrophic propeller shaft failure.

In perfect conditions with limited use, a safety factor of 1 can feasibly transfer power from the engine, through the reduction gear, to the propeller. However, boat’s are not designed for perfect conditions and rarely are they intended for limited use. Commercial vessels, offshore fishing boats, racing boats, and vessels operating in extreme environments should have an appropriate safety factor based on the projected wear and tear on the boat’s running gear.

As is the case, the American Boat & Yacht Council recommends a safety factor “ approaching five ” for inboard diesel powered pleasure boats. For “heavy commercial and racing boats”, Dave Gerr (author of Propeller Handbook ) recommends a safety factor between 5 and 8.

For more information regarding propeller shaft safety factors or to order a propeller shaft give us a call at (207) 422-6532 or send us an email through our contact page .

What is Torsional Yield Strength?

While most of the data required for the propeller shaft size calculator are readily available, the one variable least understood is torsional yield strength . For the purposes of this calculator, torsional yield strength refers to the amount of force in pounds per square inch (PSI) that it takes to “twist” the shaft by 0.2%. This 0.2% yield strength is a common measurement when discussing material strength and it has been adopted by the marine industry.

At R.E. Thomas Marine Hardware, we use Western Branch Metal Aqualoy 22 and Aqualoy 22 HS (high strength) exclusively. The minimum torsional yield of these values are published on Aqualoy’s website and are defined as per ASTM A-276, ASTM A-479, and AMS 5764 specifications.

The default value in the calculator is 70,000 psi which is the standard minimum yield strength of Aqualoy 22 shaft diameters between 1 1/4″ to 2″. For diameters over 2″ to 2 1/2″ the standard minimum yield strength is 63,300 psi . Diameters over 2 1/2″ to 3″ have a standard minimum yield strength of 50,000 psi . For Aqualoy diameters over 3″ the minimum torsional yield strength is 36,600 psi .

No matter what alloy you’re using (Aqualoy 22, Aqualoy 19, Aqualoy 17, Aquamet 22, Aquamet 19, Aquamet 17, 316 stainless, or bronze), be sure to familiarize yourself with the supplier’s reported 0.2% torsional yield strength to help ensure the accuracy of your calculations.

Gear Ratio Effect on Shaft Safety Factor:

Another important variable in our propeller shaft safety factor calculator is shaft RPM. This calculator factors shaft RPM by considering the engine’s max RPM and dividing it by the gear ratio. For example, if the gear ratio was 1.5 to 1 (often presented as 1.5:1) the engine needs to turn 1.5 revolutions for the propeller shaft to make one turn. In other words, if an engine had an max RPM of 3000, the propeller shaft would theoretically have a max RPM of 2000 (3000/1.5).

This is where things get tricky and counter-intuitive for some people; the higher the gear reduction ratio, the lower the propeller shaft RPMs will be, and the LOWER the shaft safety factor will be. One way to think about it is pulling a vehicle out of a ditch. If you hook a chain from your vehicle to the vehicle in the ditch and try to pull it out in 3rd gear very little force is applied to the chain. However, if you put your vehicle in 4-low (reducing the engine gearing down), there is a significant amount more force pulling on the chain.

If the boat’s gear ratio is 1.5 to 1, just put “1.5” in the gear reduction ratio in the calculator. If the ratio is 2.07 to 1, use “2.07”, and so on…

How to Order a New Propeller Shaft

R.E. Thomas Marine Hardware is proud to be an OEM supplier of Aqualoy 22 propeller shafts to many of the world’s finest boat builders. Call us today for a quote or for more information about our complete line of Marine Hardware. While you’re there, check out our Piranha Dual Line Cutters and our heavy duty Self-aligning shaft seals .

Disclaimer : The information on this page (including the calculator) is provided for general information only. For more specific information regarding shaft diameters, shaft safety factors, shaft alloys, and other related inquiries, please contact us directly.

5 thoughts on “Propeller Shaft Size Calculator”

Does length of shaft make a difference? I have a Yamaha 30 sailboat with a 22mm (slightly bigger than 7/8″ ) with a length of approximately 11′ bronze shaft ! The 12Hp diesel engine is in the V-Berth. Want to upgrade the engine to either 16hp or 25hp and wonder if I can stay with 22mm shaft again.

the engine is 12hp, reduction is 2:1 (prop turning approx half the speed of the engine; max engine speed is 3500; shaft is 22mm; shaft lenght is 11 feet.

We are Planning to buy propellar , but we dont have dimensions , we have only the tensile strength of 45 kgf/sqmm, how can i calculate ?

We have a crew boat with these specifications

Vessel Data

Hull Type – semi-planning/plan hull shape – Steel Builders

Boat Weight 60 ton

Waterline L 17 meters

Waterline beam 4.8 meters

Depth 1.8 Meters

Engine Data

Engine Power MTU 12V 2000 – 2X 916 KW @ 2200 rpm

ZF2500 V Gear Ratio 2.029

Propeller Data

Prop Diameter 920 mm

Pitch 1459 mm

Speed 9 Kts

We have doubts about the engine being overloaded due to the propeller was changed a long time before with the wrong specification.

According to our calculation, the propeller should be as follows:

Prop Diameter 1040 mm/ 41 inch

Pitch 869 mm/ 35.1

Estimated speed 26 Kts

Please, your kind cooperation is required to send to us the original propulsion system for the given engines and gearbox to confirm about the current propeller dimensions.

Greetings! Unfortunately, this calculator is only for calculating the “safety factor” of the propeller shaft itself. It is not designed to calculate the whether or not the engine itself is under/over propped. Thank you and good luck with your research. – RE Thomas Team

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Marine Propeller Shaft – Design And Construction

The heart of any ship lies in the engine room. Powered by large marine engines usually running on diesel oil of HFO, vessels are able to reach speeds of up to 50 kmph.

The components responsible for actually moving the ship are the large multi-blade propellers fit at the back.

So how does power transfer from the engines located deep inside the hull of a vessel to the propellers at the aft?

The answer lies in an integral mechanical component known as the marine shaft or propeller shaft.

Similar to how driveshafts work in cars, the shafts in ships also transfer rotational power from the engines to the propellers, which convert them into translational motion.

In this article, we will look at the marine propeller shaft and its various components, along with their design and construction, we will also study the working mechanism of these shafts.

How Are Ships Propelled?

The propulsion systems onboard ships power the vessel by converting rotational motion into translational motion. This is similar in theory to the famous Archimedes’ Screw mechanism credited to the Greek Mathematician Archimedes in c. 234 BC.

The rotational energy is provided by two to four marine engines located in the engine compartment of a ship. Both two and four-stroke engines are used depending on the size and required use of the vessel.

The marine engines generally run on diesel, although attempts are being made to turn to energy-friendly alternatives.

Inside the engine, pistons combust the fuel through alternating compression and expansion cycles. The combustion is achieved at ignition temperature and forces the crankshaft to make half a rotation under compression. The expansion phase completes the remaining half of the rotation.

The most commonly used type of marine engine is the reciprocating diesel engine that has a higher efficiency compared to other models. These engines can be classified into three types based on their revolutions per minute (rpm).

The three categories- slow, medium, and high speed have their own benefits based upon the type of ship to be powered.

For instance, large ships require a low speed but high torque propulsion system to power them. For such vessels, a low output speed engine can be selected.

The issue with using slow-speed engines is the large space they take up as compared to the other engines. Thus, a space-effective solution would be to install high-speed engines in the ship, and then reduce the torque before it reaches the propellers.

For this, a gearbox is a very useful component that can be used to manipulate rotational torque transfer. It is attached to the marine propeller shaft and reduces the power transmitted to the propeller.

The slow speed engines pose no problem to the transfer of torque and do not require an additional gearbox. The gearbox in the other speed engines is attached in between the intermediary and propeller shafts.

Components of Propeller Shafts

The marine propeller shaft is divided into three main components-

• the thrust shaft,
• intermediate shaft(s), and

The thrust shaft is the primary shaft emerging out of the engine. It directly receives the rotational motion from the crankshaft and rotates at the maximum velocity in high-speed engines.

For high rpm engines, the thrust shaft is further connected to other components that lie further aft.

The next component is the intermediate shaft. There is no specific restriction on the number of intermediary shafts that a ship can have. However, beyond 2 shafts, it can be difficult to service and maintain. The reason for this is the large catenary force acting on the entire propeller shaft. This force tends to deform and damage parts due to their weight.

When coupled with the large vibrational shocks that act on the shafts, there could be permanent damage to the propeller shafts. Thus, a low number of intermediary shafts are preferred. The only reason to have multiple intermediary shafts as if the engines are located far away from the propellers.

The last part is the tail shaft . It is directly connected to the propellers and lies mainly encased in the stern tube . The tail shaft is connected to the intermediate shaft by a gearbox that manipulates torque transfer. The tail shaft is built to withstand a variety of forces that may act at the stern of the ship.

The next component is the coupled bearing that connects two adjacent shafts. The coupling is achieved by virtue of joints that are usually rigid and do not flex. The coupling units are bolted to each other using high strength fasteners that can withstand a large number of vibrational stresses.

Shaft bearings are components that are used to support and bear the load of the shafts. They run along the length of the shaft and ensure smooth rotation. These bearings are constructed differently based on their location.

The last part of the marine propeller shaft system is the thrust blocks. These blocks support the propeller shafts at regular intervals. These blocks play transfer the excess power from the shafts into the hull of the ship.

As the shafts rotate at very high speeds, some amount of vibration occurs. This further leads to jarring shocks that may compromise the structural integrity of the vessel . Thus, using specialized bearings, the shocks can be dispersed over the hull of the ship.

To anchor these thrust blocks to the bed of the ship, a reinforced frame is built. There is a primary thrust block placed aft of the engine crankshaft, that disperses the majority of the shock into the hull girders and structures.

The components discussed above form the large bulk of parts that make up the propeller shafts. In addition, there are a variety of smaller parts such as sealants and bearings that serve different functions.

Design and Construction

The design and construction phase are important as it ensures structural strength. With shaft speeds reaching anywhere between 300 rpm to 1200 rpm, care must be taken to control material fatigue and reduce damages caused to the components of the ship.

The construction of the shaft bearings is important, as this holds the complete weight of the propeller shafts.

There are two main types of bearings- the full case bearing located at the stern, and the half case bearing located at the other positions.

The full casing provides a complete bearing for the weight of the shaft and is an integral part. The reason it is located at the stern is to account for both catenary weight forces, and also to counteract any buckling or reverse thrust forces felt at the aft due to the motion of the propellers. This bearing is also known as the aftmost tunnel bearing, as it encases the shaft just like a tunnel.

The other shafts only account for the weight, and hence do not require an upward casing unit. These bearings must be designed of high strength metals that do not easily buckle or deform under high strains. In addition, low levels of tolerances are expected during the manufacturing stage.

Special bearing pads are fit into slots on the connecting inner face of the bearing, such that it allows for smooth rotation. To lubricate the shaft bearing, an oil dip arrangement is carried out. By coating the rotating surface with oil from an oil thrower ring at regular intervals, a thick coat of lubrication is maintained at all times.

The coolant used to prevent overheating and subsequent damage are water circulated about the shaft bearing. This is stored in specialized tubes that run along the bearing and shaft. Tanks stored above the engine platform house coolant that is circulated around the propulsion machinery and systems.

The thrust blocks are used primarily to dampen and absorb forces from the rotating propeller shafts. These forces are redirected into specialised frames that make up the bed of the engine compartment. The energy in these frames is further distributed into the surfaces of the hull through the hull girders.

The hull girders serve as the framework on which the hull of the ship is constructed. The thrust blocks must be rigidly mounted in place to prevent any form of vibration during the course of the journey. Also, the primary thrust block can either be an independent unit that is built separately or can be integrated into the marine engines itself.

By integrating the block into the engine, it reduces the space requirements and maintenance costs while sailing. However, maintenance, while berthed, can be an issue as it would require opening the engine block casing. The casing that makes up the thrust blocks are built in two parts- an upper half that is detachable, and a lower half that supports the shaft.

The shaft is laid onto the lower block, and the upper half is then bolted into place using specialized fasteners that can absorb shock. To lubricate the rotating shaft, oil is regularly coated on to the rotating surface. This is achieved in a manner similar to that of the shaft bearings.

An oil thrower and deflector are put in place to maintain a constant supply of oil from a storage unit located on the lower half of the thrust block.

The operating temperature is controlled using cooling coils that circulate a chosen type of coolant throughout the block. It also draws coolant from the central propulsion cooling system. To absorb the vibrations and shocks, bearing pads are attached to the blocks.

They can be of two types- tilt pads, or pivotal pads, both of which are held in specialized holders built into the thrust block. The thrust pads transfer energy to the lower half of the casing that is constructed to withstand larger amounts of shock.

A thrust collar is also used to absorb thrust from the propeller shaft. The thrust blocks incorporate integral flanges that primarily help in bolting the block to other surfaces.

For instance, the block can be connected to the gearbox or engine using this flange. It can also be used to connect the engine thrust shaft to the intermediate shafts using these flanges. In case the thrust block is built into the engine block, it is made of the same casing material that the engine base plates are manufactured from.

In addition, they directly use the lubrication and coolant from the engine components itself. The integrated block is similar to the normal thrust blocks in most other features. It is interesting to note that the thrust block is integrated into the engine in most ships, except for the smaller boats that have space constraints.

The shafts themselves must be built from robust materials with high yield strength, and a lower probability of buckling. Each shaft starting from the thrust shaft must be built into small and manageable components that can be disassembled when the need arises.

In addition, seals and stuffing boxes are also built from appropriate materials that can effectively seal the inner working machinery from external water. High-grade materials are a must while manufacturing propeller shafts, as this is very sensitive equipment that needs to handle large forces.

The marine gearbox is an integral component that is attached in between the tail shaft and the intermediary shaft. It is mainly used to manipulate the torque transferred from the engine crankshaft to the propellers located at the stern. It is most commonly used in large vessels that engage a high-speed engine during operations.

The gearbox works just like an automobile gearbox, that uses a system of clutch discs and pads to control torque. Using robust gear arrangements that can withstand large vibrations, the gearbox is an integral part of ships with high-speed engines.

Lubrication is a necessity to prevent friction-related accidents or damage from occurring.

Stern Tube and the Propeller Shafts

Stern tube arrangement refers to the manner in which the tail shaft is borne by the stern tube that is located at the aft of the vessel. The stern tube is a hollow, horizontal tube that serves as the primary connection between the propellers and the rest of the vessel.

Attached to the stern frame , the stern tube acts as a plug at the rear of the vessel. The stern frame is the primary structural member that supports the stern overhang that lies above the propellers and rudder.

The stern tube houses the tail shaft of the marine drive shaft system and serves two main purposes- withstanding load and sealing the entire vessel at the aft portion.

Since the stern tube serves as the primary link between the vessel and the propeller, it must be able to withstand a tremendous amount of force that is exerted by the suspended propellers. In addition, it should provide sufficient room for the propeller hub to actually move without creating friction.

To handle the load, white metal is a commonly used material that can withstand the required loads. Sufficient lubrication is also provided within the stern tube to ensure the smooth functioning of the entire marine propulsion system.

Along with supporting the structural weight and forces of the propeller, the stern tube also needs to be able to effectively seal the vessel. It prevents water from entering through the aft and achieves this by using a combination of seals along its length. The stern tube has two main seals located at its aft and fore regions.

This serves as dual protection against any possible leaks that may occur over long periods of time. These seals can be of three main types- stuffing boxes, lip seals, and radial face seals. Stuffing boxes are built out of a variety of packing materials that are used to plug the stern tube.

Lip seals are gland seals that are used to prevent lubricants from seeping out into the water. They also serve the dual purpose of preventing water from entering the stern tube. Lastly, radial face seals extend radially out from any points of ingress and use a spring system to seal the entire structure. They are composed of two main parts that join to completely seal the rear portion.

The stern tube plays an important role in marine propulsion since it absorbs and dampens a considerable amount of power away from the propeller. In an effort to completely seal the stern section of the ship, it may also hamper the rotational abilities of the propeller shafts.

This would be counterproductive and would create a large amount of stress within the hull of the ship. Thus, being able to effectively seal the aft region without affecting the performance parameters is a major necessity in naval architecture and engineering.

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About Author

Ajay Menon is a graduate of the Indian Institute of Technology, Kharagpur, with an integrated major in Ocean Engineering and Naval Architecture. Besides writing, he balances chess and works out tunes on his keyboard during his free time.

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Prop Shaft Runout

• Thread starter Hermit Scott
• Start date Nov 13, 2008
• Forums for All Owners
• Ask All Sailors

Hermit Scott

I set up my new prop shaft on a bench with a dial indicator. I checked it at the 9 O'clock position and the 12 O'clock position. The runout is .004" in the center of the shaft. I was suprised at how easily these shafts flex. Pushing down with my finger on the threads by the prop it is very easy to get the shaft to flex out .010" to .020" in the center 24" away from the threads. Just the weight of the prop being mounted bends the shaft .001" when measured in the center of the shaft at the 12 O'clock position. The idea of having run out to with in .002" in the center of the shaft is impossible while mounted on a boat. And once it's running it would have to be pretty far out, measured in 1/16" instead of thousands. Has anyone measured the run out on a shaft out or in their boat? What are the specs for runout at the coupling with everything mounted in the boat?  

Attachments

You need V-blocks as the thomson bearings will make it hard to figure out were its really bent I would say you need less than .001 or you will get a vibration We dont have a special press BUT set up shafts in the lathe with a 4 jaw chuck and a steady rest that we place were the support bearing would be (we do mixer shafts) The hard part is how learning how much you have to bend it so it ends up true ,its the art part of it  

frank balcer

Hermit..... Hermit, .004 thousandths is not too bad. Before sucombing to the lure of big oil money, I spent 13 years in a job shop that specialized in marine running gear. For small shafts when we did run out checks we would use soft jaws to hold the coupling end of the shaft and indicate zero here. If it was a small shaft with no center we would run the steady rest at the bearing journal area. we usually shot for 2 to 3 thousandths runout. Small shafts are a pain to straighten. We used heat to straighten all shafts. Do a runout check, put the bend up, heat up and make the bend actually worse then quench with water, as high side cools faster than the rest of the shaft the metal cantracts and pushes (or pulls) the shaft back in the opposite direction. The easiest sizes to straighten were 8 inch shafts and bigger. We had a shipset of shafts for a high speed gov. boat that had 3 inch X 16 feet shafts and they required .0015 max runout. I got them straight but it took some time. Man, as I reminiss about my old job, I do miss the challenges we were given back then...... Frank  

Bailing wire? Do you think I am made of money, I can't afford bailing wire. I use coat hangers. I was standing on the duct tape so I could get a better angle for the picture. Duct tape is good for more than just water proofing hull/keel joints.  

To answer.. To answer your question more accurately, based on industry specific guidelines, I am referencing ABYC P-06 Propeller Shafting Systems This is from Table III - Straightness Tolerances (supports placed at ends of bar) Permissible variations in throw in one revolution from straightness. Up to 3' = .0025" Over 3' = .0025" Over 4' = .003" Over 5' = .003" Over 6' = .003" Over 7' = .003"  

Hermit, Just curious, how long is that shaft? Years ago I had to straighten bars after they where heat treated. I used V blocks and a small arbor press with an indicator. We applied pressure on the high points until we brought it into an acceptable tolerance. You move the V blocks closer to the bend then move them further apart and recheck. It takes patients but it can be done. I did do it on a granite surface plate. I'm not sure how flat the surface your working from should be. Looks like a welded plate used as a steady rest for your saw. Is that a Jet? If you have a Bridgeport you can use the table and use the quill as your arbor press.  

The shaft is almost 5'. The table is an outfeed table for my jet bandsaw. It's an I-beam structure. The bearings are Igus solid bearings. I just got back from the bearing company with 2 pillow block roller bearings. I guess I do need some V-blocks to do this right. The table will not flex. The bearings are self aligning, but this shaft is so flimsy if the bearings have to align at all it will flex the shaft. When I spin it by hand I have to be careful not to put pressure laterally at all or the shaft flexes. This is not acurate enough. I will try something else and see how it affects the outcome.  

Hermit, Are you discribing shaft whip rather than straightness? Machinery's handbook should give you the allowable shaft lengths between supports.  

Maine Sail Three thousandths sounds really tight for a seven inch shaft. We always used the ABS standards (American Beureo of Shipping) unless we were working a goverment contract, and I can't for the life of me remember the standard for diameters that large, im thinking .004 to .007 for low speed applications. We had a crusty old ABS inspector that served our area (Gulf Coast) and boy was he a stickler for the rules. Some of the fellows called him "Last Word" as in a last word indicator that is used to get to tenths and smaller. Man those were the days!  

frank balcer said: Three thousandths sounds really tight for a seven inch shaft. We always used the ABS standards (American Beureo of Shipping) unless we were working a goverment contract, and I can't for the life of me remember the standard for diameters that large, im thinking .004 to .007 for low speed applications. We had a crusty old ABS inspector that served our area (Gulf Coast) and boy was he a stickler for the rules. Some of the fellows called him "Last Word" as in a last word indicator that is used to get to tenths and smaller. Man those were the days! Click to expand

I have been checking the reference books that I have here and the tolerances that have been refered to seem to relate to diameters and not straightness. We always checked for straight by rolling the shaft on a known flat surface. It is not posible to find a straight piece of 3/4 inch shaft 6 feet long by supporting the ends only. It will sag under its own weight.  

It does sag under it's own weight BUT it sags uniformly. So as it rolls it should be in the same position +/- it's runout. Ross what's whip?  

Ross said: I have been checking the reference books that I have here and the tolerances that have been refered to seem to relate to diameters and not straightness. We always checked for straight by rolling the shaft on a known flat surface. It is not posible to find a straight piece of 3/4 inch shaft 6 feet long by supporting the ends only. It will sag under its own weight. Click to expand

ABYC said: Specified diameter of shafting - over 15/16 inch (23.8 mm) to eight inch (203.2 mm) incl. Specified lengths of 20 ft. (6.0960 m) and less. Supports placed at ends of bars or 42" intervals. Click to expand

As you run a shaft with no load it will run smoothly if it is well supported. When you load the shaft it should run smoothly unless the torque is pulling it out of straight. A shaft with a long over hang can start to whip or flail on the end if it is over loaded or out of balance. Sometimes the flexibility of a shaft is used for minor misalignments with proper bearing supports on each end.  

Mainesail , I think we agree that no matter how hard we try perfection is always just out of reach. ;D  

I am not an automotive engineer brother. I have never had a job outside my own company so I miss out on alot of things that are obvious to those even newly indoctrinated in a company that already has engineers working at it. I think I understand the whip concept; the little bit of mass that is outside of center of radial balance is multiplied times the speed and is a force that is outward and perpendicular to the shaft. So when the speed increases the whip increases? I am just turning this by hand as slowly as possible. I need to take this prop and have it checked for balance. This is one of those things where I have no trust in anyone that does this. They can tell me it needs some magic powder sprinkled on it and how would I know the difference. Is there a dynamic balance for props or do they just use some kind of static thing like an air bubble in a level? All this seems kind of futile with that single cylinder clatter box driving the 'perfect' shaft and 'perfectly' balanced prop.  

You could set it up on knife edges with the prop installed and get static balance. You could take it to a drive shaft shop and they could run a dynamic balance for you. In the end you have an engine on rubber mounts, your alignment can be perfect while the boat sits motionless, but when you put the transmission in gear and open the throttle everything changes. I have learned how to make things nearly perfect from one master and have learned just how bad things can be and still work well from another master. Do the best you can and it will be better than almost every installation out there.  

"All this seems kind of futile with that single cylinder clatter box driving the 'perfect' shaft and 'perfectly' balanced prop." You hit that nail right on the head. I had all my parts replaced or trued up by a marine machine shop assembled everything, lined everything up and ultimately concluded that the relationship between the engine beds and the shaft log is woefully out of alignment. So hopefully you checked your engine mounts when you pulled the engine. The problem I had could have been poor alignment of the shaft log straight from the manufacturer, or due to degradation of the beds of epoxy putty S2 used under the 4 x 4's they laid down to avoid having to do anything else to deal with the curved surface they were attaching to. I went absolutely nuts for weeks trying to figure out how to align everything correctly, thought I got pretty close, then had a whistling sound which was probably shaft related. I paid a "mechanic" for an evaluation for the opinion "I don't know." Then the noise, which only occurred at low rpm, disappeared after my 3 day cruise home. She was hauled and the orientation through the newly replaced cutless bearing...now doesn't look much different than through the old one I was told I had to replace. Due to the flexibility of hulls on the hard I think I will judge cutless bearing wear, at least on my boat, on movement and not orientation through the bearing in the future.  

Hermit - is your shaft bronze I assume it is. Bronze is MUCH more flexible than Stainless Steel. Also what was alluded to above is that you are measuring runout with the weight of the flexible shaft only supported on the ends when you measure it. You C30 will have supports with open unsupported sections much shorter. I would try your measurements on the bench with the same distance as what you have on the C30 and see what you measure. I bet it would be 1/2 what you observed  

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