Propeller Shaft Size Calculator

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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:

Where:

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.