One way to think about it is pulling a vehicle out of a ditch. 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. 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). 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. This calculator factors shaft RPM by considering the engine’s max RPM and dividing it by the gear ratio. Gear Ratio Effect on Shaft Safety Factor:Īnother important variable in our propeller shaft safety factor calculator is shaft RPM. 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. For Aqualoy diameters over 3″ the minimum torsional yield strength is 36,600 psi. Diameters over 2 1/2″ to 3″ have a standard minimum yield strength of 50,000 psi. For diameters over 2″ to 2 1/2″ the standard minimum yield strength is 63,300 psi. 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″. 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.

Thomas Marine Hardware, we use Western Branch Metal Aqualoy 22 and Aqualoy 22 HS (high strength) exclusively. This 0.2% yield strength is a common measurement when discussing material strength and it has been adopted by the marine industry.Īt R.E. 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%. 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 “heavy commercial and racing boats”, Dave Gerr (author of Propeller Handbook) recommends a safety factor between 5 and 8.įor 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. 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.Īs is the case, the American Boat & Yacht Council recommends a safety factor “ approaching five” for inboard diesel powered pleasure boats. However, boat’s are not designed for perfect conditions and rarely are they intended for limited use. 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. The higher the safety factor, the more protected your boat is from a catastrophic propeller shaft failure. Think of a propeller shaft safety factor as a protection against risk. So what is a propeller shaft safety factor?

This calculator computes an objective safety factor (design coefficient) based on user provided data.

Inboard propeller sa