Deficiencies in shaft diameter calculations

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Deficiencies in shaft diameter calculations
Miscellaneous Musings from the Technical Director

Monday, 16-APR-2007 by Donald MacPherson - Technical Director

As part of our development for the shaft sizing calculations in PropExpert over the course of the last couple of years, we contacted ABYC regarding what we believe is a significant potential problem in their recommended shaft sizing calculations (Section P-6.5). ABYC is undertaking a new review of their technical literature, and as their reviews are many years apart, I thought it important to share my observations with the marine design community at this time. The fundamental problem is with their definition of calculation power and RPM.

In their literature, ABYC simply identifies power as "shaft horsepower" and RPM as "shaft speed, RPM". They clarified this in a telephone conversation to me as rated shaft power and RPM. Using these values in the calculation, however, has the potential to result in a shaft diameter that may be inadequate. Let me explain.

The shaft diameter calculations are all about torque (power divided by RPM). The combination of rated power and RPM does not necessarily lead to a maximum torque condition, either during steady-state or dynamic operation. If an engine is overloaded due to an over-pitched propeller, by towing a net or barge, or during dynamic positioning operations, then the equilibrium steady-state condition may be something near full power but at less than rated RPM - a torque higher than at rated.

Also of concern is the dynamic case, such as when a boat is accelerated WOT to get to plane. Depending on the nature of the engine and propeller, the propulsion system can follow the maximum torque curve as the boat runs from idle to top speed. (Consider the low speed loading of a surface-piercing propeller, for example.) Somewhere during this time, it will hit the engine's point of maximum torque. This can be significantly greater than the torque at rated - especially for some of the newer EC engines. As an illustration of this, the maximum torque for a 660 hp CAT 3196 is 44% greater than its torque at rated (2168 ft-lbs vs 1508 ft-lbs at rated). Also during dynamic performance, you have to consider the effect of inertial and torsional properties, which has the potential (and I stress "potential") of greater stress on the shafting system.

In PropExpert, we provide both the "standard" calculation, but we also show the reduction in Safety Factor for the maximum torque condition. We have seen cases where a shaft sized for SF=5 provides little more than SF=2 at maximum torque. So, please take a serious look at the power and RPM that you use in your shaft diameter calculations. And I hope that ABYC will do the same.