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power the shaft “sees,” so shaft horsepower (SHP). Our shaft diameter is in inches, so we have to multiply the 33,3000 pound-feet per minute by 12 inches per foot to get 396,000 pound-inches per minute. From this we arrive at:

We can now substitute this relationship for torque in the diameter-from-torque formula we found earlier:

Here you have the 321,000. It is the result of combining the constants:

, which rounds to 321,000

We can now insert the safety factor (SF) in the formula:

This safety factor (SF) or design coefficient is included to account for wear and damage, manufacturing defects and other unknowns, which will affect the shaft over its opera-tional life. The standard constant for light gasoline pleasure craft is 2, for average yachts is 3, going up to 7.5 for com-mercial inspected passenger vessels. (See Boat Mechanical Systems Handbook for more details on the safety factor.) Built into these safety factors is a factor of 4/3rds or 1.25 for the reduction in strength caused by the keyway. Larger vessels—using interference fit instead of a keyway—can thus reduce the applicable standard safety factor by subtracting 1.25.

Having followed along this far, you may be wondering—as I have for years—why this particular expression of the shaft

diameter formula is the one commonly used. It would si plify the formula still more to extract the 321,000 (really 320,983.7) from inside the cube-root sign, or:

That would simplify the expression of the shaft-diameter formula to:

It’s exactly the same formula, yielding precisely the sam results, but—with the smaller constant—is a bit easier to Since simpler formulas reduce the chance of error, I’d re ommend that ABYC adopt this simpler expression in the next update of ABYC P-6.

2 RPMx T SHP =

396, 000

Combining the constants gives

RPMx T SHP =

63, 025 Or

63, 025 x SHP T =

RPM

3 3

3

16T 16 x 63, 025 x SHP Dia. = =

ST x ST x RPM Combining the constants gives

321, 000 x SHP Dia. =

St x RPM

16 x 63, 025

= 320, 983.7

321, 000 x SHP x SF 3 Dia. =

St x RPM

3 16 x 63,025

= 68.469, use 68.5

3 SHP x SF Dia. = 68.5 x

St x RPM

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