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Shaft size

When designing a propulsion system, the size of the shaft becomes an important consideration. Yes, there are formulas that are used:

Shaft diameter formula

D=(321000*SHP*SF/St *RPM)^ .33

Where

D = shaft diameter ,inches

SHP = Shaft horsepower

SF =Safety factor ( 3 for yachts , 8 for heavy commercial use)

St =Yield strength in torsional shear ,in PSI

RPM = Revolutions per minute of propeller shaft

Shaft bearing spacing formula

L = ( 3.21 * D/ RPM)^0.5 * ( E/ Dens)^0.25

Where

L = Distance between bearings ,in feetD = Shaft diameter ,in inches

RPM = Propeller revolution per minuteE = modulus of elasticity of shaft ,in PSI

Dens= Density of material in pounds per cubic inche.

SHAFT MATERIAL CHARACTERISTICS

Shaft material                                  Yield strength                              modulus                       densitiy lb/ci.in

Stainless steel 304                                         20000                                        28 000 000                                 0.286

Aquamet 17                                                     70000                                         28 500 000                                 0.284

Aquamet 18                                                     60000                                        28 800 000                                 0.281

Aquamet 22                                                    70000                                         28 000 000                                 0.285


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