that if more detailed calculations give you a

significantly smaller diameter, you should

check with the prop manufacturer to be cer-

tain that they don’t have to specially machine

the hub.

A 24-inch-diameter (610 mm) propeller

should have a 1.71-inch-diameter (43.6 mm)

shaft (use 1^3 / 4 in., or 44 mm) by this rule of

thumb.

You can make a more exact estimate by

dividing prop diameter by the factors from

Table 1-1. Thus a 4-bladed, 30-inch wheel

would use a 1.84-inch Aqualoy shaft (use

17 / 8 in. or 2 in. dia. Aqualoy). Or, a 4-bladed

762 mm prop would use a 46.7 mm diameter

(use a 48 mm dia. Aqualoy).

TABLE 1-1. RECOMMENDED DIVISORS

TO OBTAIN SHAFT DIAMETER FROM

PROP DIAMETER

Shaft Material

Aqualoy SS

Tobin Bronze or Monel

2-bladed propeller 14.5 18.1

3-bladed propeller 14.0 17.5

4-bladed propeller 13.1 16.3

The precise formula for determining

shaft diameter is as follows.

Formula 1-2. Shaft Diameter

or

Where

shp =shaft horsepower (assumed as 96

percent of rated brake horsepower)

kWshaft =shaft power in kilowatts

(assumed as 96 percent of rated kW at

the flywheel)

St =yield strength of shaft material in

torsional shear, in psi or kPa (kilopas-

cals; 1 kPa =1,000 Pa)

rpm =revolutions per minute at the

shaft after the reduction gear

SF =design safety factor

PART ONE: DRIVETRAIN INSTALLATIONS

321,000 403 shp 3 .0SF

70,000 psi 1,05

××

× 0 0 rpm

(^31). 74 in.,
use^3 / 4 in.dia.Aqualo

1yy
3,651
300 kWshaft 3 .0SF
482,000 kPa 1,

×

×

× 0050 rpm

(^344) .2mmdia.,
use45mmdia.Aqua

lloy
The CFR specifies a SF “approaching 10,”
which is usually taken to mean 7. 5.
Safety Factors
Light Gasoline Yachts = 2. 0
Average Yachts = 3. 0
Long-Range Motor Cruisers = 3 .5 to 4. 0
High-Performance Racing Boats = 4. 0
to 5. 0
Pilot Boats and Patrol Boats = 5 .0 to 6. 0
Commercial Inspected Passenger
Vessels = 7 .5
Example:For a vessel with an engine
rated at 420 brake horsepower (403 shp) at
2,100 rpm, and fitted with a 2:1 reduction gear
(giving 1,050 shaft rpm), the required Aqualoy
17 or 22 shaft would be
Shaft Dia.,in.
321,000 shp SF
St rpm

=^3 ××

×

Shaft Dia., mm 3,651

kWshaft SF

St rp

= ×

×

× mm

3

Formula 1-2.

Or

For a vessel with an engine rated at 313

kilowatts, flywheel (300 kW, shaft) at 2,100

rpm, and fitted with a 2:1 reduction gear (giv-

ing 1,050 shaft rpm), the required Aqualoy 17

or 22 shaft would be

Real-World Shaft Diameter

I always do the precise calculation for every

boat, but the fact is that the shaft sizes from

Table 1-1 (after rounding off for standard sizes

and fitting standard hubs) are usually so close

that I often wonder if it’s worth it. Indeed,

ABYC P-6 currently has a flat recommenda-

tion that the shaft should never be less than

one-fifteenth the prop diameter regardless of

material or propeller type. The more complex

full formula frequently gives smaller diameters,

which would be rounded up.