Dave Gerr - Boat Mechanical Systems Handbook-How to Design, Install, and Recognize Proper Systems in Boats

Though one may follow ABYC’s one-
fifteenth rule, I have questions about it.
For instance, given a 30-foot-waterline
(9.14 m), 16,000-pound-displacement (7,259 kg)
hull powered by a single 90 hp (67 kW) diesel
running at 1,800 rpm, with a 2. 86 :1 gear
(629.4 shaft rpm), standard prop calculations
would indicate a 32-inch (813 mm) 3-bladed
prop with a 0.50 DAR. Using Tobin bronze in
the ABYC formula, we’d get a 1.87-inch
(47.5 m) shaft (use 2-inch or 48 mm). This is
16 to 1, which—to meet the one-fifteenth
rule—should be rounded up to 2^1 / 8 - or
21 / 4 - inch (55 mm), as available. If we took that
exact same boat and gear and refitted
it with a 5-bladed, 0.97 DAR prop, we’d get
the same shaft diameter from the calculation,
but prop diameter would be smaller—
29 inches (736 mm). It would be delivering
the same power, torque, and even a smidgen
less thrust, but it would then pass the
one-fifteenth rule at 14.5 to 1. This appears
illogical to me. Though I can’t state ab-
solutely that shaft-diameter ratios smaller
than one-fifteenth are safe, I believe that
ABYC’s one-fifteenth rule should apply only
to 3-bladed propellers with Tobin bronze
propeller shafts. I recommend that the
diameters from Table 1-1 be considered the
minimum instead of using the simple one-
fifteenth rule.

Shaft-Bearing Spacing

The simplest rule of thumb for shaft-bearing
spacing is that bearings should be no less

than 20 times the shaft diameter apart and no

more than 40 times the shaft diameter apart.

By this rule, 1^1 / 2 - inch (38 mm) shaft should

have bearings spaced no less than 30 inches

(760 mm) and no more than 60 inches

(1,520 mm) apart. The reason for the mini-

mum spacing is to avoid excessive rigidity, but

this can be overcome in most boat installa-

tions by using a Drivesaver disk or equivalent.

Another rule-of-thumb formula follows.

Formula 1-3. Maximum Shaft-Bearing

Spacing (approximate)

or

For a 1^1 / 2 - inch (38 mm) shaft, this gives

or

The precise formula for maximum bear-

ing spacing is as follows.

Formula 1-4. Maximum Shaft-Bearing

Spacing (precise)

Ma x.Bearing Spacing, ft.

3. 21 Ds

rpm

E

=

×

×

DDens

4

3 8 mm Sha ft Dia.

3. 6

= 1 .71 m, or 1,712 mm

4. 61. 5 in.Shaft Dia.= 5 .63 ft.,or 67. 5 inn.

Ma x.Bearing Spacing, m

Shaft Dia.,mm

3. 6

=

Ma x.Bearing Spacing, ft.= 4. 6 Shaft Dia.,inn.

Chapter 1: Propellers and Shafts

Formula 1-3.

Formula 1-4.

TABLE 1-2. SHAFT MATERIAL MECHANICAL PROPERTIES

Yield Strength

Shaft Torsional Shear* Modulus of Elasticity (E) Density

Material psi kPa psi MPa lb./cu.in. g/cm^3

Aqualoy 22 70,000 482,000 28,000,000 194,500 0.285 7.889

Aqualoy 19 60,000 414,000 28,800,000 200,100 0.281 7.778

Aqualoy 17 70,000 482,000 28,500,000 198,000 0.284 7.861

Monel 400 40,000 276,000 26,000,000 180,600 0.319 8.830

Monel K500 67,000 462,000 26,000,000 180,600 0.306 8.470

Tobin Bronze 20,000 138,000 16,000,000 111,200 0.304 8.415

Stainless Steel 304 20,000 138,000 28,000,000 194,500 0.286 7.916

* Yield strength in torsional shear generally taken as 66 percent of yield strength in tension.