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

or

Where

Ds =shaft diameter, in. or mm

E =modulus of elasticity of shaft mate-

rial, psi or MPa (megapascals; 1 MPa =

1,000,000 Pa) (see Table 1-2)

Dens =density of shaft material, lb./cu. in.

or g/cm^3 (see Table 1-2)

rpm =shaft revolutions per minute

Example: For a 1^3 / 4 - in. (45 mm) Aqualoy

22 shaft rotating at 1,050 rpm, we would find

or

The longer, more precise formula usually

gives greater allowable bearing spacing,

which is often helpful.

Alignment, Shaft Material,

and Shaft-Bearing Wear

Once you’re sure the bearings are properly

spaced, you can go on to check alignment.

The stern bearing can often be surprisingly

worn by poor alignment, or a worn or dam-

aged stern bearing can cause misalignment

itself.

Properly installed Cutless bearings

should last many years. When you grab the

shaft or propeller and shake it vigorously, you

should feel virtually no play or motion. The

bearing itself should appear smooth—with-

out cracks, low spots, dents, or tears. Any of

these visual signs of wear or any wiggle in the

shaft indicate a new bearing is required. Wear

on one side or one end of the bearing indi-

cates an alignment problem.

The standard rule is that a shaft should

align with the couplings within 0. 001 inch for

every inch of coupling outside diameter, or

0 .001 mm for every mm of coupling diameter,

using a feeler gauge. Thus a 4-inch diameter

coupling should be within 0. 004 inch or

within 1/250th of an inch, and a 100 mm di-

ameter coupling should be within 0.10 mm.

I prefer Monel or Aqualoy stainless shafts.

They can be a bit smaller in diameter and a bit

lighter with similar shaft spacing than Tobin

bronze, and they’re less subject to corrosion.

(Note that Aqualoy is a trade name for

chromium/molybdenum stainless alloys fabri-

cated and sold for shafting by Western Branch

Metals. Similar alloys are available under the

Aquamet trade name and others.)

Improper alignment of engine beds or

bearings can cause incredible Cutless bearing

wear. The arrow in Figure 1-11 shows a seri-

ously degraded bearing.

Propeller-Shaft Overhang

Another source of shaft vibration is excessive

overhang at the propeller. This is an odd one.

On several occasions, I’ve examined boats

that had several inches between the forward

face of the prop hub and the aft face of the

strut bearing housing. It ought to be obvious

to anyone that this much overhang puts side-

bending loads on the shaft, which leads to vi-

bration—sometimes quite severe. Try not to

have more than one shaft diameter in dis-

tance between the hub and the bearing hous-

ing. With spurs or similar rope cutters in-

stalled, this may have to be pushed up to one

and one-half to two shaft diameters, but more

than this is just asking for trouble.

PART ONE: DRIVETRAIN INSTALLATIONS

Figure 1-11. Worn
Cutless bearing

Ma x.Bearing Spacing, m

Ds

85 .4rpm

3,984 E

=

×

×

×

DDens

4

3 .21 1. 75 in.

1,050 rpm

28,000,000 psi

0. 285

×

×

lb./cu.ft.

(^4) = 7 .28 ft.or 87. 4 in.ma x.bearring spacing
45 mm
85 .4 1,050 rpm
3,984 194,500 MPa
× 7. 889
×
×
gg/cm
(^42) .23 m, 2,230 mm max.bearing spaci
3 = nng