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

no thrust bearings.) The thrust pushes the en-

tire engine forward, and that drives the boat

ahead through the engine mounts.

Calculating Thrust Loads

The thrust loads are large indeed. You can

calculate the thrust load reasonably closely

from the following formula.

Formula 3-1. Thrust Loads

or

Where

shp = shaft horsepower = 0. 96 ×bhp

kWshaft = power in kilowatts at the

shaft = 0. 96 ×flywheel kW

e = propeller efficiency—approximately

0 .55 for displacement hulls and 0. 70

for planing hulls

Thrust, kg

198. 3 kWshaft e

0. 9 knots

=

××

×

Thrust, lb.

326 shp e

0. 9 knots

=

××

×

PART ONE: DRIVETRAIN INSTALLATIONS

Formula 3-1.

NOTE: The 0. 9 is for an assumed wake

factor to get Va (prop speed through the water

in knots). Va will be less than boat speed be-

cause the boat drags a wake along with it as

it travels. Making a 10% deduction for this pro-

vides a good-enough approximation.

Example:A 32-foot (9.7 m) express cruiser

going 28 knots on twin 300 hp (224 kW) diesels

will be subject to a thrust of 2,608 pounds

(1,184 kg) along each shaft—well over a ton.

or

Engine Mounts, Thrust,

and Couplings

It is absolutely vital that the engine mounts

are able to transmit thrust load if you aren’t

using a system with a separate thrust bearing.

It’s thus equally vital that the shaft coupling

match the engine mounts. Soft mounts

should have very flexible couplings, slightly

flexible mounts should have slightly flexible

couplings, and rigid mounts should have rigid

couplings. I’ve seen straight-shaft installa-

tions with engine mounts that were so flexi-

ble that the entire engine was pushed/rotated

forward^1 / 2 inch (13 mm) underway from the

thrust. Imagine what this did for the shaft

alignment!

The flip side of CV joints or double uni-

versals is the rigid mount. There’s no give

anywhere. If all is aligned properly, this

works just fine. But it is noisier, as all engine

vibration is transmitted directly into the

structure. Figure 3-5 shows the engine of an

aluminum ferry that had a fully rigid installa-

tion. The boat ran reasonably smoothly in

terms of the propeller noise and was other-

wise efficient. Welded metal hulls being what

they are, however, every iota of engine noise

carried through the structure.

Thrust

198. 30. 96 kW 0. 70

0. 9 knots

=

×× ×

×

=

224

28

1,,184 kg

Thrust

326 0. 96 hp 0. 70

0. 9 knots

=

×× ×

×

=

300

28

26,008lb.

Figure 3-4.
Flexible engine
mount