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

Example: Our example boat displaces

78 tons. From the chart, that indicates 0. 9

rpm for the rudder. This is minimum rudder

speed, so I usually round up slightly to ensure

crisp response; use 1.0 rpm.

We found combined torque for the two

rudders was

5,165 ft.-lb.

or

706 kgm

Then

or

This is the power required at the rudder

stocks. If this were a hydraulic system, you

could assume the hydraulic motor is about

75 percent efficient and the hydraulic ram

(cylinder) is about 90 percent efficient—

0. 75 × 0. 90 = 0 .67, or 67 percent efficiency.

So

0 .98 hp ÷ 0. 67 = 1 .46 hp

or

0 .72 kW ÷ 0. 67 = 1 .07 kW

Add 15 percent to ensure the system

always operates at somewhat less than max-

imum; you get 1.68 hp (1.23 kW). So you

would need an engine or gen-set PTO or

hydraulic power pack that delivered 1.68 hp,

say 1.75 hp (1.3 kW). The final check would

be with the steering-gear manufacturer.

Backup Steering Gear: Emergency Steering

All cruising or oceangoing vessels, which

don’t have direct tiller steering, should have

emergency steering gear. On most average

sailboats and displacement cruisers, this is

usually best set up with an emergency tiller

that sockets down onto a squared-off head

on the rudder stock, accessed through a

deck plate (Figure 12-14). Remember that

the loads can be considerable. A pair of

strong pad eyes and a pair of cleats should

be permanently through-bolted in the cock-

pit or on deck where relieving tackle can be

fastened to them and from there to the eyes

at the end of the emergency tiller. This

allows you to generate the mechanical ad-

vantage required to actually steer with the

emergency gear. Because the loads are sub-

stantial, this all needs to be carefully engi-

neered and rugged. The system also has to

have clearance to swing—something sur-

prisingly often overlooked.

Passenger vessels and larger offshore

yachts with power steering require a com-

pletely separate backup steering system.

The power source and all hose piping and

related gear for the backup system need to

be different and wholly separate. IMO

regulations require only that the backup

gear be capable of steering the boat at half

normal speed or 7 knots, whichever is

greater. The important exception to the

backup steering requirement is for twin-

screw craft that can be effectively steered

by controlling the speed and direction of ro-

tation of the two propellers. On such ves-

sels, the twin-engines-and-propeller combi-

nation itself is considered to be the backup

steering gear.

kW

706 kgm 1 .0rpm

975. 17

== 0 .72 kW

×

hp

5,165 ft.-lb. 1 .0rpm

5,250

= 0 .98 hp

×

=

PART FOUR:RUDDERS AND STEERING SYSTEMS

Figure 12-13.
Minimum rudder
rpm