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

PART FOUR:RUDDERS AND STEERING SYSTEMS

are the standard and most rugged method of

arranging a twin-rudder setup. If extreme

space considerations make it necessary, how-

ever, microprocessor-controlled systems can

control separate cylinders on each rudder

without a connecting drag link. Usually this

solution is found on hydraulically steered

catamarans, but it can be applied to mono-

hulls if necessary. Kobelt is one manufacturer

of these systems.

A Reservoir on High

A critical component for hydraulic installa-

tions—particularly those with power-assist or

powered autopilots installed—is a reservoir

tank (or headertank) built in at the highest

point in the system (Figure 12-10). This will

reduce the chance of air in the system and

make bleeding and installation much easier.

Large-Vessel Hydraulic

Steering

On larger boats, it becomes difficult to get the

mechanical advantage required with a small

enough number of turns for sufficiently quick

steering. The solution is power-assisted hy-

draulic steering. For instance, the 28-knot

twin-screw planing hull we used for our

rudder-stock calculations earlier has a

combined torque on the steering system of

5,164 ft.-lb. (706 kgm, 6,924 Nm). A review of

the manufacturer’s literature showed that we

couldn’t get down to four turns lock-to-lock

on a manual hydraulic steering system and

stay under 30 pounds (13.5 kg) at the wheel.

Accordingly, we would go to a power-assist

system. These systems use a hydraulic pump

(driven with a power takeoff, or PTO, on a

main engine) to amplify the power from the

wheel. If the power system fails, you still

have manual hydraulic steering, just slower

than ideal. A jog stick can be fitted at the

helm to directly activate the power helm

pump for quick maneuvering in close quar-

ters (Figure 12-11).

Power for Power Steering

Since larger vessels require power steering

(Figure 12-12), you need to be able to estimate

the power required by the steering gear. Mini-

mum rudder rpm (rate of turn) for boats of

varying displacement can be read off the graph

inFigure 12-13. (Under International Maritime

Organization [IMO] regulations for large ships,

the minimum rudder speed is from 30 degrees

port to 35 degrees starboard, and vice versa,

in 28 seconds or less. This is 0.387 rpm.)

Formula 12-5. Power for Power Steering

or

Where

torque= total steering torque, ft.-lb. or

kgm

rpm= revolutions per minute of rudder

motion

hp= power required at the rudder

stock(s)

kW= power required at the rudder

stock(s)

kW

torque, kgm rpm

975. 17

=

×

hp

torq ue , ft.-lb. rpm

5,250

=

×

Figure 12-10.
Hydraulic steering
with header tank
(Courtesy Kobelt
Manufacturing
Co., Ltd.)

Formula 12-5.