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

appropriate 10-degree in-angle. From then

on, the same tiller arms, drag link, and steer-

ing cylinder, installed the same way, are used.

You can use a 7-degree tiller-arm in-

angle for boats under a 10-foot (3 m) beam,

and a 10-degree in-angle for boats over

10-foot (3 m) beam. Actually, if you know the

actual turning radius, there’s a geometric

method for deriving a precise in-angle for

each craft, but such a specific turning radius

isn’t accurately knowable; for most ordinary

vessels, this 7- to 10-degree approximation

will be close enough for practical purposes.

Free-Trailing Angle

in the Slipstream

A further refinement is that rudders in the

helical flow of the water in the slipstream

don’t trail straight aft but angle slightly

(usually in) if disconnected from their link-

age and left free. You can see a somewhat

exaggerated drawing of this in Figure 12-19.

If the rudders are held straight when the

wheel is centered, the boat will go straight,

but the rudders will create a bit of extra

drag. One way to build this natural angle in

is to disconnect the linkage of one rudder

on a twin-screw boat when under way at

cruising speed and going straight. Mark the

angle the rudder takes naturally in the flow,

and hook that rudder back up to the steer-

ing gear. Repeat the process for the other

rudder.

Return to the dock, and adjust the drag

link length and attachment so that the rud-

ders are fixed at these free-trailing angles

when the helm is at straight ahead (cen-

tered). This will provide the minimum resis-

tance at speed. Combine this with the Aker-

mann setup, and you would have minimum

resistance at all speeds combined with maxi-

mum turning response.

Rudder Installation: Shaft and Propeller Removal

Shaft Removals, Anyone?

Two important practical rudder installation

considerations are shaft and propeller re-

moval. Many’s the time that I’ve seen rudders

installed just perfectly—just perfectly, that is,

if you want to force the mechanic to remove

the entire rudder every time he has to pull the

shaft. Should you ever want to bone up on

the latest nuances in sophisticated cussing,

make a point of being around on such an oc-

casion. Certainly, these are things you should

look for when inspecting a boat: Has the

builder arranged clearances for removing the

prop shaft easily? Is there clearance to pull

the prop?

Twin-Engine Shaft Pulling

On twin-screw inboard boats, there’s a simple

solution to pulling the shaft: place the rudder-

post centerline slightly off to one side of the

prop-shaft centerline so that the shaft can

slide right past the rudder. This is plain

enough, but you don’t want the rudder to be

too far from the prop center—you would be

out of the slipstream and lose steering

efficiency. A good rule is to place the rudder-

stock centerline 2.2 prop-shaft diameters

outboard of the shaft centerline, as shown in

Figure 12-20. If our boat had 1^1 / 2 - inch-

diameter (38 mm) shafts, its rudder stocks

PART FOUR:RUDDERS AND STEERING SYSTEMS

Figure 12-19.
Rudder free-
trailing angle in
slipstream