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

Chapter 12:Rudder-Stock Angle, Control, and Installation Considerations

Formula 12-1. Wheel Radius

or

The drum or sprocket radius is the radius
of the rope drum or chain sprocket, at the
steering wheel, which accepts the steering
cable. Drums can be any convenient radius
(at least 16 times rope diameter) for rope.
The smaller the drum or sprocket radius,
the greater the mechanical advantage, but the
harder it is on the rope or cable. Most modern
wheel steerers use bronze or stainless steel
roller chain over chain sprockets instead of a
simple drum. For boats up to 35 or 40 feet
(10 or 12 m), sprocket radiuses are usually 1. 1
inches (27.94 mm), and for boats larger than 40
feet (12.2 m), 1. 2 inches (30.48 mm).

Finding the Right Wheel

Trying a 12-inch (304.8 mm) quadrant radius
on our example boat works out as follows:
Using a 1.1-inch sprocket you would get

Use a 22- or 24-inch diameter wheel.

Or

Using a 27.94 mm sprocket, you would

get

Use a 570 or 600 mm diameter wheel.

You could fit a 28- or 30-incher (700 or

750 mm) if you preferred—more mechanical

advantage from a larger wheel never hurts.

Larger boats frequently have to use larger

wheels to increase the mechanical advan-

tage. In fact, you can increase mechanical

advantage by increasing wheel radius or

quadrant radius (effectively tiller length) or

both. Increasing quadrant radius also

increases the number of turns it takes the

wheel to swing the rudder, which brings us to

helm quickness or helm speed.

Quick Turn? Helm Speed

There’s another important consideration with

wheel steering: How quick is the helm? If it

takes, say, 30 turns of the wheel to swing our

boat’s rudder from hard-over port to hard-

over starboard, the steering response is too

slow—way too slow. On the other hand, if the

rudder swings from hard over to hard over

with just a half turn of the wheel, the steer-

ing response is too quick. You would tend to

turn the wheel too far and every little jerk of

your hand would cause a course change.

The ideal number of turns for different

types of craft is shown in Table 12-1.

The number of turns from 35 degrees

hard over to 35 degrees hard over—also

Wheel Radius, mm

Rudder Torque, kgm 1,000 m

=

× mm/m

Drum or Sprocket Radius, mm

13 .5kgHan

×

ddForce Tiller Length or

Quadrant Radius,

×

mm

TABLE 12-1.OPTIMAL NUMBER OF HELM TURNS BY BOAT TYPE AND LENGTH

Type of Boat LOA, ft. LOA, m Number of Turns

Runabouts and Small Power Cruisers to 30 9 1.75 to 2
High-Speed Powerboats all lengths all lengths 1.75 to 2
Medium-Speed Power Cruisers 30 to 50 9 to 15 2.5 to 3.5
Medium-Speed Power Cruisers over 50 over 15 3.5 to 4
Large-Displacement Power Cruisers* over 50 over 15 as many as 14
Sailboats 16 to 30 4.8 to 9 1 to 2
Sailboats 30 to 45 9 to 15 2 to 3
Sailboats over 45 over 15 3 to 5

*Traditional large-displacement power cruisers with cable steering can get by with a high
number of turns; however, fewer turns (between 3.5 and 5) is preferable.

Wheel Radius,in.=

Rudder Torque,in.-lb.×Druum or Sprocket

Radius,in.

30 lb.Hand ForceeTiller Length or

Quadrant Radius,in.

×

Wheel Radius,in.

3,650in.-lb. 1. 1 in.

30 l

=

×

bb. 12 in.

11. 15 in.

×

=

Wheel Radius,mm

42 .1 kgm 1,000 mm/m 27.94 m

=

××mm

13 .5 kg 304.8mm

285 .8mm

×

=

Formula 12-1.