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

called lock to lock—is found by using the

following formula.

Formula 12-2. Number of Turns Lock-

to-Lock

Where

Quadrant radius, in. or mm

Drum or sprocket radius, in. or mm

Wheel diameter doesn’t appear here. It

doesn’t affect the helm quickness at all.

Additional wheel diameter simply increases

mechanical advantage or power.

Example: For our example boat, helm

quickness works out as

or

This is right for our boat, which is about

35 feet (11 m) LOA. Playing around with

these formulas will enable you to pick the

right combination of quadrant or tiller size

and wheel and drum diameter for any boat.

Steering Cable Installation

Cable steering’s drawback is the complexity

of its installation. The cables take consider-

able loads for long periods of time. What’s

more, those loads oscillate back and forth.

Further, the cable constantly runs over

sheaves at about the same spots, which is the

same as bending it back and forth. Flexing

anything repeatedly will break it. To reduce

this potential weakness, the cables must be

run over large-diameter sheaves. Minimum

sheave diameter is 16 times the wire diameter

or greater—as large as 32 times wire diame-

ter is excellent if you can fit it. All the sheaves

and wire or rope runs must be carefully

aligned to ensure smooth operation with firm

cable tension. Often, if the wheel isn’t close

to the rudder stock in the cockpit, several

sheaves will be required on each side to make

the cable run correctly from the quadrant to

the wheel. All must line up exactly. For exam-

ple, you should be able to run a rigid dowel

through the grooves of the quadrant to the

turning sheaves on either side of the quad-

rant. The dowel must line up perfectly, with

no bend or angle at all.

If the sheaves get out of line or the cable

goes slack, the steering wire or rope can jump

out of its grooves and jam—essentially a to-

tal steering failure. Clearly, this also means

that the steering sheaves must be fastened

very securely with heavy through-bolts and

substantial backing blocks. If you’re dealing

with a boat that’s fitted with cable steering,

you should check to see that it’s equipped

with all the preceding components. There

also must be hatches, panels, and so on to

give access to all the steering-gear compo-

nents: the sprocket or drum, cable sheaves,

tensioning turnbuckles, and quadrant or tiller.

If the boat is lacking in any of this, you’ve got

a steering problem waiting to happen.

Fluid Power: Hydraulic Steering

These days cable steering is found less fre-

quently on powerboats. Hydraulics have

largely taken its place (Figures 12-4, 12-5, and

12-6). It’s not that hydraulics are especially bet-

ter; rather, they are easier to install, particu-

larly with more than one helm station. Most

sailboat skippers, in fact, don’t care for

Number of Turns Lock-to-Lock

Quadrant Radiu

=

ss

5 .14 Drum or Sprocket Rad× ius

PART FOUR:RUDDERS AND STEERING SYSTEMS

Figure 12-4. Hydraulic steering: 1 helm,

1 rudder (Courtesy Edson Corp.)

12 in.

5 .14 1. 1 in.Sprocket

2 .12 turns lock

×

= --to-lock

304 .8mm

5 .14 27.94 mm

2 .12 turns lock-to-

×

= llock

Formula 12-2.