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

PART FOUR: RUDDERS AND STEERING SYSTEMS

All hydrofoils—rudders are really hydro-

foils, not simply “boards”—are more efficient

if they are long and narrow rather than short

and squat. It’s important to be able to

describe this feature clearly, as well. It is

measured as aspect ratio, as shown in the

following formula.

Formula 10-1. Aspect Ratio

Where

Span= vertical height of rudder blade,

ft. or m

Area= area of rudder blade, sq. ft. or m^2

(NOTE: This is sometimes referred to as

the geometric aspect ratio, since the so-

called mirroring effect of the hull above the

rudder roughly doubles the effective aspect

ratioin many cases. For our purposes, we’ll

stick with aspect ratio as simple geometric

aspect ratio.)

The higher the aspect ratio, the deeper

and narrower the rudder. To quickly estimate

the area of the rudder, just measure its chord

(fore-and-aft length) at midspan (height) and

multiply by the span at midchord (the aver-

age height).

If our boat’s rudder has an area of

1 .9 square feet (0.176 m^2 ) and an average

height or span of 2.1 feet (0.64 m), its aspect

ratio is 2.32 to 1, or simply 2. 32. The calcula-

tion looks like this: 2 .1 ft.× 2 .1 ft. ÷ 1 .9 sq. ft.

= 2. 32 :1, or 0.64 m× 0 .64 m ÷ 0 .176 m^2 = 2. 32 : 1.

This is a good, deep, narrow rudder for a

powerboat. An aspect ratio higher than 2.4 or

so gives a still more efficient rudder blade but

adds excessive bending strain on the rudder

stock. Such rather high-aspect ratios should

generally be avoided on powerboats.

Sailboat-Rudder Aspect

Ratio

Aspect ratios on sailboats are somewhat

greater than the preceding. An aspect ratio

somewhere between 2.2 and 3. 5 is normal,

tending to be higher for spade rudders on fin-

keel boats. Very high-aspect rudders are

becoming common on fin-keel performance

sailboats with high-aspect fin keels. Such

craft often have rudders with aspect ratios

between 3.5 and 4. 5. Even higher aspect

rudders are found on some all-out race boats.

Generally, I don’t recommend aspect ratios

over 4.5 as best practice; the bending

loads on the deep rudder’s stock are much

higher, and improvement in rudder response

becomes increasingly less as the aspect ratio

gets very high.

Rudder and Hull Work

Together

In turning, a boat pivots roughly around

its combined center of gravity and the center

of water pressure on the hull forward.

Aspect Ratio

Span

Area

=

()^2

Figure 10-1.
Rudder plan
geometry

Formula 10-1.