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

Chapter 10: Rudder Geometry, Shape, and Size

of the leading edge, and a gently convex trail-
ing section terminating at a point. In fact, for
sailboats and displacement power cruisers,
this is the ideal standard section shape. As
vessels become faster, however, this section
becomes less effective. Above 25 or 30 knots,
the blunt rounded leading edge of a tradi-
tional airfoil section causes too much turbu-
lence in front of the rudder.
At high speed, when the rudder is put
over, this blunt leading edge virtually tears
the water flow away from the blade, leaving a
swirl of eddies along the blade surface. Such
turbulent flow generates very little force or
lift and is, in fact, a stall, just like the stalling
of an airplane wing in too steep a climb.
Accordingly, as boat speed increases, the
leading edge of the rudder should be made
sharper and sharper, and the point of maxi-
mum section thickness should be moved aft.

Airfoil Rudder Sections—

Low to Medium Speed

Figure 10-6 shows typical sections that
work well for rudders on sailboats and on
displacement- and medium-speed power-
boats (up to about 18 knots).
There is a variety of suitable airfoil sec-
tions for use on rudders, but the NACA 0010
section pictured in Figure 10-6 works well.
Such a section has the theoretical minimum
drag and highest lift in applications like this.
Table 10-1 gives the proportionate half-breadths
at various locations along the section’s chord.
At 30 percent of the chord length aft of the lead-
ing edge—the thickest point—the half-breadth
is 5.002% of the chord length, and the rudder
thickness will be twice the half-breadth.
The additional drag of a fat rudder stock
(wider than the rudder-blade section) would be

unacceptable. Accordingly, if the rudder stock

has to be thicker than you can fit inside the

0010 section, make the section proportionally

thicker by multiplying the half-breadths at each

station by the required factor. For instance, if

the 0010 section is 30 inches (76 cm) long (the

chord) and the center of the rudder stock is at

17 percent chord, the half-breadths at the stock

will be 1. 38 inches and the thickness there will

about 2. 76 inches (69 mm). The rudder stock—

to be completely inside the blade—would have

to be no more than about 2. 25 inches (55 mm)

in diameter. You may, however, need a larger

diameter. Say that your calculations indicate

you need a 2^3 / 4 - inch (70 mm) stock; then the

rudder blade should be about 3^1 / 4 inches

(81 mm) thick at the stock—at 17 percent

chord. From that we can derive our required

half-breadth multiplier as follows:

3. 25 in. ÷ 2. 76 in. chord =1. 17

or

81 mm ÷69 mm chord =1. 17

and

10%× 1. 17 = 11 .7%

Thus you need to multiply all the section

half-breadths (and the tip radius) by 1. 17. The

“ 10 ” in the “ 0010 ” indicates the basic section

has a 10 percent ratio of thickness to chord.

You would end up, in this case, with an

11 .7 percent section, or a NACA 00–11.7 sec-

tion. The fatter section will have more form

drag than the 10 percent section; however, it

will have less drag than a 10 percent section

with a rudder stock projecting outside it.

Figure 10-6. Low-speed rudder sections

NACA Sections

Objects designed to optimize lift in fluid flow (a gas or a liquid)

are termed foils. This is the same as for airplane wings, and the

standard source for rudder and keel-foil section shapes is the

work of NACA (National Advisory Committee for Aeronautics).

(NACA was subsequently replaced by NASA.) The complete se-

lection of NACA foils can be found in Theory of Wing Sections,by

Abbott and Doenhoff, published McGraw-Hill and then by Dover

Publications, 1949, 1959.