Flat Sections with
Tapered Ends
Much is made of the optimum airfoil section.
Such airfoil sections are best, assuming that
budget and construction method permit. The
fact is, however, that a simple flat section
(thick enough to completely house the rudder
stock) with the forward end rounded in an
ellipse and the trailing edge tapered (as
shown in Figure 10-6) works quite acceptably.
The increased drag is slight, and there’s little
practical difference in steering response.
Such rudders are much easier to fabricate of
laminated ply (or laminated ply and foam)
glassed on the outside. I have used this sec-
tion on many designs.Intermediate-Speed-Section
Rudders
As speeds move higher than 18 knots, the
airfoil section or flat section with tapered
ends begins to stall too soon at modest helm
angles. The effect can be accepted up to
about 25 knots, but between 15 and 30 knots
the intermediate-speed-section rudder is
optimal (Figure 10-7). The sharp leading
edge, the maximum thickness farther aft,
and the square, thick trailing edge make the
rudder work when cavitating at these higher
speeds. This rudder shape is hard to find
today and would usually have to be custom
fabricated.Wedge-Section Rudders for
High Speed
Huckins—as in the famous Huckins PT
boats—found that a true wedge-section rud-
der (Figure 10-7) gave the most reliable steer-
ing on most planing vessels cruising at 25 to
30 knots and higher. Such rudders come
nearly to a true point at the leading edge—
but with some slight thickness there for
strength—and terminated in a wide, squared-
off trailing edge. A wedge angle of about
4 degrees generally works best. Such wedge-
section rudders incur somewhat more drag at
dead center (going straight ahead) than do
airfoil-section rudders, but they are much
less prone to stalling and offer more positive
steering control at high speeds. Also, assume
the center of water force is at 40 percent of
mean chord aft of the leading edge (at hard
over).Parabolic-Section Rudders
for High Speed and
Minimum Drag
The modern variant of the wedge-section
rudder is the parabolic-section rudder
(Figure 10-7). You can see that it is similar
in concept to the wedge-section rudder;
however, instead of having straight sides in
section, its sides are curved in a gentle con-
vex parabola. Parabolic-section rudders
provide all the advantages of wedge-section
rudders, but with less drag. The difference
isn’t large, but it is enough to be worthwhile
where top speed or fuel economy is an
important factor.Real-World Comparison of
Intermediate-Section and
Parabolic-Section Rudders
Designer David Pugh reported the follow-
ing results comparing the intermediate- and
parabolic-section rudder shapes:I printed the page from your rudder draw-
ings and enlarged it to the proper scale for
our rudders. Then, I went out in the shop
and had some extra blades modified.PART FOUR: RUDDERS AND STEERING SYSTEMS
Figure 10-7. High-
speed rudder
sections