for the foil sections discussed above. Note
that the section thicknesses are half-breadths.Metal Flat-Plate Rudders
Many commercial vessels and some dis-
placement motor cruisers are fitted with
metal flat-plate rudders. Though these can
work acceptably at low speed, I recommend
against them. Flat plates are the least effec-
tive shape for generating lift (turning force)
at all angles and at all speeds. This means
more rudder angle for the same turn. It also
means slower helm response and possibly a
larger steering gear.In addition, the round rudder stock runs
outside down all or most of the height of the
flat-plate rudder blade. This causes further
turbulence and loss of steering effect.
Figure 10-9 shows a small and rather
crude stainless steel flat-plate rudder (with-
out stiffeners) on a single-screw lobster yacht.
You can clearly see the erosion of the paint
around the rudder stock from the turbulence.
This rudder, though of adequate size, gave
very unsatisfactory steering response. (Note
also that there is too much overhang from the
propeller hub to the stern bearing, and that
the trailing edge of the rudder aperture isn’tPART FOUR: RUDDERS AND STEERING SYSTEMS
TABLE 10-1. FOIL-THICKNESS FORM DIMENSIONS
Y as a Percent of Chord*
% Chord** 00-10 Intermediate Parabolic
Tip Radius 1.100 NA NA
0.00 0.000 0.410 0.410
1.25 1.578 – –
2.50 2.178 – –
5.00 2.962 1.338 1.727
7.50 3.500 – –
10.00 3.902 2.212 2.519
15.00 4.455 2.986 3.043
20.00 4.782 3.641 3.421
25.00 4.952 4.167 3.691
30.00 5.002 4.554 3.892
35.00 4.920 4.803 4.056
40.00 4.837 4.930 4.205
45.00 4.625 4.948 4.350
50.00 4.412 4.871 4.492
55.00 4.108 4.715 4.628
60.00 3.803 4.493 4.754
65.00 3.428 4.220 4.867
70.00 3.053 3.911 4.968
75.00 2.620 3.580 5.062
80.00 2.187 3.242 5.152
85.00 1.697 2.906 5.238
90.00 1.207 2.572 5.324
95.00 0.672 2.240 5.408
100.00 0.105 1.908 5.492
Boxes Indicate Region of Max. Thickness
* Y is the half-breadth thickness as a percentage of chord length at each location along the
rudder-blade section. In a 00-10 section, the half-breadth at 30% chord length would be
5.002%× chord length. Thickness is twice the half-breadth.
** “% chord” refers to percentage of chord length back from the rudder’s leading edge.