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

Chapter 10: Rudder Geometry, Shape, and Size

well faired. Both can cause vibration and tur-
bulence, and both can lead to loss of speed
and reduced rudder response.)
The sole advantage of a flat-plate rudder
is that it’s cheap and easy to build. This is
the reason they’re so common on workboats.
To my mind, this is false economy, as steering
control is integral to the crew’s experience
of a boat during every moment of operation,
and crisp, reliable helm control improves
safety. Regardless, flat-plate rudders do
work and are acceptable if cost-saving is
important.
Rudder-blade thickness should be calcu-
lated per the following formula.

Formula 10-4. Rudder-Blade Thickness

or

Where

S= stiffener spacing, in. or mm

kts= boat speed, knots

Maximum allowable stiffener spacing is

Max. Stiffener Spacing, in., steel or stain-

less= 6 + (plate thickness, in.× 48)

or

Max. Stiffener Spacing, mm, steel or stain-

less= 152. 4 + (plate thickness, mm× 48)

Max. Stiffener Spacing, in., aluminum=

4 + (plate thickness, in.× 32)

or

Max. Stiffener Spacing, mm, aluminum=

101. 6 + (plate thickness, mm× 32)

Example:Let’s say we have an 18-knot

boat, with a steel flat-plate rudder:

Try^3 / 8 in. (10 mm) plate. Then

6 + (0. 375 in.× 48)= 24 in. maximum

spacing

or

152. 4 + (10 mm× 48)= 632 mm

maximum spacing.

Try 10-inch (260 mm) stiffener spacing:

or

Stiffener thickness should be the same as

the rudder-blade plate thickness.

Stiffener height (thickness athwartships)

on either side of the rudder blade should be

0. 6 × diameter of the rudder stock

or

0. 06 × mean rudder chord

whichever is greater

Though the stiffeners are commonly of

constant height from leading edge to trailing

edge (rounded at the ends), they can be

tapered. If tapered, the maximum height is

centered at the rudder stock and continues

for 15 percent of mean chord fore and aft.

The stiffeners can then taper to 60 percent of

maximum height at the leading and trailing

edges.

Plate Thickness, mm 2. 54

Skts

666

=+

×

Plate Thickness,in. 0. 1

Skts

666

=+

×

Figure 10-9. Basic flat-plate rudder

Plate Thickness,in. 0. 1

10 in. 18 kts

666

0

=+

×

= .. 37 ;use 3/8in.,OK

Plate Thickness, mm 2. 54

260 mm 18 kts

666

9

=+

×

= ..56 mm;use10mm,OK

Formula 10-4.