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

Chapter 11: Rudder-Stock Size, Construction, and Bearing Specification

NOTE: Nm has to be multiplied by 1,000 to
get Nmm.
Silicon bronze would be superior for any
boat that wasn’t aluminum or steel. The sili-
con bronze would not have stainless steel’s
susceptibility to pitting corrosion. Using
60,000 psi (413 MPa) UTS for bronze, the USS
is 36,000 psi (248 MPa). Entering this in the
preceding formula would give a required di-
ameter of 2. 59 in.—use 2^3 / 4 in. (65.7 mm, use
70 mm) silicon bronze.
NOTE: For convenience, we assumed
that this was a single-screw planing hull. In
practice, this would be most uncommon for a
boat of this size. Most (but not all) boats with
bearings above and below their rudder blades
are displacement or semidisplacement craft.
In this case, the water force on the rudder
would have to be calculated using a coeffi-
cient of lift (CL) of 1.2, as indicated in For-
mula 11-2 for standard rudder water force—a
significant increase in water force, but of
course, at lower operational speed.

Rudder Bearings and Rudder Ports

Rudder Bearing

Loads—Spade Rudders

All rudders must have at least two bear-
ings—an upper bearing and a lower bearing
(see Figures 11-1 and 11-2). Later we’ll take a
look at rudder bearings that are below the
bottom of the rudder blade (see Figure 12-21).
Here we’ll examine the loads on the upper
and lower bearings of a spade rudder. For
such rudders, the lower bearing is just inside
the hull bottom, while the upper bearing is
well above this in the hull. Within practical
limits, the higher the upper bearing can be in
the hull the better, as this reduces the bearing
loads.
Find loads (reaction forces) on rudder
bearings by using the following formula.

Formula 11-7. Load on Rudder Bearings

Where

P =water pressure on rudder blade, lb.

or kg

L =distance from upper to lower bear-

ing, in. or m

a =distance from center of water pres-

sure to lower bearing, in. or m

R1 =reaction force on upper rudder

bearing, lb. or kg

R2 =reaction force on lower rudder

bearing, lb. or kg

NOTE: R2 is negative because it operates in

the opposite direction of R1. R1 plus R2 must

cancel out to equal –P.

Example:For our example spade rudder

(Figure 11-1), we’ve already found that rud-

der pressure is 8,512 pounds (3,839 kg). Re-

ferring to the drawing, we see that a = 23. 8

in. (604 mm) and L = 29. 8 in. (757 mm). Then

NOTE: –15,310 lb. +6,789 lb. =–8,512 lb. The

net reaction in the bearings equals the load

(the rudder pressure) applied in the opposite

direction.

Or

R1 =3,063 kg× 9. 8066 =30,037 N

R2 =–6,902 kg× 9. 8066 =–67,685 N

NOTE: –6,902 kg +3,063 kg =–3,839 kg. The

net reaction in the bearings equals the load

(the rudder pressure) applied in the opposite

direction.

R1 3,063 kg

3,839 kg 0.604 m

0 .757 m

==

×

R1 6,798 lb.

8,512 lb. 23. 8 in.

29. 8 in.

==

×

Lower rudder bearing:R2

P

L

=− (L a)+

Upper rudder bearing:R1

Pa

L

=

×

Formula 11-7.

Dia.,mm

16 3,462 Nm 1,000 mm/m

352 N/mm

4

= 2

××

π

SSF

3

5 mm, use 60 mm 316L

stainl

⎛

⎝⎜

⎞

⎠⎟

= 85.

eess steel

R2

3,839 kg

0 .757 m

=− () 0 .757 m 0+.604 m =− 6,90 2 2kg

R2

8,512 lb.

29. 8 in.

=+=−−() 29. 8 in. 23. 8 in. 15 5,310 lb.