If your rudder fails, and somebody's will, you need a way to steer the boat. Does not have to be a rudder, but it really does have to work.
Excerpt from Transpac Questions and Answers
Bill Lee, Wizard Yachts Ltd.
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Q: My boat has a long graceful stern overhang with a small traditional transom. I
see no way to fit an emergency rudder to meet the emergency steering
requirement of section 4.15.
A: The actual requirement is “alternative methods of steering” of which an
emergency rudder is only one.
Another approach is to permanently mount an extra pin for the spinnaker pole on
the stern close to centerline with a universal toggle. In case of rudder loss, the
spinnaker pole is fitted on to the pin. 4 controls are needed, that being port,
starboard, up, and down. For port and starboard, run lines from the outboard end
of the pole through snatch blocks and to the cockpit winches. For up, swing a
spinnaker halyard around the mast and attach it to the end of the pole. For down,
attach a suitable amount of anchor chain to the end of the pole. You are not
going to keep seriously racing with this system, but if your main rudder breaks
you are probably done racing anyway. Two benefits of this system are that it is
not prone to breakage, and there is little risk of loosing the parts during
installation.
Regardless of what alternative method of steering are chosen, it is imperative
that the owner and crew have tested the system away from the dock and are fully
confident that it will in fact steer the boat.
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Bill Lee, the Fast is Fun Wizard. Legendary Santa Cruz boat builder whose outof-
the-ballpark successes include the record-setting Merlin as well as numerous
other boats optimized for Pacific Coast conditions. Currently matches boats to
people from his Wizard Yachts office in Santa Cruz
by Paul Kamen
PACIFIC CUP 1998 - Preparation Seminar No. 1
Emergency Rudder Design and Construction
Berkeley Yacht Club February 28 1998
Build blade like surfboard. Thick blade for strength and light weight. Moderately rough surface okay.
Keep gudgeons well separated to keep upper gudgeon lightly loaded.
For swim step transom, use stern pulpit to support top gudgeon.
Foam blank - "Lastafoam" medium density urethane foam boards available from Svendsen's in 1.5" x 4' x 8' sizes or cut fractions at $8.59 per square ft.
Epoxy: TAP Plastics 314 marine epoxy resin ($50.25/gallon) and 143 slow hardener ($33.35/half gallon). Or West System epoxy (West Marine or Svendsen's).
Glass: "Knytex" from Tap Plastics, or similar. This is a mat-cloth combination totaling 25.3 oz. per sq. yard. $12 per 36" of 50" wide material. Selvege tape lapped around leading and trailing edges. (Tech. contact at TAP: Russ Miller, manager at San Leandro, 510-357-3755.)
Rules for fiberglass/resin/foam work:
1) Always make a test patch
2) Cut glass carefully to size before mixing resin
3) Use a very good particle mask
Allow full rotational degrees of freedom at lower gudgeon during deployment. Only one bolt in rudder and one bolt in transom, fitted loosely. Additional bolts added after top gudgeon is in place to establish alignment.
F = A * Cl * 1/2 * RHO * V^2
F = force (lb)
A = area below transom (ft^2)
Cl = Coeff. of lift (use 3.0 to allow for pumping transients)
RHO = density of water (1.9905 slugs/ft^3)
V = design speed (ft/sec)
(1 knot = 1.6878 ft/sec)
F = 8.5 * A * V^2
F = force (lb)
A = area below transom (ft^2)
V = design speed (knots)
[example: 1 ft. x 4 ft. blade, 7 knots: F = 1,666 lb.]
M = bending moment (ft-lb)
L = distance from lower gudgeon to tip (ft)
F = maximum blade force at design speed
[example: L = 4 ft, so M = 3,332 ft-lb)]
Required "section modulus" = M*12/10,000 (the 12 is to change moment from ft-lb to in.-lb)
[example: SM required = 4.0 in^3
SM = section modulus (in.^3)
W = width of blade (in.)
T = overall thickness of blade (in.)
t = thickness of core material (in.)
[example: blade is 12" wide (but use 10" to account for shaping), core is 1.5" thick: By trial and error, use T = 2.02". SM = 4.02 in^3. So required thickness of fiberglass = 1/2 (2.02 - 1.50) = 0.26 in.]
FU = force on upper gudgeon (lb)
M = Bending moment at lower gudgeon (ft-lb)
D = distance between gudgeons (ft)
[example: For D = 6.0, FU = 3,332/6 = 555 lb]
F = force on blade (lb)
FU = force on upper gudgeon (lb)
[example: FL = 555 + 1666 = 2221 lb.]
For pins in double shear (as in turnbuckle clevis pins) use safety factor of 5 and look in rigging catalog for appropriate turnbuckle size. Or use allowable shear stress of 6,000 psi for same result.
A = 1/2 * FP/sigma (for double shear)
sigma = allowable shear stress (use 6,000 psi for 316 stainless)
FP = force on pintle (upper or lower, lb)
A = required area of pintle pin (in.^2)
Solve for required pin diameter = sqrt(4 * A / PI)
[example: A = 1/2 * 2,221/6,000 = 0.1851 in.^2; pin diameter = 0.486 in., use 1/2 in. diameter pin for bottom pintle. For top, 1/4 in. diameter is sufficient, but use 3/8 in. for easier alignment.]