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From: William Kidd (no email)
Date: Sun Aug 07 2005 - 20:05:57 EDT
While not particular to a Westsail 42 here is what I used to deal with this question:
main factors determining rudder torque are: vessel maximum speed, rudder surface area, and maximum rudder angle. The formula is torque = (1.67 X rudder area in sq. ft.X speed in knots squared X (0.37 X fore to aft length of rudder in feet) This formula will work for a rudder without a balanced section configuration. If the rudder has a balanced configuration then you must take into consideration the distance in feet of the total blade width afore the rudder post. (Subtract the length afore from the total length)
An example:
Boat maximum speed 9 knts
Width of rudder blade = 1.9 feet
Width of balance section = 0.3 feet
Ht. of rudder blade = 3.3 feet
Torque = 1.67 X 1.9 X 3.3 X 9x9 = 848 lb ft X
(0.37 X 1.9) – 0.3 = 0.403
848 X0.403 = 342 ft lbs.
In the above example a cylinder having a capacity of about 6.5 cu. In. capacity would work. The size of the steering pump in cu. In. per revolution is what determines the no. of turns lock to lock. In this example a pump of 2.5 cu. In./revolution would yield 2.5 turns lock to lock and one with 1. 25 cu.in./revolution would require about 5 turns lock to lock.
Hope this is of some help.
Steven Hodgson <> wrote:Greetings,
I need to calculate the rudder torque in order to determine the size
of hydraulic cylinder in a hydraulic steering system for a Westsail 42.
Does anyone have access to the formula or know the appropriate size of
cylinder and pump for a W42?
Thanks,
steve
Steve Hodgson
Box 51, Roberts Creek
B.C. Canada V0N2W0
Arohanui
Secret Cove Marina
VE7 LKS
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