RUDDER

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DOGBREATH

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Good Day All, I am thinking of increasing the size of my rudder to improve steerage. The boat is 4-5 tons, 28ft long.and semi-displacement. The current rudder is 18" high by 12" wide. I can only increase the width. Any ideas on a new size? The rudder is flat steel plate. Many thanks.
 
There is a rule of thumb that the rudder on a displacement yacht should be around 15% of the under water area. You don't say what your steering problems are. A flate plate rudder is in itself inefficient and you might get improvements by building a new one with a streamlined shape. If it does not already have it, some balance area forward of the stock/pivot point can often lighten the load and improve steering. What have others done on your design of boat?
 
Thanks Tranona. The problem I have is having to constantly adjust to keep her straight. The steering feels light. There is no play in the cables or helm.
 
Quote:
"The problem I have is having to constantly adjust to keep her straight."
Yep, that's what rudders are for.
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Just noticed that your boat is semi-displacement - assume motor boat rather than sail boat so rule of thumb not necessarily relevant. Motor boats generally have as small a rudder as possible to reduce drag. Difficulty in keeping a straight course is probably more to do with hull design and trim than rudder size. Is the wandering in displacement mode or at speed?

One good thing with steel plate rudders is that you can always weld a bit extra on and try it!
 
Sounds like hull trim wrong - probably too much weight in bows. Try moving weight aft before altering the rudder
 
Thanks for all the input. Not enough horses to get her on the plane, 8 knts about top speed. Never thought about trim, as she seems to sit nicely. Are trim tabs a waste of time at low speeds? Would still like to extend rudder if anyone has an idea about dimensions.
 
If it has a planing hull shape that might explain the wandering at displacement speeds. A keel or skeg if it has not got one already will probably have more effect on straight line stability than a larger (in your case only longer, not deeper) rudder. Is the original builder/designer still around to ask advice?
 
Hull trim is almost certainly the problem: you have too much weight forward. Get some poly bin liners, and load about two dozen of them with sand or shingle - around 10kg per bag, or whatever you find easy to handle to total around 250kg.

Load them aft in your boat and see whether it solves the problem. It does? Now move them amidships about 2 at a time until you find the steering starts getting unmanageable again. Bring enough back to give nice positive steering again, then that is near enough the amount you need to take off the bows. If you cannot restore trim by shifting ballast or gear, then you need to arrange permanent ballast aft, usually in the form of iron bars secured in the bilges. You cna of course just leave te poly bags securely stowed aft.

You get the same effect on an rowing or outboard rigid dingy if you put too much weight forward and trim down by the bows - can easily become quite impossible to steer straight. Try it and see if its the same effect as you have in the boat.

Trim tabs on a planing hull are used to lift the stern and flatten the angle to stop it digging in on the plane. They would do nothing on a displacement hull.
 
Thanks Harry, There is hardly any weight in the bow, just a couple of berths. Fishing boat, so usually 3-4 anglers out on the aft deck.
 
" There is hardly any weight in the bow, just a couple of berths. Fishing boat, so usually 3-4 anglers out on the aft deck. "

That may be the problem then - too much weight aft, so the bows do not grip? Try getting your anglers to go up forward into the cabin, and see if that helps. I had a 25 ft boat like that once - virtually uncontrollable until I got round to filling up the large water tank in the bows. Problem solved.
 
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