Join Date: Feb 2000
Location: Annapolis, Md
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This was my response to an earlier discussion where a fellow wanted to convert a boat from a post hung to a skeg hung rudder that discusses some of points under discussion,
Changing a boat from a post hung to a skeg hung rudder has been done in the past. I once had a commission to design a modification to a foul- handling ketch with a metal sharpie or powerboat style counterbalanced rudder. This rudder was converted to a stern post- mounted outboard rudder with a partial skeg. The project was made easier by a number of things; first of all going to an outboard rudder with tiller simplified much of the process. We didn't need to build a larger rudderpost tube or add internal floor frames. Secondly the boat was a double ender allowing us to add an oak stern post that stabilized the skeg. Lastly the propeller was fully supported with a short skeg. We added a strut and lengthened the propeller shaft.
This approach would not work on a boat with a reverse transom. To do this on a boat with a reverse transom you will need to do an awful of work. You would need to build a new rudder with a larger diameter post. In one of the BB’s, I had run some numbers on the loads on counterbalanced spade vs. a non- counterbalanced skeg hung the torsion loads (in reverse) on the non-counterbalanced required a bigger rudder post than the spade rudder. You would need to find a way to solidly attach the skeg and distribute to loads into the boat. You would probably want to pass a vertical tube through the bottom to the underside of the deck and then build a fore and aft knee and athwartships floor frame. The skeg would be constructed around this post and glassed into the hull. I would not ballast the skeg in any way as will alter trim.
As far as whether this is a good idea or not, I would say it is not a good idea in any shape of form. To begin with the skeg will alter the balance and handling of the boat. In calculating the balance of a spade rudder the rudder does not figure in, only the canoe body and keel are used. But a skeg becomes a part of the lateral resistance. This would move the center of lateral resistance aft probably give you some lee helm. That is not a good idea from a safety standpoint. When you get hit with a puff of wind you want the boat to try to head up rather than head off toward the knock down.
More significant to this discussion is the question, “why would you want to do this? I have been sailing now for over 35 years. This whole business of an inherent advantage of a skeg or long keel over a post-hung spade is hogwash in the real world. Each have some advantage but properly engineered a spade rudder is handier in almost all counts. I have owned or had use of all three types of keel and rudder. I have found that skeg hung rudders have all of the disadvantages of both with few of the advantages of the spade. I have rarely caught a line in a prop or rudder but the worst case of that was a line that was caught in a long keel cutter.
As I have pointed out, the biggest loads on the rudder occur in backing a boat and while skeg hung rudders would logically seem stronger than a spade this is not always the case. To begin with the gudgeons and pintles on many production boats are such that the pin is a single sheer hinge rather than a double sheer hinge to facilitate removal of the rudder. A double sheer hinge is monumentally stronger. It gets worse if there is enough vertical play and the rudder can be driven upward enough to put the pin in bending. The whole strength of the skeg hung rudder is dependent on these hinges.
But beyond the simple mechanics of the hinges, the skeg itself is very difficult to build well. The skeg is one of those chicken and egg things. Skegs were originally molded as a part of the hull but fear that a damaged skeg could sink a boat led to some boats being built with a skeg built as a separate unit and glassed on. That didn’t last long. But the problem with the skeg molded as apart of the hull is that it is very hard to build. Remember you are trying to get a fully saturated glass resin matrix 3 or 4 feet down from the canoe body in a space too small for a man to stand in or other wise reach easily. I have helped out on a boat that had the lower pintle pull loose from the skeg. What we found was that the pintle casting was through-bolted through essentially dry glass with a small pond of resin at the bottom. There was a lot of dry glass and a lot of resin but no GFR to speak of.
The other problem is at the joint between the skeg and the hull. Typically, today the skin of the skeg turns sharply into the bottom of the counter. There is often a lot of layers of glass across the top of the skeg and the reinforcing around the rudder post, but even on some very well built boats there often isn’t any real structure to distribute the large moment at the top of the skeg. You would expect to see ring frames or floor timbers. Often there is only thick glass and that thick glass is flexing constantly. Fiberglass is very prone to fatigue and this is the perfect condition for fatigue, a stiff object pushing up and down perpendicular to the hull surface.
Does this make a Spade hung rudder any better? No, but it can be engineered better than many a skeg hung rudder is actually constructed. I personally like counterbalanced post hung rudders with their lighter steering loads and greater efficiency. I know the old canard about catching pot warps and the fear that they will bend or be damaged, but for the kind of onshore sailing that I do I like them better.
Lastly to better bearings and seals: Harken makes a great series of rudderpost bearings. They are designed for anything from a 30 or so footer on up to very bid boats. At least some of the Whitbread boats used the Harken bearings on their massively loaded post-hung spade rudders. The seal issue is one that relates more to the design of the steering system. If you insist on a wheel with the quadrant below decks or worse yet below the water line, then there isn’t must to help you. But if like me you prefer a tiller (I heard that groan) then it is easy to install a watertight tube between the deck and the top of the rudder post tube. On my Kirby I had a large diameter clear plastic hose inside the hose I had a couple snap ties which were enough to temporarily support the rudder if the rudder head connection failed.
By the way, who ever said that a post hung rudder was cheaper was mistaken. The torque and moment resisting rudder posts on a post hung rudder are far more expensive to build than a strut and its gudgeons and pindles.
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Curmudgeon at Large- and rhinestone in the rough, sailing my Farr 11.6 on the Chesapeake Bay
Last edited by Jeff_H; 11-30-2009 at 03:43 PM.