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Old 07-06-2013
Brent Swain Brent Swain is offline
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Re: Spade rudders vs blue water passage making

Quote:
Originally Posted by JonEisberg View Post
Seems to be a commonly held perception, that "the forces" are somehow generally larger on a spade rudder, relative to a traditional attached rudder, or skeg hung...

Among the most critical "forces" one should be considering in terms of rudder design for an offshore boat, are those that are transmitted from the rudder, to the helm, through the various components (quadrants, cables, etc) of the steering system... These are the things that are most likely to be overworked to the point of failure during an extended passage, rather than a complete failure of the rudder itself... And, a balanced spade rudder, being the most 'efficient' in hydrodynamic terms, will result in a far 'lighter' helm, and will as a rule introduce considerably less load into the overall steering system, than more traditional low-aspect 'barn door' style rudder attached/trailing a keel or skeg...

My boat original rudder was skeg hung, and as a result was not balanced, the shaft being very close to the leading edge... About 10 years ago, I rebuilt it with a new, far more substantial shaft. And, with the generous and superb advice of our resident expert Jeff H and others, I eliminated the skeg, and built up the leading edge of the rudder to 15-18% of the chord length forward of the shaft - the generally accepted optimum point to achieve hydrodynamic 'balance' in a rudder...



The result was a MASSIVE improvement over the original, unbalanced design... And I was reminded last winter after running a Cape George cutter south - a boat of equivalent size and displacement to mine - with its attached rudder requiring a huge tiller that swept the entire length of the cockpit, how much heavier the loads can be on such a comparative 'barn door', and how much less efficient they can be... Making a course correction in a heavy quartering sea in such a boat, required FAR more force than what would have been required in my own boat, with a more 'nimble' split underbody configuration with a balanced spade...

Personally, I think that a partial skeg extending perhaps 30% down the length of an otherwise high-aspect, balanced rudder, represents the ultimate overall solution in a bluewater rudder... Rich H is spot on in his earlier comments, it's not rocket science to properly engineer and size the components of a spade rudder, it's just gonna cost you... And, many of the failures of rudders we hear about on passage, were in boats that might not have been designed for such service to begin with, and were 'under-built' in this regard... whenever the "Production vs. Blue Water" debate rears its ugly head, I think rudder engineering is the principal category in which many production boats fall short, and where problems are likely to develop...

Something along these lines - a partial skeg, and a partially-balanced rudder - is what I'm trying to describe...


Catalina 27s could use this kind of change, as their rudders are extremely unbalanced, with a huge part of them well behind the shaft. Puting a gudgeon and pintle at the bottom of that partial skeg would greatly increase the strength of it, by taking much of the load off the shaft, and puting it on the skeg, which could be greatly overbuilt, without any penalty for doing so. It would also eliminate the chance of a line getting between the skeg and the rudder.
Tank tests have shown that a vertical or slightly raked foreward rudder is more efficient, and less likely to stall, than one raked aft.

Last edited by Brent Swain; 07-06-2013 at 11:37 PM.
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