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  #21  
Old 07-06-2013
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Re: Spade rudders vs blue water passage making

A friend bought $1200 worth of air tools at Boeing surplus for $125 . Last time I was there, unused drillbits, with the plastic coating still on them, were $1 a pound. What do you call someone who buys the same stuff in a hardware store, at retail prices, in the same town?
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  #22  
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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.
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Last edited by Brent Swain; 07-06-2013 at 10:37 PM.
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  #23  
Old 07-06-2013
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Re: Spade rudders vs blue water passage making

"Tank tests have shown that a vertical or slightly raked foreward rudder is more efficient, and less likely to stall, than one raked aft. "

Did they really write "foreward"? Better check your references. I think the word is "forward".

Present the data and make it current. CURRENT. Back up your words with reality. I know that is a challenge for you.

Oh Brent, there you go again making things up again.
Open those eyes. Look around. It won't hurt. You are determined to be blind.

You had to build a Pipe Dream before you figured out the rudder was in the wrong place.
I was a kid when I saw that design and I knew it was an antique. Of course the rudder was in the wrong place. Thank goodness I did not have to build the boat to figure that out.
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  #24  
Old 07-07-2013
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Re: Spade rudders vs blue water passage making

Interesting stuff! Especially for someone with an "old" 1977, Bob Perry designed spade rudder on an amazingly well balanced little boat, the I-28. I have never sailed a boat with a nicer helm. That said, with the boat on the hard I will be dropping the rudder for a cutless bearing replacement. Time to do a shaft inspection, too!! I have already repaired the skin of this boat's rudder. When I got the boat the entire surface of the rudder was spyder cracked. I assumed it had filled with water and frozen. It had been on the hard for several years in New England. The cracks were evenly distributed. I drilled some holes and drained a lot of water. I ground down the cracks, not through the skin, filled them with an epoxy resin and tediously teased into place fg mat, then added a layer of fg mat in epoxy resin over the entire skin. I carefully sanded and faired the entire surface leaving a continuous epoxy/mat layer surface covering.

The stuff that drained out of the rudder the first year was septic. I was going to drill out some large plugs for inspection at that time but decided to see how things looked after a season of coastal sailing. I did inject foam into the holes I had drilled using a long feed tube placing the foam deep in the holes. I used a long 3/8 bit and drilled up from the bottom edge of the rudder as well as some holes up high. I repeated the hole drilling and foam injections for a couple of years and quit after no drainage or apparent "wasted" core showed up. No signs of the original surface cracks have ever appeared. I closed the holes with epoxy paste each time. It has worked so far!

Now I guess it is time for an exploratory dissection since I can take the rudder into my shop and a careful look. Where should I expect to find signs of failure as I inspect the rudder shaft?

Bob, have you ever heard of one of your I-28s loosing a rudder? Any suggestions?

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  #25  
Old 07-07-2013
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Re: Spade rudders vs blue water passage making

Down:
Never heard of an I-28 losing it's rudder.
I'm here to help any time I can.

I appreciate your kind words on the balance of your boat. I will go to bed now with that thought in my mind.

Bob P.
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  #26  
Old 07-07-2013
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Re: Spade rudders vs blue water passage making

There's a Sun-27 on my dock; another sweet-handling little boat. I think this Perry fellow may know a thing or two about yacht design.
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Re: Spade rudders vs blue water passage making

Quote:
Originally Posted by bobperry View Post
But today I am equally as confident that the spade rudder is best for any boat.
This shows that C&C and Pearson were ahead of their time in adopting the use of spade rudders in mass production coastal cruisers.
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Re: Spade rudders vs blue water passage making

James:
Nathaniel Herrshoff was using spade rudders on some of his his designs.
I think he predates Pearson and C&C.
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