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  #11  
Old 07-03-2013
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Re: Spade rudders vs blue water passage making

Dropped it out of my boat about 5 yrs ago. did a new cutless bearing, PSS seal. On my oday the post is held up only by the through bolt on the steering quadrant. Digging a hole in the ground was necessary for it to drop out completely.. thing is heavy too!
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  #12  
Old 07-03-2013
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Re: Spade rudders vs blue water passage making

Quote:
Originally Posted by capecodda View Post
RichH...that's interesting. I don't think I've ever met anyone who dropped their rudder and did the inspection you describe. Sounds like a good idea before taking a long offshore trip on a less than new boat.

My recollection isn't perfect, but I think this rudder shaft was around 4" in diameter solid stainless. LH did lots of work in Taiwan, so I wonder about the quality of stainless.

I know Fontaine Yacht design has also done some of these shafts in carbon fiber. In talking with them, it was not only weight but also less bend, which reportedly made the bearing work better.
Solid shafting will not merit anything as essentially all or most of the bending stress is carried in the first few % of depth along the surface of the material. Crevice corrosion AND bending stresses which promote fatigue (cracks) because the endurance limit was exceeded .... once it penetrates into the macrostructure will cut even 'solid' metal like a knife through butter; same as how a puny little girl could rip apart an old fashioned thick phone book. When it comes to 'cantilevers' you want 'good' not 'solid'.

The reason pintle/hinge hung rudders are preferred for 'blue water' is they are less subject to bending stress because of their inherent (hinge) 'support'. Cantilevers are always 'bending'.
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Old 07-03-2013
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Re: Spade rudders vs blue water passage making

Quote:
Originally Posted by RichH View Post
Solid shafting will not merit anything as essentially all or most of the bending stress is carried in the first few % of depth along the surface of the material. Crevice corrosion AND bending stresses which promote fatigue (cracks) because the endurance limit was exceeded .... once it penetrates into the macrostructure will cut even 'solid' metal like a knife through butter; same as how a puny little girl could rip apart an old fashioned thick phone book. When it comes to 'cantilevers' you want 'good' not 'solid'.

The reason pintle/hinge hung rudders are preferred for 'blue water' is they are less subject to bending stress because of their inherent (hinge) 'support'. Cantilevers are always 'bending'.
good thread guys...I have been postponing my rudder repair alot...I guess because I probably need to haul out...I never liked spade rudders and have never owned a boat with one...after reading this discussion and realizing the forces at work I will probably never own one...for the type of sailing I like to do...but if I was a racer or stayed solely in the bay I would certainly not rule them out.
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Old 07-04-2013
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Re: Spade rudders vs blue water passage making

Quote:
Originally Posted by capecodda View Post
Yea, I agree with that. Forces are big on a spade, and in fact my current boat has a skeg hung rudder...although given the limited offshore we do, the primary advantage is catching a few less lobstah traps:

Quote:
Originally Posted by souljour2000 View Post
I never liked spade rudders and have never owned a boat with one...after reading this discussion and realizing the forces at work I will probably never own one...for the type of sailing I like to do...but if I was a racer or stayed solely in the bay I would certainly not rule them out.
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...



Last edited by JonEisberg; 07-04-2013 at 08:41 AM.
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Old 07-04-2013
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Re: Spade rudders vs blue water passage making

Forces on the rudder when it's skeg or keel hung are twisting forces.
Forces on a spade rudder are both twisting and bending so the spade rudder will require a much stronger stock. Of course the greater the span of the spade rudder the greater the bending moment. Increase in chord length increases the twisting moment.

I favor carbon fiber rudders for my new designs. I have never had a failure with one. The build process of the carbon fiber rudder results in a monocoque structure with the blade and the stock being unified into essentially one piece. Given the strength of CF it is easy to apply a very conservative safety factor without getting a huge rudder stock. The modest size of the CF rudder stock, even when conservatively designed, means the designer has more freedom in choosing the foils used. The CF stock also has the advantage of being easily tapered to fit within the chosen foil with max stock size at the hull where the bending moment is greatest. With older, s.s. steel stocks the required stock diameter often drove the foil selection.
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  #16  
Old 07-04-2013
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Re: Spade rudders vs blue water passage making

Yes ,rudders of any kind are often underbuilt but a rudder well built on a skeg is far stronger than one without a skeg. The advantages in peace of mind of overbuilt rudder fittings on an overbuilt skeg far outweigh any disadvantages of doing so. The weight difference is that of an extyr case of beer or two in the stern. While spade rudder boats do a lot of offshore cruising , I would not feel safe cruising in one ,over a boat with a well built skeg and rudder fittings, especially with all the Fukashima debris floating out there.. A low aspect ratio skeg can eliminate the chance of fouling your rudder as well.
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Old 07-05-2013
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Re: Spade rudders vs blue water passage making

From everything I have read... a Skeg or keel hung rudder is also less likely to stall. The Skeg and Keel hung rudders wind up more like the flaps on an airplane wing. While the Spade rudder does allow for faster low speed turning.. if you crank her over at speed, you risk a real chance of stalling and no turning at all.

But.. it is a matter of just knowing your boat. I know in my Seasprite 23, I -have- to turn the rudder harder over to tack as compared to the Grampian 23 my parents had
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  #18  
Old 07-05-2013
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Re: Spade rudders vs blue water passage making

"a rudder well built on a skeg is far stronger than one without a skeg"
Another silly statement from Brent Swain.
"Stronger in bending?
"Stonger" in twisting?
" Stronger" in your mind?

Engineering would say both rudders can be equally strong depending on the stock specs and the skin specs.
I know you don't like numbers Brent but sometimes an understanding of them can help.

I have designed more skeg hung rudders than any other designer. I now skegs. In 1973 when I designed the Valiant 40 I gave it a skeg hung rudder. I was attacked from all angles, "How dare you put a skeg hung rudder on a cruising boat". The arguments the skeg hung rudder are almost word for word what I hear now used against the spade rudder. But I was right back then and in no time it was very hard to find an offshore crusing boat without a skeg hung rudder. I know skegs. I know the arguments. Even Halber-Rassy went to skeg hung rudders.

But today I am equally as confident that the spade rudder is best for any boat. I notice that Halberg-Rassy now uses spade rudders in all their new models. I don't expect everyone to agree with me. That does not change my opinion. But I have a lot of spade rudder cruising boats out cruising and I have yet to have had a problem with one rudder.

So if you want to argue with me over rudder designs just keep in mind that for years almost every boat I designed had a skeg hung rudder. I know skegs and I know how they behave. I know their strengths and their weaknesses. I have designed both types.

If you are interested in my approach to rudder design I wrote an article for GOOD OLD BOAT about rudder design. I enlisted the help of several other designers in writing this article so a well rounded approach could be presented.
You can find the article in the July/August 2010 edition.
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Last edited by bobperry; 07-05-2013 at 01:55 PM.
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  #19  
Old 07-05-2013
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Re: Spade rudders vs blue water passage making

A friend had to replace the bearings on his Dash 34 with a spade rudder which had not all that many miles on her, and has never been offshore. It was expensive.
You can make a skeg as strong as you please, without a serious size or weight penalty, then use some greatly oversized gudgeons and pintles, with no real penalty. The weigh difference between going for 1 inch pins and1 1.2 inch pins is negligible, but the safety factor goes up tremendously. With a spade rudder you have more serious limitations.
As Bob points out ,a skeg hung rudder has a twisting load on ,over a much shorter distance ( the chord) than the cantilevered force on a spade rudder ( the length, a much greater leverage). The load on the hull bearing is huge ,and the catilevered load on the shaft at the top of the rudder, compared to the spread out loads on pintles, as it is on the shaft, and thus far more prone to fail on a spade rudder. If you have a pintle on the bottom of a partial skeg, it puts the bending load on the much stronger blade, rather than on the shaft .
Bob. What do you use for sleeves where the carbon fibre wears on the bearing? Bronze? Titanium , more affordable in Seatle and near Boeing Surplus? It seems that crevice corrosion under the sleeve would rule out stainless.
Then there is always the concern of fouling a line between the rudder and the hull, which is common sense which doesnt show up in computer calculations. A good low aspect skeg eliminates this problem, a liability on spade rudders.
On my first boat, a pipe dream designed by Kinny, in my early 20's ,I visualized a skeg hung rudder, 6 ft futher aft from the keel hung rudder specified. Then I thought "I have zero sailing experience, so I better stick to what the highly regarded designer specified. After sailing her accros the Pacific I did exactly that, a huige improvement.
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  #20  
Old 07-06-2013
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Re: Spade rudders vs blue water passage making

Brent: You are asking me to school you on rudder design? Can't see that happening.
There are good bearings and not so good bearings. Just because the bearing fails does not make the design of the rudder fail. I use,,,,,,,bearings exclusivey with great success.

Don't shop at Boeing Surplus Brent. That's more your style.
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