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Old 06-04-2009, 10:08   #121
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Many thanks for that. The MS was design to be a heavy boat with big tanks and long range. I gave it an overall shape that would result in the longest possible DWL for speed and deep deadrise midsection for a slow motion roll. Of course with that keel it won't go to weather very well but that's not what the client wanted.
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Old 06-04-2009, 10:30   #122
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Bob,
For what it is worth, probably nada, I liked the rudder arrangement much better when I saw the line drawing as opposed to what was shown on the earlier sail plan drawing. I don't like rudders without a skeg under them. I guess many sailors would disagree.

As far as the ability to go the weather, you can't have everything.
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Old 06-04-2009, 11:13   #123
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Cadence:
Skegs have advantages but so do rudders sans skegs. I have probably designed more boats with skeg hung rudders than any othger designer so I think I know skegs. In this case I opted for half a skeg so the rudder below the skeg could have some balance area to it. For me, this boat is just too big not to have some balance to the rudder. With half a skeg I can still manage three bearings on the stock. Left on my own I probably would have gone for an all carbon spade on this boat but I would prefer not to open that can of worms on this thread. I think it has been beaten to death already.
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Old 06-04-2009, 11:44   #124
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Bob,
Thanks for the input on balance. Would it not be possible to have the leading edge of the fully skeged rudder extend forward of the axis? seems to me about 15%?

Great concideration. No one wants an unbalanced rudder. Your the expert.
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Old 06-04-2009, 12:51   #125
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Cadence:
It might be but then the skeg would be directly in line with the flow over the rudder and I can't see how that woud be good thing. That's one of those things that you would have to try to find out. I don;t recall seeing that detail before so I suspect it does not work or it would heve been used more. It is an interesting idea though. I'm just not a big fan off skegs for a variety of reasons that I think you can find on other threads here.

Keep in mind that any rudder on a conventional full length skeg is "unbalanced". As boats get bigger they need some "power assist" so helm pressures do not get overwhelming.
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Old 06-04-2009, 13:22   #126
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Bob,
I don't think I had explained it right. A leading portion would catch water flow or prop wash to compensate for the pressure on the trailing portion of the rudder. Most power boat rudders are not piveted on the leading edge? I'd have to go back and research the subject. By skeg I an referring to the keel extention to the lower pivetal piont of the rudder.

It's always a pleasure hearing your comments.
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Old 06-04-2009, 14:37   #127
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Cad:
No, we are talking about the same thing. I also design power boats, I just don't brag about it. I just can't see a rudder, using my favorite NACA foil type, that would "mate" up to the hind end of a skeg in a graceful and effective manner. I suppose if the skeg were three feet away from the rudder that would work but not up close. I like to see the rudder working in clean water where it will be the most effective.

But go ahead and try it and let me know how it works out. I love fooling with things like that.
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Old 06-04-2009, 15:10   #128
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Bob,
I see somewhat what you are saying.

You are speaking of a more hydrodynamic rudder than I.

Power boats other than trawelers aren't my pref.. With a waterline aft tapered, tapered may not be correct canoe stern ,similar to your MS, with a square stern for room. Also for lift once the following sea has caught up with you.

Thanks for letting me plck you brain.

At my age, I guess it's academic.

Cheers,
Cad

Stern wave I don't like, for ecomony, nor being pushed down a following sea.
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Old 07-04-2009, 12:25   #129
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Bob,

I thought about it over night. Your naca foil rudder acting with the leading skeg section is actually a wing. Relying on the flow to create a vacuum on on side. How does that work with min. forword motion? Probably as a plan old slab rudder.
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Old 07-04-2009, 20:40   #130
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Cad;
Good point and the reason I like to come in "hot" so I keep plenty of water flowing over the foil. I treat all rudders like wings and I use NACA 0012 with max thickness at 30% chord. I vary the balance percentage between 10% and as much as 17% depending upon the size of the boat.
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Old 08-04-2009, 11:49   #131
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Bob,
Thanks for the education. I couldn't find anything on NACA0012 as far as hydro dynamics but could find it on aerodymanics, about the same other than viscosity and I'm sure drag.

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Old 09-04-2009, 09:33   #132
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Cad:
I use THE THEORY OF WING SECTIONS by Abbot and Doenhoff.

I've used that rudder foil for years and have yet to be shown there is a better one.
But if you find one let me know and I'll check it out.

If you use a full skeg and maintain a constant thicknews to the trailing edge of the skeg there is just no way you can preserve the rudder foil while accomodating that contant skeg/rudder junction. If you go with a constant foil section you will end up with what we used to call a "golf tee" rudder. It's amazing complex and difficult to build. I think Brit Chance did a few. I have always gone with the constant trailing edge skeg geoemtry in deference to buider issues. It works but a pure spade is more efficient and with all carbon construction I can design a monocoque rudder/stock/blade far stronger than any skeg hung rudder of traditional construction.
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Old 09-04-2009, 11:26   #133
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Bob,
Thanks again. I can see what you are speaking of as far as loosing foil efficiency when behind a skeg. I did find some hydrodymanic NACA 0012 info.. At a quick glance, you loose the effect of leading edge and place the compression forward of the rudder? You seem to sold on the strength of carbon construction.
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Old 10-04-2009, 12:33   #134
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Bob,
I think we got off the original subject but I've enjoyed it. May have boared some others. I went back and looked at rudder balance, we were speaking of the same thing.

What was it you said Niles replied? I changed the avitar to one I build from scratch. BRG design.
Do you have any Niles stories about Bruce? I noticed his site does not have this design on it? Was a good easily driven design.

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Old 10-04-2009, 14:19   #135
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Sorry I asked after looking at the BRG web site.
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