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 Post subject: Centreboards ans Rudder shapes ans profiles
PostPosted: Mon Oct 11, 2010 10:11 am 
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Joined: Tue Jun 02, 2009 8:22 pm
Posts: 98
I've been looking at making a new rudder and centreboard for the new IC project and need advice from all you wizz kids out there. Here is the problem:
Centre board / rudder leading edges according to Frank Bethwaithe should be near vertical ,however there is a trend that in many highspeed class the leading edge of both foils is tapered back towards the trailing edge in a circular arc from about half height. A lot of existing canoe rudders have this feature already. Also check out any pictures of the F 40 cats rudders. Stern hung canoe rudders apprear to be near parallel.
Is this largely fashion or has there been a break through ? You do not see any aeroplane wings with these profiles.


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 Post subject: Re: Centreboards ans Rudder shapes ans profiles
PostPosted: Mon Oct 11, 2010 3:17 pm 
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Joined: Fri Dec 22, 2006 10:32 pm
Posts: 522
Location: United Kingdom
With my numerous attempts at rudder design using Alistairs very narrow i found when mountd vertically the rudder was very twitchy and stalled out very easily on a high speed reach. I made a new one and canted it back by about 20 degrees and found it to be a lot more stable and much less probe to stalling. I did make a few other adjustments however so this was not a controlled experiment but I the rudder does have a much better feel to it now and certainly hess much less load on it as my pivot problems have now gone as well.

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 Post subject: Re: Centreboards ans Rudder shapes ans profiles
PostPosted: Mon Oct 11, 2010 9:26 pm 
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Joined: Fri Aug 01, 2003 2:26 pm
Posts: 164
Airplane wings operate in a simpler environment.
Plan form is one of the really obvious variables, but probably one of the least significant. Symmetrical shaping, area and section choice are all a bigger deal. I think practicality trumps any small improvements that might be made. Although we certainly are using much smaller boards then when you and I started in the class!
The reason for a step taper in the board is to have the board fill the hole in the bottom of the boat. I do this for what I deem to be the reasonable range of settings, which is something like half the span.
A 50% taper also seems to be a good idea.
I used to taper the bottom around the maximum thickness, such that that the max t was in a straight line. However I would off set the leading edge taper and trailing edge taper to approximate something like an elliptical plan form.
The point of tapering the leading edge and keeping the trailing edge straight is to sweep the max camber line ( which makes the blade a bit more stall tolerant) and ( in theory) make the tip vortexes a little bit better.
On the rudder, you need to have enough meat to get the shaft installed. If you want a minimally this rudder (which is something I like) it helps if the top 150mm or so is parallel sided and aligned with the shaft. If you sweep the leading edge aft, you can get the center of pressure far enough back that you can put the shaft at the maximum thickness of the blade up top and not end up with an over balanced rudder. I also like deep skinny rudders because I like to keep their influence on the wake to a minimum. Something in the % of displacement the rudder blade is at any section and the curve of areas haunts my sleep.
SHC

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 Post subject: Re: Centreboards ans Rudder shapes ans profiles
PostPosted: Mon Oct 11, 2010 10:05 pm 
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Joined: Mon Sep 12, 2005 7:45 pm
Posts: 757
Location: United Kingdom
Steve Clark wrote:
Something in the % of displacement the rudder blade is at any section and the curve of areas haunts my sleep.

I'm not at all sure what you mean by that Steve. Please could you elaborate?


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 Post subject: Re: Centreboards ans Rudder shapes ans profiles
PostPosted: Wed Oct 13, 2010 8:42 pm 
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Joined: Fri Aug 01, 2003 2:26 pm
Posts: 164
Jim, it is probably a mash up of a number of different theories that I have got hold of part of but not all of them. and then it's a way I can describe what I see happening around a hull and do something about it.
I guess the source is the Whitcomb Area Rule, which more or less says that the volume of the fuselage and appendages of an airplane should be the same a s a Sears-Haack body. For me this turned into the goal of having the curve of areas stay fair even when the volume of the board and rudder are added in. This way the hull and appendages work as one body instead of 3 bodies and is more efficient. The board isn't much of a problem because it is usually at the biggest part of the boat and so is a relatively small addition to the immersed volume at that station. The rudder, on the other hand is back at the shallow pointy end where it easily can be more than 50% of the immersed volume. When the rudder or daggerboard are "too big" you see their wake within the wake of the hull and I postulate that the interference of wakes is slow.
So as rudders get further aft, I try to make them skinnier and smaller because I think they are less drag that way and I like to think that a clean uniform wake is an indication that the flow over the hull is more right than wrong.
SHC

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