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post #1 of 6 Old 08-14-2008 Thread Starter
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Rudder Rake

Just wondering about the relative hydrodynamic efficiencies of the rudder rake angle. I notice that many daysailers and smaller boats have the bottom of their rudders raked aft while most larger boats have vertical rudders. And older full-keel boats have the rudder raked forward. I assume the vertical is the most efficient, but what are the relative merits/problems of raked rudders (assuming the boats are well balanced with their rudders as designed?)

My boat has a traditional raked transom with an outboard rudder, causing the rudder to angle forward from top to bottom. She balances well, I have no complaints other than the enhanced weed catching capability. As an experiment, I tried adjusting the kick-up rudder blade so that it was vertical but this put too much area behind the pivot point and created so much weather helm as to be unmanageable.

Henry
Chiquita - 1974 Macgregor Venture of Newport 23

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post #2 of 6 Old 08-15-2008 Thread Starter
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Thanks for the link. I'm still trying to digest the information it contains. I don't think it actually answers my question as the rudder angles it deals with are "angle of attack" -- i.e. how far off the centerline is the rudder turned to produce the highest amount of lift and the least drag. My question is about the fore and aft rake of the rudder.

It did confirm my intuitive sense that a longer rudder provides more lift and less drag than a shallow one. I recently replaced my rudder blade with an Idasailor foil. I could have had it cut to the original length but opted to use the full 48" blank, which is almost a foot longer than the original. I can always cut it down later if it causes any problems, but so far so good.

To simplify my original question: Ignoring the benefit of increased rudder draft, would my boat perform better if the rudder was vertical instead of raked forward? Why or why not?

One additional piece of information: the boat has a swing keel which rakes aft. Combined with the forward rake of the rudder, the profiles of the two together form an open ended "V"?

Henry
Chiquita - 1974 Macgregor Venture of Newport 23

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Last edited by heinzir; 08-15-2008 at 11:02 AM. Reason: Clarification
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post #3 of 6 Old 08-18-2008
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Henry, 'cause nobody else has pitched in, after doing a fair bit of research a while back before modifying our rudder, I'll add my $0.002 - at current exchange rates for what it's worth..

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Originally Posted by heinzir View Post
To simplify my original question: Ignoring the benefit of increased rudder draft, would my boat perform better if the rudder was vertical instead of raked forward? Why or why not?

One additional piece of information: the boat has a swing keel which rakes aft. Combined with the forward rake of the rudder, the profiles of the two together form an open ended "V"?
Assuming the keel position doesn't change, the thing that matters most is the Center of Effort (CoE) and it's proximity to the turning axis of the rudder - basically a straight line through the pintles. To keep the boat going in a straight line, the lift from the rudder (rudder force) must equal the lift from the keel, less any sideways forces acting on the sails - one reason yachts need bigger rudders than stinkpots!

Anyhoo, assuming a rectangular blade shape and a typical NACA cross-section, the CoE will be approximately 1/3 of the way back from the leading edge ie. just forward of the center of the blade.

If the CoE was precisely on the turning axis of the rudder, you would have the least amount of turning force on the tiller (ie. it would feel sloppy). If the CoE is too far from the turning axis of the rudder, the turning force on the tiller would be too great one way or the other (too far aft = weather helm; too far forward = lee helm). As it turns out, you want a bit of weather helm (to be safe), so most boats have the turning axis right at or very near the front of the rudder blade - certainly within the first 1/3rd.

It then follows that if you move the rudder toward the keel, for a given amount of rudder force the turning rate would increase (ie. the boat turns faster) if you move it back, it turns slower.

Thus, if your rudder is currently raked forward, I suspect that moving it to a vertical position (CoE slightly further aft) would make little or no difference, but would probably not be better.

Apologies for the long reply..

Cameron

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"Honestly, I don't know why seamen persist in getting wrecked in some of the outlandish places they do, when they can do it in a nice place like Fiji." -- John Caldwell, "Desperate Voyage"

Last edited by Classic30; 08-18-2008 at 03:40 AM.
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post #4 of 6 Old 08-18-2008
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If the designer did his job, the rudder should at an angle which is efficient, and balanced so the helm is not heavy. We used to adjust the rudder angles when racing Hobie Cats, and it was amazing how much a little change made in performance. I also remember observing the Olson 30's evolve from early models which drove like trucks to later models which were balanced, with the rudder post angled slightly more forward. Of course, the builder claimed that all the boats were the same 'one-design'. Same thing was happening with J-44's, where the owners had to buy replacement rudders or get weightlifters to steer.
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post #5 of 6 Old 08-18-2008 Thread Starter
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Cameron,
Thanks for the explanation. I was not planning to make any changes to the rake angle anyway. I would have to add an extension to the lower pintle attachment to make it more vertical. I was just curious if the forward angle of the rudder was detrimental to performance since it is opposite the aft angle on my daughter's Sunfish and many other small boats.

Don,
Thanks for the reply. Macgregor boats were built with price, not performance as the main criteria. My 23 has a sharply raked transom with an outboard rudder which makes the rudder angled as well. They also made 17', 21', 22', 24', and 25' models with more or less plumb sterns and vertical rudders. Interestingly, my 1890s "character" boat will outsail them all!

Henry
Chiquita - 1974 Macgregor Venture of Newport 23

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post #6 of 6 Old 08-19-2008
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Quote:
Originally Posted by heinzir View Post
Cameron,
Thanks for the explanation. I was not planning to make any changes to the rake angle anyway. I would have to add an extension to the lower pintle attachment to make it more vertical. I was just curious if the forward angle of the rudder was detrimental to performance since it is opposite the aft angle on my daughter's Sunfish and many other small boats.
It's a complicated design problem that sorts the good Naval Archi-types from the bad, I guess (note: I'm not one).

My guess is that the aft angle on some small boats has nothing to do with performance, but is intended to (a) improve slow-speed manoeverablity in surf and (b) minimise damage to the blade upon hitting the beach. I say this because higher-performance dinghys most always have vertical rudder blades and are pigs to turn on approach to a beach.

A forward-raked rudder like yours might be more manouverable, but you better not hit anything with it!

Happy sailing!

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"Honestly, I don't know why seamen persist in getting wrecked in some of the outlandish places they do, when they can do it in a nice place like Fiji." -- John Caldwell, "Desperate Voyage"
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