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Discussion Starter · #1 ·
Hello,

I have been looking for a thread or other info on rudder rake in boat design but I cannot find much about it.

I am curious about the effect of rudder rake and why/how designers scale this variable in the design of the boats.

Most rudders are more or less vertical on modern boats/dinghies. Older boats with transom hung rudders are raked. Is that simply to conform with the shape of the transom where it is attached?

Why are other boat with keel attached rudder raked likewise.

One of the reasons I am curious is that my boat (a Columbia29) has a fairly extreme rake in the rudder and I am trying to understand why. Intuitively it seems inefficient, pushing a lot of water up for no reason as it is being turned.

But obviously I am neither a boat designer nor a fluid dynamics specialist so I can easily be completely wrong.

Can someone instruct me?

Florent
 

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Remember you're a womble
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A certain Mr George Cuthbertson has this explanation. Don't know if that's relevant or not. I'm sure a certain Mr. Perry would have something to say as well.

GC: Red Jacket was tank-tested at Stevens Institute. The model was showing a very strong weather helm, which I couldn’t understand. So Peter Desaix said, “Why don’t you tuft the model and have a good look at the water flow?” So we tufted it with strings and so on, and it was obvious there was a strong cross flow under the counter as the boat heeled. It was from the windward side to the leeward side, because the rudder was impeding the cross flow, and the cross flow was driving the stern to leeward and the bow to windward and giving the appearance of weather helm. We took the rudder off the model, ran it again, and the cross flow was just the same, but the boat balanced fine. That told me to absolutely minimize the connection between the rudder and the hull. That’s why for years we had a scimitar-shaped rudder to minimize the breadth of rudder at the hull to allow the cross flow to pass under the counter unimpeded.
 

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A certain Mr George Cuthbertson has this explanation. Don't know if that's relevant or not. I'm sure a certain Mr. Perry would have something to say as well.
That Cuthbertson quote is talking about the 'contact area' of the top of the rudder against the hull, not really about rake or lack of it. Our old Viking 28 had a then-typical 'scimitar' rudder seen on many C&C designs.. subsequently (in the late 70s and still today) most rudders' upper edges are in close contact with the hull (working as an 'end plate'???) - in many cases the top of the rudder appears specifically shaped to match the counter profile amidships.

So it appears the thinking about that changed significantly since then..

I'd guess that the rake seen on attached rudders is driven by the transom angle and/or the desire to remove some lateral area underwater by cutting back the trailing edge of the keel.. but the opposite exists too, notable the CS 27 whose rudder matches the angle of the reverse transom, and to a lesser degree the C&C 27MKIV... (and maybe the Hunt Paceship 26?)

On boats with a flat run, like our former M242, - another boat with minimal clearance between top of rudder and hull - the stock was 'raked' sightly but really at 90 degrees to the run of the underbody.. a nice feature that allowed us to 180 the rudder when backing up for better control.
 
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Remember you're a womble
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I meant to include another quote from the same article:
I also tended to sometimes over-rake the rudder. I was interested in getting the lateral plane as far aft as possible.
 

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Florent, you make a valid point.

I recall reading about a boat --I think it was one the Hiscock' Wanderers-- with a dramatically sloping rudder post, that steered so poorly that they had to make MAJOR modifications. I have a picture in my mind of the prop aperature way in front of the relocated rudder post, and what looked like a jury rigged rudder. Somebody made a mistake in that design.

This was the result of a one off design (I believe they were determined to have a transome hung rudder) that was "corrected" once the problem was seen.

I wouldn't have thought that your problem could be as bad as this, or it would have been picked up before production. But I have no experience with the Columbia 29, so obviously can't say.

I guess the reason for the design is aesthetics and perhaps ease of construction given a full keel.
 

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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.
 

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I try to get my rudders as far aft as practical and l like to keep the rudder stock perpendicuar with the hull profile in the way of the rudder. I am working with a rudder foil I know and I would like the flow of the water to be perpendicular to the rudder stock.
 

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Discussion Starter · #8 ·
Interesting.

So what I am gathering from all this is that the reason could be part tradition from when boats used transom hung rudders and part to reduce the keel length (since the rudder is attached to it) with the result of reducing the wet surface for speed as well as tuning the balance of the boat (again given the hypothesis that hanging the rudder to the keel is the design choice from the start)

I'm not saying my boat is bad, but I am curious about the philosophy behind the design.
 

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Sea Sprite 23 #110 (20)
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While Mr. Perry will gladly go on about how fins are better than long full keels.. he is right. If you look at the long full keels you will notice that many of them have a cut away forefoot and a highly raked rudder.

Look at this half hull of the boat I own.. a Sea Sprite 23.



Look at the shape of the full keel.. now shorten it from bow to stern.. you wind up with a fin. The designers of the day knew they had to shorten the keel to keep wetted surface down, but had to work with the technology and knowledge of the day, so they cut down the forefoot and angled the rudder back as far as they could to do so
 

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So.. here's an experiment.. ;)




Remove your existing rudder and refit with the one shown ( opposite rake and spade :eek:).. then tell us how different she feels/behaves.... ;) :)
 

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Discussion Starter · #11 ·
Haha. No way...

If I ever run out of projects maybe i'll work on a transom mounted spade rudder with a fitting to reduce the rake (essentially a windvane mount). Then I'll let you know the results.
 

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Sea Sprite 23 #110 (20)
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I do not mind the reverse rake (if I did, I wouldn't own a "crab crushing full keel" boat) and I like the protection the keel gives the rudder
 
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