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 Post subject: rudder stalling
PostPosted: Sun Apr 28, 2013 10:05 am 
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I am sailing a nethercott and am slowly getting better and am doing less swimming, but could do with some help. heres the problem.

In about 10 to 15 knots of breeze and going upwind, when a gust comes along I get a bit of weather helm and then the rudder stalls. the boat slows dramitcally and I am left on the end of the seat not going anywhere.

I have tried easing the main and trying to steer out of it, with limited success.

So am wondering if I should change the rudder shape or make it bigger?

Any one got an easy fix?


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 Post subject: Re: rudder stalling
PostPosted: Sun Apr 28, 2013 3:23 pm 
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Joined: Mon Sep 12, 2005 7:45 pm
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Location: United Kingdom
Smoothing and polishing the rudder might make a bit of difference, depending on how rough it is, and its readily done at home in the evenings and its never a bad thing to do.
But if you can keep her flatter that will help, IC rudders are pretty tolerant of heel and so on, but there are limits.


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 Post subject: Re: rudder stalling
PostPosted: Sun Apr 28, 2013 8:12 pm 
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The rudder does look a bit sad, so I will give it some tlc. the boat isn't heeled that far over when it stalls , the deck on the leeward side is just at water level. I didn't think the rudder angle was that great either when it let go may be 15 degrees.

I will get the thing looking a bit more pretty and try again. I have a new one in progress which is a different shape and a bit bigger, and am wondering if I am doing the right thing

thanks for the help


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 Post subject: Re: rudder stalling
PostPosted: Mon Apr 29, 2013 7:17 am 
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Joined: Mon Sep 22, 2003 7:28 pm
Posts: 225
Location: United Kingdom
Hi Bill
Post a picture of the old one and you will get plenty of advise, they do stall out from time to time but should not really happen too much, normally down to rudder shape.

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 Post subject: Re: rudder stalling
PostPosted: Mon Apr 29, 2013 10:23 am 
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Joined: Thu Jul 17, 2003 10:08 am
Posts: 148
Location: United Kingdom
Yes, I used to have this problem...

I made a bigger and cleaner foil and things seem to work much better now. I also put the shaft point a little further forwards to say 35% chord rather than the 45% it was before, giving it a bit more feel.

I do reckon that a smaller foil is obviously a bit faster, but no doubt that a bigger foil is easier to use when you are learning.

I think a bit of the issue (may be) that most IC rudders are pretty 'balanced' and you don't get as much 'feel' from the tiller. I think when you are learning it is easy to over-steer, when you need to learn to have a very gentle hand.

cheers

eib

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 Post subject: Re: rudder stalling
PostPosted: Wed May 01, 2013 8:12 am 
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Thanks for the advice,
I have patched up the old rudder, and am working on a new beast. the blade is a tad bigger and deeper, and hopefully has a better shape. I have also moved the rudder shaft further forward so it is less balanced. I did find that once going at a decent pace the boat was very twitchy, I hope by moving move blade behind the shaft it will need more force to move it when going quick. It is so lite to touch at the moment that I'm not sure if the boat has weather helm or not.


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 Post subject: Re: rudder stalling
PostPosted: Wed May 01, 2013 8:24 am 
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Joined: Thu Jul 17, 2003 10:08 am
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Yes, that was my experience with my first rudder.


Then new rudder, whether because of being slightly bigger or bringing the shaft forwards gave the boat much more 'feel' and I found this much easier to sail with. Whether it is faster/slower I just don't know...

I have always been curious why IC rudders seem to often be 'over-balanced' or with the shaft at 45-49% or as near as damn it the middle. What is the advantage to this? JimC? DaveC? others?

eib

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 Post subject: Re: rudder stalling
PostPosted: Wed May 01, 2013 2:24 pm 
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Joined: Mon Sep 22, 2003 7:28 pm
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Hi
Not sure why pins are put so far back, perhaps because of structural issues with getting the pin in? In any case the further forward the better in my experance, and a bit of rake on the front edge so a good amount is behind the pivot point. Have not had this style of rudder on a canoe for some years but from memory about 600mm long 200mm wide, or am I far out? my blade off the back sits about 600 deep in the water and is 130 wide tapering to 40mm, but works fine when 50mm up.

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 Post subject: Re: rudder stalling
PostPosted: Mon May 06, 2013 11:19 pm 
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Joined: Fri Dec 22, 2006 10:32 pm
Posts: 511
Location: United Kingdom
I've gone through many rudder designs and came the conclusion that for through the hull designs the status quo works best. This is basically a NACA 0012,14 profile and using a pivot which needs to be a 16mm OD carbon rod filled with epoxy and carbon going as far into the blade as possible - this is what you need to take the load which if the rudder stalls at full pelt equates to about 400kg of load at the pivot point where it enters the hull. This is a big load which is why this shaft design has been adopted as the standard. The other problem is stalling when the boat heels and the blade ventilates. These parameters give us the design features we have. Small rudders are fast but do not work well in a breeze when the boat slows through a tack. Laminar flow blades stall badly with horrible effects and put massive loads on the rudder. The only bearing surface which works as the rudder shaft comes through the rudder stock is stainless steel. An 18mm od tube slid over the carbon tube above the top of the rudder. The placements of the shaft is then limited by the thickness of the blade but raking the blade backwards does seem to stabilise the foil at speed and also reduces tendency for weed to get caught on the leading edge.

Lack of feel is more to do with the profile,I have found, thinner blades stall at speed and low speeds but are much lower drag. Raking the blade backwards seems to help feel by placing as much rudder behind pivot point as possible.

Placing the rudder on the transom is completely different on which I can not comment.

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 Post subject: Re: rudder stalling
PostPosted: Sat Aug 24, 2013 10:32 pm 
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Joined: Sun Apr 28, 2013 12:01 am
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thanks for all the information. over the winter I have remade the old rudder and also made a new one. I also went through all the photos I had of the boat sailing, and it appeared that in every photo I had a lot of weather helm. so I have now played around with the mast rake and have got the mast standing a lot more upright (less rake).

I got my crash test dummy (son) to take it for a test sail, hardly any wind but it looked faster, the helm is somewhat twitchy he tells me, but he was sailing a Europe the day before. best speed was 14kph wind was very light.

Not sure what to do about the twitchiness .


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 Post subject: Re: rudder stalling
PostPosted: Sun Aug 25, 2013 9:10 am 
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Joined: Fri Dec 22, 2006 10:32 pm
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Location: United Kingdom
Jim

From my experience this is caused by having too fine an entry on the leading edge of the rudder or the pivot is too far back.

Make sure you have a very rounded entry. If it is parabolic and sharp as per a laminar flow profile you seem to get stalling on this edge at high angles of attack. You can check and avoid this by making only subtle changes in rudder angle when sailing in flat water. More abrupt changes in angle will make the rudder feel vague and can start a full stall where you get plumes of water flying out the back of the boat.

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