Weather helm/rudder design

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nathanlee

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Hi all,

Last year I suffered quite severe weather helm in high winds (30+). A strong gust would round the boat up, and seemingly nothing I could do would stop it. The helm became very heavy, and spilling the main didn't seem to help once the "round up" had got going.

From this I've decided that the main needs a third, deep reef, but on top of that given some further research I think the rudder has a design flaw.

The rudder on my Corribee is a balanced spade rudder. It's balanced not far off 50%.

I've read an account of a chap fitting a newbridge skeg moulding to his boat and it doing wonders for the issues I've described, but I am reluctant to go down this route since fitting a skeg is going to be a serious project.

As such, I've decided to experiment with the rudder. I've made (am making) myself a new one with slightly more surface area (nothing excessive) and intend to mount it with a refresh bias aft, of about 2/3. I'm hoping that moving the centre of effort aft a bit will help.

What are your opinions? Am I doing the right thing?

Cheers,

Nathan
 
Years ago, I modified the rudder on my 33ft 3/4 Tonner with very good results. In my unqualified way, I beleived the problems were caused by the rudder configuration.

Rudder was raked aft, with a square shaped foot and a full-length skeg. The more she heeled (e.g on a 'shy' kite reach) the rake of the rudder meant that it was trying to lift the stern rather than steer. Eventually it would 'stall out' and a windward broach ensued.

I removed the skeg. I then modified the rudder to an elipse (Spitfire wing) shape. I gave it about 15% extra area, mostly forward of the stock to make it balanced, but also at the foot where the elipse was added to the squared foot.

Finally, I moved the rudder tube slightly forward and brought it more 'upright' to alsmost eliminate the huge rake that was previously there.

The results were dramatic. Much better control, steering not stalling when heeled. Oh yes, and a better Rating (waterline measured to the rudder stock;)

So, based on my (amateur) experience, I guess what you need on your rudder is more draft, more area and possibly less rake.

Hope this helps.
 
Before you go down the route of building a new rudder, why not modify the existing one with a ply sheath with a solid leading edge,shaped, and increasing the forward edge or extending the after trailing edge in the proportions you think are right, after all, a few self tapping screwholes are easily filled to try an alternative shape. Rudolph above suggested that the leading edge was the improvement in balance so perhaps try that option first when increasing the area.

I assume that you've already checked mast rake etc?

Did they ever build the Corribee with skegs from scratch?

ianat182
 
ianat182 said:
Before you go down the route of building a new rudder, why not modify the existing one with a ply sheath with a solid leading edge,shaped, and increasing the forward edge or extending the after trailing edge in the proportions you think are right, after all, a few self tapping screwholes are easily filled to try an alternative shape. Rudolph above suggested that the leading edge was the improvement in balance so perhaps try that option first when increasing the area.

I assume that you've already checked mast rake etc?

Did they ever build the Corribee with skegs from scratch?

ianat182
Click to expand...

She's out of the water at the moment so difficult to test. To be honest, it's as quick for me to just knock up a new rudder out of a couple of sheets of ply and some epoxy. If it takes me a few goes, then so be it.

I hadn't check the mast rake. Rig tuning is, well, I think the aforementioned statement demonstrates how much I know about it :) Could you point me in the direction of some articles, if you know of any?

They did build the Corribee with a skeg. The later ones all had them. Indeed, I have a letter to the original owners group from a chap that described my symptoms to a tee, then managed to source a new skeg moulding from Newbridge at the boat show (obviously this happened a while ago). He fitted the skeg and apparently it made a huge improvement, which leads to me believe it's the rudder design.

However, on the skeg'd Coorribee's, the rudder is not balanced so there's more area aft; hence my initial thoughts about extending it.

I have, as described in the above post, created an elliptical trailing edge, but not fettled with the depth.
 
Excessive mast rake can induce more weather helm,although poor setting/balance of the sails is often a cause too.
Your intention to put an extra reef in the main is a good idea.

The amount of mast rake set varies from boat to boat, but on yours I'd guess that 6" aft would be a good starting point .
Place a heavy weight on the main halyard so that it is vertical and check the distance at the mast foot to halyard.
To reduce rake tighten the forestay, and take up tension on the backstay until the rake is as you want it.
Sight up the mast luff groove to ensure that there is no sideways bend. A slight bend aft ,at the top of the mast is acceptable.
Check shrouds are in tension.Sight up the mast again if you adjust either or both shrouds for sideways bend. Check that the mast is vertical laterally too.No tilt.
Sail boat and adjust as necessary, aft rake for pointing and coming up head to wind when the tiller is freed.
A light steering should still have slight weather helm.

All mast adjustments should be done when afloat as the hull will change shape slightly when not resting on its keel.

The above is the way I do my rigging on a masthead rig,others may differ.
 
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I would agree with the comment on mast rake; my stag 28 was very bad until I decided to go back to basics and see what I could do. I used the Ivar Dedekam book as a guide and it was one of my good decisions; it's easy to do and you learn a lot. Mast rake changed from about 2.5 degrees to about 1 and the bend fiddled with - the transformation has made it a better balanced boat to sail and it now just heels in the gusts. The biggest difference I noticed was that the tiller pilot has a much easier time of it now and will work in conditions that would previously overwhelmed it, which make single handed sailing a lot easier... Obviously it may not be the whole answer to your problem but I'd try it first - it might make more difference than you think and it would be gutting to change your rudder then find a bit of fiddling with the rigging screws would have fixed it...
 
Just seen the advert on 'Kudu' ,was checking for your type of rig,just in case it was a 15/16ths job with swept spreaders......!
Lovely set of pics there, should have no trouble selling her.
 
I made a new rudder for my Jaguar 27 last winter and it has made a huge difference.
The old rudder was effectively 'stalling' at the slightest provocation and was a real pig in anything over a 4. She would screw up into the wind and the rudder was just acting like a brake!
I agree with what others have said regarding checking / setting the rigging, and would do definately do this first. I had done this very thoroughly on mine before deciding that the rudder was the ultimate cause of my problems.
Actually, the previous owner had her re-engined with a larger prop which was (in my opinion) too close (1") to the rudder.........so the new rudder design increased this gap (5").
The original rudder design had about 11% balance, I increased this to 17% on my new design. I used a NACA 0012 profile. I also increased the aspect ratio.

I found this article very useful reference when I was designing my new rudder: http://www.proboat-digital.com/proboat/200512/#pg78
You might find this article interesting too: http://www.jonnymooresailing.co.uk/rudder
 
Ubergeekian said:
He's not selling any more - recent blog post.
Click to expand...

Aye, apologise. A technical hitch has meant I couldn't remove the ad from the front page. I will this evening.

Note for geeks: Technical hitch being; I got a new computer and hadn't got around to importing the project into Eclipse, or setting up the FTP details in Filezilla :p
 
I don't believe that extending the rudder will materially alter the centre of lateral resistance - unless it's a very big extension !

What it will do is make the rudder 'heaver' and coarser. If you know that a skeg cures the problem, then that will be the optimum solution.
 
I'm not sure I completely follow the geometry of either the existing or the proposed rudder. However I am pretty sure that changing rudder balance won't affect weather helm or sailing efficiency, all it will do is change the amount of force on the tiller.
 
Hi Nathan, Before going down the rudder route I would check mast rake, as Iain suggesed. I've had exactly your problem (not Coribee) and it proved to be the source of the weather helm. A couple of points: you may need to slacken/readjust shrouds to avoid inducing forward mast bend; measure mast rake on the water with added payload/boat level to lines.
 
Nathan

A skeg is the way to go really but then you gotta build it and grind and bond it to the grp underbody of the boat, yuk, then source and fit a bottom rudder pintle etc ( I was never really sure with mine wether the rudder was actually supporting the skeg or t'other way round !).You could certainly wobble the skeg a wee bit....

So just make a flat 'plate' rudder from 2 pieces of ply, as you say. What you want is increased size from front to back ( a 'lower aspect ratio' rudder of enlarged area). The balance area forward of the rudder post should not really be more than 15pc of the overall, otherwise it will be very tiring to helm, too lively, wanting to wander all over the place, pushing AND pulling the tiller as you try to steer along.

I would be tempted to make it really quite big, test sail, dry out on the beach, cut it down a bit, test it again etc until you're happy. You have become the R and D bloke for 'Kudu marine', really! Good luck.
 
nathanlee said:
Hi all,

Last year I suffered quite severe weather helm in high winds (30+). A strong gust would round the boat up, and seemingly nothing I could do would stop it. The helm became very heavy, and spilling the main didn't seem to help once the "round up" had got going.

From this I've decided that the main needs a third, deep reef, but on top of that given some further research I think the rudder has a design flaw.

The rudder on my Corribee is a balanced spade rudder. It's balanced not far off 50%.

I've read an account of a chap fitting a newbridge skeg moulding to his boat and it doing wonders for the issues I've described, but I am reluctant to go down this route since fitting a skeg is going to be a serious project.

As such, I've decided to experiment with the rudder. I've made (am making) myself a new one with slightly more surface area (nothing excessive) and intend to mount it with a refresh bias aft, of about 2/3. I'm hoping that moving the centre of effort aft a bit will help.

What are your opinions? Am I doing the right thing?

Cheers,

Nathan
Click to expand...

Firstly it does sound like a case of being overpowered and nothing else. You mention that the rudder is very heavy which suggests that it hasn't stalled, so more area is not really going to help (just make it heavier). Best cure as you say is to get a third reef in the main.

Secondly moving the rudder CoE aft is only going to make the rudder feel even heavier. If it is already too heavy then you need a rudder more balanced rudder not less. Although you mentioned that you think it is at 50%, I personally would be surprised if that is the case because a perfectly balanced rudder would be very light, have very little feedback and hence make the boat very sensitive on the helm. Go too far and you arrive at an over balanced rudder so when you let go of the tiller it rapidly moves to one side of the other - often happens with propwash.

A skeg would help moving the overall CLR (centre of lateral resistance) aft, but a lot of work.

By all means have a go at different rudders if it is easy to do, but reducing sail does sound like the answer here.
 
ianat182 said:
Did they ever build the Corribee with skegs from scratch?
ianat182
Click to expand...
Yes (though they were actually made from grp, not scratch...) The later mk2 and the mk3 had skegs fitted, with the added benefit of a bottom fixing for the rudder which prevented it falling off when the clamp bolt at the top of the rudder post failed.
 
Doh. Just reread the OP, note to self: Answer the question Blueboatman.

Yup, You need a skeg- this puts more underwater hull (but not rudder) lateral area aft of the combined centre of effort of the sails.

In boat design the idea of the relationship between the underwater profile and where to plonk the mast and sails in order to get balance is known as 'Lead" ( as in to lead astray, not the ballast bit).
Have a google around a bit, there should be articles out there on Lead/hull balance and how to measure the lead for your actual boat from nothing more than a cardboard profile of the boat and rig taken from a Corribee website..

Something ain't right because the Corribee hull is imo beautifully undistorted, perfectly balanced and light on the helm.

Of course any boat will develop weather helm as it heels more and more. Can you sail the boat flatter by reefing earlier, using a small flat jib vs part-rolled genoa which tend to be baggy?

If all else fails, moveable drinking water ballast wing tanks Nathan ?
 
rb_stretch said:
FAlthough you mentioned that you think it is at 50%, I personally would be surprised if that is the case because a perfectly balanced rudder would be very light, have very little feedback and hence make the boat very sensitive on the helm.
Click to expand...

The centre of lift on an aerofoil is generally about 1/3 of the way back from the leading age ... I'd expect a rudder pivoted halfway along the chord to be unstable. Perhaps I'm missing something.

Blueboatman said:
Something ain't right because the Corribee hull is imo beautifully undistorted, perfectly balanced and light on the helm.
Click to expand...

Kudu is a Mk 1 Corribee - did the hull change significantly for the later ones or just the deck/coachroof?
 
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