Rudder Experts Opinions

[Lakesailor]

I've decided to alter the rudder on my Seahawk 17ft. As shown in my video it stall at large angles of heel and I want to improve it's performance in that situation.
The discussion about not needing to heel is not relevant. This is the improvement I am trying to achieve.

Currently the boat has the standard and rather strange rudder. It was designed for sailing on the broads so I assume it is meant to be shallow and powerful at low speeds.

The rudder is shown as it hangs on the transom which is slightly underslung. The two blue lines show alternative new leading edges..
1: Vertical
2: Following the transom angle

Which would be the better? I imagine that removing the large paddle section on the aft part of the original rudder and bringing the leading edge forward may alter the amount of weather helm I have. At present there is very little weather helm and under some circumstances even lee helm.

I have a blade, I made for another boat, which I am going to use as a test-piece which will be 6" deeper than the original and almost parallel-sided. The forward edge is on the right of the picture.

This should stay deeper in the water when heeling and shouldn't break the flow over the surfaces as much as the rather wide original, which at 40° or so heel will be mainly bearing down on the water rather than trying to divert it.

Is my thinking right? once I've tried it and found an improvement I'll make a new rudder with better fittings so that I can leave it on the transom all the time. These dinghy fittings are not up to winter storms.

 

[Seajet]

The second, proposed rudder blade looks a lot better but the big question is why heel to 40+ degrees ?!

If you feel you really have to, things like 'fences' - small strips to keep the flow over the keel rather than going off the end, and / or 'vortex generators' ( tiny triangular things like aircraft tail fins, in your case about 1/2 -3/4" proud ) might well help, they keep the layer over the rudder foil active.

I still think a more efficient mainsheet system - or if in gusty conditions the old trick of sheeting the main fully out then pulling the whole purchase in by hand ( very good for Man Over Board no matter what size boat BTW ) would be your best bet, the 40+ heel seems like a big clue; but I would fancy the Plan B rudder.

 

[ianat182]

I assume that the newer blade will be a lifting type, and if so it will be adjustable by the amount of rake you permit; light airs a bit more rake, vertical for the macho windward stuff,with the advantage of lifting out altogether, or raising the blade to go into the shallows.

The shape of your original is similar to Wayfarer original blade, that I replaced with your near identical size and shape blade. The dinghy became very light on the helm, and only needed a shallow angle to turn/tack in the raked mode.

Another method might be a daggerboard type retracting vertically,reducing depth when in shallows or at your mooring.

ianat182

 

[Lakesailor]

but the big question is why heel to 40+ degrees ?!

The discussion about not needing to heel is not relevant. This is the improvement I am trying to achieve.

Do you go snowboarding to be efficient? No. I go sailing for fun.

Thanks for the ideas on fences.

 

[Lakesailor]

I assume that the newer blade will be a lifting type,

I could do that but on the lake it isn't necessary as the shallows are marked and everywhere else is deep. I was just going to make it a simple design. The finished new one won't have the enormous side cheeks on the headstock that the original has.

 

[boguing]

With a shallow rudder at large heel angles you're not getting stalling but ventilation. Low pressure side gulps air from the surface and traction is gone.

A deeper rudder will fix that. If you feel that you have heavy/light weather helm, then play with the fore anf aft location of the CofE. If not, leave it as is.

 

[Lakesailor]

OK. I have put the new blade in the original headstock and will give that a try before committing to a whole new rudder.
(Transaltion: If it works it'll stay like that for years to come)
Whilst I haven't given it the full foil profile it does have a nicely bevelled leading edge and trailing edge.

Interstingly the previous picture of the new blade earlier in the thread was taken just after epoxying. It has only been on a boat for about 6 weeks before I replaced it with a longer one (different boat to mine).
You can see the effect the sunlight has had on the wood, with epoxy having no UV protective properties. The other boat had a deeper headstock than mine. I have coated the whole assembly with some Quick Dry varnish as an experiment. If you read PBO there is a guy in there reckons acrylic is the bees knees. I know we had some bunk ladders made at the activity centre and used this varnish for them. The volunteer who varnished them was a bit cack-handed and I decided to rub them down and re-do them. I gave up quickly as the coating was as hard as nails.

I've aligned the blade vertically, which is the line of the aft edge of the headstock.

Now I need some decent breeze to try it.

 

[boguing]

Looks much better.

If helm balance was OK under normal coditions previously, I'd rake the new one back by about twenty degrees. But as we are debating a few inches movement of the CofE of the rudder, against several feet of moment around the CofE of the boat, it's time to take a flask of coffee out and do some intense research. Beats working.

I was slightly surprised to see that the total rudder depth didn't increase by as much as I thought it would. But it is a large improvement. I used to race a boat that suffered from ventilation, and found that you could often save it by dropping the tiller to leeward (to re-establish flow) and then pull it back up moderately quickly to avoid the wipe-out.

 

[Lakesailor]

It has increased the depth by 6"
That was as far south as I would be confident in fitting the blade I had into the headstock. If it doesn;t quite do the trick I will make a longer blade. As it will then be deeper than the keel I may pivot it "just in case".

I'm waiting for a decent bit of breeze (about 12-15 knots) to try it again. Wednesday sunny and 13 mph rising

 

[VicS]

You did not fancy or do not need a small balance area?

Below is the basic design I have in mind for when/if I get around to making a new rudder for my Sea Wych. Also a design published some years ago for modifying the original swept back style rudder and more or less what I have at present.

You did not fancy or do not need a small balance area?

Below is the basic design I have in mind for when/if I get around to making a new rudder for my Sea Wych. Also a design published some years ago for modifying the original swept back style rudder and more or less what I have at present.

....

 

[Lakesailor]

Well I could have aligned it with the transom so it would have canted forward a touch, but I don't have any weather helm to speak of even with the old shovel hanging out the back, so I'll see how this goes.

 

[Robin]

IIRC from dinghy racing days a deeper more vertical rudder blade is more efficient but stalls at a smaller angle so go easy on the tiller action or fit some limit stops? Many moons ago I made a new transom hung rudder for my then Halcyon 23 and it looked pretty much like your new one, except I did put a bit of balance area in. Like yours it also replaced a lifting plate one with a fixed blade, but I kept it just a tad shorter than the keel.

 

[William_H]

Rudders

Hello Lakey
Your new rudder is obviously vastly better than the old.
Weather helm should not be significantly changed by the rudder shape. More area will slightly reduce weather helm by giving more lateral resistance at the back. But forget that aspect.
The new rudder will give a much lighter rudder action. The area of the old rudder being well aft of the pintle line will have twice the leverage of the new rudder being shorter pintles to back. Further any area ahead of the pintles will give a balance effect. Roughly 10% area ahead of pintles will balance out 10% aft of the pintles reducing tiller pressure by 20%. That is a good amount.
The deeper the rudder the better at heeling angles. The boat will pivot on the chine roughly when it heels so rudder will rise out of the water so having less area in the water and closer to the surface for the above mentioned ventilation. Of course nothing can help the fact that at 45 degrees heel half the rudder power is trying to lift the stern and half is trying to move the stern to windward. Only canted twin rudders (windward rudder lifted) can help there)

Yesterday I had a typical example of how I needed rudder power which I have improved by deepening the rudder on my 21fter. We were running big spinnaker the course turned shy in about 16 knots of wind. On 3 or 4 occasions the boat heeled (dumped the main of course) but no help. The necessary action is to turn downwind. But of course the heeling gave viscious weather helm (despite huge sail area forward). It was a battle of forces with rudder just winning to get the bow down wind. It is a point a bit like a balance act you feel like it can topple either way. I know from experience that if the wind/spin win then the boat slews to windward water pours in the cockpit and mast nearly touches the water. The boat stops and the opposition catch up. All very exciting. nail biting. Not an ideal situation but one which occurs from mistakes in wind expectation or too much courage.
This whole problem was improved dramatically when I increased ruder depth by about 50mm and increased area at the front by 1 cm to increase balance. Now it is about 30cms parallel chord and about .8 metre deep (below hull)

Back to rudders. You might certainly in theory put a eliptical bottom on that new rudder. This can reduce the tip vortices. Hence drag. (as in spitfire wing tip) The square tip gives very clearly defined flow one side to the other across the tip while an eliptical (semi circular shape) gives vortices from different points which I suppose in some cases oppose one another.
If possible add to the depth rather than reduce area by shaping. However of course for safety the rudder should be somewhat shallower than the keel in case of grounding. It would be a shame to lose area where you8 want it down deep.
I have sailed mine and other boats with swinging rudder not down and forward enough. The effect is dramatic with large rudder load and very poor steering. so I imagine you would get a huge improvement with new rudder. Of course you don't want a fully balanced rudder as you need some load to feel the pressure. To discourage too much rudder angle.
Lastly the down side of deep rudder is that the leverage on the pintles becomes a worry with huge unsupported side loads when turning. Balance doesn't reduce the side loads or drag just your weight on the tiller. I have had bottom pintle bolts sheer and I constantly have a wear problem with the rudder in water on mooring. I tie the tiller off but motion tries to move the rudder on every wave or sheer of the boat with wind. good luck olewill

 

[Lakesailor]

Yes, I have the same concern about the pintles as they are only dinghy items. I aways ship the rudder when leaving the boat on the mooring, so any wear or breakeage will only happen when I am sailing.
On my Foxcub I left the rudder on all the time and it tore a lower pintle mounting from the transom. Not too much of a problem except the holes were partly below water!

 

[jwilson]

I've decided to alter the rudder on my Seahawk 17ft. As shown in my video it stall at large angles of heel and I want to improve it's performance in that situation.
The discussion about not needing to heel is not relevant. This is the improvement I am trying to achieve.

Currently the boat has the standard and rather strange rudder. It was designed for sailing on the broads so I assume it is meant to be shallow and powerful at low speeds.

The rudder is shown as it hangs on the transom which is slightly underslung. The two blue lines show alternative new leading edges..
1: Vertical
2: Following the transom angle

Which would be the better? I imagine that removing the large paddle section on the aft part of the original rudder and bringing the leading edge forward may alter the amount of weather helm I have. At present there is very little weather helm and under some circumstances even lee helm.

I have a blade, I made for another boat, which I am going to use as a test-piece which will be 6" deeper than the original and almost parallel-sided. The forward edge is on the right of the picture.

This should stay deeper in the water when heeling and shouldn't break the flow over the surfaces as much as the rather wide original, which at 40° or so heel will be mainly bearing down on the water rather than trying to divert it.

Is my thinking right? once I've tried it and found an improvement I'll make a new rudder with better fittings so that I can leave it on the transom all the time. These dinghy fittings are not up to winter storms.

I owned a Seahawk about 35 years ago and about the first thing I did (sailing in the Bristol Channel) was change the rudder. I absolutely needed a lifting blade, so built a new dinghy-style but much heavier and stronger lifting rudder in reclaimed teak chemistry bench. Mine was about a foot deeper and straight down, much like your photo.

It was in every way an improvement, but be warned. With the old rudder the rudder stalled out and the boat would round up at about 20-30 degrees heel. This stopped you driving the boat hard in gusty conditions. With the new one you could keep her going in a gust heeled much more, and keep control, which let you keep plenty of sail up for the lighter moments. However once in Weston Bay close under Brean Down an extra-strong downdraught gust knocked her down flat (spreaders picked up seaweed). She righted OK before I had to climb out onto the keel stub and centreplate, virtually no water below even with the hatch open.

Same knockdown situation a year later off the North Devon cliffs approaching Combe Martin, offshore wind and big downdraught gust through a valley.

Good little boat. Took mine from the upper Bristol Channel out and around everywhere from Clovelly to up into the Irish Sea. Kept her about 4 years.

 

[oldharry]

The new rudder looks a big improvement on the original, but I agree with VicS's comment about giving it a bit of balance, I think you would find further improvement. Possibly not enough to be worth another new rudder if the present one works well enough.

There is another side to this question, which possibly even you would not be able to resolve, and that is hull shape: heeling to 40 degrees is not in the designers brief, and by the time she is that far over the shape of the waterline in the water will have radically changed. Nearly all the buoyancy will be on the leeward side of the hull centreline, and the hull itself will be stalling, so that even a massively larger rudder will not be able to control it. Rather the same effect as having one front brake fail on the car while the other locks up, except the waterline shape will be forcing it to turn towards the 'missing' buoyancy. The modern design tendency to placing the maximum beam well aft creating a 'pear shaped' waterline is intended to counteract this. IIRC the Seahawk hull has quite traditional lines, with the point of maximum beam quite well forward, forcing the hull to pivot up wind as she goes further over. This was regarded as a safety feature in those days, forcing the boat to spill wind when over-pressed.

Good luck anyway, the new rudder will certainly improve things I think.

 

[Lakesailor]

However once in Weston Bay close under Brean Down an extra-strong downdraught gust knocked her down flat (spreaders picked up seaweed). She righted OK before I had to climb out onto the keel stub and centreplate, virtually no water below even with the hatch open.

Yup, that thought had occurred to me. However I will practice playing the main a bit more to stay in balance. I used to use the main like that in my 14ft clinker dinghy as, being an open boat with no buoyancy, I wasn't keen on dipping the lee rail under.

My thinking with his rudder is that it will hopefully mean I won't keep being brought up during a gusty sail as that really annoys me.
But being knocked down is also very annoying!

 

[Lakesailor]

heeling to 40 degrees is not in the designers brief, and by the time she is that far over the shape of the waterline in the water will have radically changed. Nearly all the buoyancy will be on the leeward side of the hull centreline, and the hull itself will be stalling, so that even a massively larger rudder will not be able to control it.

Yes. That is something I just have to accept. It's an old boat for a bit of fun. I'm just trying to delay the inevitable rounding-up moment and if the new rudder helps there I'll be well pleased.
I don't normally go out for a sail when it looks marginal, but on the lake things can change in minutes. Whilst I don't have anywhere to go in a pressing hurry, I do like to be in control if possible.

 

[lw395]

The new shape should be better.
Balance probably not required on a small boat like this, it can give less feel in lighter airs.
Regarding the pintles, I think it pays to think that they may break if you run aground, but will break in a cheap fixable way without sinking the boat....
Something has to give, best it's not the transom.

 

[Lakesailor]

It certainly is better. New Thread