Want your opinion on rudder design.

[gedaggett]

So my rudder has a crack leading through half of the top section and the Previous owner did not have a prop guard on his outboard so there is a fair amount of chewing taken place on the lower section. So I am redoing the rudder. I have a friend who is an engineer and racer/cruiser who has a very fast expensive custom built boat. He designed for me a rudder with foil and swept back design. It is in designed to balance the boat and prevent stalling and weather-helm. Should offer me a much more neutral tiller. My question is have any of you seen a mod like this and know what the end result was.


Thanks

Gary

[Freedom77]

Looks good. When I built my rudder out of teak a million years ago I put a "kicker" on it. This is a forward extension of the bottom part of the rudder that extends forward under the hull. It makes the rudder more balanced and gives you less weather helm. My scanner is not working or I would send you a picture. If you have a fax I can give you an idea of what it looks like.

[heinzir]

I don't want to sound negative but I would not go with a swept back rudder for this boat. This will increase, not decrease weather helm. You will be fighting the tiller with both hands when the wind picks up. I speak from experience. Whenever the line holding my rudder blade in the fully extended position is the tiniest bit loose the rudder blade will swing back slightly (like a swept back rudder.) I notice the effect immediately in sharply increased weather helm. Straightening the blade immediately cures the problem. You can prove it for yourself. The next time you are out on your boat, let the rudder kick up a little and lock it in that position. You will not like the way she handles.

Making the rudder cross section in the shape of a foil is correct. Check out the Idasailor.com web site and you will see that their replacement rudder blades all have a NACA foil shape. The rudders for the Ventures are all straight, not tapered, and certainly not swept back.

The idea of adding a few inches to the forward edge of the rudder is a good one. Putting a little rudder area ahead of the pivot point (pintles and gudgeons) will help to balance the rudder and make steering much easier. This is a mod made by many Venture owners.

Henry

[seahouse]

Hi Gary –

I have not seen a mod like this but I would expect that this change would increase the forces required to turn the rudders, and therefore the forces exerted on the entire steering system.

As Freedom says, a forward facing horn (IIRC called “dynamic horn balancing”) would help balance this, but the design and profile has to be right or the hydrodynamic equivalent of aerodynamic flutter can happen. Might not sink your boat, but it did in fact crash planes in the early days.

Be aware that the ratio of the surface area (profile) on each side of the pivot axis should be kept within certain tolerances, and with this new design that will change dramatically as the boat heels and the top of the rudder leaves the water.

I would also expect that the “kick up” forces would be greater at any given speed than those with the stock rudder, but the airfoil shape, creating maybe less drag than the stock profile, might partly compensate for this.

Before putting a lot of time, money and work into this I would suggest doing some experimenting with some plywood mockups and do a “proof of concept” first.

Regards - Brian.

[Freedom77]

I agree with Seahouse. I copied my "spade" rudder from someone else who had great success. I think he worked on the design of the space shuttle? If I can get a photo to our moderator maybe he can post it on line so you can all see my magical mystical rudder Fair Winds and Full Sails...

[Catigale]

Send me the pic Freedom,ill post....

[cookwithgas]

So my rudder has a crack leading through half of the top section and the Previous owner did not have a prop guard on his outboard so there is a fair amount of chewing taken place on the lower section. So I am redoing the rudder. I have a friend who is an engineer and racer/cruiser who has a very fast expensive custom built boat. He designed for me a rudder with foil and swept back design. It is in designed to balance the boat and prevent stalling and weather-helm. Should offer me a much more neutral tiller. My question is have any of you seen a mod like this and know what the end result was.

Hi Gary:

Sometimes things can be "over-engineered." I am a professional engineer (at least according to the State of Texas), but even if I wasn't I could tell you that sweeping the rudder backwards only makes it harder to steer (increases weather helm). All my boats have been on the smaller side with kick-up rudders. Any time you don't get the rudder or rudders all the way down, it becomes very hard to steer. Once down, all is good. Now that I have a kick-up rudder on my V222, the same thing happens when the rudder is not all the way down. This is most noticeable on a Hobie catamaran where the rudders are kicking up all the time when beaching or starting off. You can feel it right way without looking if a rudder is not locked down. I learned this before I had ever heard the term "weather-helm". When I was a young boy I bought a Hobie 14 and sailed it almost every day. Before launching that boat, I had never set foot on a sailboat. I knew nothing of sailing terms but learned a lot about how to make a sailboat get from point A to point B. I'm still learning - I learn something every time I take the boat out.

Have fun,

Aubrey Scott in Texas

[Currie]

Whether or not you create an imbalanced rudder by sweeping it back depends on the axis of its rotation. The picture doesn't show the axis, so it's hard to tell how much, if any, the rudder will exagerate the feedback/feel of the steering (both weather-helm and lee-helm). Swept-back rudders are usually applied to traditional displacement sterns that gently curve upwards as the hull is led out of the water. Since the water also follows that curve, the rudder often rotates on an equally swept-back axis - so there is no exageration of feedback. I'm not sure that's appropriate for a Mac hull which has more of a powerboat stern.

For the record, relieving the feel/feedback of weather-helm, doesn't relieve the weather-helm - it's still there. The only way to achieve the right amount of weather helm, is to balance the sails, centerboard/daggerboard and amount of heel. An imbalanced rudder that creates a hard-steering in all steering situations is not creating weather helm.


my $0.02

Cheers

~Bob

[heinzir]

Whether or not you create an imbalanced rudder by sweeping it back depends on the axis of its rotation. The picture doesn't show the axis, so it's hard to tell how much, if any, the rudder will exagerate the feedback/feel of the steering (both weather-helm and lee-helm).
~Bob

This is an outboard rudder. The pintles are attached to the upper board. They extend out about two inches. The axis of rotation, then, is at the pintles and gudgeons, about two inches forward of the upper blade.

Henry
Chiquita

[Freedom77]

The Venture 22 has a slight reverse transom. The biggest amount of strain is exerted on the lower, below water, pintle. If the rudder is "kicked" aft it will produce weather helm and will also bight less into the water when healed over even to the point that the rudder could come out of the water. The "spade" rudder on the other hand will produce less weather helm or as currie says more balance and will give better steerage. Voice of experience. Fair Winds and Full Sails...Old Salt.

[gedaggett]

Thanks for the insight guys. I am going to recondition the old rudder and make this new design and sea trial them both and see which is best. I will let you know this spring after splash and trial.

Thanks,

Gary