Rudder shapes

I built a rudder for my new IOM, very long looked somthing like the newer big racing boats. I do not think it is working good for me as it rounds up very easyily it also does not want to turn down wind as well. Can anyone link me to a web site that shows a bunch of ruddr profiles. If not I will just make a few and try them out?


There is a lot to argue in favor of short, fat rudders. Long skinny ones have a lot of frontal area, whereas short fat ones have less frontal area and more wetted surface. There is a funtional compromise somewhere in between.

For more than you want to know about rudders, dial up Go to forums, use search function and find all kinds of rudder stuff. This is generally a big boat forum but there is an enormous amount of information about damned near everything, some of which is applicable to our models.

Hi Andrew,

Can you post a picture of your rudder, side view and top view.

It sounds like the rudder is not biting into the water. It could be stalling at small angle of attack. (problem with the choice of foil section)

How far back is the rudder post as a % of cord width. It should be about 20%.

How thick is the rudder blade?

What kind of airfoil?

Also what kind of wind strength? how much were you heeling?

The problem you describe is unusual. It is more frequent to hear the opposite. . .too much rudder, especially downwind.

Before you give up on this rudder & build a new one, you may want to consider the possibility that it is not the rudder causing the problem. From the symptoms you describe above, it very well could be a matter of excessive weather helm caused by an imbalance between the center of effort (CE) of the sails and the center of lateral resistance (CLR) of the hull/keel/rudder. Try raking the mast forward a bit & see if that makes a difference. Tuning of the main/jib sail sheeting trim, draft, etc will also have an effect.
Bill K

What kind of air foil? well I do not know the rudder was made by eye! I will take photos tonight when I get home. It was quite windy, max for the A rig I had on, and do not have a B rig.


Here are pictures of rudder and boat.

Hi Andrew,

My first impression is that the rudder is a very high aspect. As such it is more prone to stall than a shorter, wider rudder.

Take a look at Sails etc (Graham Bantock) rudders.

I have used his rudders on various different boats and classes and find them to be a good compromise between frontal area and wetted surface area. I also tend to choose a larger size than might normally be needed - I like to keep better control in higher winds rather than have the minimum drag possible.

John, (if you are the same John that emailed me the drawing of the rudder?)
I printed out your fin drawing and that is it beside my fin. Thank you , I have already started glueing up a new one.


Yes, same John:zbeer:

Thank you for the new rudder shape. I have the new one glued up and will shape tonight with belt sander then glass next week. The lower profile rudder worked good in lighter wind conditions, it also really throws the boat around down wind, or it just gets thrown around by the power of boat downwind. I see people use carbon fibre on there rudders? Is there that much twist? I have not tried but I was just thinking of sanding the plywood and bulsa blank, then seal it up then paint no glass?


My experience with rudders is that the location of the shaft is important. You want some of the rudder surface ahead of the shaft, and it acts like power steering (the pressure ahead of the shaft cancels the pressure aft of the shaft - a balanced rudder). However if the shaft is a bit too far back then the front of the blade ‘oversteers’ the back and the boat will become very ‘grabby’ , especially downwind.

I suspect that carbon rudders are easier to mass produce that glass ones, can be lighter and stiffer, don’t absorbe water, and look sexy. For home production a balsa blank, covered in glass is plenty stiff enough and still quite light.

My concern about just painting a balsa rudder is that water may get through the paint and cause the wood to swell. I would try to seal it as best I could.

John, I just thought about it and I will glass it, considering I was going to seal it with epoxy it will be easy enough to lay som glass on it. The drawing that you sent me , is the rudder shaft work good where it is?


It should be fine. You want about 20% of the area ahead of the shaft but no more.

Finished new rudder a couple of weeks ago and sailed boat today, lots more control. Just have to get my thumbs working slower.

Hi Andrew, pleased to hear that you are making good progress.

There are a couple of items you could look at to help the steering. The best, if you have a programmable transmitter, is to set the rudder for dual rate at the full travel end. My transmitter has a dual rate capability and after the start (where I want max rudder), I can set the rudder to a reduced % throw by flicking a switch.

Another option is reduce the overall throw on the rudder by moving to a more outer hole on the rudder bell crank, or a closer one on the rudder servo crank.

Thanks for the tips,
I have the poor mans ranger 2n so not programable. I have the throw mid way, and I do not think that is the problem, it is just time behind stick. The other problem I have is I can only get out a few times then winter set in and I need to do somthing so I start thinking that maybe a diferent hull design would be better so I think I will build another IOM, I just have to find someone local whom I can give the old hull to, (less servo’s) to build the fleet.