Adding a Rudder Tab

[machone]

While welding is going on, I'm thinking about adding a small piece of metal to the rudder, forward of the post to reduce the force for a new self steering system I'm adding. The idea is similar to that used on the rudder of aerobatic aircraft to reduce control forces.


Has anybody done this and does it work?

 

[Otter]

What design of boat is it? Just a caution that our yard had to help someone who'd done something similar and had overbalanced the rudder making it almost impossible to steer, they said 10 degrees of rudder was enough to turn it back the way it was going - but that it did circles very nicely!

 

[maxi77]

While welding is going on, I'm thinking about adding a small piece of metal to the rudder, forward of the post to reduce the force for a new self steering system I'm adding. The idea is similar to that used on the rudder of aerobatic aircraft to reduce control forces.


Has anybody done this and does it work?

You need to do some sums on the forces involved, too much balance can cause problems, you need some one who understands rudder force calculations

 

[Tranona]

Balanced rudders are quite common with anything up to 15% of the total area in front of the pivot point. Usually done to reduce loads on the tiller. Difficult to say whether it would work on your particular boat. It could make the helm very sensitive to small angles of movement.

 

[VicS]

I have done it to my rudder. 10% to 15% is the generally recommended balance area. I split the difference and made it as near as I could to 12.5% but I reduced the original blade size so that the final result was the same total area as the original.

It has reduced the effort required as expected and the whole boat seems to handle better but I am sure its just that the steering is easier.

The people who race their boats dont like the loss of "feel" though

 

[William_H]

I race my little boat and I do like the reduction in helm forces that came when I added more balance to the rudder. (and area too) there is still plenty of feel. If it is a steel rudder )welding) then you need to add a folded/curved piece of sheet to maintain the foil shape as far as possible.
You need to measure the chord of the rudder and determine where the line of the pivot extends to. You can then determine if it has any balance at all. Then as said add 15% or so to the front perhaps 20% if there is no balance at the moment.
Most aircraft have a trim tab at the trailing edge of the control surfaces. This is often adjustable from the cockpit. It reduces the force required to zero on the surface to maintain straight and level. I t works by producing lift at the trailing edge which then moves or tries to move the whole rudder (or aileron or elevator).
Balance of a control surface is different and is often in the form of a part of the rudder which extends beyond the fin and forward of the body of the rudder ie area of rudder forward of the hinge. (pintle) same again on aileron and elevator. This is not to be confused with mass balance which is a weight mounted forward of the control surface to minimise tendency to flutter and also to affect dynamic response.
An interesting but useless bit of additional information for aircraft is that loads on the control stick or yoke must be carefully adjusted by area balance and control ratios to be light enough for a pilot to control at high speed but not so light that he can easily apply full movement which at high speed could overload the airframe. (tear the tail off) All in FAR 23 or FAR 25 design standards.
good luck olewill

 

[machone]

Add a tab

Thanks. I think I'll give it a go I think. The boat is an old steel dutch domp kruiser. The rudder is just a flat steel paddle, all aft of the pivot. If the worst comes to the worst I can always chop off anything that I add. I was thinking about making a foil section but it's a bit complicated and expensive!

 

[Rossynant]

On those dutch boats rudders often were made such as to enable prop removal, or things like that. Better make sure, if this rudder is not transom mounted
Also clearance from prop, there is a minimum distance necessary for prop to work properly (and for rudder not vibrating your teeth off...)

 

[Davegriff]

Handling improved vastly with no noticable problems when I added about 10% balance area to the rudder of my fin keel Jaguar 25.