Balanced Rudder - How big?

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Simes

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Hi Folks,

Our Schooner needs a balanced rudder, it is transome hung very big and heavy. how do I decide how large the balance area should be? None of my books talk about this. I can move the rudder pintles further aft to give more space between the transom and the leading edge. The rudder is tiller controled so need some help.

Many thanks

Simes
 
[ QUOTE ]
how do I decide how large the balance area should be

[/ QUOTE ] I have seen a figure suggested but i cannot remeber where. I think it was 10 to 15% . I modified mine but I cannot remmeber what balance area I opted for 12½% I think.

Sorry thats not very helpful.
 
Vics sounds about right. 12% should give you a helping hand, the max thickness of a foil is around 25/30% from the leading edge. But pivoting there would give you no feel at all, as the rudder would be totaly balanced.
Andrew
 
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This picture of my partially balanced rudder may give you some ideas. Yes balance is good. If you increase the area ahead of the pintle by 15% then you balance out 15% of the area aft of the pintles so reducing rudder load by 30%. Probably a good amount.

I would suggest you add a strip of material on the leading edge of the rudder in the area under the hull if there is room.
Alternatively if you cut away the rudder body at the pintles so that the rudder comes forward or if there is no room mount the hull pintles out on blocks from the hull. (In effect moving the pintles towards the middle of the rudder.)

Don't forget that a balanced rudder means actually more side loads on the rudder and so it needs to be very strong. (with sideways loads) good lcuk olewill

PS you might consider filling in any curve at the front bottom making almost square leading edge at the bottom. This is the area can work for you without adding tiller load.
 
G'day Simes,

We have done a few of these and 12% is good, you may have to hook it to go under the transom, also take care to retain an airfoil shape for best performance.

What material is your existing rudder made of?

Hope this helps.

Avagoodweekend......
 
The thing to watch out for is where you move the center of pressure on the rudder to. If I remeber correctly the CP is about 1/3 back from the leading edge of a rectangular rudder, and must remain aft of the pivot point to remain stable. Equally it must be aft of the pivot point to retain some 'feel'.
 
Hello Folks, The rudder is made of solid Mahogany, glass and epoxy sheathed. is about 2 feet wide and around 5 inches thick, is 7 feet high. the tiller is 7 feet long and is almost impossible to move at 5 knots.
When you are talking about 12%, is this 12% of the width, 12% of the total area, 12% of the wetted area?
Does the shape of the leading edge play an important part?- I was thinking of rounding the edge to be more hydro-dynamic.
Greater side loads are not a problem th e pintles and gudgeons are massive, I just need to reduce the loading, we would like to fit a Monitor next year.
Thanks for all the help.

Simes
 
!2% is added to the wetted area measured from the shaft (stock) centre line.

You should also duplicate any taper in the blade (side view). Leading edge should be reinforced and rounded, also ensure the new class extends at least 120mm aft of the shaft when glassing.

Avagoodweekend......

[ QUOTE ]
The rudder is solid Mahogany, glass and epoxy sheathed. is about 2 feet wide and around 5 inches thick, is 7 feet high. the tiller is 7 feet long and is almost impossible to move at 5 knots.
When you are talking about 12%, is this 12% of the width, 12% of the total area, 12% of the wetted area?
Does the shape of the leading edge play an important part?- I was thinking of rounding the edge to be more hydro-dynamic.
Greater side loads are not a problem th e pintles and gudgeons are massive, I just need to reduce the loading, we would like to fit a Monitor next year.
Thanks for all the help.

Simes

[/ QUOTE ]
 
As you are talkin about the movement of the center of pressure forward it is clearly the area of the rudder. The rudder is an aerofoil, it turns the boat by generating lift on the blade whch moves the stern of the boat. The point at which that lift operates nominally is the center of pressure. If your rudder has an aerofoil shape then ideally you need to extend that shape into your extension, if it is just a flay board then adding the same thickness and rounding the leading edge will do. You should be trying to make your extension look as if it was original not just a stuck on bit.
 
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