What size is YOUR balance blade?

[MtB]

The balance blade on our rudder is too big, I think.

 

Symptoms are the tiller, if let go whilst cruising, will move all the way across to one side unaided, right to the limit of travel. Pulling it back to centre is major, two handed effort. Steering the boat around a bend is a weird experience - Just stop holding the tiller in the centre and let it move to one side by itself, then pull on the tiller quite hard to keep it in the right place and prevent it moving further across and turning the boat ever more sharply. Our last boat was the opposite, i.e. what I call normal - I had to PUSH the tiller across to one side to go around a bend and continue pushing gently to hold it off-centre or the boat would eventually straighten up.

 

I actually measured the rudder last time the boat was out and the main blade of the rudder is 29½" front to back. 9" of this is balance blade (forward of the pivot pin) and 20½" is main blade aft of the pivot pin. Too much balance blade I think. Do any other posters here know the dimensions of their rudders?

 

The boat is coming out for blacking tomorrow so this is a perfect opportunity to slice some metal off the forward edge of the balance blade to fix this problem. Question is, how much to cut off? I'm thinking of trimming it back from 9" to 7". Does the board think this will be about right, or would 1" off be better to start with? I don't want to cut too much off as it's easier to cut metal off than put it back on. What's your view?

 

Cheers, Mike

[Proper Job]

It depends on the size of your prop as well

 

An 18" prop will give 50% propwash on the (9")balance blade and 50% on the main blade. Effectively the rudder will be just about perfectly balanced.

 

Anything larger will give you a positive effect on the main blade and some 'wash past' on the balance blade. If you get what I mean?

Edited May 8, 2009 by Proper Job

[steelaway]

The balance blade on our rudder is too big, I think.

 

Symptoms are the tiller, if let go whilst cruising, will move all the way across to one side unaided, right to the limit of travel. Pulling it back to centre is major, two handed effort. Steering the boat around a bend is a weird experience - Just stop holding the tiller in the centre and let it move to one side by itself, then pull on the tiller quite hard to keep it in the right place and prevent it moving further across and turning the boat ever more sharply. Our last boat was the opposite, i.e. what I call normal - I had to PUSH the tiller across to one side to go around a bend and continue pushing gently to hold it off-centre or the boat would eventually straighten up.

 

I actually measured the rudder last time the boat was out and the main blade of the rudder is 29½" front to back. 9" of this is balance blade (forward of the pivot pin) and 20½" is main blade aft of the pivot pin. Too much balance blade I think. Do any other posters here know the dimensions of their rudders?

 

The boat is coming out for blacking tomorrow so this is a perfect opportunity to slice some metal off the forward edge of the balance blade to fix this problem. Question is, how much to cut off? I'm thinking of trimming it back from 9" to 7". Does the board think this will be about right, or would 1" off be better to start with? I don't want to cut too much off as it's easier to cut metal off than put it back on. What's your view?

 

Cheers, Mike

 

 

Hi Mike

A couple of pictures that may help you decide.

The balance plate need not be as big as you'd imagine.

Although I like it big enough for the prop wash to hit some of the balance blade on full lock

 

 

Johnathan Wilson rudder

 

 

 

Mine built 4 years ago by Soar Valley

 

 

 

Alex

[Speedwell]

Hi Mike

That sounds far too overbalanced to me. Mine is overbalanced enough to go hard over (about 45 degrees in my case) if let go, but it is certainly not a 2 handed job to get it back. It has some advantages when doing "3 point turns" as I can leave the helm hard over while I control the engine and watch how the boat is turning. I have never been brave enough to remove any metal although I would prefer less overbalance. In your case I think I would try the 1 inch first for the reason you give. You could also look to see how much of your prop is covered at maximum turning angle, which should be 45 degrees despite the number of people you see with their tillers over at 90.

Arthur

[RobinJ]

The balance blade on our rudder is too big, I think.

 

I actually measured the rudder last time the boat was out and the main blade of the rudder is 29½" front to back. 9" of this is balance blade (forward of the pivot pin) and 20½" is main blade aft of the pivot pin. Too much balance blade I think. Do any other posters here know the dimensions of their rudders?

It will depend on the size of the prop!

A guesstimate is that the balance blade should be at most around 70% of the prop and probably no more than a third of the main blade (which seems about right for yours). At 9" thats about 24" for the prop?

It will also depend on things like rake too.

 

I was having problems turning with a 17" prop and a balance balde of only 4". It was increased to nearly 8" (which seemed a bit big), but it has made a great difference to turning and only pulls gently when it gets to about 30 degrees of tiller.

[Gibbo]

I had exactly the same problem with Lionheart. Identical symptoms. Yes it was overbalanced. Everyone said so.

 

The first time I had her docked for blacking I had 30% of the front removed. The result? Obsolutely no difference whatsoever. None.

 

Gibbo

[ditchcrawler]

I had exactly the same problem with Lionheart. Identical symptoms. Yes it was overbalanced. Everyone said so.

 

The first time I had her docked for blacking I had 30% of the front removed. The result? Obsolutely no difference whatsoever. None.

 

Gibbo

Interesting, I have a "Marque" boat as well and had the same problem, chopped a bit off the front and added some to the back.

[MoominPapa]

I had exactly the same problem with Lionheart. Identical symptoms. Yes it was overbalanced. Everyone said so.

 

The first time I had her docked for blacking I had 30% of the front removed. The result? Obsolutely no difference whatsoever. None.

 

Gibbo

 

What angle does the tiller axis make to the vertical? On Melaleuca the bottom bearing is significantly further forward than the top bearing, which means the moving the rudder sideways lifts its centre of gravity. That generates a significant centering force which is independent of hydrodynamic effects, it centers the rudder quite effectively even if the prop, boat and water are all still.

 

This also has the advantage that in reverse, when the hydrodynamic forces are trying to pull the rudder to the extreme of it's travel, the weight of the rudder is still trying to centre it. At high reverse power, the tug-o-war is won by the prop, but it does moderate the tendency to slam sideways.

 

MP.

[Gibbo]

What angle does the tiller axis make to the vertical? On Melaleuca the bottom bearing is significantly further forward than the top bearing, which means the moving the rudder sideways lifts its centre of gravity. That generates a significant centering force which is independent of hydrodynamic effects, it centers the rudder quite effectively even if the prop, boat and water are all still.

 

This also has the advantage that in reverse, when the hydrodynamic forces are trying to pull the rudder to the extreme of it's travel, the weight of the rudder is still trying to centre it. At high reverse power, the tug-o-war is won by the prop, but it does moderate the tendency to slam sideways.

 

MP.

 

I dunno what angle it's at. With everything stationary the tiller stays roughly where it's put. So that would seem to imply that whaver angle it's at is roughly balanced by the blade, swan and tiller. How that translates to use when moving is beyond my capacity.

 

Gibbo

 

Interesting, I have a "Marque" boat as well and had the same problem, chopped a bit off the front and added some to the back.

 

Ok, but...............

 

Did it make any difference?

 

Gibbo

[dave moore]

Hi all

My good friend Graham Edgson at Norton Canes Boatbuilders reckons that the flight plate in front of the rudder stock should be about 20% of the area behind. Hope this helps. Graham regularly replaces rudders and swan necks to improve handling on a range of other boats and most customers are amazed at the improvement in handling.

Cheers

Dave

[Timleech]

Hi all

My good friend Graham Edgson at Norton Canes Boatbuilders reckons that the flight plate in front of the rudder stock should be about 20% of the area behind. Hope this helps. Graham regularly replaces rudders and swan necks to improve handling on a range of other boats and most customers are amazed at the improvement in handling.

Cheers

Dave

 

One fifth is the po;pularly quoted figure (20%).

Part of it is down to personal preference.

I hate steering a boat with too much balance, would far rather have too little than too much.

 

Tim

[ditchcrawler]

Ok, but...............

 

Did it make any difference?

 

Gibbo

I can now let go of the tiller, walk forward and take the exhaust off and get back to the tiller before we go up the bank.

First year out for blacking I had I think an inch take off the front, then I made an extension about 6 inches wide to bolt/clamp on the back of the rudder. I then progressively trimmed this extension until I got the balance I wanted. The misses found an extra 6 inches to much. We probably ended yo with 2.5 -3 inches which I had welded on next time out for blacking.

[Gibbo]

...........The misses found an extra 6 inches to much.............

 

 

Gibbo

[MtB]

It will depend on the size of the prop!

A guesstimate is that the balance blade should be at most around 70% of the prop and probably no more than a third of the main blade (which seems about right for yours). At 9" thats about 24" for the prop?

It will also depend on things like rake too.

 

I was having problems turning with a 17" prop and a balance balde of only 4". It was increased to nearly 8" (which seemed a bit big), but it has made a great difference to turning and only pulls gently when it gets to about 30 degrees of tiller.

 

 

The prop is 21" in diameter. 70% of 21" is 14.7". Are you saying my balance blade is too SMALL?!?

 

Rake is almost zero. Here's a photo.

 

 

Perspective in the photo makes the balance blade look smaller than it actually is. The measurements I quoted in my original post are accurate. The rudder looks quite out of proportion when viewed square from the side.

 

Based on the posts so far I think I'm inclined to take at least 3" off!

 

Cheers, Mike

Edited May 9, 2009 by mike bryant

[MtB]

Based on the posts so far I think I'm inclined to take at least 3" off!

 

Cheers, Mike

 

 

Right, a follow up post now the boat has been out of the water and the rudder removed and modified.

 

Here are some before and after pics:

 

Before

 

 

After

 

 

 

In the end I decided to remove 2" from the leading edge of the balance blade and replace the lost area by adding 2" to the trailing edge of the main part of the rudder.

 

The results are excellent! The tiller feel is 'normal' now i.e. I have to push it whilst turning the boat (instead of pulling it to stop the rudder moving sharply across to full deflection).

 

An unexpected improvement it the behaviour of the boat in reverse. I now have a degree of control in reverse and like my last boat I can steer it backwards almost indefinitely in calm windless conditions although quite a bit of speed is needed to get enough control.

 

Cheers, Mike