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Rudder Balance advice please


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Having read all the threads on rudder balance I'm sure the rudder on my 57 foot boat is over balanced. If the tiller is released at canal cruising speed the rudder immediately swings fully over and the boat is in the bank in about 15 seconds and only 100% concentration will keep it on a straight course. Having done all the measurements I calculate the balance to be about 33% at present. In an effort to improve the steering I want to reduce the balance to approx 25% when out for blacking next month. This will involve either (1) cutting 1.5 inches off the leading edge of the balance blade or (2) adding 3.5 inches to the trailing edge of the rudder blade to give the required 25%. Could anyone advise which is the best option to take ?????? Thanks.

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I think the perceived proportion is 20% of flight plate in front of the rudder stock. The builders I work with do this, our boat ( 60' Norton Canes tug ) handles beautifully and steers well in reverse. I'd be inclined to reduce the flight plate rather than add extra to the rudder. Good luck!

 

Dave

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I think the perceived proportion is 20% of flight plate in front of the rudder stock. The builders I work with do this, our boat ( 60' Norton Canes tug ) handles beautifully and steers well in reverse. I'd be inclined to reduce the flight plate rather than add extra to the rudder. Good luck!

 

Dave

Thanks for your comment Dave, I just thought that reducing from 33% to 20% might be a little ambitious for the first attempt.

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Thanks for your comment Dave, I just thought that reducing from 33% to 20% might be a little ambitious for the first attempt.

 

The downside of being too timid in shortening the flight plate is the unstable tiller behaviour persists, albeit less fiercely.

 

I had 2" cut off mine for the same reason, and 2" welded onto the trailing edge of the rudder to keep the blade area the same, and it worked beautifully.

 

The only downside to trimming too much from the flight plate is more effort is needed to push the tiller over when turning. No great hardship in my opinion, and you can really feel the feedback from the rudder. Working boat rudders generally had no flight plate at all, after all.

 

 

MtB

 

 

(Splling edit.)

Edited by Mike the Boilerman
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Our Hudson has not much in front of the pivot so tiller forces can be a bit on the high side but let's face it, most of the time you are going fairly straight. The up-side is that it goes dead straight for long periods with hands off the tiller. I much prefer that to the boat we used to borrow, which had a much lighter tiller but was off into the bank as soon as you blinked.

 

If you are going to only remove rudder in front of the pivot, you are of course reducing the overall rudder area. So unless it is already overburdened with rudder power I would go along with Mike's suggestion and add at the back what you take off at the front. Make sure your fender(s) is/are adequate for the more protruding rudder.

Edited by nicknorman
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One consequence of reducing the flight plate is that less of the propwash is caught on high deflection of the rudder, so turning power is reduced despite adding to the trailing edge.

 

If the flight plate is totally removed then no more than 50% of propwash is available to act on the rudder, as half will miss the rudder completely. A secondary effect of my mod is that the whole rudder blade is now 2" further away from the propeller, but this is a price worth paying as the appalling tiller instability I previously had has completely gone.

 

 

MtB

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One consequence of reducing the flight plate is that less of the propwash is caught on high deflection of the rudder, so turning power is reduced despite adding to the trailing edge.

 

If the flight plate is totally removed then no more than 50% of propwash is available to act on the rudder, as half will miss the rudder completely. A secondary effect of my mod is that the whole rudder blade is now 2" further away from the propeller, but this is a price worth paying as the appalling tiller instability I previously had has completely gone.

 

 

MtB

Negative or lea-helm is particularly highly dangerous in a sailing boat too. Let go the helm for any reason and the boat will swing off down wind out of control, whereas with proper positive weather helm, let go the tiller and the boat rounds up and luffs head to wind, sails stall, boat stops and all is calm.

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Lovely terminology Bizz.

 

Wish I knew what it all meant!

Lea or negative helm is when the tiller is pushing against you all the time, particularly when sailing upwind. It's called lea helm because the rudder is being forced over to lea-ward or loo-ard ie down wind side of the boat which gunwale is lower when healing to to the wind (dangerous). Weather or positive helm is when the tiller is gently trying to pull away from you and the rudder is trying to turn towards the weather or into the wind side of the boat, the high gunwale side, (correct and not dangerous).

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Hmmmm still none the wiser...

 

I've never noticed any of this happening when sailing REGINALD or ALDEBARAN up the Shroppie...

biggrin.png

Negative helm on a motorboat is a bit similar to that on a sailing boat. The rudder- tiller direction force is unpredictable and will push and pull on your arm one way or another all the time which makes it very uncomfortable and difficult trying to steer straight, as you say, let go the tiller and the boat immediately dives round to left or right.

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when you have an unbalanced rudder then correct balance can be obtained by changing the ratio between fore and main sail areas.

Possibly if you installed a decent sized bow sprit (plus associated dolphin striker) on Reginald and rigged a flying jib ???

 

 

edit to add or conversely a mule mizzen ???

Edited by John V
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when you have an unbalanced rudder then correct balance can be obtained by changing the ratio between fore and main sail areas.

Possibly if you installed a decent sized bow sprit (plus associated dolphin striker) on Reginald and rigged a flying jib ???

 

 

edit to add or conversely a mule mizzen ???

Or by moving the mast or a deep keel backwards or forwards to balance the mast-keel-rudder ratio. Small amounts of imbalance can be corrected by just raking the mast forwards or backward a bit,(tuning).

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Or by moving the mast or a deep keel backwards or forwards to balance the mast-keel-rudder ratio. Small amounts of imbalance can be corrected by just raking the mast forwards or backward a bit,(tuning).

Sort of how a windsurfer steers, tilting the mast backwards or forwards. Back to head into wind, forward to turn tail to wind.

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I had the same problem as the OP and cut two triangles off the front of the rudder one at the top and one from the bottom to reduce the ratio to 25%. It made a huge difference to cruising and because the front of the rudder is a crude curve with the centre being 'longer' than the top and bottom it still deflects most of the prop wash. With hindsight I think 20% may be better but I can cut that another time, it is much easier to cut metal off than to add it back. Now when I let go of the tiller instead of careering madly towards either bank, the boat now gently curves (due to propwalk so it is predictable) and takes several minutes before contacting the bank.

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Whether you add or cut off depends a lot on whether the rudder is big enough as previously said. What a lot of people overlook is not just the area, but the shape - adding to the top and bottom of the rudder can make a huge difference to how the boat handles as, like altering the balance plate, this affects how much of the propwash the rudder deflects more than the length of the rudder (past a certain point). If you add any to the top, just make sure you have enough clearance to lift the rudder out of the cup. Good rudders drop down behind the cup almost to skeg level.

 

Don't take the 20% too literally either - it's a good starting point but is dependent on how close the rudder is to the prop.

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Whether you add or cut off depends a lot on whether the rudder is big enough as previously said. What a lot of people overlook is not just the area, but the shape - adding to the top and bottom of the rudder can make a huge difference to how the boat handles as, like altering the balance plate, this affects how much of the propwash the rudder deflects more than the length of the rudder (past a certain point). If you add any to the top, just make sure you have enough clearance to lift the rudder out of the cup. Good rudders drop down behind the cup almost to skeg level.

 

Don't take the 20% too literally either - it's a good starting point but is dependent on how close the rudder is to the prop.

Just checked my rudder on my 58foot new build I am fitting out and its trailing edge is 500mm the forward edge is 100mm with 200mm to the prop, does that sound about right before I launch it.

 

Neil.

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Rudder sounds about the same as mine but I think 200mm to the blade seems a little too much.

 

Or at least mine is about that too and it feels intuitively too much. I expect others more knowledgeable than me will now pile in saying I'm wrong!

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The problem is that so much of this is relative to other factors but assuming a 17" or 18" dia prop is going to be fitted it I'd be happy with that.

 

My own boat has about the same gap but a 22" x 22" 80%BAR prop which causes some interesting gyrations on the tiller (think car steering wheel with an out of balance front wheel) and conventional wisdom says that this is because the distance to the rudder is not sufficient for a prop of that size. The balance on this is also way too small for most people's taste, but vibration would get worse if enlarged, and I'm used to it. An advantage of a small balance is that you can feel more of what is going on in terms of water flow round back of the boat.

 

18281527581_2938619c80_b.jpgrudder by Rose Narrowboats, on Flickr

Note the rudder is at an angle in this shot, so the gap appears a bit bigger than it really is.

 

PS I maintain that it is coincidence that I have bad shoulder!

Edited by Rose Narrowboats
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The problem is that so much of this is relative to other factors but assuming a 17" or 18" dia prop is going to be fitted it I'd be happy with that.

 

My own boat has about the same gap but a 22" x 22" 80%BAR prop which causes some interesting gyrations on the tiller (think car steering wheel with an out of balance front wheel) and conventional wisdom says that this is because the distance to the rudder is not sufficient for a prop of that size. The balance on this is also way too small for most people's taste, but vibration would get worse if enlarged, and I'm used to it. An advantage of a small balance is that you can feel more of what is going on in terms of water flow round back of the boat.

 

18281527581_2938619c80_b.jpgrudder by Rose Narrowboats, on Flickr

Note the rudder is at an angle in this shot, so the gap appears a bit bigger than it really is.

 

PS I maintain that it is coincidence that I have bad shoulder!

 

 

 

Hmmm that gap looks much the same as mine (which I thought was too large). And my blade is 21 x 19.

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