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Old 23-08-2012, 13:51   #16
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Re: How to adjust rudder alignment

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Originally Posted by colemj View Post
Ackerman angles can only be set on rudders that are physically tied to each other by a tie-bar. The angle is set for turning by offsetting the tiller bar connection on the rudder post - not toeing in the rudders on centerline. Toe in will result in drag with no benefit in turning.

Mark
48ft x 19ft plywood cat with assymetrical waterline plane hull shape (aircraft aerofoil shape, with the "wing bottom" side outboard),

Don't know if it works with the boat he described but since it is assymetrical, similar to a Hobie 16 the racers have determined that a little toe in is faster. Might not be similar as the 16s also run with a lot of weather helm to deliberately load up the rudder to reduce leeway.

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Old 28-03-2018, 09:38   #17
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Re: How to adjust rudder alignment

Quote:
Originally Posted by colemj View Post
Ackerman angles can only be set on rudders that are physically tied to each other by a tie-bar. The angle is set for turning by offsetting the tiller bar connection on the rudder post - not toeing in the rudders on centerline. Toe in will result in drag with no benefit in turning.

Mark
I just replaced a rudder on my Fountaine-Pajot Belize43 with a factory fresh rudder. (Long story - don't ask.) the design is such that there is no adjustment or alignment possible. The tiller (which does have several degrees of bend toward center) is attached to the rudder with two set screws that fit into machined grooves in the shaft. The two tillers are joined by an aluminum tube with no adjustment. With the wheel centered, I can measure ~1" difference between leading and trailing edges of my rudders. Does anyone know if this is the way FP designed it to be? I don't really see how it could be otherwise as there is just no way to change things except by replacing (or re drilling) the connecting tube.
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Old 29-03-2018, 05:51   #18
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Re: How to Adjust Rudder Alignment

I can't imagine any benefit to having rudders toed in, other than to increase drag. I would be tempted to cut the tie-bar and insert a sleeve so that it can be adjusted.

Concerning Ackerman angles, I understand why they are used on cars as the wheels are more or less fastened to the pavement and during a sharp turn would create friction and tire wear. On a boat, however, this is not the case and most of our rudder adjustments while sailing are on the order of a few degrees either way. Even when tacking the angles are not more than 35 degrees and the rudders are free to slip through the water. I suppose there may be some theoretical advantage but I would imagine the benefits are minimal. While maneuvering in port the rudders are usually amidships and steering is done with the props.
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Old 29-03-2018, 07:40   #19
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Re: How to Adjust Rudder Alignment

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I can't imagine any benefit to having rudders toed in, other than to increase drag. I would be tempted to cut the tie-bar and insert a sleeve so that it can be adjusted.

Concerning Ackerman angles, I understand why they are used on cars as the wheels are more or less fastened to the pavement and during a sharp turn would create friction and tire wear. On a boat, however, this is not the case and most of our rudder adjustments while sailing are on the order of a few degrees either way. Even when tacking the angles are not more than 35 degrees and the rudders are free to slip through the water. I suppose there may be some theoretical advantage but I would imagine the benefits are minimal. While maneuvering in port the rudders are usually amidships and steering is done with the props.
As I said in post #16, I don't know if it applies to the boat in question or in the type of sailing it will be involved in, but for the Hobie 16 the racers have determined toe in is faster. It's for a unique set of circumstances.
The situation is you spend more time close hauled than reaching in a buoy race.
You have no center/daggerboard and asymmetrical hulls.
You have deliberately created weather helm with mast rake.
This results in the lee rudder angled to sail in a straight line.
Which gives the rudder an angle of attack reducing leeway.
Also been suggested that it continues the curve of the waterflow lines off of the asymmetrical hull resulting in lower drag (personally ??).
And it results in the weather rudder being straight on inline.
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Old 29-03-2018, 07:54   #20
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Re: How to Adjust Rudder Alignment

If he could cause it to slip by hand it is WAY to weak, like 10 times. That is what caused the loss of the Alpha a few years ago (clamp slipped).

Alignment is not the problem!
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Old 29-03-2018, 11:08   #21
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Re: How to Adjust Rudder Alignment

You can have Ackerman without a solid connection, it is designed into the steering by the angle of the idler arm in relation to the centre line of the rams motion. Mine are set "push/pull". In a turn, as one pushes it moves away from 90 and the other pulls towards 90 giving the pull rudder less rotation for the same travel of the ram.

Ackerman steering will provide less drag for a given turn.

I wondered about toe in/out. Interesting the post on Hobie 16's Cal. Maybe the additional stability created by toe in makes for better flow around the hulls?

Toe out would be easier on the autopilot by responding quickly to smaller steering inputs but a bit more drag. Toe in would be slower to respond and a bit more drag.
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Old 21-05-2018, 14:16   #22
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Re: How to adjust rudder alignment

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Originally Posted by dmtparkerLcsk53 View Post
I just replaced a rudder on my Fountaine-Pajot Belize43 with a factory fresh rudder. (Long story - don't ask.) the design is such that there is no adjustment or alignment possible. The tiller (which does have several degrees of bend toward center) is attached to the rudder with two set screws that fit into machined grooves in the shaft. The two tillers are joined by an aluminum tube with no adjustment. With the wheel centered, I can measure ~1" difference between leading and trailing edges of my rudders. Does anyone know if this is the way FP designed it to be? I don't really see how it could be otherwise as there is just no way to change things except by replacing (or re drilling) the connecting tube.
If it looks like my Mahe 36, which from your description sounds identical, you'd be surprised at how little the rudder shaft turns and how much you can change the position of those set screws despite the groove. Give it a try, you can easily move at least 10 degrees of rudder on a side.
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Old 21-05-2018, 14:34   #23
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Re: How to Adjust Rudder Alignment

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Originally Posted by mikereed100 View Post
I can't imagine any benefit to having rudders toed in, other than to increase drag. I would be tempted to cut the tie-bar and insert a sleeve so that it can be adjusted.

Concerning Ackerman angles, .... On a boat, however, this is not the case and most of our rudder adjustments while sailing are on the order of a few degrees either way. Even when tacking the angles are not more than 35 degrees and the rudders are free to slip through the water. I suppose there may be some theoretical advantage but I would imagine the benefits are minimal....

Of course, every cross bar cat is set up this way, as are dual-rudder sport boats. It does help in tacking speed, and there is simply no reason not to, since all it requires is a slight off-set angle on the tillers. Really simple. Not electronic.


(If the rudders are free to slip sideways they arn't working very well. Their sole function is to resist. They have no other function.)
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Old 21-05-2018, 20:26   #24
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Re: How to Adjust Rudder Alignment

Rudders steer by generating lift. To generate lift there must be an angle of attack. ie, there must be slippage.

The difficulty in setting up Ackerman geometry on a cat is knowing how much slippage there is. And is it always the same amount? And what point do the hulls pivot around? Is it always the same point?

If you get the geometry right, it could reduce drag. Get it wrong and drag could increase. IMO it's more likely to be wrong than right.
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Old Yesterday, 00:32   #25
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Re: How to Adjust Rudder Alignment

I have a small trailer cat and as I designed it myself I was wondering about ackerman angles. On a car there is little slippage and you can work out the correct ackerman for the different radius of each steering wheel when turning.

Our cats are similar and I wanted to work out the ackerman angle on my little cat. So I tacked her a number of times using only the inside rudder and looked at the angle the other rudder was at zero angle of attack. I then repeated tacking with the outside rudder and marked the position it held itself at as well. This gave me zero angle of attack for a normal tack on both rudders. Then I shortened the ackerman angle so that both rudders had a reasonable (about 5 degrees) angle of attack.

It made a big difference to the little Jarcats I sometimes hung out with. One owner thought Ackerman was rubbish and found he couldn't tack as well as the other Jarcats. After installing Ackerman the boat tacked better.

BTW the little cat tacks like a dream. She is the only cat I know that can sit stationary, stalled about 30 degrees off the wind with no jib up, then you give two quick tugs to windward and gently sheet in the main and she bears away. Never misses a tack and she is more like a tri or mono than a cat. She does have biggish rudders though.

When I raced Tornadoes there was a lot of talk of toe in, but we flew hulls all the time and the windward rudder was rarely much in the water.

I think that there are issues with hydraulic steering that mean you can move one rudder and it does not slip on the steering arm. There usually is some bypass valve or something. On my mums cat and a friends as well, we had to adjust this every now and then as the rudders would pull themselves out of true. So it is probably tight on the mount but the hydraulic calibration that is out.

cheers

Phil
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