How to Adjust Rudder Alignment

[cal40john]

Originally Posted by colemj

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.

John

[dmtparkerLcsk53]

Originally Posted by colemj

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.

[mikereed100]

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.

[cal40john]

Originally Posted by mikereed100

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.

[thinwater]

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!

[Dave_S]

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.

[14murs14]

Originally Posted by dmtparkerLcsk53

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.

[thinwater]

Originally Posted by mikereed100

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.)

[44'cruisingcat]

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.

[catsketcher]

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

[Mike_Thor]

Hi all, this is an old thread but rather than create a new one with the same general topic here goes.

The above thread seems to have a number of basic assumptions which are not entirely correct and notwithstanding the general principle of different rudder AoA (angle of attack) there does not appear to be an outcome. I have read a little on the various issues when applied to motor vehicles and in particular to the racing fraternity - suffice to say it becomes extremely complicated given that this toe-in [as I understand it Ackerman Theory ('AT) when applied is really about 'dynamic toe adjustment' (i.e. not a fixed amount as some of the above would imply)].

In any case over the last two or three decades 'AT' has been in and out of favour. I believe that given the 'heal' of monohulls, the hull 'flying' of some cats etc. then the same complex issues will be present as found by the car racing environment. In summary 'what appears to be obvious is not in practice - even for the informed.

We have a medium sized catamaran with a degree of performance and I am now in need of re-installing the mounting system for the steering. This is a very good Hydrive system and we have had no leaks and no issues for the last three years with some heavy sailing involved. We have a 'bypass' valve mounted on the helm to allow for adjustment of the rudders to 'align' them around a known 'reference'. The reference is provided by the Raymarine rudder angle indicators, one for each rudder. The rudders (centreline) are a reasonable 6.8 m apart.

Does anyone have any geometry that was used in setting up rudders to take advantage of varying AoA based on % of turn angle, which in practice I seem to understand is actually setting the angle of the tiller away from parallel as compared to the other tiller.

Given that we have hydraulic rams on each tiller is there an 'offset' or angular displacement which can give the same effect?

From observations there is definitely a lot of turbulence (drag) created by the much higher AoA on the inner rudder in a tight turn than the wake created by the outboard rudder. We use asymetric power to assist with tight turns but this does not address the above principle.