Catamaran steering.

[Rohorn]

Hi.....In a previous rambling post a few months back I was aking about hydraulic steering for my Dean cat, and I'm moving forward on that project. However, I'd like to ask you cat enthusiasts about setting up the "toe in" or "toe out" of your rudders.
My existing archaic steering mechanism drives the port tiller. A tiller bar with metal u/j's at the ends links to the starboard tiller. There's a sort of Ackerman geometry, i.e, the outer rudder, in a curve, cuts a shallower angle than the inner.
I'm nerving myself to run a test with the tiller bar disconnected. This would leave the starboard rudder to follow the flow with least resistance. Could I assume this to be the correct setting to reconnect the tiller bar?.
Also, if when steering like this under varying conditions of sail, and power, this setting changes, I propose changing those metal u/j's for flexible rubber joints such as those used at the bottom of some windsurfer masts. This would allow the rudders some relative movement to find their most comfortable "toe-in" setting automatically under any conditions, and hence reduce drag. Has anyone else tried this?
Polite comments appreciated.
Cheers.....R


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[Birdseye]

the rudders on my prout were set parallel at the factory. there is sideslip on any foil as the boat moves through the water, so I wouldnt have thought that it mattered.

<hr width=100% size=1>this post is a personal opinion, and you should not base your actions on it.

 

[Jools_of_Top_Cat]

mine are slightly toe'd in, but the is so much play on them in reality it wouldn't really matter if I set them parallel out toe'd.

I am not sure that I would like to add extra flexibility to the system, but an expert I am not, on this one I just followed instructions. Though I only set them up by eye, she holds a good course under motor with the wheel centred so it can't be too bad.



<hr width=100% size=1>Julian

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[chippie]

I visit a multihull site 'themultihull.com' which had quite a discussion on this matter, it seems that no consensus was reached.

The geometry of Ackermann steering is determined by extending a line out through the back axle,(in plan view) and then extending a line out perpendicular to each front wheel . Where thes lines meet on the line through the back axle is the toe out in the turn, usually measured at 20 degreesof turn.

This is done on cars to get a true rolling radius for all points of contact with the road. A boat has no equivalent to the rear wheels (if we regard the steering rudders as the front wheels) , so setting out the geometry becomes a guess at best and pointless at worst in my opinion.

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[Keith]

I know on the small racing cats ( dart etc) they have about a quarter inch toe in, apparently this improves the steering response because as i understand it, if you turn to port from an ahead setting the port rudder would become neutral and the stbd would generate the turn and vice versa, on my oceanic i will this winter fit a threaded end fitting so i can adjust the tiller bar for toe in, unfortunately i have just put two small yanmars in and the gearboxes both turn the same way so she turns quick one way and is not so hot the other i'm hoping the toe in will help alleviate some of the problem, also be careful if you are running your boat with the tillerbar disconnected as on my boat she would only turn one way in that condition ( i lost the skeg and the rudder through hurricane damage).........keith

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[Rohorn]

Hi guys, thanks for your comments so far.
Another reason for my interest in steering adjustment is that after replacing my 1gm10 saildrives with 2gm20s, Cool Cat is sailing slower. OK, sure there's a bit more weight and bigger props, but I'm hopefully (naively) looking elsewhere. During the work the tiller bar was disconnected and the adjustment lost. I set her up again parallel but have this feeling that it could be better.
Julian may have a good situation as he says "there's so much play" that the rudders can do their own thing. Have you watched that play fluctuating in and out as the boat is manoeuvered? How does it go? Also, do you have one motor or two?
Keith, thanks for the warning about tiller bar disconnection. One will be careful. Interesting the idea that perhaps the outer rudder determines the curve...at least on beach cats...I'd mindlessly got it the otherway round.
Putting in the rubber u/j's and some simple stretch measurement system might show whats really going on here. If the rudders do fight one another under certain conditions, it would be good to know when and how. Ah, this is the real essence of sailing for some of us....(!)
Chippie, thanks for the multihull site...I guess that's run by the multihull magazine. I haven't suscribed to that since my Wharram days. I'll have a look.
Cheers .....R

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[boatmike]

At sailing speeds no one has ever proved that it makes any difference whatsoever. What will make a difference however is anything that causes drag when going in a straight line. I personally set mine parallel for that reason. If you want to play games with geometry leave the rudders parallel and set the tiller arms at an angle. this will cause the movement to be progressively unequal on each rudder. This has been done in the past by some purists but no one has ever proved it makes a difference.

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[Talbot]

another poster has alluded to the discussion on www.multihull.com about ackerman steering. I initiated tat discussion, and very much came to the understanding that it would improve slow speed manoeuvrability. The best method that was proposed for the actual angle was to take a line across the bows from the point that is on the waterline. Find the midpoint and then take a line back to the pivot point of the rudder. That would be abt the right angle. On a couple of scale drawings this angle (on 2 different boats) is about 10 degrees. This is not toe in or toe out, but the angle towards the centre to be used for the steering rod (from your description you already seem to have this set up) Thus the connecting bar is shorter than the distance between the rudder posts. When the ruddders are straight ahead, they are parrallel, when they are at an angle, the inner hull turns at less angle than the outer - this helps to stop the skid, which boats like mine with no ackerman angle suffer from.

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[Jools_of_Top_Cat]

Play in my rudders

I am not sure that play is a good thing or not, but mine comes from the fact each rudder is loose on its 3 pintles. If I tighten the bolts it is still loose side to side but too tight to turn, so have to live with a little play. This is maybe 1/2 inch at the end of the blade.

Then you have the tie bar, which uses pins to connect to the rudder, need to be fairly loose in order to keep steering light, another 1/4 inch maybe less.

In reality when sailing if you study my rudders you will see a slight wobble, and both seem to have the same wobble, they do not appear totally independent of each other. This is only when she is going ahead, when the rudders are turned and pressure applied from the cable they do stiffen up.

I would never sell this as a good system, I have been wondering how to rectify it.

<hr width=100% size=1>Julian

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[snowleopard]

letting one rudder turn freely won't give the ideal setting as then you have one rudder doing all the work and the other idling whereas you should have both contributing to the turning.

to get an ackerman-type effect with the inside rudder at a steeper angle than the outer, the tillers should be attached to the rudderstocks in such a way that the tillers point inwards when the rudders are straight ahead.

my hydraulics give rise to toe-in which has to be corrected from time to time. when they are out of true the reduction in speed is considerable.

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[insider]

Hi Roghorn, when I lived in South Africa [the home of dean cats and multitudiinous powered cats] the way we used set up the small outboard powered boats was with about a 50 mm toe in front to back on the outboards, but on the larger in board , shaft driven units we let the vessel decide by sailing or driving with the tie bar disconnected and then adjusting the tie bar to suit the rudder possition that it sat naturally in, while traveling in a straight line , [sorry to state the obvious ]
any way that worked ok or of course you could contact Peter Dean at
http://www.deancatamarans.co.za/
or peter@deancatamarans.com
best regards

<hr width=100% size=1>Logical progression is fault finding tool in every tool box.
Regards,
Insider

 

[Rohorn]

Hi....thanks for info, snowleopard and insider. Snowleopard, you raise a point about hydraulic steering I guess using two cylinders, one for each rudder, which might with time get out of phase, as it were. In my case I'll retain the tiller bar, to keep the rudders tied together, hoping that will be enough to keep the hydraulics balanced.
I'm still rather attached to the idea of some controlled convergence or divergence under different sailing conditions, allowed by using the rubber u/j's. If the rudders want to fight, let 'em, in the interest of reducing drag. As insider says, start off by driving the boat straight ahead with the rudders free and assume that's the correct position to clamp the tiller bar. Then watch what happens to the rubber u/j's..
My Ackkerman geometry must be very nearly as Talbot describes, so I'm reassured about that.
Many thanks for an interesting discussion,
Cheers....R

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[snowleopard]

tiller bars

if your boat will allow it, it's definitely the way to go. my complicated setup with twin rams and equalising valves is only necessary because my rudders are mounted too far behind the bridgedeck to allow a solid link between the tiller arms. don't consider it if you don't have to!

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[Birdseye]

Re: tiller bars

I would bet my boots that any perceived slowness in a straight line was related to extra weight rather than rudder geometry. on my cat, simply filling the water tanks is good for a half knot speed reduction!

could the larger props also be having an effect

<hr width=100% size=1>this post is a personal opinion, and you should not base your actions on it.

 

[Keith]

Re: tiller bars

your probablyright, when i went to yanmars from volvo MD7s (best move i ever made) i lost 240 pounds out of the stern of the boat, what a difference it made to the boatspeed!!!! feathering props next and then i think the boat will be as fast as i can economically make it.......keith

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[snowleopard]

drag

no, there was no illusion about the drag caused by toe-in. i was getting 7 knots under power so i re-aligned the rudders and it immediately went up to 8.5 knots. the toe-in was around 20 degrees prior to the correction. (the alignment process is done at the wheel and takes less than 30 seconds)

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