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Old 19-02-2013, 08:13   #1
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Self Steering by Windvane for Cats

Hi all, I am looking to talk with folks who has experience building their own windvane selfsteering systems particularly for Catamarans. After some basic research, it appears that the basic version of a trim-tab windvane coupled directly behind the main rudders would work for me. My question is: do I build one for each rudder to compensate for the weaker steering force of trim tabs? and what is the best linkage ideas to use. Right now, I am thinking to keep it light and simple and cheap, so probably use two disks with a drop in pin at the coupling for each vane to "put it in gear so to speak". Queston: How to calibrate/calculate the number of wholes to drill in these discks. Should I do 36 ie every 10 degrees of compass? I read that the trim-tab vane itself can be attached with simple dinghy gudgeon and pintles, directly behind the main rudder. That sounds ok but other ideas. Also I found out that when in reverse, the trim tabs should be locked "inline" behind the main rudder. My main question for now however is whether I should build a system for each outboard rudder on my catalac 8M or just one? Not interested in other van designs as trim-tab is best for my boat. I have a center outboard and arch with solar panels in the way and dinghy, so the only option is to mount the systems directly behind/on top of each rudder. My boat sails really slow so dont give me the story about cats being too fast, mine is not :-)

Any advice on how to calculate the size of the a) vanes b) trimtabs also would be nice to know from experienced members.
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Old 20-02-2013, 08:01   #2
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Re: Self Steering by Windvane for Cats

As I understand, wind vane steering is not recommended for cats. Autopilots work best.
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Old 20-02-2013, 11:31   #3
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Re: Self Steering by Windvane for Cats

Please look at WINDPILOT - Photos
They can work on catamarans too. I am interested to hear from folks who use the trim tab version on each of two outboard rudders that might work on a Catalac 8M.
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Old 20-02-2013, 12:46   #4
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Re: Self Steering by Windvane for Cats

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Originally Posted by Imoya View Post
Please look at WINDPILOT - Photos
They can work on catamarans too. I am interested to hear from folks who use the trim tab version on each of two outboard rudders that might work on a Catalac 8M.
Interesting set up. Still looks to have some tiller pilot attached.

On a Cat it's just so easy to just use the autopilot.
Never found a need for a wind vane on a cat
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Old 20-02-2013, 14:47   #5
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Re: Self Steering by Windvane for Cats

Another windpilot example on a cat. WINDPILOT - Photos / WharramTiki38
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Old 20-02-2013, 16:31   #6
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Re: Self Steering by Windvane for Cats

Imoya,

Jim Brown designed a windvane for use on his Searunner designs. IIRC details could be found in the Searunner Construction Manual. Might be worth a look. I don't think you would need one for each rudder to compensate for the weaker steering force, but a vane on each rudder would allow you to choose the one in cleaner wind when on a close reach.
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Old 20-02-2013, 17:00   #7
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Re: Self Steering by Windvane for Cats

Mechanically, yes, one can certainly fit a wind vane to a cat. In many ways.

Functionally, one may find the steering erratic. Here is a simple test. Go sailing on a pleasant day. Tie a telltail somewhere aft about where the windvane might be. Then steer the boat ONLY by looking at the tell tail direction - holding the telltsil angle constant by steering. Have someone else watch for traffic and observe the course keeping performance.

You may find that dead downwind performance is reasonable. Do some tests a broad reaches. Try beam reaches.

If it is easy to hold a course then the windvane will work. If not....
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Old 20-02-2013, 19:36   #8
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Mechanically, yes, one can certainly fit a wind vane to a cat. In many ways.

Functionally, one may find the steering erratic. Here is a simple test. Go sailing on a pleasant day. Tie a telltail somewhere aft about where the windvane might be. Then steer the boat ONLY by looking at the tell tail direction - holding the telltsil angle constant by steering. Have someone else watch for traffic and observe the course keeping performance.

You may find that dead downwind performance is reasonable. Do some tests a broad reaches. Try beam reaches.

If it is easy to hold a course then the windvane will work. If not....
I think that's the reason for having a windvane on each side, to get the vane
Hopefully into clean wind.
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Old 21-02-2013, 09:09   #9
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Re: Self Steering by Windvane for Cats

Check the Wharram website. They offer plans for a vane and have gotten good results using a trim tab on only one rudder. A boat with good directional stability has a edge with vanes.
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Old 31-05-2013, 10:07   #10
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Re: Self Steering by Windvane for Cats

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I think that's the reason for having a windvane on each side, to get the vane
Hopefully into clean wind.
It is not about how "Clean" the wind is but the apparent wind direction. This changes when the boat accelerates and then it does not matter where the wind vane is.
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Old 31-05-2013, 10:50   #11
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It is not about how "Clean" the wind is but the apparent wind direction. This changes when the boat accelerates and then it does not matter where the wind vane is.
This would hold true on some cats but probably not the slower cruising cats. Still holds true that if your bridgedeck cabin is blocking the apparent wind then the vane won't work, thus the reason for two vanes.
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Old 31-05-2013, 11:27   #12
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Re: Self Steering by Windvane for Cats

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Originally Posted by Imoya View Post
... I am thinking to keep it light and simple and cheap, so probably use two disks with a drop in pin at the coupling for each vane to "put it in gear so to speak". Queston: How to calibrate/calculate the number of wholes to drill in these discks.

...Any advice on how to calculate the size of the a) vanes b) trimtabs also would be nice to know from experienced members.
Imoya - You are definitely up for some challenges. First you need to calculate the size of the trim tab, which is relative to the ruder size. Then how far behind the rudder you should mount it. That is math, the easy part. The hard part will be how to construct and mount the vane and the mechanism to drive the trim tab(s) with a minimum amount of friction.

You can refer to the link below for reference... on Page 97 its mentioned that servo-pendulum is more appropriate.
Best of luck

http://windpilot.com/n/pdf/bookeng.pdf
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Old 07-12-2013, 12:25   #13
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Re: Self Steering by Windvane for Cats

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Originally Posted by Cavalier MK2 View Post
Check the Wharram website. They offer plans for a vane and have gotten good results using a trim tab on only one rudder. A boat with good directional stability has a edge with vanes.
Wharrams typically have small low deckpods minimizing disruptions to wind flow.
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Old 07-12-2013, 15:45   #14
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Re: Self Steering by Windvane for Cats

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Wharrams typically have small low deckpods minimizing disruptions to wind flow.
If you look them over and think about it a bit you will realize that they can be mounted anywhere in good airflow using a cable control system.
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