Wind vane on Multihull

[roblanford]

Has any one had any experience using a wind vane on a cat or tri?I am considering adding one to my trimaran.I have heard that the changing apparent wind on a multi makes it difficult to keep the vane tuned. Is that a real concern or just a theoretical worry?Thanks for any info!Robert

[mark_morwood]

I too have been wondering about this, and would like to hear of anyone's real experiences, in my case for a 48' catamaran.

I do not have any first hand experience, but have seen the following from a couple of windvane companies:

http://www.selfsteer.com/faqs/faq.php?ID=146

Self Steering - Hydrovane Selfsteer also has several references throughout the site to use on a multihull.

Mark.

[ess105]

I am also interested in this question as we are wanting to liveaboard and cruise our cat. When I've spoken to manufacturers at boat shows they've always been keen to sell yet I am unconvinced you can overcome the barrier presented by the salon on certain points of sail.

[md7a]

Wish I could offer real experience, but I can offer this:

Jim Brown's Searunner trimarans were designed from the start for a self-steering wind vane hooked to a trim tab on the rudder and the ancient catalog that came with my tri includes a number of photos and descriptions.

I'll try to attach a scan...

Chris White's book The Cruising Multihull suggests that the Brown design works well on other multihulls as well.

[Steve Rust]

Adding to what md7a posted, the apparent wind issue seems to be more of a problem in light air but in most conditions a windvane on a multihull should perform well. Trim tabs mounted on the main rudder seem to be the most common type but an outboard rudder is needed for this. For those with a spade rudder using a vane connected to an auxilary rudder with a trim tab would be an option. I think Autohelm makes a unit consisting of the vane and an auxillary rudder and it can also be bought separatly for those with an outboard rudder. All you need to supply is the trim tab. Chris White mentions in his book that the servo pendulum type commonly used on monohulls may develope to much power at the higher speeds. He also mentions that he used the Jim Brown designed windvane on his Searunner 31. Beam reaching in trade wind conditions was like riding on rails; there is no course variation. The Brown design is a vertical axis design so the air vane is faily large compared to a horizontal axis vane like the Autohelm. The vertical axis vane is a simpler mechanism so much easier to construct for the DIY'er. Mounting a vane on a trimaran is the same as a monohull so there is little problem there. I have never seen a windvane on a cat and I am sure there may be some added complication withthe twin rudders. Maybe the guys with the cats have some experience with this. I built a vertical axis vane with trim tab for my 22 foot monohull not because I need it but I was just curious. It worked pretty well but I had the added complication of a kickup rudder and being a trailor sailor it was something else to put up and take down. I have two rudders for the boat one with trim tab and one without. Another benefit of the rudder mounted trim tab is that it can be coupled to one of the cheap push pull type of autopilots as the tab requires very little force to move.
The diagram is the Brown design vane from the Searunner Construction manual

[Evan]

Wind vanes work well on trimarans (if they are not TOO fast) because they don't have a big saloon cabin in the way. Cats are not so good on some points of sail because of the wind shadow of the cabin. Also, if you have a faster multi, you will get an apparent wind effect that causes the vane to steer an "S" shaped course as the boat accelerates in the puffs.

[Troutbridge]

I had a Hydrovane on a mono. I also had a large sprayhood, which created a 'low pressure' area in the cockpit and vicinity of the steering vane. With the sprayhood up the windvane wouldn't work, with it down, no problem! I enquired about a Hydrovane for my new boat (BB385) but wasn't too convinced by the answers, it's a good unit but a lot of money to spend to find out that a high cabin creates similar problems to the sprayhood, and you can't take the cabin down! Interested in hearing from others who have tried it, though.

[44'cruisingcat]

Fast multi's can have the apparent wind at say, 45 degrees, but the true wind could be anywhere from 50 to 100 degrees, which would make a wind vane fairly innefective. On slower boats they should work fine though.

[Troutbridge]

I may be wrong here, but I was under the impression that wind vanes are set to the apparent wind, not the true. Also, the Hydrovane worked just fine downwind, even dead downwind (provided gybe preventers were set up of course). Thinking this through a bit, 'my' Hydrovane would alter course quite dramatically if the wind shifted (not suprising, they work by maintaining a constant angle to the wind) so if a multi accelerates (and decelerates) faster than a mono, with a resulting change in wind angle to the blade, then there will be significant changes in direction. The upshot would be that you would get almost constant small changes in course, but crossing an ocean with predominently NE (or whatever) Trade Winds then the over-all course should average out (even in a mono the wind vane was not a set and forget item, it paid to periodically check that you were going in the right direction). Where the WV might be a pain would be on a coastal trip, where frequent short wind shifts are experienced. A mono tends to 'even' these out, a multi might not. And, there's still the question of whether or not a high cabin would interfere with the airflow over the blade.

[44'cruisingcat]

Originally Posted by Troutbridge

I may be wrong here, but I was under the impression that wind vanes are set to the apparent wind, not the true. .

Which is exactly my point. A fast cat might have the apparent wind at a fairly constant angle while the true wind angle (and thus the course) could move considerably.