Typical course deviations with self steering?

[dustman]

What's the typical course deviation in degrees of a typical self steering system?


I'm exploring the option of building a biplane wing sail rig for a small catamaran, and it appears that the choice of profile and thickness has a significant impact on the range of effective angle of attack. So it would be good to know the range of angles of attack I would be seeing with a self steering system.

[Orion Jim]

Perhaps 4-5° total in relatively calm conditions, up to 18 knot winds, and a sea state to match. Beyond that it is the sea state which most affects course deviation, although it still stays close or the same. My experience is with a full keel cruiser with a Monitor windvane over several years and 12-15 thousand miles.
Never forget that the windvane follows the wind, not a course per se, when the wind shifts it happily follows the wind shift but keeps the same relative angle to the wind.

[HankOnthewater]

Like Orion Jim said above, about 5 degrees swing either way (without the wind changing direction) is acceptable for me. If it is more then either the sails are not set properly for that wind angle, or the seastate is throwing the boat of course, and the latter is particularly happening with following seas.

[NevilleCat]

Windvane self steering is not usually recommended on performance catamarans as a fast boat tends to drag the apparent wind forward as it accelerates which can cause problems.
Having said that, for cruising boats with the possibility of using a trim tab on the rudders, the late James Wharram has a simple design on his website which has been used very successfully on some of his boats.

[dustman]

Thanks all, that's pretty much the info I needed.


I'd likely just be using a tiller pilot. I guess as far as sail trim is concerned that could be an issue if left unattended for too long, since it steers to a course rather than steering to the wind. I'd love to have a wind vane system but they are pretty expensive.

[Overdue]

Dustman,


If you haven't watched, interesting videos at Youtube about a guy, Christian Williams, uses a wind vane when single-hand solo sails from LA to Hawaii and back, more than 2,000 miles. I am sure he is constantly monitoring and adjusting his track.

[dustman]

Originally Posted by Overdue

Dustman,


If you haven't watched, interesting videos at Youtube about a guy, Christian Williams, uses a wind vane when single-hand solo sails from LA to Hawaii and back, more than 2,000 miles. I am sure he is constantly monitoring and adjusting his track.

Yes, I watched that but at the time I didn't pay attention to that aspect, perhaps I should watch again. Thanks. His videos are great.

[NevilleCat]

https://m.youtube.com/watch?v=HwEejk7bhCw

[Hasbeen]

I had an extremely reliable wind vane system on my yacht, that steered me for about 52,000 miles of my 53,000 miles wandering. I had a small compass that could be read from underneath screwed to the deck head above my bunk. I found the heading would vary by about 20 degrees on most passages with the wandering of the wind, even in the trade wind belts.


It did not take long to get a feeling for the center of the shifts, so the boat made good my desired heading, while wandering either side of it.