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Old 27-04-2013, 17:07   #1
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Self steering

In the past I have built a few servo-pendulum type self steering gears with success ranging from 'totally useless' to 'rather good, really'.
My current vessel has a transom hung rudder so is very suited to a rudder/trim tab arrangement driven by either a vertical or horizontal air vane. The internet has not been of much help in determining the trim tab to rudder size relationship - the best advise on offer being that the trim tab should be approximately 20% of the rudder chord. This is all fine and dandy for a rudder with a parallel leading and trailing edge but not for my rudder shape which kind of reminds me of a pregnant woman..
Can anyone help me, perhaps with a formula to calculate the optimum trim tab shape and area relative to the rudder area?
Thanks, Chris
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Old 27-04-2013, 17:45   #2
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Re: Self steering

You might want to find information on the Quartermaster Windvane, and scale it up to match the tonnage and helm pressures of an H28. The Quartermaster was designed for Folkboats, Contessa 26, and similar boats. It is very simple and should be cheap to build, and worked a treat on my Contessa for 9000 miles.____Worth a try!!!!____Grant.
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Old 27-04-2013, 18:22   #3
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Hi Chris

The H28 is a great boat we had one for 20 great years. Have you tried the h28 club site? h28.org.nz if you ask your question there someone might be able to help.
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Old 27-04-2013, 18:39   #4
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Re: Self steering

You might try finding the center of lateral resistance of your rudder. The simplest way to do that is to trace the shape of your rudder on some foamcore. You then find the balance point by...balancing the foamcore on the edge of a table. When it is just so, you have found the center of your trim tab. Then calculate the area of your rudder and build a trim tab equal to 20% of said area. the horizontal dimension of the trim tab will be 20% of the cord of the rudder as measured at its balance point. As for the mounting hardware, you be the judge.
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Old 27-04-2013, 19:28   #5
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Re: Self steering

John Letcher's "Self Steering for Sailing Craft" has enough formulas to make a geek tear out his hair. I'm sure they're useful if you can decipher them. The book can still be found (out of print) on book sites like half.com, etc. One downside to the trim tab sort of steering is that when not in use, the trim tab is still there. If you lock it in place, it essentially makes your rudder 20% bigger, which may be disadvantageous to your steering. If you let it idly flap it, well...idly flaps, which can be distracting. For my money, the servo-tab sort have the advantage of being pulled out of the water and harm's way and fouling when not in use.
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Old 27-04-2013, 19:40   #6
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Re: Self steering

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Originally Posted by NZGrant View Post
Hi Chris

The H28 is a great boat we had one for 20 great years. Have you tried the h28 club site? h28.org.nz if you ask your question there someone might be able to help.
IIRC there was a trim tab self steering article on that site.
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Old 27-04-2013, 20:58   #7
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Re: Self steering

Thank you for the replies and the info. I have the rudder removed at the moment for maintenance, so will follow Sam Plan B's advise on calculating size/area.
Benz, I had considered the additional rudder surface area by adding a trim tab and its effect - I am thinking that I may reduce the rudder by 10 - 15% to compensate. Ressearch indicates that rudder size is usually dictated by low speed manouvering requirements and that there is more than enough area for steering whilst underway. I could design a system to lock the trim tab in line with the rudder to get back usuable area for low speed work, ie manouvering in a marina, and whilst going astern.
If all this sounds a bit over the top, I should explain that I have a life long interest in self steering arrangements. My wife calls it an obsession but I think that is a little harsh.
Chris
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Old 27-04-2013, 22:43   #8
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Re: Self steering

I would be hesitant to mess with the basic rudder. Hershoff(sp?) designed great boats, and there might be lots of times when you dont really need the vane on the boat. One of the reasons that I liked the Quartermaster was that it could be removed in 5 minutes with a cresent wrench, and didnt require any modification to the rudder. It didnt have any effect on backing, because it would free swivel if not engaged. It simply clamped onto the rudder head, and one control line(engage or disengage) to a point on the tiller that was easy to reach. Of course there was a safety line in case you snagged something and it swung up. Simplicity in a vane is a good thing._____Grant.
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Old 27-04-2013, 22:57   #9
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Re: Self steering

I just did a search on our forum and found a thread called Quartermaster windvane. I made a few posts on the thread, but someone else posted some photos which show one fairly well. _____Grant.
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Old 27-04-2013, 23:48   #10
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Re: Self steering

Grant, would love to see photos of the Quartermaster, and any additional info, but have difficulty finding my way round the site..
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Old 28-04-2013, 08:04   #11
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Re: Self steering

I should add a few clarificatons. Trace only the submerged portion of your rudder to find the CLR (center of lateral resistance). I agree with others that it is not wise to alter the rudder itself. If your self steering gear does not work out you will want to start over from a known base line, i.e. the rudder as designed. Also, you might ignore the area of the trim tab because if it comes loose and is lost you will want the full rudder area as designed.

I agree that John Letcher's book, Self Steering for Sailing Craft, is a real classic. It is a little hard to find these days since it was published in the 70's. You can also try The Windvane Self-Steering Handbook by Bill Morris. That one is in print.
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Old 28-04-2013, 10:34   #12
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Re: Self steering

Since I am a techno dinosaur it may be hard to make my explanation clear, but here goes. When you bring up CF there are lots of things you can click on, and the search feature is on one of the blue bars that runs across the screen. Not the top blue bar! If you look at all of that stuff you will see where it says Search. A window will open that you type in Quartermaster self steer. It will bring up about half a dozen threads, and one will be Quartermaster info. One of the posts has a link to a photo of an old Quartermaster handbook and a couple of photos. It might not be enough to build one, but will give you a better idea of what they are like. Now, if I had computer skills that were not stuck in the last century, I could just post a link instead of all this excess verbiage. Oh Well, such is life._____Grant.
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Old 28-04-2013, 10:40   #13
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Re: Self steering

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Originally Posted by gjordan View Post
Now, if I had computer skills that were not stuck in the last century, I could just post a link instead of all this excess verbiage. Oh Well, such is life._____Grant.
What I've found that works: go to the linked site, and simply copy & paste the URL of that page into your reply message.

I consider it "providing links" and do it a lot to avoid having to retype answers to frequently asked questions and/or to point skippers to areas of further research they can do themselves.
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