Trim tab linkage for a kick-up rudder blade - SailNet Community
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post #1 of 24 Old 03-25-2008 Thread Starter
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Trim tab linkage for a kick-up rudder blade

I am hauled out in St. Augustine, and one of the things I have to replace is the rudder blade. It's a 32-foot cutter ketch with a transom-hung rudder. It has a rudder blade that kicks up if the boat hits shallow water (as does the centerboard). I want to add a wind vane (a simple horizontal-pivot arrangement with push rod linkage to trim tab) controlling a trim tab on the rudder blade. The thing that has me stumped is how to set up the trum tab control linkage so that it can cope with the rudder blade kicking up. The pivot range on the trim tab has to be 20 degrees or so each way. Has anyone seen or heard of such a thing?
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post #2 of 24 Old 03-25-2008
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It might help if you said what kind of boat it was... since someone who owns that boat may have done something similar. The problem is that the trim tab linkage has to deal with both the pivot and the shifting of the trim tab's physical location when the rudder kicks up. Do you have a photo of the rudder setup??

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post #3 of 24 Old 03-25-2008
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Have you contacted the manufacturer/designer of your vane? They may already have engineered a solution.


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post #4 of 24 Old 03-25-2008 Thread Starter
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The boat is a 1989 Chris Morejohn 32. It's a sharpie hull, cold-moulded, fiberglass over plywood. 2-foot draft, does 6-8 knots out on the ocean. Chris is currently pondering the same question.
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post #5 of 24 Old 03-26-2008
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Surely the shaft that turns the tab is vertical and attached to a horizontal plate. Is there a problem making that attachment able to turn vertically fore n aft with a pivot bolt through a bracket?
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post #6 of 24 Old 03-26-2008
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Most kickup rudders have a serious problem with trim tabs for wind vanes. The kickup rudder has a pivot point, and the trim tab needs one as well... however, the pivot point for the trim tab has to accommodate the movement caused by how far from the rudder's pivot point the trim tab is located. So it generally has to slide as well as pivot—or otherwise accommodate the horizontal and vertical components of the rudder kicking up. Making something like this isn't easy or simple, especially if you're trying to make it so that it doesn't interfere with the windvane or rudder's functions. It also has to allow for transmitting the turning movement past the pivot mechanism without damaging the pivot mechanism or torquing the trim tab's shaft out of alignment.

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Originally Posted by chris_gee View Post
Surely the shaft that turns the tab is vertical and attached to a horizontal plate. Is there a problem making that attachment able to turn vertically fore n aft with a pivot bolt through a bracket?

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post #7 of 24 Old 03-26-2008
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Quote:
Originally Posted by ClubOrlov View Post
The boat is a 1989 Chris Morejohn 32. It's a sharpie hull, cold-moulded, fiberglass over plywood. 2-foot draft, does 6-8 knots out on the ocean. Chris is currently pondering the same question.
Is Chris the designer of the vane as well? If not, I would contact the vane designer/builder.

The best solution might be to avoid a vane that uses a rudder trim tab. Otherwise it just seems like you are needlessly complicating what could be a fairly simple installation, since most vanes already have a kick-up rudder feature. Consider an outboard version of the Cape Horn:

Outboard models


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Last edited by JohnRPollard; 03-26-2008 at 09:22 AM. Reason: spelling
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post #8 of 24 Old 03-26-2008
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There are a lot of different vane designs, some use trim tabs, some use auxiliary rudders, some require neither, and some use a servo pendulum blade. You're right in that if he doesn't already have the wind vane, he'd probably be better off getting one that doesn't require a trim tab—ideally, he should get one that doesn't require an auxiliary rudder or pendulum servo blade either, since either would cause problems with grounding in some situations.

However, the trim tab, servo pendulum blade and rudders are generally required for a wind vane to generate sufficient turning force to counter the helm on some boats. If his sharpie doesn't have high helm loads, he might be able to get away with a simple wind vane, that doesn't require the amplification of a trim tab, servo or aux rudder.

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post #9 of 24 Old 03-26-2008 Thread Starter
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Well, the designer of the wind vane is me - I don't have the cash to shell out for a Monitor or an Aries. Plus, I am replacing the rudder blade (core rot) so why not add a trim tab while I am at it? There are two reasons to add the trim tab: one is that I want a wind vane, for going against the wind for long periods of time without having to man the helm; two is that I have a Simrad tillerpilot, which is cheap enough for me, but too wimpy in heavy seas, so I want to link it to the trim tab instead of the rudder itself.

The pivot of the trim tab will be back of the pivot of the rudder itself. Trim tab blade will be about 20% of the area of the blade. I want a linkage that will disengage when the rudder kicks up and reengage when it goes back down. Since the swing of the trim tab is only +/- 20 degrees or so, I am think of putting a horizontal steering plate right above the pivot point of the rudder blade with a v-slot in it, and having the trim tab have an interesting-shaped protrusion at its top that sits in the v-slot of the wishbone plate when the rudder blade is all the way down. When the blade kicks up, the protrusion should gently slot itself into the bottom of the rudder body, which consists of two vertical boards that hold the rudder blade, holding the trim tab in the same plane as the rudder blade instead of flopping side to side and causing (even more) erratic steering. (Steering gets quite interesting when the blade kicks up; as in, get your heel on the tiller because your arms are too small.)

The designer is working with a later version of the same transom-hung rudder design (a better one: more balanced) so we'll probably end up with slightly different designs.
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If you have a proper steering vane, you won't have to worry about the strength of your tiller pilot in heavy going. Those are the conditions when the steering vane typically will work its best, and the tiller pilot won't be used.

A lot of folks have hooked up tiller pilots to steering vanes,in order to provide light air and compass course-keeping capability, with low power consumption. But I seem to recall it's most commonly done with servo-pendulum types. The tiller pilot replaces the input from the windvane, and the servo pendulum provides the power to turn the main rudder. The servo pendulum is usually kick-up style.

Personally, I would not want a permanent trim tab affixed to the main rudder if I had other options. I would want the ability to get all the steering vane components out of the water when I didn't need it.

Hartley sells plans for a DIY steering vane:

Hartley windvane self steering plans


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NEVER CALLS CRUISINGDAD BACK....CAN"T TAKE THE ACCENT
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