Trim tab linkage for a kick-up rudder blade

[ClubOrlov]

I am looking for a design that I can execute myself, out of stock components, in a week or two. Since I am redoing the rudder blade, it makes sense to make the trim tab part of the effort. Having a trim tab provides the same sort of amplification as a servo-pendulum, but without any fancy stainless steel gears.

I expect that the trim tab will get a lot of use: hand-steering, especially in heavy following seas; hooked up to the tiller pilot, for precise compass courses when sailing free, and hooked up to wind vane, for beating for days on end (headwinds seem to be attracted to me as fleas to dogs). So, I think I am OK with having the trim tab integral to the design, especially as it can be secured in line with the rudder to provide extra turning force when maneuvering.

I think I have all the pieces more or less thought up: a 1'x4' horizontally pivoted wind vane paddle with a pushrod linkage to the trim tab, which is 20% of rudder blade. It's the coupling between the two that's giving me headaches.

[sailingdog]

You need to read the following books:

Letcher's "Self-steering for Small Sailing Craft",
Bill Morris's "The Windvane Self-Steering Handbook",
Gerard Dijkstra's "Self-Steering for Sailboats" and
Bill Belcher's "Wind-vane Self-Steering: How to Plan and Make Your Own".

BTW, I would highly recommend not making a trim tab part of the rudder. If the trim tab breaks, you're going to lose a good part of your rudder, since it makes up 20% of the rudder. You'd be much better off IMHO making either an auxiliary rudder or servo pendulum type wind vane if your boat requires a windvane with a power amplifier device.

[ClubOrlov]

I got three of the two books on order, but I bet there's nothing in any of them about linkages to trim tabs on kick-up rudder blades.

I think that I will make the trim tab integral to the rudder blade, because that's the only place to put it. The blade itself is going to be built like the one it replaces: 2 layers of 3/4" marine plywood, 2 layers of 1/2 oz matt, and 2 layers of 10 oz cloth. I'll make the trim tab the same way. The pivot for the trim tab will probably be a 1/4" stainless steel rod. The rudder blade will have a steel shoe bolted to the bottom of it, for weighing it down, and for hitting things with when kicking up. The rudder can probably deal with 1kt collisions with the dock when backing up. Actually, I think I've done that, with no ill effects. This boat is built like an icebreaker, and so I'll just keep with the style of it when replacing the rudder blade.

Now, if only I knew how to design the linkage... I am hoping against hope that somebody out there in Sailnet-land knows of a design that I can at least lift some ideas from!

[sailingdog]

The problem is that the linkage for the trim tab is going to have to be way too complicated to be reliable. The parallax problem caused by the fact that the rudder will pivot around a specific center, and the trim tab has to rotate and bend around that motion...then if the rudder can pivot while kicked up, as mine can, the trim tab's mechanism has to be able to deal with that or be destroyed the first time the rudder turns when it kicks up.

If you mock this up with a #10 envelope and a pencil as the trim tab hinge axis, you'll see what I mean... You can't have it rigid to turn the trim tab, and still be able to deal with the rudder when it is kicked up.

You're going to waste a lot of time, money and effort trying to develop this, and then probably end up designing a windvane that isn't using a trim tab. Then you'll probably regret engineering the trim-tab into the rudder as an integral part.

[sailingdog]

Here's an illustration of the problem. The dot-dash-dot line is the trim tab hinge axis, the black circle is the rudder kick-up pivot point.

[ClubOrlov]

I did a rough cardboard mock-up of an idea. None of the dimensions are exactly right; it's just to illustrate the principle.

The trim tab is made up of two sections. The lower section is connected to the back of the rudder blade using gudgeons and pintles and to the upper section using a U-bracket and a through-bolt. The upper section's upper end slides through a slot hinged to the back of the transom-hung rudder body. Its middle section is V-shaped and mates with a fork at the end of the trim tab tiller, but only when the rudder blade is down. As the rudder blade comes up, the upper section of the trim tab backs away from the fork. Note that when the blade is down, the axes of the trim tab's top section slider pivot and the bottom pivot are aligned, allowing the trim tab to turn, but as it comes up, the axes are no longer aligned, constraining the trim tab to the same plane as the rudder.

Omitted from the model is the windvane linkage. This would be a bell crank permanently connected to the wind vane tiller about 20% back of the rudder pivot point (adjustable to get the right balance of responsiveness and damping), as shown here:
.

Here are photos of the model in operation.

This design achieves all of my objectives: make the trim tab integral to the rudder blade; disengage automatically when the blade kicks up; stay in the plane of the rudder when kicking up; and, be really really simple to build.

Reactions?

[JohnRPollard]

Orlov,

Clever design and nice sketches.

I know you indicate the mock-up dimensions are not to scale. How far out of scale are they? I ask because the surface area of the trim tab seems insufficient to usefully steer the boat. Hopefully there's more surface are than the drawings suggest, or possibly your boat is so well balanced that tiny variations in rudder pressure will induce steerage?

Also, I will wonder aloud about the plan to connect the tiller pilot. As I mentioned previously, most steering vane/tiller pilot rigs use the power of the vane to steer the boat on a compass course provided by the tiller pilot, so as to reduce power consumption and wear/tear on the tillerpilot. Your plan seems to be to attach the tiller pilot to the trim tab, and give it less rudder surface area with which to steer the boat. Maybe I'm missing something, but wouldn't that make the tiller pilot work harder?

Finally, I still don't like the idea of mucking up the flow over the rudder with a trim tab and all the pintle/gudgeon mounting hardware. Again, I would want to be able to get the self steering rig out of the water when I didn't need it.

Sorry to come across so critical - I don't mean it personally. I just see some issues with the plan and thought I'd mention them for consideration. You seem to have a knack for this, so I sincerely hope you prove me wrong on all counts and I look forward to hearing how the project goes.

[ClubOrlov]

The surface area of the trim tab will be 20% of the effective surface area of the rudder; both the upper and the lower sections of the trim tab will do work. Based on my research, this ratio is traditional and conservative.

The trim tab's action is pretty similar to what the servopendulum achieves - it's a sort of power steering system that deflects the flow, using the motion of the boat to turn the rudder. It will make it possible for the wimpy Simrad (no insult intended; it's a wonderful device, just not that powerful) to drive this boat in quartering seas (which currently require yours truly to man the helm for hours on end, sometimes to the point of utter exhaustion).

As far as the pintle/gudgeon business, I am thinking of eliminating them with a set of interlaced tabs between the back of the rudder blade and the front of the lower section of the trim tab, run through with stainless steel tube and nylon bushings, and staked together with stainless steel rod. That's how the whole rudder is hung on this boat. Built like a tank, I tell you! The rudder even has a swim step built as part of it.

The guy I bought this boat from said: "Most of the steering on Hogfish is done with the feet." At the time, I thought he was being funny; not any more. I've spent too many hours tucked into the cockpit, with both feet on the tiller. This boat NEEDS power steering, which is what I hope to get out of the trim tab, in addition to all the other advantages (more steerage when maneuvering, wind vane, ability to use autohelm more of the time).

[JohnRPollard]

Originally Posted by ClubOrlov

As far as the pintle/gudgeon business, I am thinking of eliminating them with a set of interlaced tabs between the back of the rudder blade and the front of the lower section of the trim tab, run through with stainless steel tube and nylon bushings, and staked together with stainless steel rod.

That sounds like a big improvement for flow/drag -- good idea.

Please post results when you get it up and running. Real photos are always helpful for visualizing.