Placement of Windvane Steering System.

[El Pinguino]

The trim tab doesn't turn the boat... it turns the rudder which turns the boat.......

[Terra Nova]

The trim tab doesn't turn the boat--it only has to deflect the rudder, which turns the boat.

[delmarrey]

The faster the water passes a rudder the smaller it can be for turning. Race boats have large rudders for light air maneuvers. Also large rudders become part of the COE vs COR.

Have you ever noticed small fins on airplane wings? A tab is for minor adjustments. A wind vane only keeps it on course, not for turning the boat.

[Rustic Charm]

Originally Posted by delmarrey

The faster the water passes a rudder the smaller it can be for turning. Race boats have large rudders for light air maneuvers. Also large rudders become part of the COE vs COR.

Have you ever noticed small fins on airplane wings? A tab is for minor adjustments. A wind vane only keeps it on course, not for turning the boat.

Thank you. That makes sense to me. I fly small planes as well.

[Rustic Charm]

Originally Posted by El Pinguino

The trim tab doesn't turn the boat... it turns the rudder which turns the boat.......

Terra Nova has said the same thing. If my main rudder is to be locked in place in the centre of the boat, I take it the trim tab won't be turning the rudder?

I know from watching some windvane video's there are many styles of wind vanes set up on boats with tillers that have lines going to the tiller , which then turns the tiller/rudder.

But, with a trim tab attached to the back edge of the rudder, how is the rudder turned?

[JPA Cate]

There was a fad, long out of use, for trim tabs on keels, on the aft end. I think where the communication is dropping out here is that "trim tab" can refer both to the trim tab on an auxiliary rudder style wind vane, and to the paddle that's in the water for a pendulum style windvane. Or at least, I've heard that usage.

With the auxiliary rudder style windvane, the trim tab steers that rudder, not the main ship's rudder, which is usually locked to just take out the weather helm, not amidships, except in very light air.

[Rustic Charm]

Originally Posted by Ann T. Cate

There was a fad, long out of use, for trim tabs on keels, on the aft end. I think where the communication is dropping out here is that "trim tab" can refer both to the trim tab on an auxiliary rudder style wind vane, and to the paddle that's in the water for a pendulum style windvane. Or at least, I've heard that usage.

With the auxiliary rudder style windvane, the trim tab steers that rudder, not the main ship's rudder, which is usually locked to just take out the weather helm, not amidships, except in very light air.

Thanks Ann, I'm needing in the next few months to come up with a solution to fix my auto pilot problem. Boats coming up river to Launceston today to slip it in a couple of weeks. My problem is that I have an electric auto pilot ram, which works exceptionally well in all but large waves and swells. But as my boat is 36 foot and about ten tons, it's just way to inadequate for all weathers.

So, I'm looking at putting a trim tab on the large transom hung rudder, then the auto pilot connected to the shaft of the trim tab and locking the main rudder in place. This I hope will get me sailing again next summer. Then later I can add a windvane upper unit to the trim tab when I can afford it.

[Pauls]

You are not understanding the function of the trim tab. When mounted on a rudder the purpose of the tab is to turn the rudder. It only takes a small force to turn the tab. The tab creates a much larger force which is applied to the back of the rudder, so the rudder turns. You do not lock the rudder in place.

Yes, you are right that this set up will greatly reduce the load on your autopilot. Using a trim tab allows a very small autopilot to helm a large boat.

[Rustic Charm]

Originally Posted by Pauls

You are not understanding the function of the trim tab. When mounted on a rudder the purpose of the tab is to turn the rudder. It only takes a small force to turn the tab. The tab creates a much larger force which is applied to the back of the rudder, so the rudder turns. You do not lock the rudder in place.

Yes, you are right that this set up will greatly reduce the load on your autopilot. Using a trim tab allows a very small autopilot to helm a large boat.

Why then do some set ups talk about 'locking' the main rudder in place so it won't move?

If I'm connecting an electronic auto pilot to the trim tab, if I don't 'lock' the main rudder in place, then it will just flap around.

[Pauls]

The setups which talk about locking the main rudder in place are only those which are using an auxiliary rudder/trim tab. You are not doing this. You are using your main rudder.

The rudder will not flap around. It will be controlled by your autopilot, which drives the trim tab, which controls and drives your rudder.

This is a very old, well established, common control technique. Do a search, you will find lots of examples and info.

[Rustic Charm]

Actually, I'm getting the hang of it I think.

This link is good for different types. WINDPILOT - Tips

All I want to do is lock the large main rudder, and put an electronic auto pilot directly on to a long thin trim tab to keep the boat on course. From what I'm understanding this will work quite well until I can afford an upper wind vane system to add to it. If anyone knows this won't work please tell me.

[Rustic Charm]

Originally Posted by Pauls

The setups which talk about locking the main rudder in place are only those which are using an auxiliary rudder/trim tab. You are not doing this. You are using your main rudder.

The rudder will not flap around. It will be controlled by your autopilot, which drives the trim tab, which controls and drives your rudder.

This is a very old, well established, common control technique. Do a search, you will find lots of examples and info.

Do I still put the electronic auto pilot directly on to the trim tab shaft?

[delmarrey]

Originally Posted by tedsherrin

Actually, I'm getting the hang of it I think.

This link is good for different types. WINDPILOT - Tips

All I want to do is lock the large main rudder, and put an electronic auto pilot directly on to a long thin trim tab to keep the boat on course. From what I'm understanding this will work quite well until I can afford an upper wind vane system to add to it. If anyone knows this won't work please tell me.

To do that you'll have to lock the rudder in a position where it is already sailing a desired course depending on the weather helm, then you can set the tab pilot. If you change course, you'll have to readjust the rudder/tiller position. The rudder will over power the tab if not in the course position.

[Pauls]

Maybe this is just a nomenclature thing. What you want to do is NOT called a trim tab. You are describing building an auxiliary rudder and powering it with your autopilot. Yes, that will work. If you make the auxiliary rudder balanced it will reduce the load on your autopilot significantly. Just don't call it a trim tab.

You're a pilot. Trim tabs apply a force to the control surfaces. The control surface of your boat is the rudder.

You're building a second rudder.

[Jim Cate]

Originally Posted by tedsherrin

Thanks Ann, I'm needing in the next few months to come up with a solution to fix my auto pilot problem. Boats coming up river to Launceston today to slip it in a couple of weeks. My problem is that I have an electric auto pilot ram, which works exceptionally well in all but large waves and swells. But as my boat is 36 foot and about ten tons, it's just way to inadequate for all weathers.

So, I'm looking at putting a trim tab on the large transom hung rudder, then the auto pilot connected to the shaft of the trim tab and locking the main rudder in place. This I hope will get me sailing again next summer. Then later I can add a windvane upper unit to the trim tab when I can afford it.

G'Day Ted,

It's Jim jumping in here... I've been following Ann's comments and have become a bit confused about your setup!

Are you saying that you have both a "main" rudder and a large transom hung rudder? If so, perhaps the scheme that you propose (with the main rudder locked) might work, but the differences in dynamics between boats makes it difficult to predict. What is the general shape of your hull? Fin or full keel, flat or deep canoe body, "main" rudder placement and size, etc?

Also, do you realize that for a trim tab to turn a rudder, that rudder must be of nearly fully balanced design... or the trim tab must be located well behind the trailing edge of the rudder in order to gain leverage? Most transom hung rudders have little or no balance area, and the physical awkwardness of the long extension arm is hard to deal with.

Finally, in my personal experience, rudders, whether main or auxiliary, that are driven by trim tabs have a significant lag time between actuating the tab and the rudder responding. That lag time means that wave induced yaw has more time to develop before rudder correction is applied, and this can lead to oscillations in the course steered, sometimes increasing in magnitude with every swing. Not nice! This was observable in the tab driven aux rudder wind vane that I designed and built for our previous boat. Some experimentation with wind-blade angle and drive ratios for the tab seemed to eliminate the building-up of oscillations, but did not eliminate them. It was just a fact of life under certain sea conditions. One of the advantages of the servo pendulum design is that when the boat yaws, the pendulum is forced to the side even before the wind blade can respond to the new apparent wind direction, and thus begin to apply correction to the rudder angle very early in the yaw process.

It has been a few years since we've been up the river to Launceston... will you be slipping at the YC or is there now a slipway/travel lift associated with the marina (which was not there when we last visited)?

Cheers,

Jim