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Old 24-04-2014, 17:33   #61
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Re: Placement of windvane steering system.

Quote:
Originally Posted by Pauls View Post
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.
Ok. Thanks.

'IF', I decided to use this auxiliary rudder, as a true 'trim tab' on a plane, and let the rudder loose to turn, the electronic auto pilot would have to turn the Trim tab in the opposite direction than the rudder wouldn't it? Can you program electronic auto pilots to do that?
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Old 24-04-2014, 17:39   #62
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Re: Placement of windvane steering system.

Ted--YOU DON'T LOCK THE RUDDER!!!

The trim tab (on that system) applies a small force to the edge of the rudder (like the trim tab on an airplane elevator), which moves away from the small force, turning the boat.
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Old 24-04-2014, 17:50   #63
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Re: Placement of windvane steering system.

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Originally Posted by Jim Cate View Post
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
I've found a picture of the stern. I hope it's attached. I'm not sure what being 'balanced' is. I can tell you that in rough seas, my rudder is hard to turn and after a six hour stink my arms are killing me. I got caught out in Bass Strait on Easter Sunday with a broken auto pilot and had to make my way back to Bridport in 30 knott winds with a two meter swell and it was a bitch. Lol.

I think I know what you mean by 'lag time' but would not the electronic auto pilot attached to the trim tab/auxiliary reduce the lag as it will be operating off a flux gate compass?

In Launceston I'll slip at Tamar Marine, not the Yacht Club. Tamar Marine has had a haul out slip for more than 30 years.
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Old 24-04-2014, 17:52   #64
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Re: Placement of windvane steering system.

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Originally Posted by Terra Nova View Post
Ted--YOU DON'T LOCK THE RUDDER!!!

The trim tab (on that system) applies a small force to the edge of the rudder (like the trim tab on an airplane elevator), which moves away from the small force, turning the boat.
Ok, then the auto pilot would have to be programmed to push and pull in the opposite direction wouldn't it?
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Old 24-04-2014, 18:11   #65
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Re: Placement of windvane steering system.

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Originally Posted by tedsherrin View Post
I've found a picture of the stern. I hope it's attached. I'm not sure what being 'balanced' is. I can tell you that in rough seas, my rudder is hard to turn and after a six hour stink my arms are killing me. I got caught out in Bass Strait on Easter Sunday with a broken auto pilot and had to make my way back to Bridport in 30 knott winds with a two meter swell and it was a bitch. Lol.

I think I know what you mean by 'lag time' but would not the electronic auto pilot attached to the trim tab/auxiliary reduce the lag as it will be operating off a flux gate compass?

In Launceston I'll slip at Tamar Marine, not the Yacht Club. Tamar Marine has had a haul out slip for more than 30 years.
OH dear...

That picture explains why your steering is heavy!

Balance, in rudder-speak, means that there is a portion of the rudder blade area below the WL that is in front of the axis of the gudgeons and pintles. When you turn that sort of rudder, a portion of the force of the water going by works to aid turning the rudder against the force against the main part of the blade... that latter force is what is making your arms tired! Spade rudders are usually partially balanced, and provide a lighter helm. Skeg hung rudders (and that is sort of what your setup is) can have some balance area if the rudder extends below the skeg, and has some area forward of the hinge axis (that's how our last two boat's rudders were mounted).

But the bottom line is that your rudder, in its current configuration, can NOT be operated by a trim tab mounted on its trailing edge, unless you hung it a long way behind the trailing edge, and even then I'm doubtful if it could be made to work. If you were willing to do some major mods, you could cut off the bit of narrow skeg that extends below the second gudgeon and add some balance area to the forward side of the rudder blade. Then a trim tab might well work to drive it. Some educated calculations would be required to work out the necessary added area.

I'm sorry to bring this up, but it would be a disservice to do otherwise.

And since you reminded me, I do remember the slip at Tamar Marine (a very good chandlery, too).

Cheers,

Jim
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Old 24-04-2014, 18:13   #66
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Re: Placement of windvane steering system.

Quote:
Originally Posted by tedsherrin View Post
I've found a picture of the stern. I hope it's attached. I'm not sure what being 'balanced' is. I can tell you that in rough seas, my rudder is hard to turn and after a six hour stink my arms are killing me. I got caught out in Bass Strait on Easter Sunday with a broken auto pilot and had to make my way back to Bridport in 30 knott winds with a two meter swell and it was a bitch. Lol.

I think I know what you mean by 'lag time' but would not the electronic auto pilot attached to the trim tab/auxiliary reduce the lag as it will be operating off a flux gate compass?

In Launceston I'll slip at Tamar Marine, not the Yacht Club. Tamar Marine has had a haul out slip for more than 30 years.
That type of rudder can't be balanced. To balance a rudder a portion of it has to be forward of the pivot point. Yeah that's an arm killer.

If you were to change the design you could move the pivot point aft and balance out the rudder a bit to make it easier to steer. I see why you want an auto pilot.
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Old 24-04-2014, 18:13   #67
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Re: Placement of windvane steering system.

That's a simple adjustment.

Your boat is a perfect setup to add a trim tab, because of the aft-hung rudder. You will simply hang a tab hinged off the back of your rudder.

Edit to add that I have sailed on a Bristol Channel Cutter, with a rudder setup similar to yours, and it self-steered beautifully, even in fairly light air, using the trim tab added to the back of the existing outboard rudder.

Further, it is possible to control the trim tab from the cockpit so you needn't have to deal with a heavy helm again.
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Old 24-04-2014, 18:16   #68
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Re: Placement of windvane steering system.

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Originally Posted by Terra Nova View Post
That's a simple adjustment.

Your boat is a perfect setup to add a trim tab, because of the aft-hung rudder. You will simply hang a tab hinged off the back of your rudder.
Errr, TN, you might want to rethink that statement. Or read the two posts immediately above yours.

Jim
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Old 24-04-2014, 18:28   #69
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Re: Placement of windvane steering system.

Jim--this is the simplest kind of rudder system to add a trim tab. Please read my edit.

Hopefully owners of transom-hung rudders with trim tabs will pipe up. The famous racer Jester had such a system IIRC.
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Old 24-04-2014, 18:35   #70
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Re: Placement of windvane steering system.

This is the principle.

Edit to add that a transom-hung rudder offers the very simplest opportunity in boats for adding trim tab self-steering.

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Old 24-04-2014, 19:02   #71
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Re: Placement of windvane steering system.

TN, it sounds great when you say it fast, but I stand by what I said: Ted's rudder will not be amenable to a simple trim tab driving it.

And while I have not personally sailed a BCC, three long range cruising friends have them, and they have all felt the need to fit a servo pendulum vane system because of the heavy steering loads under strong wind conditions. I wonder why?

I'll drop out of this discussion now, unless further questions come up that I feel competent to address.

Jim
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Old 24-04-2014, 19:09   #72
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Re: Placement of windvane steering system.

@ tedsherrin

A trim tab is like a rudder with its own tiny little tiller - the whole assembly hanging on the trailing edge of the main rudder in the same way that the main rudder hangs on the trailing edge of the boat.

Imagine you're motoring ahead and nobody's touching the main tiller. Pushing the tiny little trim tab tiller to port forces the trim tab out into the water flow on the starboard side of the main rudder. Water pressure on the trim tab's starboard face forces it - and thus the trailing edge of the main rudder - to port.

A pendulum oar type of servo between a windvane and your tiller should work much better (ie have much more power) than a trim tab type hinged to your (non-balanced) rudder.
Same basic principle - windvane turns the oar shaft, water flow pushes on one or other side of the oar forcing it to swing sideways, linked lines pull the tiller to bring the boat back on course.
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Old 24-04-2014, 20:04   #73
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Re: Placement of windvane steering system.

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Originally Posted by Terra Nova View Post
This is the principle.

Edit to add that a transom-hung rudder offers the very simplest opportunity in boats for adding trim tab self-steering.

And the photos of an actual instalation are at #35.....

several points to note....1/ this is attached to an aux ""semi balanced"" rudder and ... 2/ you will need to damp the main rudder ( as shown using the stub tiller in the pix) otherwise she will oversteer more than just a bit.....
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Old 24-04-2014, 21:21   #74
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Re: Placement of windvane steering system.

From the builder's own site.
Fay Marine vane steering plans

"when the Fay range of yachts from 32 to 44ft were designed they all had transom hung rudders. This makes using a trim tab on the main rudder easy. If I had any other type of yacht without a transom mounted rudder, I would endeavour to use an auxiliary rudder with a trim tab...

Linking a small tiller autopilot to a trim tab makes very inexpensive electronic steering possible on even large yachts."
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Old 24-04-2014, 21:44   #75
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Re: Placement of windvane steering system.

A trim tab system is at its worst with a following sea. The overtaking waves will force the tab to steer the opposite way as the vane is trying to get the vane to steer. The faster the overtaking wave, the greater will be wrong input to the rudder. Once the wave passes, the reversal of the water flow over the tab will try and slam the rudder hard over in the opposite direction exacerbating the yawing. Pendulum servo vanes are somewhat effected in the same way but they will still steer the boat as long as there is enough relative wind for proper steering input.

IIRC There was at least one airplane that the controls didn't directly move some of the control surfaces directly but indirectly through tabs on the aft end of the control surface.

The input force of a trim tab increases as the waterflow increases with boat speed through the water. Because the tabs are at the aft end of the rudder there input force is increased by the distance they are from the hinge point. For an unbalanced rudder, all you have to do is find the happy place where the lever force of the tabs distance from the hinge and the force of the water on the tab generate the proper amount of power to turn the rudder and steer the boat.
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