Raymarine St4000 with Trim Tab Problem

[Rustic Charm]

Originally Posted by atoll

the problem is with your current set up is that the trim tab is on a different axis to the rudders axis,and the ram is set to steer to the axis of rudder pintle,if you could move the trim tab axis forward so it had the same axis as the rudder pintle then it would work,but moving the ram is easier

in the diagram the new trimtab control arm , would steer as the old one did ie,turning the trim tab from left to right on the end of the rudder,thus steering the rudderwith the auto pilot.

Yes, the trim tab is on a different axis to the rudder.

The ram from the drive is set to steer the trim tab, not the rudder.

I would have thought that moving the trim tab axis to aligne with the rudders and to move with it defeats the whole purpose of having a trim tab. All that would do is extend the rudder depth i would have though

[Rustic Charm]

Originally Posted by Alan Mighty

Atoll's idea has merit, but RC (the OP) would be doing research & development to re-invent the wheel instead of doing what has been proven by myself and others to work.

I don't have to hand a photo of what works on Led Myne.

What I have is similar to this:That's a Freehand Self-steering system. A small black tiller pilot, probably a Navico, is mounted on a bracket attached to a taff rail. It connects to a peg on a truck on the tiller of a trim tab. You can see that the truck has a wing nut that allows the peg to be moved fore and aft. That allows the peg to be placed at either:

(a) the "null point", with neutral damping of movement of the trim tab tiller;
(b) further aft of the null point, so the tiller is damped to produce understeer; or
(c) forward of the null point, so the tiller leverage arm is longer than the null point, producing oversteer by amplifying the action of the autopilot.

If you're not afraid of words, read Eric Baicy's "Build your own Trim Tab Self-Steering System", which you will find as a PDF at svsarana.com

I've attached a graphic from Eric's 33 page PDF. See trim tab tiller length.jpg, about 40 KB.

What Eric calls, in that graphic, 'neutral damping' is what Larry Pardey called the "null point" or "differential", the point on the trim tab tiller which is in line with the gudgeon-pintle axis of a transom-hung rudder and which produces no amplification of the autopilots action on the trim tab.

What Eric calls, in that graphic, 'positive damping', is any point on the trim tab tiller that gives an tiller lever arm shorter than the null point. In that area, the action of the autopilot on the trim tab is 'positively' damped.

What Eric calls, in that graphic, 'negative damping', is any point on the trim tab tiller producing a tiller lever arm longer than the null point. In that area, the action of the autopilot on the trim tab is amplified by leverage. No damping. So the boat oversteers.

The Raymarine tech who advised a longer trim tab tiller arm was mistaken.

If RC's autopilot is causing oversteering, he needs to damp the steering action positively.
how do i do this?

No need for R&D. Larry Pardey did the development a few decades back. Eric Baicy rediscovered it and wrote about it.

If the tech guy was wrong, not really his fault as he was relying on me explaining to him what it was doing.

[atoll]

Originally Posted by Rustic Charm

Yes, the trim tab is on a different axis to the rudder.

The ram from the drive is set to steer the trim tab, not the rudder.

I would have thought that moving the trim tab axis to aligne with the rudders and to move with it defeats the whole purpose of having a trim tab. All that would do is extend the rudder depth i would have though

sorry that was a bad example what I meant was having a second pintle in alignment with the rudder pintle,and a lever to the trimtab control arm.

though I am not sure if this would work,as it would have the same problem as before,thinking about it now.

best would be to mount as per my first diagram.

see diagram

[Alan Mighty]

Originally Posted by atoll

@alan mighty

in your diagram the tiller pilot is behind the axis of the rudder pintal,making the tendancy to over steer when the rudder moves minimal,plus being able to adjust the control arm lenth so the tiller pilot steers almost on the axis of the trim tab negates the over amplification we see on rustic charms set up.

That's not my diagram. It's Eric Baicy's. And he (and Larry Pardey) are correct - a shorter trim tab tiller gives positive damping.


I can only point you again to Larry Pardey's book. Or the PDF you can download freely from svsarana.com.


And for the importance of being able to move the attachment point along the tiller of the trim tab so it is aft of the null point, read p. 29 of Eric Baicy's PDF. Here's a small clip of that diagram again and the following paragraph:

[Alan Mighty]

Originally Posted by Alan Mighty

That's not my diagram. It's Eric Baicy's.

Eric Baicy's diagram is a redrawing of one in John S. Letcher, Self-Steering for Sailing Craft, 1974.


For Letcher's original diagram, see Letcher on trim tab tiller damping.jpg, (about 33 KB).


You can read Chapter 7 on Oversteering, from John S. Letcher's book, for yourself at: Jesterinfo.org.


And here's Peter Christian Förthmann (Self-Steering Under Sail) on the importance of damping (both mechanical damping in the linkage to the trim tab and electronic damping set in the autopilot):


"Principle 1: More damping equals better steering (although obviously not to the point where the system is so well damped that it does not move at all). Conceiving and building a system which properly balances damping and steering is the toughest challenge before any windvane steering designer. Systems must be powerful but must deliver their power in a controlled way.


"Principle 2: The less damping there is built into the system, the more additional measures the helmsperson will have to take to offset this steering deficit and cajole the system up to a level where it can cope with a particular boat. This entails not only maintaining perfect sail trim but also reducing canvas early to cut the steering demands placed on the windvane gear. Poorly damped systems make particularly hard work of reaching and downwind courses and often surrender full control to the elements.


"Principle 3: With no damping at all, self-steering is only possible if sail trim and sail area are so perfectly set that the boat steers straight ahead entirely of its own accord. Of course if your boat tracks along a straight line all on its own you might as well jettison the windvane gear altogether. Completely undamped systems can steer properly at just a few specific wind angles and are only really suitable as an aid to steering."

[Rustic Charm]

Originally Posted by Alan Mighty

Eric Baicy's diagram is a redrawing of one in John S. Letcher, Self-Steering for Sailing Craft, 1974.


For Letcher's original diagram, see Letcher on trim tab tiller damping.jpg, (about 33 KB).


You can read Chapter 7 on Oversteering, from John S. Letcher's book, for yourself at: Jesterinfo.org.


And here's Peter Christian Förthmann (Self-Steering Under Sail) on the importance of damping (both mechanical damping in the linkage to the trim tab and electronic damping set in the autopilot):


"Principle 1: More damping equals better steering (although obviously not to the point where the system is so well damped that it does not move at all). Conceiving and building a system which properly balances damping and steering is the toughest challenge before any windvane steering designer. Systems must be powerful but must deliver their power in a controlled way.


"Principle 2: The less damping there is built into the system, the more additional measures the helmsperson will have to take to offset this steering deficit and cajole the system up to a level where it can cope with a particular boat. This entails not only maintaining perfect sail trim but also reducing canvas early to cut the steering demands placed on the windvane gear. Poorly damped systems make particularly hard work of reaching and downwind courses and often surrender full control to the elements.


"Principle 3: With no damping at all, self-steering is only possible if sail trim and sail area are so perfectly set that the boat steers straight ahead entirely of its own accord. Of course if your boat tracks along a straight line all on its own you might as well jettison the windvane gear altogether. Completely undamped systems can steer properly at just a few specific wind angles and are only really suitable as an aid to steering."

Ok, I'm 'starting' to follow this, though it's given me a headache. Am I correct I need neutral or positive dampening. That is, the point to which my auto pilot ramp attaches to the trim tab tiller has to be behind or in line with the axis of the rudder?

[Snowpetrel]

Originally Posted by Rustic Charm

Ok, I'm 'starting' to follow this, though it's given me a headache. Am I correct I need neutral or positive dampening. That is, the point to which my auto pilot ramp attaches to the trim tab tiller has to be behind or in line with the axis of the rudder?

Exactly right. Atols system works as a neutral damped system. Your current system is negitively damped hence the wild steering. The best systems have a strong positive damping action.

[Rustic Charm]

Originally Posted by Snowpetrel

Exactly right. Atols system works as a neutral damped system. Your current system is negitively damped hence the wild steering. The best systems have a strong positive damping action.

Thanks for the phone call Ben, helped me grasped what you are all talking about.

I'm starting to think that following Atoll's suggestion might be the easiest way to make a change. BUT, then I come up with this idea.

Can you have a look and tell me if this will work? It means I would only have to re-engineer the trim tab tiller and not change the mounting bracket for the auto pilot. ???

[Rustic Charm]

Here it is in a picture

[Snowpetrel]

Sorry Ted, it isnt going to work either. You might just get away with an offset tiller the reciprocal of what you have drawn (facing fwd rather than aft) but this will cause slight differences between port and stb movements. This may not be a problem in practice, but if you want to get it up and running quickly a temporary plywood and hoseclamp mockup and test will show you what does and doesnt work. Id probably give atols simple system a try first. It will certainly work much better than what you have at the moment, and if you make the transverse tiller long enough you can vary the effective rudder gain by changing where you attach the autopilot ram.

[Rustic Charm]

Just a reminder of what my rudder system looks like

[Rustic Charm]

Originally Posted by Snowpetrel

Sorry Ted, it isnt going to work either. You might just get away with an offset tiller the reciprocal of what you have drawn (facing fwd rather than aft) but this will cause slight differences between port and stb movements. This may not be a problem in practice, but if you want to get it up and running quickly a temporary plywood and hoseclamp mockup and test will show you what does and doesnt work. Id probably give atols simple system a try first. It will certainly work much better than what you have at the moment, and if you make the transverse tiller long enough you can vary the effective rudder gain by changing where you attach the autopilot ram.

Do you mean like this?

[Rustic Charm]

sorry about being a pain with everyone, but I really don't know anything about this stuff.

Ben, what about this one? I'm just trying to avoid having to change too much. If this will work I can make up nice Stainless bracket to use the current mount off the tow rail.

[Snowpetrel]

Originally Posted by Rustic Charm

Do you mean like this?

Yes, thats what I was thinking. I assume thats not to scale. Id make it slightly longer than you drew it, nearer neutral feedback.

There are issues with overcentre things happening if the rudder turns too far, so test it with full rudder angles and fit rudder stops if needed to prevent nasty damage to the tab or the ram.

If you just u bolt on a big triangle of plywood over the entire trim tab tiller bit you can drill and adjust the ram attachment to perfect the ratios and feedback. When you have a combo that works build it from expemsive shiny stuff.

Personally id try Atolls approach first.

[Snowpetrel]

Also, you are goimg to have issues with the ram not being at its midships position due to the lomger distance. I cant remember if the raymarine is smart enough to quickly work out the "permenant helm" setting?