Raymarine St4000 with Trim Tab Problem

[Alan Mighty]

Drat! Looks like ST4000, ST4000+, and ST4000 MkII have subtle differences in the firmware. I guess a Raymarine tech would have known that and asked about version/date of manufacture etc.

And that means that for your ST4000+ you are right and am I was wrong.


In your case, you do need to reverse the drive wiring to reverse the operating sense.


See ST4000plus operating sense reversal.pdf (about 90 KB).


Note the remark about changing the Rudder Damping in the Dealer Setup.


My apologies for wasting your time.

[Rustic Charm]

Originally Posted by Alan Mighty

Drat! Looks like ST4000, ST4000+, and ST4000 MkII have subtle differences in the firmware. I guess a Raymarine tech would have known that and asked about version/date of manufacture etc.

And that means that for your ST4000+ you are right and am I was wrong.


In your case, you do need to reverse the drive wiring to reverse the operating sense.


See ST4000plus operating sense reversal.pdf (about 90 KB).


Note the remark about changing the Rudder Damping in the Dealer Setup.


My apologies for wasting your time.

To the contrary, you have definately not wasted my time. I think the other recommendations you made will probably be spot on. i couldnt take it out this afternoon as i used the clubs tender to get to her. Ill check the set ups next week when ill have plenty of time.

[Rustic Charm]

I have rung the Raymarine tech guy and after a lenghty discussion trying to describe what is happening, he's suggested two things. Firstly to reduce the rudder gain down to 1. This may help. But, he suspects there might be a secondary problem if the movement required of the trim tab is much less than required of a rudder. If this is the case he thinks it might be a mechanical problem in that the trim tab arm (or tiller) needs to be extended as the trim tab is reacting much faster in turning the boat than would be expected. Afterall, the auto pilot is designed to operate a rudder not a trim tab. I suspect he's on the right track with this.

Ill try the rudder gain first anyway.

If i have to extend the trim tab arm, it will be problemstic as ill be moving it into the cockpit, which i certainly dont want to do.

[Jim Cate]

RC, if you need to keep it out of the cockpit, I reckon that some system of intermediate bell cranks and tie rods can be conjured up to reduce the motion of the trim tab tiller as much as you need. Because the forces are so low, the linkages need not be very robust... kinda Mechano set stuff!.

Jim

[Rustic Charm]

Originally Posted by Jim Cate

RC, if you need to keep it out of the cockpit, I reckon that some system of intermediate bell cranks and tie rods can be conjured up to reduce the motion of the trim tab tiller as much as you need. Because the forces are so low, the linkages need not be very robust... kinda Mechano set stuff!.

Jim

Im not sure what your suggesting

[atoll]

I would have thought that the auto pilot servo/ram needs one end to be mounted to the tiller arm itself,then to the trim tab,NOT from a fixed point on the deck as it would be if it were steering the tiller.

with the configuration you have,every time the trim tab tries to steer the rudder,as soon as the rudder moves the righting movement applied by the auto pilot is counteracted by the rudder turning the trim tab.

make a bracket to bolt onto the tiller arm for the servo,then your system should work

[Rustic Charm]

Originally Posted by atoll

I would have thought that the auto pilot servo/ram needs one end to be mounted to the tiller arm itself,then to the trim tab,NOT from a fixed point on the deck as it would be if it were steering the tiller.

with the configuration you have,every time the trim tab tries to steer the rudder,as soon as the rudder moves the righting movement applied by the auto pilot is counteracted by the rudder turning the trim tab.

make a bracket to bolt onto the tiller arm for the servo,then your system should work

Seriously i cant grasp how that would work o

[atoll]

Originally Posted by Rustic Charm

Seriously i cant grasp how that would work o

think about it!
,with the auto pilot attached,but turned off when you move the rudder the trim tab stays in the same orientation with the ram,but the rudder is pointing in a different direction.

what you want is for the trim tab to remain in the same orientation with the rudder,when the rudder turns.

the trim tab control arm needs mounting at right angles for the ram to attach to,and you need to build a bracket on the tiller for the other end of the ram/servo to attach to.

trust me,this is you problem

[Rustic Charm]

Originally Posted by atoll

think about it!
,with the auto pilot attached,but turned off when you move the rudder the trim tab stays in the same orientation with the ram,but the rudder is pointing in a different direction. yes, i can see that

what you want is for the trim tab to remain in the same orientation with the rudder,when the rudder turns.i dont understand this. The whole idea i would have thought of a trim tab is to help move the rudder. The trim tab therefore moves in the different direction to the rudder, which then forces the rudder to move. This bit works well

the trim tab control arm needs mounting at right angles for the ram to attach to,and you need to build a bracket on the tiller for the other end of the ram/servo to attach to.

trust me,this is you problem

I think id need to see this in a sketch because i just cant grasp what your suggestng. And the only pictures i can find doing a goole search are similar to my set up.

[atoll]

Originally Posted by Rustic Charm

I think id need to see this in a sketch because i just cant grasp what your suggestng. And the only pictures i can find doing a goole search are similar to my set up.

give me a minute and I will do a sketch.

in the mean time at the moment with the configuration you have;it is like standing on a skateboard and trying to steer a car rolling down hill.

what you need to do is get behind the wheel and steer the car,and ditch the skate board!

[atoll]

here you go!

[Rustic Charm]

Originally Posted by atoll

here you go!

Thanks Atoll, i can now picture what your suggesting.

Id be interested to see what others think.

I can grasp how that operates, but i would have thought that the moment the rudder moves under the pressure of the water on the trim tab, that the rudder moving would then throw the trim tab even further away. I guess im questioning how it would recover to go the other direction?

Its also odd that i cant find any pictures of this set up?

[atoll]

Originally Posted by Rustic Charm

Thanks Atoll, i can now picture what your suggesting.

Id be interested to see what others think.

I can grasp how that operates, but i would have thought that the moment the rudder moves under the pressure of the water on the trim tab, that the rudder moving would then throw the trim tab even further away. I guess im questioning how it would recover to go the other direction?

Its also odd that i cant find any pictures of this set up?

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.

[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.


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

[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.

in engineering terms what rustic charms set up is like,is having 3 universal joints on a shaft when only 2 will work in alignment.