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Old 30-03-2016, 05:16   #91
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Re: Raymarine St4000 with Trim Tab Problem

In this picture ive found the auto pilot is directly on to the trim tab and it seems the main rudder is tide to stop it working. Im wondering if this is what i should have tried. The trim tab basically becomes a small but conventional rudder.
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Old 30-03-2016, 05:39   #92
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Re: Raymarine St4000 with Trim Tab Problem

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Originally Posted by GILow View Post
So I am coming in a little late on this, but I would like to contribute something because I have been down this path recently.

What is being proposed should work, but as acknowledged, it may not have the right kind of feedback to prevent hunting.

So I would like to step back a bit, and remark on something about the tiller pilot software. From my own personal experience of reinventing that particular wheel, I discovered that the TP software must work on the assumption that the rudder position has moved with the movement of the tiller pilot. This seems obvious, but consider this, the corollary is that the TP does not think the rudder has moved BACK to the centre until the TP moves it back. With this basic assumption, a whole lot of the parameters of the TP software are defined.

So for the TP software to work optimally, you really want the activation of the trim tab to move the tiller to a point and HOLD IT THERE, until the TP reverses the stroke. Any other system is going to defeat whatever self learning systems are programmed into the TP software, and probably cause it to hunt much more than is good for it.

To do this you need a very strong imperative in the design that will make the rudder want to slavishly track the position of the TP arm.

Jim hinted at it in post #19, and I assume he discovered this principle when designing and building his own trim tab powered auxiliary rudder windvane. You need a system that brings the rudder over and holds it in that position until the "signal" from the vane is cancelled. The simplest way I found of doing this with a trim tab rudder is to place the pivot point of a SECOND longer control arm slightly FORWARD of the pivot point of the rudder, with a more of the second control arm length forward of the pivot point than aft of it. This has two effects. It will reduce the movement of the trim tab in relation to the movement of the TP ram, and it will cause a self-centring of the trim tab as the rudder swings across, because the forward arm of the second lever is quickly moved to a "neutral" position relative to the rudder by the movement of the rudder arm. Also, if the rudder "overshoots", the same mechanism will reverse the trim tab to bring the rudder back into alignment.

A little fiddling with my favourite prototyping tool (Lego) found a nice apparent balance with the pivot point of the second arm approximately half the distance forward of the rudder pivot point as the trim tab pivot point is aft. After that, a 1:4 length ratio of the longer arm about the pivot point also seemed to "feel" about right, but this is clearly one of those cases where you drill half a dozen holes in the second pivot arm and move the attachment point of the TP ram until the whole system responds as you want it to.

What is really clear from the prototype is that this system will utterly and totally self correct. If you have some Lego I suggest you give it a go and see for yourself.

This will allow the TP software to work exactly as though it were connected to the rudder, and make full use of whatever A.I. was programmed into it to improve its response and minimise actuator time.

Matt
This has got to be the ultimate version. And I have seen at least one system rigged up this way. Its more complex, with more moving parts but has lots of advantages in terms of possible adjustments to get it working just right.

Many years ago when I was a kid I remember making a lego trim tab system just like that to try and visualise how the feedback worked. It taught me alot.

Maybe you could video it working and post it here so Ted can see exactly how the feedback works.
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Old 30-03-2016, 05:47   #93
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Re: Raymarine St4000 with Trim Tab Problem

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Originally Posted by Rustic Charm View Post
In this picture ive found the auto pilot is directly on to the trim tab and it seems the main rudder is tide to stop it working. Im wondering if this is what i should have tried. The trim tab basically becomes a small but conventional rudder.
It looks to me like this system has exactly the recomended positive feedback ratios, with the trim tab tiller swinging about 20% aft of the main tiller pivot and should work pretty well as a proper trim tab.

Not sure running a trim tab as a rudder would work well in any sort of wind or sea state.
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Old 30-03-2016, 05:57   #94
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Re: Raymarine St4000 with Trim Tab Problem

Quote:
Originally Posted by Rustic Charm View Post
In this picture ive found the auto pilot is directly on to the trim tab and it seems the main rudder is tide to stop it working. Im wondering if this is what i should have tried. The trim tab basically becomes a small but conventional rudder.
I don't see it working that way. To my eye, the trim tab is leveraging the tiller post directly, probably supported at the bottom on the trailing edge of the rudder.. And, at a quick glance, that system also self centres on the movement of the arm, but I have been coding for a few hours now and my eyes are too blurry to find the bits of Lego to test the theory tonight.
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Old 30-03-2016, 05:57   #95
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Re: Raymarine St4000 with Trim Tab Problem

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Originally Posted by Rustic Charm View Post
In this picture ive found the auto pilot is directly on to the trim tab and it seems the main rudder is tide to stop it working. Im wondering if this is what i should have tried. The trim tab basically becomes a small but conventional rudder.
Be serious, RC.


Whoever designed your yacht drew a rudder with the necessary hydrodynamic profile to complement your hull.


Get out your copy of Norman Skene's Elements of Yacht Design (or any equivalent) and read about a Flettner rudder.


The function of your Flettner rudder (aka trim tab) is to turn your rudder.


So why have a Flettner rudder? Why did Anton Flettner bother inventing the jolly thing?


1. A Flettner rudder saves power: you only need a tiny force to turn the trim tab. The trim tab then turns the rudder. So you helm your boat manually by a tiller to the trim tab with much less effort (your muscle power, or an autopilot) than manual work on your rudder tiller.


2. A Flettner rudder reduces torsional stress in the pintles or post of your main rudder. And may (if you avoid oversteering by providing the appropriate mechanical and electronic damping) give you a steadier helm. Remember that forces on your rudder come from both the tiller and the sea (waves striking the rudder, the hydrodynamic forces).


Those advantages are not produced if you lash your tiller. Trying to helm your boat with a fraction of your rudder's hydrodynamic profile is a recipe for disaster.


Here're a couple of pages from Norman Skene. Trim tabs (or Flettner rudders) are established, proven technology on yachts. Don't play silly buggers.
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Old 30-03-2016, 06:04   #96
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Re: Raymarine St4000 with Trim Tab Problem

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Originally Posted by Alan Mighty View Post
Be serious, RC.

Whoever designed your yacht drew a rudder with the necessary hydrodynamic profile to complement your hull.

Get out your copy of Norman Skene's Elements of Yacht Design (or any equivalent) and read about a Flettner rudder.

The function of your Flettner rudder (aka trim tab) is to turn your rudder.

So why have a Flettner rudder? Why did Anton Flettner bother inventing the jolly thing?

1. A Flettner rudder saves power: you only need a tiny force to turn the trim tab. The trim tab then turns the rudder. So you helm your boat manually by a tiller to the trim tab with much less effort (your muscle power, or an autopilot) than manual work on your rudder tiller.

2. A Flettner rudder reduces torsional stress in the pintles or post of your main rudder. And may (if you avoid oversteering by providing the appropriate mechanical and electronic damping) give you a steadier helm. Remember that forces on your rudder come from both the tiller and the sea (waves striking the rudder, the hydrodynamic forces).

Those advantages are not produced if you lash your tiller. Trying to helm your boat with a fraction of your rudder's hydrodynamic profile is a recipe for disaster.

Here're a couple of pages from Norman Skene. Trim tabs (or Flettner rudders) are established, proven technology on yachts. Don't play silly buggers.
Ok ok, appropriately repremanded i get the whole Flettner rudder concept. It was quite obvious when i first tried it. It's the mounting the auto pilot and getting it to work that is frustrating the hell out of me.
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Old 30-03-2016, 06:10   #97
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Re: Raymarine St4000 with Trim Tab Problem

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I see what you have drawn, and this 'counter' would mean the auto pilot will operate in the same direction as the main tiller. So no need to reverse the control head wires.

But i dont understand how all of this counters it's desire to over reaact is it is now?

Are you suggesting Atoll's will not fix the problem ive been having?

I am not sure if Atoll's system will fix your main problem, it probably will, but I don't see how it can self centre on the position established by the TP ram.

And I think this is important because I am more concerned with making the most of the A.I. in the TP system. I spent days coding the software for my home made T.P. and in doing so I developed a real respect for autopilots that "learn" as they go. The number of registers you have to maintain in hardware to adapt the TP behaviour is a PITA, particularly when using limited hardware like the Picaxe chip I used for mine.

Snowpetrel suggests I make a video of my Lego system in action. I will try it tomorrow when there is better light.

FWIW, I actually think the fabrication of my design is far, far simpler than what you are trying to build. One pivoting arm, could be made from timber, pivoting on a SS bolt, with a slot on the short end to engage a pin fixed to the existing trim tab arm. The TP ram can stay where it was originally, and a few holes drilled in the long end of the arm can be used to adjust the mounting point of the TP ram once you are under way. Downside would be drilling a hole in the existing tiller, but you could avoid that by lashing the pivot bolt to the side of the tiller.

FWIW, I made superb tiller for the Austral 20 using a nice big single block of Kapur. It was strong as iron, but lovely and light, plus a very nice rich red/brown colour.

Matt
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Old 30-03-2016, 14:28   #98
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Re: Raymarine St4000 with Trim Tab Problem

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Originally Posted by Snowpetrel View Post
It looks to me like this system has exactly the recomended positive feedback ratios, with the trim tab tiller swinging about 20% aft of the main tiller pivot and should work pretty well as a proper trim tab.

Not sure running a trim tab as a rudder would work well in any sort of wind or sea state.
Yep it's the 'positive' aspect of being aft of the rudder axis that i temporarily forgot about. It's the whole positive aspect we are hoping will fix my problem arnt we?

Tonight ill whip up another ply bracket so that the trim tab tiller faces aft, just so i can try it. If it doesnt immediately indicate it will fix the problem, then ill go back to using the current trim tab tiller and mount the pin further back 'aft' of the rudder axis. In fact, ill just drill holes in three positions so that I mount the bolt on pin and try different positions. Then ill figure out someway to 'temp' mount the AP on the outside of the transom, as thats where it will need to go in order to get a positive aspect. For security ill need someway of tieing it in to..
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Old 31-03-2016, 00:29   #99
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Re: Raymarine St4000 with Trim Tab Problem

Sounds like a plan. Good job you didnt get it all nicely welded up ehh.. maybe copy from that pic you posted and make a temporary extension aft for the unit if need be out of plastic plumbing fittings. Plastic plumbing fittings and cable ties score top marks from me!
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Old 01-04-2016, 00:16   #100
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Re: Raymarine St4000 with Trim Tab Problem

Well, i took the day off work to go and test this new contraption. And I have some good news and some bad news.

The GOOD it works, in principle.

At least I can see its going to work. The problem is that it wonders up to 45 degrees. It helps with playing around with the rudder gain. 1 is nuts and its too slow to react. 5,6,7 and its starting to be quick enough in reacting but it then kicks the ram off the spigot. Ill post a video of this soon. But, i think it's just a case of playing around with a solid enough contraption, settings, and with distance out from the trim tab (what you guys called the positiveness i think) and also probably need to check the computers accuracy setting as it might be set on holding a strict course.

The BAD, is that making it out of ply on the trim tab end is simply not strong enough and kept bending in the wood. Then it just keeps falling out as you will see in the video.

But, i definately think this is going to work. The other thing I noticed is that if the boat is only going slow, it doesnt really work at all, but if im going at about 4 + knotts then it seems to work better, but again within about 45 degrees and then it swings back.
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Old 01-04-2016, 00:22   #101
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Re: Raymarine St4000 with Trim Tab Problem

So, is the whole ram moving with the tiller in that setup?


Sent from my iPhone so apologies for weird autocorrect typos.
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Old 01-04-2016, 00:31   #102
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Re: Raymarine St4000 with Trim Tab Problem

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So, is the whole ram moving with the tiller in that setup?


Sent from my iPhone so apologies for weird autocorrect typos.
Yes it is Matt, this is Atoll's recommendation. And as it's moving with the ram, then the ram motion is the same as if i had it on the rudder, so I had to reverse the wires on the control head back to their origional direction.

Another unexpected positiveness to this system is that I can take over in an emergency just by hitting the 'standby' mode. I dont have to disconnect the ram to take over.

Ive got a video to show you guys if I can work out how to upload it.
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Old 01-04-2016, 01:23   #103
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Re: Raymarine St4000 with Trim Tab Problem

Keen to see the video. Sounds like the plywood is to thin. Can you screw a doubler under it? Frankly I am supprised the loads are that high, all the trim tab systems ive used have been fingertip. But then the 4000 ram does get a bit brutal when it decides to leap into action...

Is it +/- 45 degrees heading, or rudder angle? If its heading I would be a bit concerned...

How well does the system work when its directly conected to the tiller? Mine used to steer her pretty well. In flat water it would only swing a few degrees.
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Old 01-04-2016, 01:41   #104
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Re: Raymarine St4000 with Trim Tab Problem

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Keen to see the video. Sounds like the plywood is to thin. Can you screw a doubler under it? Frankly I am supprised the loads are that high, all the trim tab systems ive used have been fingertip. But then the 4000 ram does get a bit brutal when it decides to leap into action...

Is it +/- 45 degrees heading, or rudder angle? If its heading I would be a bit concerned...

How well does the system work when its directly conected to the tiller? Mine used to steer her pretty well. In flat water it would only swing a few degrees.
I can't seem to upload the video off mu ipad. Ill have to do it shortly from my computer instead.

The reason it kept damagkng the thin ply was simply because it didnt know what the end stops of the max ram extention and return is. So everytime it reached it's limits it would pop out if the spigot, which was loosening it. I think there is a way to set this up. Ill be reading the manual tonight.

It's got wbout a 30 degree sweep that it does from left to right. So when i put it on dead North heading it will swing from say 345 to 15 degrees, or maybe a touch more.
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Old 01-04-2016, 02:36   #105
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Re: Raymarine St4000 with Trim Tab Problem

30 degrees, +/- 15 deg isnt as bad as +/-45 but it still isnt great. Hopefully the video might shed some light on whats going on.

I assume it was in the river, in flat water with out the nasty tidal eddies you get in the river in places, or much gusty winds. It should be nearer +/-3 deg in flat water motoring.
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