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Old 08 May 2012, 14:13   #1
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Automatic Engine Trimming?

Is it possible, or am I barking up the wrong tree?

Have been wondering whether a hydraulic or pneumatic ram, attached to the transom and resting (perhaps in some sort of cradle) against the leg of the outboard, could be used to create an automatic trimming of the engine.

My guess is that the pressure on such a ram would be greatest when the boat is at low speed, ie not on the plane (trimming the motor in), and that this pressure would reduce, effectively allowing the motor to trim out, as resistance reduces on the plane.

I'm guessing I must have missed an obvious reason why this does not (so far as I can see) appear to have been done before.

Please put me out of my misery before I waste any cash trying it!
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Old 08 May 2012, 14:45   #2
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I'm not sure why you'd want to? The optimum trim for the engine isn't directly related to speed if that's what you're suggesting.

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Old 08 May 2012, 17:30   #3
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I think your theory is slightly back to front too. The force against your transom is greater when on the plane than at displacement, otherwise you would not need to increase the revs/power to get onto the plane. As a result your auto trim system would be trimming the engine in when planing not out.
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Old 08 May 2012, 17:37   #4
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Quote:
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I think your theory is slightly back to front too. The force against your transom is greater when on the plane than at displacement, otherwise you would not need to increase the revs/power to get onto the plane. As a result your auto trim system would be trimming the engine in when planing not out.
Really? I'd figured that the boat has greater resistance when in displacement than when on the plane. I appreciate the prop is generating greater thrust as the revs rise, but assumed (perhaps incorrectly) that the reduction in drag would more than compensate for this.
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Old 08 May 2012, 17:45   #5
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I'm not sure why you'd want to? The optimum trim for the engine isn't directly related to speed if that's what you're suggesting.

Sent from my portable speaking device using Rib.net
Whilst I appreciate that the optimum trim is not directly speed related, I do know that my Rib rides better at low speed with the engine trimmed in, and better on the plane with it trimmed out a couple of holes. Trouble is, with manual trim I end up setting a compromise position.

The Smart Tabs I fitted recently have certainly improved the situation by helping to keep the bow down when not on the plane, and by getting it on the plane at lower speeds than previous, but it's still not ideal. The struts on the Smart Tabs got me thinking as to whether a similar set up between the leg and transom might be the answer.
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Old 08 May 2012, 17:59   #6
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Technically there is a period of time when you are just over the hump and on the plane where you could drop the revs slightly and still remain planing, but friction increases with speed, so overall, the faster you go the greater resistance there is and therefore pressure against the transom. True natural displacement (as in a sailing boat) requires very little thrust. Once you go beyond natural displacement speed the thrust required increases tremendously until you plane when it levels off somewhat, but never significantly reduces.
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Old 08 May 2012, 18:35   #7
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Okay, thanks Erin. I knew there must be a reason why it wouldn't work. Cheers!
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Old 09 May 2012, 03:44   #8
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Quote:
Originally Posted by Erin View Post
I think your theory is slightly back to front too. The force against your transom is greater when on the plane than at displacement, otherwise you would not need to increase the revs/power to get onto the plane. As a result your auto trim system would be trimming the engine in when planing not out.
That's interesting? I had also assumed that once on the plane, there is less hull making contact with the water as it skims across the surface, hence less drag, although there will be other issues such as aerodynamics kicking in.
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Old 09 May 2012, 04:37   #9
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The big power guzzlers on the plane are holding the boat up on the plane (esp. with a Deep V) & dragging the gearbox / leg through the water!



Couple of thoughts:

- the actual trim ram is an easy one - a lot of outboards already have them fitted, so all you would need is an automatic method of firing the up / down realys instead of the usual two switches.

- without repeating all of the above, but because of all those variables you would probably need as part of your control circuit:
a water speed sensor
a fore / aft clinometer
a throttle position sensor
rev counter
trim sensor (for feedback)

You would then need to do a fair few test runs to gather data on boat angle & speed at various trim, throttle, speed & load (i.e weight i nthe bow or stern) settings.

Then you can buy a 5 PLC and a couple of op amps from Maplin, and having worked out the optimal trim angle for various throttle / speed settings program in a curve (with feedback loop form the trim sender) and then overlay a boat angle modifier (to, for example handle the moment when you have 4 pie loving mates sat at the back and the settings you spent weeks optimising go out the window).

Probably also worth having an "E-stop" button in case it gets confused or the trim angle sensor packs in (which appears to be quite a common failure) & tries to motor the engine clean out the water or plant your nose into thre back of a wave at 30 knots.....

So yes, it's feasible on paper, but probably easier just pressing the switches on your throttle lever!
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Old 09 May 2012, 18:31   #10
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LOL! If 4 x pie-loving mates sat in the back it would sink, and if I had buttons on the stick I wouldn't have been considering an alternative!
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