Automatic Engine Trimming?

[NSS]

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!

[martini]

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

[Erin]

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.

[NSS]

Originally Posted by Erin

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.

[NSS]

Originally Posted by martini

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.

[Erin]

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.

[NSS]

Okay, thanks Erin. I knew there must be a reason why it wouldn't work. Cheers!

[Rokraider]

Originally Posted by Erin

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.

[9D280]

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!

[NSS]

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!

[Cookee]

Originally Posted by NSS

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!

I think we've hit the nail on the head here - buttons on the stick!

Or to put it another way NO!

[9D280]

o-o-o-o-o-o-k. if that's your problem find the nearest outboard breaker with a dead set of Suzi remotes and buy a PT equipped lever from them. (it's always the body that breaks so levers are ten a penny)

If you don't have the extra cables in the engine loom for the up / down switches, a length of 3 core from B&Q aand a set of biullet connectors form your locaal auto factors will make a quick lead, but beware the "domestic" cable will corrode at 10 times the speed of the official suz wiring so you'll liklely have to replace it every couple of years.

[Searider]

You'll often hear about boats having a "hump" speed ot "climbing over the hump" onto the plane.

The resistance of a planing hull is made up of two components, form drag and frictional resistance.

Form drag is in effect the effort required to push the boat through the water - displacement mode.

Frictional resistance rises as a function of speed (squared iirc).
and immersed area.

As speed rises the form drag increases exponentially until the boat can generate enough lift to start planing. Once it starts planing this form drag reduces. Frictional resistance will of course continue to increase as speed increases.
As speed increases immersed area should decrease a little but not much.

The resistance curve you get rises quickly at high displacement speeds, forms a "hump", drops a little before rising again.

Resistance = Thrust, so the auto trimming function would only work just as you have made it onto the plane, possibly trimming out a little. If you continue to increase speed it will trim back in as the load increases.

[NSS]

Originally Posted by 9D280

o-o-o-o-o-o-k. if that's your problem find the nearest outboard breaker with a dead set of Suzi remotes and buy a PT equipped lever from them. (it's always the body that breaks so levers are ten a penny)

If you don't have the extra cables in the engine loom for the up / down switches, a length of 3 core from B&Q aand a set of biullet connectors form your locaal auto factors will make a quick lead, but beware the "domestic" cable will corrode at 10 times the speed of the official suz wiring so you'll liklely have to replace it every couple of years.

Thanks 9D280, I didn't realise that was all I needed to do. The local breaker doesn't have any Suzi levers with PT buttons so I've ordered a brand new set direct from Suzuki and paid for express delivery. With luck it should be here in time for me to fit over the weekend.

[Cookee]

Originally Posted by NSS

Thanks 9D280, I didn't realise that was all I needed to do. The local breaker doesn't have any Suzi levers with PT buttons so I've ordered a brand new set direct from Suzuki and paid for express delivery. With luck it should be here in time for me to fit over the weekend.

Yamaha ones are totally sealed - don't know about the Suzuki ones ..........

[NSS]

Originally Posted by Cookee

Yamaha ones are totally sealed - don't know about the Suzuki ones ..........

No idea, but is the PT mechanism invisible on the Yamaha like it is on my Suzuki? ;-)

[9D280]

Originally Posted by Cookee

Yamaha ones are totally sealed - don't know about the Suzuki ones ..........

EH? 1 bolt undoes lever, two screws undo the "anti drop into gear accidentally" lever, which in turn reveals the switch?

Are we talking about the same thing here?



The levers & remotes are slightly swappable as well - I used a Tohatsu lever on a Yam box before I got a switch for my Yam remotes. (and I got the whole lever as well!)

Originally Posted by NSS

No idea, but is the PT mechanism invisible on the Yamaha like it is on my Suzuki? ;-)

Ha-a-a-a-ang on a mo. You do have a PT unit on your engine? I kind of assumed you were just looking at a fancy way of controlling it......

[NSS]

Originally Posted by 9D280

Ha-a-a-a-ang on a mo. You do have a PT unit on your engine? I kind of assumed you were just looking at a fancy way of controlling it......

Nope, as I said in post 5 of the thread, it's a manual trim. Suzuki don't make a DF25 with PTT. And there was I thinking you were trying to wind me up (for asking such a dumb question in the first place) by suggesting I only needed a lever with a switch ;-)

[Cookee]

OK forget I said anything at all - I think I was on the same page as 9D280!

[9D280]

Originally Posted by NSS

Nope, as I said in post 5 of the thread, it's a manual trim. Suzuki don't make a DF25 with PTT. And there was I thinking you were trying to wind me up (for asking such a dumb question in the first place) by suggesting I only needed a lever with a switch ;-)

Sorry. Must learn to read my own language sometime.....

It might be worth having a trawl through Brownspoint Marine's Microfiches. I bet the DF25 clamp / swivel assy is also used for other engines. Hopefully bigger engines that have PT fitted.

If I get time Later I'll have a quick peruse. The DT25 also doubled as a 30, which assuming a similar "bits share" these days a 30 should be big enough to warrant PT as an option....