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Old 20 April 2006, 07:48   #1
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trimming engine

OK this is a question that will probably make most people thing "doh what is he like" but...

Not yet having done the RYA powerboat courses (not available here) I am learning things mostly by trial and error at the moment, but one thing is puzzling me.

I have found on my RIB that unless you have the engine trimmed quite a long way up, at any sort of speed you get a huge rooster-tail of spray from the stern coming up at an angle of about 60 degrees, at anything over about 15 knots, so much that it comes up the side of the tubes and a certain amount comes into the boat.

This happens with the engine trimmed in the horizontal position or anything below that, but it doesn't make sense to me - it should (I would have thought) be the opposite, in other words it would spray water everywhere if the engine was too far up and the prop was near the surface. In fact several people asked me in the first few days if I was running with the engine trimmed too far up, because of the spray. I wasn't, in fact it was about in the horizontal position but since then I have been running it with the engine trimmed a couple of notches up on the gauge, to avoid this happening. I have never noticed it on any other boats.

I was under the impression that for best speed you would normally trim the engine down a bit, to lift the stern up out of the water a bit more and get it more up onto the plane, but you just can't do this on mine as it sprays too much. I have done some experimentation on mine and in fact there is a slight maximum speed advantage if you trim the engine down a bit (i.e. forcing the stern into the water), only about 2kt or so though.

The only thing I can think of is that it is some relationship between the height of the cavitation plate on the engine, and the bottom of the transom. If you can imagine taking something flat like a garden spade and drawing it across the surface of a pond at high speed, the way the water would spray out the sides of the spade, that is the best description I can think of. I can't check the height of the cavitation plate against the keel until I take the boat out of the water, but having done a bit of digging on Google it looks like the cavitation plate should be set at the same level as the keel?

I had 4 other people in the boat the other weekend, all were sitting on the tubes behind me, and it seemed to reduce this a lot having the weight at the stern, I tried it trimmed down a bit and it didn't do this. Most of the time I have been running solo, with just me (drivers seat is just forward of centre and of course the weight of the console is in front of that). No extra fuel cans or anything.

Any thoughts, or suggestions? It seems that the motor should perhaps be set a little lower on the transom, but there is no space to do this.

Or should I just carry some fat people round in the back?

comments please

thanks
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Old 20 April 2006, 08:54   #2
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As you put on the power and get the boat up on the plane, start to trim the engine up (out). This will have the effect of lifting the bow and help to 'unstick' the hull from the water. You should gain speed, and you can then back off the throttle should you wish.

There are *far* more experience folk on this board, but I thought I would add my 2p worth, as I understand it.

--Andy
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Old 20 April 2006, 09:31   #3
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Sounds like your motor may be mounted a little low on the transom.
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Old 20 April 2006, 09:44   #4
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Hi Stephen

Some of what you typed is a bit confusing to me - and I'm not sure if its just the wording or not so ill start from scratch.

As I understand it.

Trimming an engine in or down is moving the leg of the engine closer to the transom and will have the effect of lowering the bow of the boat. It will also get the boat on the plane quicker and make it handle better in sharp turns. You should start from standing with the engine trimmed in/down and also use this setting when heading into an oncoming chop as it forces the steepest part of the V of the hull to pierce the waves.

Trimming an engine out or up is moving the leg of the engine away from the transom and will have the effect of raising the bow of the boat. It will make the boat much harder to get on the plane and make the boat much lighter in turns and more likely to skip sideways. It does however when at speed lift a lot of the boat clear of the water resulting in far less drag. If the engine is trimmed out/up too much the prop will start to ventilate and you will have to trim back in or throttle down to get it to bite again. You trim out for maximum speed as the lower drag allows the boat to move faster. Also when in a following sea trimming out helps prevent the bow from stuffing into the back of the next wave.

Typically you will start from a standstill fully trimmed in/down then once the boat is up on the plane trim the engine out/up gradually - you will hear the engine rpm rise as the drag is reduced and therefore load on the engine decreased and the speed will pick up. This will continue as you trim out/up until there will be a change in the way the engine sounds, a rise in revs but a drop in speed - this is ventilation setting in and this point will represent the highest point you can trim out in the current conditions - so you trim back in/down slightly to get the prop biting again and this should be your top speed. Remember this point on the trim gauge and you can easily return to it later.

Sorry if that was all "teaching granny to suck eggs" but i wasn't sure how much you already knew.
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Old 20 April 2006, 11:10   #5
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Roy,

thanks for that, the answer is I don't know much (so no eggs being sucked! only had it a few weeks and no previous experience, done about 15 hours now) though what you say is pretty much what I had assumed, apart from the bit about what setting to get max speed but I see the logic there now you have pointed it out.

The problem is that at the moment I am having to run with the engine trimmed up/out all the time otherwise you get this spraying effect. If you trim it in/down there would be water everywhere (even cruising at say 3200rpm/20kt which is what I tend to use for not-too-scary fuel consumption) and it is what is causing this that is puzzling me, it must be the effect of the cavitation plate being at or near to the surface of the water.

On the trim gauge what appears to be the "normal operating range" on the gauge has a centre mark (engine horizontal) and 3 marks either side, where I am having to run it is 2 marks up towards the "fully up/out position" if that makes sense. Anything below that, water spraying everywhere. I'll try and take a photo next time I am out to show people what I mean.

I can't fault the pickup from a standing start though, even when trimmed out at the position I have been running it in most of the time, when there is just me in it if you opened it right out from a standing start I would think you'd get to max speed (about 35kt) in probably under ten seconds, though I haven't timed it. I don't know how that compares to others but since I figure I could lose any irritating passengers over the back with a good shove on the throttle I guess it is OK

At high speed say 25kt+, would the cavitation plate on an outboard normally run just below the water surface behind the boat, just above it, or just skimming the surface?

Maybe it is just down to the weight distribution i.e. when you put a fat b&^*&%d like me up the sharp end, it doesn't like it
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Old 20 April 2006, 11:24   #6
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You haven't got loads of animals living on your hull, have you?
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Old 20 April 2006, 11:25   #7
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Most boats have the weight far too far back as the heaviest thing in the boat (the engine) cant be moved. so having weight up the front is usually a good thing.

As far as the cavitation plate goes - it should be roughly lined up with the keel of the boat when the engine is trimmed in - this basically means that it is at the surface when the boat is on the plane as the water streams off of the underside of the hull in a fairly straight line before it starts to "spring" back and form the wake.

If your engine is mounted slightly high or low (ie cavitation plate above or bellow the level of the keel) this may cause the effect you are experiencing but you are then beyond my realm of experience Ive not had much experience with trying engines at different heights my self but there are plenty of people here on RIBnet that have.

I would suggest taking a few photos of the engine leg trimmed in and out etc showing its position relative to the transom and keel and post them here - someone will be able to help.
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Old 20 April 2006, 11:26   #8
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Sounds like the engines too low or the shaft too long and its the drag from your lower leg that is sending spray everywhere.
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Old 20 April 2006, 11:52   #9
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There's an echo here!
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Old 20 April 2006, 12:01   #10
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Thanks - yeah I might do that, take some pics when I get it out of the water in a couple of weeks and see what people think.

The odd thing is though, that if you sit four people in the back the problem disappears, which I would have thought wouldn't be the case if the engine was set too low (it should get worse if you load it more).

I guess it has been like this from new as the supplier would have installed the engine before shipping I think.
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