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Old 28 August 2007, 13:44   #1
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Trim level for max speed

hi all, I canīt thank you all enough for the tips I have been getting from this forum. Iīve just ordered a RIB cover from Rib shop, got a box of 303 protector, been remote flushing my ob with no tap, cut a new hatch with a router, launch spot on most times.....& I could go on.

Anyway, today the Med was flat calm so we rushed out there to see how fast we could go. Until now itīīs been really choppy as the heat of the day sucks in the winds producing a large lumpy sea which in a 5m boat isnīt much fun.

So today, perfect conditions, sunny & flat calm, no wind. Usually we cruise at 4k rpm tops which keeps us on the plane so not to be slowed down by waves or wash. Today we got to 5500 rpm & got far more speed. So we started playing around with the trim having read several posts here to see how fast we can go.

So my question: does a RIB go fastest when it is flattest on the water? ie, with least drag? & do you know by trimming around & seeing the speed fluctuate? We got to 32 on the speedo with a Ribeye 5m Yam 80 & I think thatīs about all there is, 2 of us salad dodgers & a full fuel tank. Or do you trim up to get max hull out the water leaving I assume just the ob leg? & if you do the latter & hit a freak wave will the boat wipe out & dunk us in the sea?

Cheers all, much appreciated
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Old 28 August 2007, 13:49   #2
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I have a 4.7m Capelli Tempest with 60hp Yam, and found that it needed to be trimmed up quite a way to get max. speed in flat calm seas.

Basically, I found that the best speed came (33 knots) when trimmed up as high as possible - you can actually look back at the engine and see how close the prop is to coming out of the water. As long as the prop is fully in water and is not cavitating, or overspeeding, you are fine. This is keeping the hull out of the water as much as possible.

The boat will be slightly less stable if you hit wash or waves, so be aware of that.

Trimmed fully in the boat will only do 28 knots.
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Old 28 August 2007, 13:54   #3
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hi, thanks for speedy reply. Is it that the ob leg is additional drag that when trimmed right up you get a few extra mph? ie, you want to get as much of the hull & ob out the water?

Ta for tip on stability too, last place I am looking at max revs is the prop....
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Old 28 August 2007, 14:57   #4
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For Max speed trim the motor out as far as possible, when either the prop starts sucking in air (you'll here it) or the boat starts to chine walk too much and becomes unstable. you've trimmed up too much, also get the fuel level as low as possible and cut out the pies
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Old 28 August 2007, 15:54   #5
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Yep my Humber is faster by a few knots when you trim it right up, not really measured it but I would say maybe 3-4kt extra over the "flat" position. It doesn't really chine walk (or perhaps I should say it hasn't yet!) but it does feel noticeably more "twitchy" at top speed with the motor trimmed right out. It cavitates like hell if you try and turn like that with any power on though! It also hurts more when you hit waves, though I can't say I have tried a max speed run in rough conditions as it would be a bit daft...
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Old 29 August 2007, 04:26   #6
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As a racer I feel as qualified as anyone to answer this one! The above answers are spot on - keep trimming up until the revs start going up and the speed starts going down and then trim in a little - this will be your maximum trim angle for that setup. Of course if the boat starts to become unstable before that - back off or trim in! Keep a good look out for unsettled water, wakes etc as the more you trim out the more unstable the boat will be.

To get more speed you could move items towards the back of the boat like your anchor or your salad dodging mate - you might be surprised how much effect that will have! Then of course there is raising the engine a bolt hole at a time, changing props ...........
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Old 29 August 2007, 04:58   #7
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We had a 5m Playtime with 80 Yam... We had the engine lifted to the top hole since the way it was delivered gave pretty poor performance, lifting to the top jole and a SS 4 blade prop made it mauch more responsive and changed the engine note and character of the engine -well recommended

With the 4 blade SS prop the maximum speed was 34mph and from memory with the stock 3 blader it was 36mph.
BTW ours would rev to 6000rpm
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Old 29 August 2007, 09:42   #8
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Quote:
Originally Posted by TNDB View Post
BTW ours would rev to 6000rpm
2 or 4 stroke?
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Old 29 August 2007, 09:52   #9
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F80 - 4 stroke
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Old 29 August 2007, 09:54   #10
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Quote:
Originally Posted by TNDB View Post
With the 4 blade SS prop the maximum speed was 34mph and from memory with the stock 3 blader it was 36mph.
BTW ours would rev to 6000rpm

But slower
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