Trim level for max speed

[RibinSpain]

hi all, I can´t thank you all enough for the tips I have been getting from this forum. I´ve 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 it´´s been really choppy as the heat of the day sucks in the winds producing a large lumpy sea which in a 5m boat isn´t 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 that´s 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

[bernithebiker]

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.

[RibinSpain]

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....

[Johnwin]

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

[BogMonster]

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...

[Cookee]

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 ...........

[TNDB]

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

[Nos4r2]

Originally Posted by TNDB

BTW ours would rev to 6000rpm

2 or 4 stroke?

[TNDB]

F80 - 4 stroke

[bedajim]

Originally Posted by TNDB

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

[9D280]

No PTT on mine , but echoing the "load postitioning" comments, I have 2 30L Hulk tanks, and although my speed is only "perceptively" more when I empty the bow tank first, the handling is far more responsive & wheel lighter. (must get a GPS.....)

I have also played with the pin position, and can also agree with the trim up comments, granted I have to fiddle about in the water whilst hanging over the transom to make the adjustment! I've left mine in the top notch, although do occasionally bring it down one hole if I'm solo with only one tank aboard.

Granted it's a relatively small RIB, so loosing 30L of fuel from the bow does raise the nose quite a lot!

[TNDB]

Originally Posted by bedajim

But slower

4 bladers knock a few mph off the top end but give better low end thrust and supposedly a more "stable" mid range (can't say I noticed that bit)

[RibinSpain]

thanks for the advice & I looked at the ob setup today, it sits on about the middle or lower hole. If raising up a hole or 2 gives faster top speed, what is this at the expense of? thanks, don´t want to rush into raising it & finding it´s more unstable at mid range, which is where we are mainly using it.

Cheers all

[codprawn]

Rising it up will give you less acceleration and more ventilation of the prop in turns etc. having said that it all depends on the boat - some like the outboard higher than others. That is why some people use a transom jack.

[TNDB]

Originally Posted by RibinSpain

thanks for the advice & I looked at the ob setup today, it sits on about the middle or lower hole. If raising up a hole or 2 gives faster top speed, what is this at the expense of? thanks, don´t want to rush into raising it & finding it´s more unstable at mid range, which is where we are mainly using it.

Cheers all

There seemed to be zero ill effects from relocating the engine to the top hole. It actually changed the noise of the engine, it went from sounding sort of "drowned" to sounding quite "rorty" and powerful.
Don't forget I did this in combination with a new 4 blade SS prop - I never tried the standard ali prop once the engine had been raised but suspect the SS prop was an integral part of the set up. I would talk to Steel Developments if you intend to raise the engine, tell them what you have, engine revs etc and see what they say.....

[Chris]

Originally Posted by RibinSpain

thanks for the advice & I looked at the ob setup today, it sits on about the middle or lower hole. If raising up a hole or 2 gives faster top speed, what is this at the expense of? thanks, don´t want to rush into raising it & finding it´s more unstable at mid range, which is where we are mainly using it.

Cheers all

What hole the engine is set to is irrelevant surely? what you need to look at is the cavitation plate in relation to the bottom of the transom. if the cav plate is about an inch above the bottom of the transom then you probably won't want to raise the engine any further without risking cavitation however if the cav plate is at the same height as or lower than the bottom of the transom then you may be in a position to do so.

Am i right in saying this?

Chris

[TNDB]

Originally Posted by Chris

What hole the engine is set to is irrelevant surely? what you need to look at is the cavitation plate in relation to the bottom of the transom. if the cav plate is about an inch above the bottom of the transom then you probably won't want to raise the engine any further without risking cavitation however if the cav plate is at the same height as or lower than the bottom of the transom then you may be in a position to do so.

Am i right in saying this?

Chris

I think you are right in what you are saying, but because I had exactly the same boat/engine package then I guess the engine would sit at the same height on his boat as mine hole for hole ?
The top hole did put the cavitation plate right level with the bottom of the V

[9D280]

I assume we are all talking about the same adjustment here? The pin is the one that the engine "sits" against when pushing the boat along, and the anti- lift catch engages on? Raising the cav plate above the transom surely would involve unbolting the engine from the transom & puting in packing pieces / drilling more holes to raise it?

I ask because altering mine will simply alter the engine angle relative to the transom, or put another way, alter the vertical thrust angle of the engine. Being a SR, if I tuck it to the bottom hole, then as the transom in far from vertical it means the prop is trying to cimb out the water, stern gets pushed up, and the bow goes down. (quite unnervingly so!) The top / second top holes mean the engine is vertical - ish and so something approaching equilibrium is met!

I reckon mine needs another 20 or so horses to get it planing enough to get it high enough out the water enough to worry about the prop popping out, chine walking and all the other nice problems you guys have been talking about! What's your Engine like in the HP dept compared to the boat's rated max? If you're miles awaylike me then I guess that's one of the reasons you've not had any ill effects?

[BogMonster]

mine is 115hp boat is rated for 150hp and feels like it could easily handle more though I would not like to add a hell of a lot more weight on the stern and then be in a following sea...

[Chris]

Originally Posted by 9D280

I assume we are all talking about the same adjustment here? The pin is the one that the engine "sits" against when pushing the boat along, and the anti- lift catch engages on? Raising the cav plate above the transom surely would involve unbolting the engine from the transom & puting in packing pieces / drilling more holes to raise it?

No I was talking about raising the engine vertically up the transom which on larger engines is done by moving the transom bracket up a hole as they have about four holes to choose from.