Outboard Height

Wayne Rogers · 13 July 2004, 20:19

How high can you fit an outboard on the transom???
What effect will it have on performance???
Will it handle any different at top-end speed
Will it effect speed???
Will it make plan any different???

Sorry for all the questions.

Having problems with it feeling unstable at speed, outboard is offset 35mm
Also when I trim in the Rib dives to port badly,is this due to the engine being to low.

Please help, any suggestion greatfuly recieved.

Thanks

Turbodiesel · 13 July 2004, 20:34

To start with the engine;s cavitation plate ( the one just above the prop) should be in line or just above the bottom of the hull.
If you using a Stainless prop you can usually go one hole up.
You will know if its to high because it will cavitate in a straight line if your running light and cavitate lots when your turning.
If the engine is to low it usually tends to make the boat bow steer, you might find the boat will lean the wrong way when turning.
What sort of revs are you getting out of the engine? You should be able to get 5500rpm max from a 90hp, when trimmed out, do you have to trim it along way out to get it to rev up?
If the engine is to low it will effet your speed as there is more drag in the water and you probably wont be able to rev to the max rpm without trimming out so far that the boat becomes unstable.

Hope this helps

Turbodiesel · 13 July 2004, 20:36

Sorry, just read that your 90 is a four stroke!!, you should be able to get the revs to a maximum of 6000rpm (5800 rpm is probably better though so its not running at max revs all the time)

quinquarimarine · 13 July 2004, 20:57

[QUOTE=Turbodiesel] To start with the engine;s cavitation plate ( the one just above the prop) should be in line or just above the bottom of the hull.

Sorry Turbo but do disagree. For general seakeeping the cavitation plate should be at a min in line with the hull or up to an inch below. Raising it will achieve optimum revs on an over-proped or under revd engine but the ideal is to have more than req power to achieve revs thus you can drop engine to reduce caviation.

Raising engine is false economy for fuel, engine life and stability long term.

If you are racing and in flat calm waters then I agree with Turbo but otherwise not keen.

Cheers

John
www.quinquari.co.uk

Wayne Rogers · 13 July 2004, 21:11

Thanks for the suggestions,what do you think it is that makes her lean to port if I trim in,it's very severe when it does it,does it at low speed on the plane and at high speed 40-45 MPH
I'm running a alloy prop at present engine revs are spot on.

Dirk Diggler · 13 July 2004, 21:48

[QUOTE=quinquarimarine]

Quote:

Originally Posted by Turbodiesel

To start with the engine;s cavitation plate ( the one just above the prop) should be in line or just above the bottom of the hull.

Sorry Turbo but do disagree.

Sorry quinquarimarine but do disagree, your suggestion is ok for the 12 seater minibus style rib that you favour, but for general use you wouldn't go wrong following turbodiesels advice.

tue · 13 July 2004, 23:26

When I re-fit my engine it is going to be 2 holes up. Apparently this increases top speed. improves handling + something else I can't remember. I'll try and find a thread I started a couple of weeks ago.

tue · 13 July 2004, 23:38

found it http://www.rib.net/forum/showthread.php?t=5284

Pete7 · 14 July 2004, 06:29

Tue, what it might do for you is allow a higher transom so the water doesn't slop in when you have divers getting kitted up always a problem on mine.

Pete

Turbodiesel · 14 July 2004, 09:23

The reason you can go up one hole when running a stainless prop is because,
Stainless props can run in less water, they have thinner blades so they create less turbulance in the water and have better grip. they dont bend anywere near as much as ally props. They also grip the water much better because they are much more cupped than ally props which give better performance.
If we ever change a ally prop to a stainless the engine should be lifted by one hole and the pitch should go up by one.
This is of course all in theory!! usually its very much trial and error. If we are rigging a boat at Barrus we will take it out and prop test it to make sure its all set up ok. Thats ok for us because we have all the props we need, i think for most people they dont have the endless amounts of props they can try!!

niels · 16 July 2004, 14:26

And if the tubes are out of the water at speed you must place the engine a bit ( 1 to 1,5 inch) to starboard!!! I never hear anyone about that. Why?

Pete7 · 17 July 2004, 00:03

Neils, the idea of the offset is to counter the torque from a single prop, however all it did on my ribtec was cause the boat to sit uneven when she was winched onto the trailer so we put the engine back on the centre line with no ill effects.

perhaps on a light weight speed boat with a big hp engine there may be some advantage, but most ribs are not in that league.

Pete

arnevandalsem · 18 July 2004, 00:53

Excuse me pete7,

about the offset of the engine, I own a ribcraft 6.8 powerd by a honda 225 and I have an offset off 1.5" (about 4 cm) and I would like it too be more to the starboard side off the boat. If I sail fast at little waves the boats still tends to lean to the port side. and I don't think the offset has anything to do with trailering!!!

Best regards Arne

Pete7 · 18 July 2004, 07:16

What I said was the only effect I could notice good or bad was that it made the boat sit on the trailer at an angle. I didn't notice any difference when the boat was used on the water.

Moving the engine wasn't difficult, why not have a go at it.

Pete