Stability Problem

[neil.mccrirrick]

Picked up my new engine in Oban at the weekend. I managed to get it fitted and running while I was up there. Took it out on the water and big WOW factor. The old engine was a Mariner 75 about 20 years old, this is 2001 90 merc, take off was amazing WOT was about 42 mph and the prop is shagged. (new prop waiting to go on)

One problem though, I moved the trim about and in certain positions I was canted severley over to the left, but still going straight, I need to throttle off damn quick then set off slow and re trim to get rid of the problem. Not a nice experince at all.

The engine has hydrofoils on it, could that cause this? I didn't have time to get them off and go out again, be good to know if you think that may be the issue.

Or could it be the engine is at the wrong height? Its currently as low as it can go, which is how the Mariner was.

Boat is a 5m humber, not bow light or heavy, when I was on the plane and stable, cornered and planed fine, it was only when I slowed, retrimmed and set off again.

[JSP]

Can't help much there Neil but glad you got the motor ok.

[chewy]

Chine walking?

http://www.kencook.net/Chinewalk.html

I noticed when the Atlantic's got Yam 75's rather than the Evinrude 70's it walked alot more when trimmed out.

[codprawn]

Could be the fins or a knackered prop. As the engine produces a lot more power it is possibly torque effect - is the engine offset at all?

[neil.mccrirrick]

No I don't think so, it didn't want to "bounce" from one side to the other, it just wanted to cant to the left, like there was too much weight to the left, big time.

I wondered if it was torque from the engine? And if it set off slightly at an angle, if the hydrofoils would then keep the pitch at that angle? I could be talking bollox here?

So I'm thinking torque sets it off at a slight pitch, hydrofoils keep it like that and make it worse, (I never went above about 15 mph like this before I throttled back, definately not powering or steering out if it).

Without the hydrofoils it would self right?

I guess the answer like most things is to just experiment, proper stress when it gors though, it makes you feel like a capsize is on the way, (it never actually got that bad, i.e. no water in the boat, I think my missus screaming in my ear made my adrenalin pop up a level :-) )

[Bigmuz7]

Any pics ?... did it stay on the left, or slap to the right aswell ? *edit* Ah you've just answered that,.. any pics then ?

[neil.mccrirrick]

Originally Posted by codprawn

Could be the fins or a knackered prop. As the engine produces a lot more power it is possibly torque effect - is the engine offset at all?

Prop has got a lump out of it, I'll be changing that anyway. The engine should be fine, its gone back exactly where the mariner 75 was same holes the lot, and the weight of both engines is more or less the same give or take 10 pounds or so?

[martini]

Was it really flat calm? Sometimes one side of the hull can get "stuck" to the water in those conditions. Probably something to do with surface tension and all that.

[codprawn]

Originally Posted by neil.mccrirrick

Prop has got a lump out of it, I'll be changing that anyway. The engine should be fine, its gone back exactly where the mariner 75 was same holes the lot, and the weight of both engines is more or less the same give or take 10 pounds or so?

Yes but as the engine is a lot more powerful it will tend to lean to one side - it may not have mattered with the older engine. Basically some engines are mounted an inch or 2 off centre to counteract the torque. This could cause it to chine walk as well.

But first sort out that prop and take those fins off!!!

[neil.mccrirrick]

Originally Posted by Bigmuz7

Any pics ?... did it stay on the left, or slap to the right aswell ? *edit* Ah you've just answered that,.. any pics then ?

Here you go, don't know if this will help?