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Old 30 March 2009, 10:51   #1
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Stability Problem

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.
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Old 30 March 2009, 11:12   #2
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Can't help much there Neil but glad you got the motor ok.
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Old 30 March 2009, 11:20   #3
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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.
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Old 30 March 2009, 11:39   #4
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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?
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Old 30 March 2009, 11:45   #5
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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 :-) )
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Old 30 March 2009, 11:46   #6
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Any pics ?... did it stay on the left, or slap to the right aswell ? *edit* Ah you've just answered that,.. any pics then ?
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Old 30 March 2009, 11:48   #7
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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?
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Old 30 March 2009, 11:53   #8
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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.
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Old 30 March 2009, 11:54   #9
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Quote:
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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!!!
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Old 30 March 2009, 11:54   #10
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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?
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Old 30 March 2009, 12:01   #11
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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!!!
Ok, Prop coming off as are the fins, JSP if you are about Easter weekend a trip from Knott End might be in order? At best we can have a laugh, at worst you can fish me out of the drink :-)
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Old 30 March 2009, 12:18   #12
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Neil
I do not think it is your fins as i have them on my mariner 150. On looking at your pics I cannot but think that it could be the position of your cavatation plate relative to the bottom of your boat, have you checked it?

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Old 30 March 2009, 12:22   #13
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fiddle with the trim tab, that should go a fair way to sorting it out.
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Old 30 March 2009, 12:23   #14
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Does the cavitation plate need to be higher or lower than the stern of the boat? if its lower can that cause this?
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Old 30 March 2009, 12:39   #15
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[QUOTE=neil.mccrirrick;288450] Does the cavitation plate need to be higher or lower than the stern of the boat? if its lower can that cause this? [/QUOTE

Neil it needs to be level with bottom of the hull so the water flows undisurbed from the hull over the plate and passed the engine. It is worht a check especially with the fins on.
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Old 30 March 2009, 12:46   #16
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Ok thanks, I'll check that too. Its likely to the weekend after next before I can get back out on the water, I'll let you know if I get the problem resolved.

Thanks all
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Old 30 March 2009, 12:49   #17
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Ok, Prop coming off as are the fins, JSP if you are about Easter weekend a trip from Knott End might be in order? At best we can have a laugh, at worst you can fish me out of the drink :-)
Sounds good to me Neil! Will PM you next week
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Old 30 March 2009, 12:52   #18
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fiddle with the trim tab, that should go a fair way to sorting it out.
Can the trim tab do this? I thought it would just affect a drift in direction?
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Old 30 March 2009, 12:57   #19
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just found this on another site

Under most conditions, the engine will be trimmed fairly close to the transom. However, once on plane, the engine can be trimmed slightly further away from the transom to obtain maximum efficiency. Experiment with trim until you obtain a comfortable ride based on the boat's load and sea conditions.

Large propellers on high horsepower engines normally create port list conditions on "V" hulls. If desired, the installation of trim tabs will correct this condition. The installation of a transom mounted transducer on the port side will also reduce port list.

I'll be fiddling with the trim tab a lot then :-) as slim tim advises
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Old 30 March 2009, 13:03   #20
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Aye the wee tab which is also an annode behind the prop he means as it looks like it has no offset to me, however I dont think thats your only problem, and it generally only improves the torque against your steering anyway but its woth kicking it round a notch or two.... its impossible to tell, but there might me some mis alignment in your engine that the fins are exacerbating, take them off as others have said, get that horrible prop off and go from there
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