Help needed with instability since eng fitted

[inj doc]

Hi Guys
I am having handling problems with a Humber Destroyer 5.5, which was fitted with a yamaha 75 2 stroke with no problems whatsoever but engine was ageing & left a choice to the fitters to replace. It was fitted with mecury100 4 stroke & whilst running in under half throttle no problem, but now if you open it up as soon as you touch a wave the boat goes into an uncontrolable Zig-Zag & you have to shut engine instantly otherwise it will throw you out the boat & I reckon the boat would screw itself into the water. Its very frightning!. Ok on calm water until it hits a wave. I had about an inch of freeplay in the cable steering & thought that was the cause & just had hydraulic steering fitted & no free play but no better. I can only discribe it as simular to a tank slap on a motorcycle. Has anyone come across this problem & how to resolve it as the shop said the steering would fix it!
I would appreciate any help & advice please.
Regards Inj Doc

[wavelength]

we have a couple of similar set ups (Ocean pro 5.5/merc 90 fourstroke and a 6m destroyer/ 90 suzuki four stroke). Trim is all important, with the smaller ocean pro in particular. Sounds like it might just need trimming up somewhat but of course I dont know the full story and you may have already been there and done that to no effect. Both of ours are excellent sea boats, are in constant use and are no problem but the do need to be correctly trimmed to the conditions on the day.

[wavelength]

conversely it might need trimming down Extreme up trim to get that last half a knot or so on the ocean pro with minimum weight on board can see it become unstable at speed and a little less trim will cure it.

[codprawn]

Sounds like chine walking - often caused by incorrect engine height.

[al40]

I agree - sound like Chinewalking. My SR4 did it if engine was trimmed out and was especially noticeable if there was a slight chop to set the boat a bit off balance. Trimming in pretty much removed the problem.

What trim setting are you using?

It may just be that the new engine gives enough power to allow it to chinewalk. With my SR I had no probs with a 25 must noticed it when I went up to a 50.

[fast fred]

the center of gravity has moved on the hull, which is makin it PLOW,
( chine walkin is when the hull teeters from one side to the other) which is not what you have said, move some stuff from the front to the back of the hull.

[wavelength]

I doubt swopping a 75 two stroke for a 4 stroke 100 has move the CoG appreciably forwards. Quite the opposite I would have thought. We often have "man sized" students who move about the boat altering its CoG sigificantly on courses, and on the next course they may all be lightweights. Moving a bit of gear from here to there would not be a realistic or worthwhile excercise whilst the trim used properly sorts it all out.

[inj doc]

Hi guys
Would like to thank those who made contributions on instability since new engine fitted. With the advice given am returning too boat yard for them to look at & will then have another go at trimming
Will let you know the out come
Regards Doc

[Gordon McMath]

Try moving the engine up,it could be riding on the gearbox.

[jwalker]

I think Doc's description of the symtoms may be causing confusion. As Fast Fred says from the descripion it does sound as though the boat is ploughing and attempting to hook a bit but, as Wavelength says, one would expect the 4 stroke engine to be heavier and the CG to move rearwards which would tend towards chine walking instability. And as Wavelength says, one wouldn't expect a bit of movement of the CG to affect the boat in a way which couldn't be trimmed out. However, since we are talking about a Humber Destroyer, I'll relate a wee story. My previous boat was a 6.5m Destroyer and we owned it for a good number of years. I felt it was a good handling boat and it was stable at 50mph but it did need fine control at that speed. Anyway, it was sold to a group of divers and it was arranged that we would launch, go a trip and recover so that they could get the hang of the system. The boat was normally set up to carry 2 persons and the seating was towards the rear although the fuel was forward of the consol and a pile of chain and an anchor was kept in the bow locker. So off we go, myself driving and my wife sitting beside me and 3 big divers standing behind and holding on, half a tank of fuel and no anchor up front. Well, it handled like a bu**er, chine walking at anything approaching 40mph. Trimming down brought back some stability but nothing I did made me confident once the speed increased.
Plainly it was possible to tip the balance of what was a good handling boat by making it stern heavy.

I wonder whether this and Doc's lack of experience could be contributing to the instability he is experiencing, especially if the boat is light and carrying little equipment.