Stability Problem

[chewy]

Does it do it regardless of how far in or out the engine is trimmed?

[slimtim]

Originally Posted by Bigmuz7

...and go from there

Or not as will be the case without a prop

You could also make sure there is a bit of offset of weight onto the starboard side, battery being the prime candidate.

[neil.mccrirrick]

Tried to offset the weight when it happened the third time, as I was expecting it, with me sat off to the right. Didn't level it at all, which is what made me think it was the flow of water over the hydrofoils that was creating the list.

Plus if I trimmed the engine out this seemed to remove it, I say seemed as I was tad nervous of turning at speed, (only me and my wife out and no kill switch, so if I'd gone over she was on her own).

[neil.mccrirrick]

Originally Posted by chewy

Does it do it regardless of how far in or out the engine is trimmed?

No engine trimmed out seemed to remove the problem, leastways it let me get to WOT and slow turn no probs

[mike-m]

Neil,

Sounds to me like you are driving the boat with too much "nose down" trim. This will give an unpleasant feeling of the boat trying to "twist" to port and it will also be in grave danger of bow steering (bow digs in and boat pivots violently around it).

Try trimming the nose down but NOT fully (those hydrofoils will magnify the effects of trimmer movement quite a bit), then steadily open the throttle. As the boat goes "over the hump" and the bow drops, you will hear the engine rpm increasing without further throttle movement. At this point you should be trimming out/nose up and you will find the boat will behave more normally. Once you have established a comortable trim position, try trimming in and out and notice what happens to your bow spray - the more you trim in/down, the further forward you will see it coming from under your boat, the further out/up the further back it goes. If you trim out/up too much at speed, the boat will start to "chine walk" - possibly quite aggresively, given that is a lot of horsepower for the size of boat.

I think the suggestion to remove the fins is probably a good one, at least until you have become comfortable with the trim effects of your new engine. Engine offset doesn't sound like the root cause here, so leave it alone until everything else is checked?

The cavitation plate should be level with the bottom of your transom in an ideal world, but half an inch lower will not adversely affect you and may improve your "grip" in turns.

That prop is shot - get a decent stainless one and enjoy yourself!!

This is all IMVHO, so if I am teaching you (or anyone else) to suck eggs, please excuse me....

Regds

Mike

[Chris]

Originally Posted by neil.mccrirrick

Does the cavitation plate need to be higher or lower than the stern of the boat? if its lower can that cause this?

1 - Throw that prop in the bin
2 - Loose the Fins
3 - Lift the engine on the transom so that the vent plate is 1" above the bottom

Once you have done the above have a fiddle with the trim tab however it would not be causing the issues you are having I suspect as much as anything that it's the fins and height of engine as well as you not trimmming it out quick enough.

[Hightower]

Originally Posted by Chris

Neil to get the best economy and speed out of the engine this is something you need to experiment with I run mine 2" above the bottom however I could probably go to 3"

I would start out about 1" above the bottom and see how it performs, if you loose grip in corners etc then drop it if it grips well then either stick with it or you could even go to 2"

I just raised my engine buy not only looking at the speed and holeshot but by assesing how she cornered too. I stopped going up when I had to start to trimming in when cornering.

[Chris]

Sorry hightower I edited after I looked at the Pics he posted on the previous page.

That prop looks like it made a pretty good depth sounder

[alystra]

If it's any help, my 5.3 Destroyer with similar engine runs straight and level at any speed and trim. Engine offset 2" to starboard.

Maximum speed I've had is 42 knots, trimmed right out (i.e. as far as the engine will go) in calm water. Not much more than a ripple will cause chine walking when trimmed like that, as will accelerating too quickly from say, 30 knots, as the sudden torque makes the boat heel and then she chine walks.

This is no problem for me. First, there is no way I can afford to run the engine at the 5000 plus revs. needed to go that fast, and second, trimming in slightly cures the chine walking, though the speed drops below 40 knots.

At my usual cruising speed of 23 knots (3600 revs.) there's never a problem. When we bought the boat, Humber said the engine was the maximum power and weight the boat was designed for and I do think she's a little stern heavy. Still, the 90 hp means I can fill the boat with people and still get a reasonable cruising speed without thrashing the engine - and my wallet - to death.

[willfinch36]

I've sort of got a problem like this - prop is fine though, probably because I have hydrofoils on? when you get up to top speed and trim the engine up, if you steer hard left then right and then left again the boat will feel very unstable as if it could easily flip, dont really want to take the foils off though as there will be a few holes in the engine where they were... but might try it...

[chewy]

Originally Posted by willfinch36

I've sort of got a problem like this - prop is fine though, probably because I have hydrofoils on? when you get up to top speed and trim the engine up, if you steer hard left then right and then left again the boat will feel very unstable as if it could easily flip, dont really want to take the foils off though as there will be a few holes in the engine where they were... but might try it...

You always not steer hard left then hard right. Most boats when trimmed out will feel heavier than normal when turning.
If you trim it in a bit for turning it'll turn alot quicker and sharper.

[neil.mccrirrick]

Originally Posted by mike-m

Neil,

Sounds to me like you are driving the boat with too much "nose down" trim. This will give an unpleasant feeling of the boat trying to "twist" to port and it will also be in grave danger of bow steering (bow digs in and boat pivots violently around it).

Try trimming the nose down but NOT fully (those hydrofoils will magnify the effects of trimmer movement quite a bit), then steadily open the throttle. As the boat goes "over the hump" and the bow drops, you will hear the engine rpm increasing without further throttle movement. At this point you should be trimming out/nose up and you will find the boat will behave more normally. Once you have established a comortable trim position, try trimming in and out and notice what happens to your bow spray - the more you trim in/down, the further forward you will see it coming from under your boat, the further out/up the further back it goes. If you trim out/up too much at speed, the boat will start to "chine walk" - possibly quite aggresively, given that is a lot of horsepower for the size of boat.

I think the suggestion to remove the fins is probably a good one, at least until you have become comfortable with the trim effects of your new engine. Engine offset doesn't sound like the root cause here, so leave it alone until everything else is checked?

The cavitation plate should be level with the bottom of your transom in an ideal world, but half an inch lower will not adversely affect you and may improve your "grip" in turns.

That prop is shot - get a decent stainless one and enjoy yourself!!

This is all IMVHO, so if I am teaching you (or anyone else) to suck eggs, please excuse me....

Regds

Mike

Mike, I think you are right, I was coming to that conclusion while I was on the water, plus having read up on hydrofoils they apparentkly "lift" the stern.

My usual set off routine is to be fully trimmed in, to give max manoevrability coming from shallow water and to then set off get on the plane and then trim out to reach cruising speed etc.

The port list was always at the start, I was hitting the trim button immediately to lift the engine, and throttling back. From standstill setting off again, this seemed to remove it, although not completely, i.e 90 degrees to a small swell re created it again.

I think some experiments with trim and a trim bar, (is that what its called), to prevent the engine going too low, may well fix this.

[9D280]

Yep, I had identical problem with the old Yam on mine. Had trim tabs (one bent!) when I got it, removed them, which improved it a bit, and then played with the trim bar. Unfortunately the "shortbread block" saga then reared it's ugly head so I never got to a final conclusion, but trimming out did seem to improve matters at speed. The engine was mounted dead centre.

Maybe it's a Humber design feature!

[DonMacN]

sorry if this is hijacking the thread a wee bit, but...

I'm about to have a new motor fitted and have been reading with interest about the location of the cavitation plate, whether 1" 2" 3" above the bottom or whatever folk have experimented with.

Sorry if this is an obvious question, but how do you play around with that? Does that mean drilling new holes in the transom or is there another adjustment somewhere? If it's new holes, I'd have thought putting a selection of holes so close together would be bad news?

[JSP]

Originally Posted by DonMacN

sorry if this is hijacking the thread a wee bit, but...

I'm about to have a new motor fitted and have been reading with interest about the location of the cavitation plate, whether 1" 2" 3" above the bottom or whatever folk have experimented with.

Sorry if this is an obvious question, but how do you play around with that? Does that mean drilling new holes in the transom or is there another adjustment somewhere? If it's new holes, I'd have thought putting a selection of holes so close together would be bad news?

The mount bracket will have a few mounting holes to set it at different heights.

[DonMacN]

Ah! OK, thanks. The old engine's gone, so I'd nothing to go out an look at. I'd hoped it would be something like that.