Porpoising Boat

[thepato]

I recently purchased a new outboard for my boat and have had problems with the bow bouncing all over the place. I fit a 96kg 40hp 4 stroke to my 200kg 4m Edgetracker. I then had some small trim tabs welded to the bottom of my boat and I thought this had fixed it but on the weekend the I could not get the bow to stop bouncing. I don't want to fit a hydrofoil if I can avoid it but would this fix the problem or do I have any other options? Seems like it's a fairly common problem. I would like to hear what has worked for others.
Thanks

[dogsbody]

Gday Thepato

Had the same problem with my first boat 3.85 v-punt 25hp.Tried trimming the motor in and out but to no avail. I put on a foil and problem solved,lost a touch of top end speed but much quicker out of the hole and felt more stable in turns. Did'nt mind losing a bit of speed you can only go as fast as the chop will allow. You may have to bite the bullet and fit one. I was glad i did. Hope this helps.

Dave.

[2iar]

Probably best to look at the height of your engine first, before fitting a foil. I had the same problems and raised the motor a notch, and it's much, much better.

As a rough guide, your cav plate should be level or just above the bottom of the transom. I'd guess yours is a little bit low.

Good luck,
Mike

[BaysideMarine]

I would be thinking that 100kg engine on the back of a 200kg hull is the main problem.

[Roughasguts]

Yeah BM, has a point I have 127 kg hanging off the arse of my 460 Kg hull.

She doesn't bounce up and down but she certainly is bum heavy.
I think your motor hull combination would certainly be fighting the odds.
With that Motor being 50 % the weight of the hull, that balance point is way to aft.

Put pods each side of the motor would help. Or lots of 20 litre containers of water up front, to move that center of gravity forward.

[thepato]

Thanks guys,
I think I'll talk to my dealer and look at giving 2iar's suggestion a go as it is not too much trouble raising the Yamee a notch. This will lift the engine 20mm. As far as the weight goes I have about 50 litres of water in the well and two deep cycles up the bow so I've already got about 70kg of extra weight up front. I'll let you know in a couple of weeks how it goes.
Patrick.

[Noelm]

why would you get 'some small trim tabs welded to the hull" but not want to fit a foil? that may fix the problem, though I am thinking it may be more of a problem than that.

[thepato]

Noelm,
I'm looking at upgrading the boat within the next two years so I don't mind making alterations to the boat but I would prefer not to touch the outboard. I really dont feel like drilling holes in the cav plate of my new investment.

[FNQCairns]

yeah don't drill any holes, 2iar is right on the money, porpoising is not the result of a too light bow/heavy stern it's the result of too much drag (engine leg) and slip (propeller), raising the engine allows the bow lift proponent in thrust to overrule the leg drag and keep the bow up -no porpoising. Very basic equation used world wide in power race boats of every kind.

Foils only fix symptoms and besides who would run one low enough to be in the water at planing speeds :D

Only 2 caveat here: have you ruled out a hull distortion? if not try to discount this first and also ensure your prop is in good condition as any increase in slip from a dodgee prop will not help the situation.


cheers fnq

PS got enough negative trim? -wedges?

[thepato]

I ended up raising the motor a notch but that didn't make any difference at all so the next step was to add the wedges for more negative trim. I also dropped the motor back down a notch to it's original position. Took it out for a run with no weight up the front of the boat and it ran beautifully. Hopefully the problem is now fixed.
Thanks for your suggestions.

[Smelly]

Thopeto - Bennetts trim tabs are your answer.

Check em on web.

[peeler]

good result

[mark221263]

I had a similar problem with a 16ft glass boat when trimmed up, fitted a permatrim foil and all was fixed. Planed slower, no porposing and also felt lie a longer boat ride wise.

Cheers Mark