16' Half Cab Porpoising, Advice needed

[fender202]

Hi all, have had my boat for around 1 year and while it gets up on the plane and goes well, I do notice in any sort of chop the boat is a bit up and down in the nose causing me to either back the speed right back or trim the nose down using lots of fuel and putting the boat on a semi plane.
I have noticed it does a lot better with a couple of passengers sitting on the vee berths at the front. I have my fuel and batteries at the back, so obviously I'm at a disadtvantage to start off with. I have a trim fin on the motor.

It is absolutely inpractacle for me to mount the fuel in the bow and an underfloor mount costs more than I am prepared to spend on this particular boat. (Its about a 1980 vintage NOVA 4.7m)
Anyway, I never worried much, just figured thats the way it was even though people I have known have told me what a classic hull these are and I really think it should be performing better.
I noticed a couple of jerry cans fit perfectly under the front berths so I was wondering if this would be considered a sensible fix, one with my spare fuel and 25l of water in the other one. Or should I just be slowing down and trimming the nose down.
Any help appreciated

[blaze]

really answer your own question
TRIM
dont be of the belief that you are saving fuel by having a poorly trimed boat, trim your boat correctly (you have already said this fixes the non problem), enjoy your boating and relise that at the end of the day fuel is really only a small cost of owning a boat in the big piture.
cheers
blaze

[MulletMan]

Chuck a couple of bags of sand up the front of the bow and as Blaze said, work on the trim!
Might be worth a look to see if the outboard leg is adjusted to the innermost holes on the transom bracket.
If it is one or two "out" then this will make the bow up pitching more pronounced.
Might be wrong but didn't that old Nova come without a deep V?
If so, then like its mate the Carribean of that era, the lack of a good V keel up front won't be helping you to keep the nose down.

[fender202]

Oh its very much deep vee with very high sides and really beamy, that's why I like the boat so much, very roomy fishing platform for it's size. As for working on the trim, thats what I thought I was doing? For the first 6 months I was driving the boat around, I had the nose trimmed right down. I was driving around on the river one day and I trimmed up instead of down by accident. The boat just popped oput of the water and soared! I thought I had it on the plane before but the revs just picked right up and the boat just skimmed across the water and then I reallised I hadn't been on a plane at all. That was fine in the river with nice calm conditions but as I say, once out in a bit (even a little bit!) of swell it starts see sawing, just like it's a bit light in the front. Once it starts doing it, it seems to get worse until I trim right down and slow down. I have a trim guage and when it is set to level or in the middle, thats when the boat pops un on the plane. Before when I had been driving around with the nose down to counterbalance the boat it was pointing 45 deg below this. ANyway, was out in the bay yesterday, a little wind about 15 knots and just a bit of a bump on the water. Was finding it hard to maintain speeds of over 20 mph without the rocking horse effect. I got my partner to sit up the front on the vee bunks and immediately could do 30mph cpmfortably and stably

[vertico]

i might suggest a foil ?

[fender202]

have a foil

[seatime]

What about the length of the leg, not sure if too long or too short would make it porpoise, but the wrong leg length combined with trim might be the cause? I drove an 18' half cab years ago that did the same thing, we worked out the leg was too long. This could vary between hull shapes, just a thought, cheers.

[blaze]

still reckon ya trimming tooooooooo much and creating the problem.

just cause the front of the boat is down doesnt mean its not planing, I think what you are experiencing is the normal operations of a poorly trimed boat.
When trimming
when the motor revs up and you start to "fly" just trim down a little (forget about your gauge, trim needs to be adjusted by revs and feel)
cheers
blaze

[fender202]

I hear where you're coming from but when the boat is properly trimmed it skates across the water and the revs pick right up. On smooth water its fine. If there is a bit of chop and I trim the nose down it still goes nice and smooth and is probably still planing but the revs and speed drop noticeably

[fender202]

I hear where you're coming from but when the boat is properly trimmed it skates across the water and the revs pick right up. On smooth water its fine. If there is a bit of chop and I trim the nose down it still goes nice and smooth and is probably still planing but the revs and speed drop noticeably. Its really just like a see saw effect with someone heavier on one end. Obviously with most of the weight in the back it tends to favour tipping back.
I'll put it this way. I have tried setting the trim in all different ways and to cut through roughish water at a reasonable speed, and I'm only talking 25-30mph at a very minimal chop, I have to trim the nose right down and while it still goes well it feels like the motor is working a lot harder. With someone sitting in the front, and the nose trimmed higher the boat skims and is a lot smoother. Again my question is it a feasable fix to add some weight to the front?

[skippa]

My way of viewing Trim is in your ride comfort. Just because one thing works for smooth water doesn't mean its right for all sea conditions.

I run a 18ft DeepVee glass cuddy, on a smooth sea I trim it right out (up) with just the bum in the water. Sure the motor works a lot easier cause there's a lot less hull in the water which equals less drag. This is obviously going to give the best performance and economy.

The idea of a DeepVee is to 'cut' thru the water/waves ( smoother ride/comfort), but it can't do if its still trimed up and out of the water. Trim is a balance between both which you have to decide; yes a boat will always porpoise if its light on the bow or trimmed up to much for the sea conditions.

As far as weight distribution goes, it has to be balanced along the boat, no good just having all the weight at the stern, front or sides for that matter too. Larger Deepvees will also be helped with trim tabs too as they tend to lean into the wind which results in loosing the balanced (level) plane, eliminating the benefits of the DeepVee design.

As the old saying goes, any boat is a good boat on smooth seas. Its a power boat with 2s outboard and its going to use fuel. Some days it will use more, some days less.

Short answer is Trim and balance.


Cheers,
Tony 8-)

[fender202]

Wouldn't adding a little weight to the front balance the weight in the back out?