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hull tuning

R

RickMc

Well-known member
Hey guys, I have a 4 cyl Lycoming on a 12 ft fiber glass hull. Before I added 5/16 poly to it it would hop badly in deep water at speed.It has a trim tab that would settle it right down. After I added the poly it rode hard on the nose as soon as I got on plane. After reading some post and looking into it I saw the back edge of the poly,along the transom was pulled down. I sanded the poly down and the ride got better. I recently was able to get the boat up off the trailer and run a string line under it. Where the rigging bolts in on the stringers and in the center stringer where I have 3 jacks, I can get it all flat. My problem is about a foot outboard of the stringers and a little over a foot forward of the transom, there is a spot on both sides that is about 3/16 pushed up. Any ideas on how to fix this? Thanks Rick
 
Your not going to like the answer but not much sticks to poly after the fact!
Build it up flat before poly or suffer another bad poly job because the hull prep was not done before new poly?

Kind of a no brainer with a water hose and good eye or a even a flash like at night. Your hull was rippled or hooked but you installed poly as it was.
Pull it. Flatten the hull. reinstall it!
As far as better or worse, 3 jacks kind of tells the tale to a bad bottom and jacks are easy. New straight bottom, not so much!
 
Prototype said:
Your not going to like the answer but not much sticks to poly after the fact!
Build it up flat before poly or suffer another bad poly job because the hull prep was not done before new poly?

Kind of a no brainer with a water hose and good eye or a even a flash like at night. Your hull was rippled or hooked but you installed poly as it was.
Pull it. Flatten the hull. reinstall it!
As far as better or worse, 3 jacks kind of tells the tale to a bad bottom and jacks are easy. New straight bottom, not so much!
Click to expand...



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Sounds as if poly was too hot when installed, ask me why I know! Both of the 2 new boats I've put it on plowed, and bad, had to get creative and Jack them around, you might need to build a custom truss to Jack out the areas you found convex with the string.
FWIW a solid straightedge is superior to string, plus a good run down an asphalt road will hone her flat.
 
I was really surprised the way it rode after the poly. It was completely different. Direct opposite. The bottom seemed flat upside down on the lift before I installed it. The glass is really thin in the rear third of the hull. 1/4 or less. That’s a big part of why I installed the poly. It’s pretty rocky on my part of the river. It did require a fair amount of heat to bend the poly around the chimes and I’m sure my first time installation isn’t perfect but what is the fix now? I thought of heating it up and trying to bend it down. Or unbolting the back 1/2 and adding shims or something. I did consider the dragging it a few miles down the asphalt. But I’ll be down to the bolts before I get it flat. What next ? Thanks Rick


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Can you raise the back of the motor? Possibly a few fender washers? You might get it up some.
Reality is the poly is acting like a wedge on the bottom.... and that is hard to overcome.

Adding another jack over the low spot in the hull might be an easier fix... as stated.

Good luck
AirHog
 
John Fenner said:
Sounds as if poly was too hot when installed, ask me why I know! Both of the 2 new boats I've put it on plowed, and bad, had to get creative and Jack them around, you might need to build a custom truss to Jack out the areas you found convex with the string.
FWIW a solid straightedge is superior to string, plus a good run down an asphalt road will hone her flat.
Click to expand...

I've never heard of too hot poly install? Normally it's too cold and ripples even the strongest bottoms?
Interesting thought about plowing though with a new boat and new hot poly. They should run real free with correct motor angle and weight setup.
Did hot poly install ripple a brand new hull with a new flat bottom twice? Or just make them plow?
How did they run before they were new?
 
No ripple effect at all, it sucked the bottom up so tight that the bulkhead top rail had an 1/8" crown at center on 1, the other I had to put .100" shims under the sheet metal truss style bulkhead. Finally got it to break free on top end, they were never run without poly.
 
I did both in direct overhead sunlight, rivet holes were off each side a good 1/2" when cold, a full inch up front, so it was too hot in my book, I think I will avoid poly anymore, too much to do with it for the money.
 
My experience is if you have a slight concave in front of the transom and bolt/ screw or rivet it tight to the hull it will act like a trim tab and force the nose down.
 
John Fenner said:
I did both in direct overhead sunlight, rivet holes were off each side a good 1/2" when cold, a full inch up front, so it was too hot in my book, I think I will avoid poly anymore, too much to do with it for the money.
Click to expand...

I do see your point but also wasn't there! Normally a week or two in good heat to wrap the hull, plus straps and 2x4 to wrap up or down the chime.
I did notice my first ever poly job did the same thing but also noticed why later with it's first wake encounter. I terminated the poly too low on the bow even cut at a 45 and it caught a wake to spray the whole boat. Figure it was also plowing until up on plane and running free on it's flat bottom. Added a bow section of poly and the john deere plow turned into a sea rat.

Who ever said a hook in hull is a trim tab is funny and comedians are highly needed, but not in airboating!
 
Hey guys , I'm pretty sure it wasn't over heated . No direct sunlight anywhere near here that time of year. haha. I'm pretty sure what happened was we didn't have the whole sheet hot enough. We worked it in the best we could with a heat gun and a propane torch .Pulling the poly around the chines with longer bolts . Apparently while we did that it pushed up on some less supported parts of the hull just in front of the transom and outboard of the stringers and rigging mounts. Moving forward I'm going to fab up some trusses and add 2 more jacks. This should flatten it out and make it very adjustable.( Now 5 jacks in a 12 foot hull) The funny thing is that I've been trying to tune the nose high hopping characteristic out of this thing sense I bought it. Its amazing to me how different this boat was after the poly. Anyway thanks for the replies Rick
 
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