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Porpoising here we go again

12ftgso480

12ftgso480

Well-known member
Read through the threads, found lots of thoughts, ideas opinions on how / why my boat is bouncing at a higher speed. This is a 15' aluminum hull with new poly. We got the bottom pretty close but I'm sure we still have some dips or "waves" on the bottom. This is a pretty fresh build for me, I've added a gear box, moved rigging and seat forward. To me is seems like I'm riding somewhat low in the back high in the nose before we plain out, at speed or 20ish I'm guessing we develop a pretty violent bounce. If anything my motor is sitting slightly high in the rear compared to runners. I do have room to take my 40 gallon fuel tank and move further forward. Can I get some current thoughts and thinking of where to start here. I've read about shimming poly, bending back lip, and then the last resort trim tab. I would love for someone to explain exactly whats happening when you start to propose, is your nose being force down or up ? Ect.
 
So u could get a trim
Tab and be done ! That lip bending could work but it dont last jmo
Dont shime the poly .. thats crazy you obviously have a issue more then likely your bottom . Do u have Jack’s.? Also post pics
 
No jacks can you post a pic of what your referring ti?
 

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Gary S said:
High in the back is some of it.
Click to expand...
^^ This is where to start before you start moving things around.

Waves in the poly can forever wreak havoc, hopefully that will not be the case.
Were it me, I would lower the rear of the engine and hope to resolve the problem first.

If that doesn't work, then you have another issue to pursue.
In any case, lacking other info, this is always the place to start

Don't know if your search got back this far, but if not, this will likely help:

Hull Tuning
 
Deano said:
^^ This is where to start before you start moving things around.

Waves in the poly can forever wreak havoc, hopefully that will not be the case.
Were it me, I would lower the rear of the engine and hope to resolve the problem first.

If that doesn't work, then you have another issue to pursue.
In any case, lacking other info, this is always the place to start

Don't know if your search got back this far, but if not, this will likely help:

Hull Tuning
Click to expand...
That's a very good read. Thank you guys for the info. Does anybody have a picture of the jacks that have been mentioned ? I do know we are high in the back so we'll give that a try first and will fallow up on post.
 
Well the thrust angle isn't helping but,,,,,, take that back lip behind transom and knock it downward.
Set a piece of thin Masonite or 1/8" plywood edge directly under transom plate across the width of the hull on level concrete, smack lip down till the absolute rear is touching the concrete.
 
John Fenner said:
Well the thrust angle isn't helping but,,,,,, take that back lip behind transom and knock it downward.
Set a piece of thin Masonite or 1/8" plywood edge directly under transom plate across the width of the hull on level concrete, smack lip down till the absolute rear is touching the concrete.
Click to expand...
I thought the first step was to take it to the local Amazon warehouse with a big concrete pad, set the boat on ground, connect a tow strap to your truck and do a little drifting/water skiing? Once that is done, then the redneck trim tab if required.

Those gravel shoals will do the same eventually and stern lip shape becomes more important with age, hate to start there.

I seem to remember a poly install project this past winter in Oklahoma. Nothing makes heat like summer. UHMW expands about 0.0001 in/in per degree F. Assume an 80 inch wide sheet that is 0.009 inch/degree. Now assume a poly install at 60 Deg F and a hull temp of 90, 30 degrees delta.

That's 1/4" of excess poly across the width.

Fresh poly install while its cold!
 
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Slidin Gator said:
I thought the first step was to take it to the local Amazon warehouse with a big concrete pad, set the boat on ground, connect a tow strap to your truck and do a little drifting/water skiing? Once that is done, then the redneck trim tab if required.

Those gravel shoals will do the same eventually and stern lip shape becomes more important with age, hate to start there.

I seem to remember a poly install project this past winter in Oklahoma. Nothing makes heat like summer.

Fresh poly install while its cold!
Click to expand...
Well, that is another part but,,, gotta start with that rear lip first, leading cause of porpoise in these type hulls, when they weld the transom plate in, it sucks up that tab due to the pulling of the weld.
 
Little update guys! Had a couple buddies go for a ride with me totaling close to 350lbs in the front. We hit alot higher speed before any bounce. I lowered motor close to 3/8" in the rear the next day to get level with the runners. Don't know where we got so far off from the mock up block but I'm hoping this was alot of our issue. Gator I agree there's nothing like summer sun but we did our best to keep it warm and was able to catch 2 70° days during install so hoping like hell that's not our issue 😅 gonna run it over the weekend so it's to be continued. I know I get all shapes and sizes of responses on here but Thank you all for the input and willingness to help. I have been stoked with this 15' rig so far minus the bouncing. Truck norris cam seems to be doing well with our 2.1:1 gear and my R two blade. We have a 2800 cruise rpm, we're taking off on sand bars feathering the throttle 3500 off and on to 4kish. I bought my prop and box when I thought we were gonna small block this rig and went ahead and ran it all after finding a good deal on this 6.0.
 
Ok everyone. Update time ! I've lowered my motor where it is sitting slightly lower in the rear compared my runners. It seemed to help some with a higher speed before it started to bounce. I had my passengers move to the very front or "nose" of my boat and it eliminated all porpoising( probly totalling 350 lbs ). A friend at work is telling me I need to continue lowering my engine in the rear. Another very complicated option is to try and move my rigging further forward. My fuel tank can be moved nearly 2' or so without moving my rigging at all. Do we think by lowering the motor more in the rear is a viable solution? Thoughts? Also the bottom of this hull was not built flat, if you look closely to my picture the center is the lowest point, making it a very slight v bottom in a sense. Very very slight v. Thanks!
 

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I've always thought that the fuel tank should be at or near a neutral point in the hull because it's weight changes as you burn gas. You are in the point of "tuning" the hull. My guess is you have a bulge in the bottom of the boat. I had an old Panther hull very similar to yours that rode on the nose bad when I first put it together. This is where the jacks help, because mine had a hook in the bottom. Keep adjusting your engine angle if you have the room. Just a little at a time. (1/4" or so) on the trailer, if you can put a straight edge on the bottom of the boat to check for the bulge or confirm that the bottom is flat that could help as well. My cousin has shims between the back edge of his poly and the hull because he has a bulge in his fiberglass hull that he can't fix. It does help if you can't get it with engine angle. Also, the bulge I'm talking about would be from back to front of the hull. Transom forward.
 
John Fenner said:
Engine should be level with bottom, as I said above, knock that rear lip down and slide on.
Click to expand...
I off load dry so much I'm trying to find a solution to the real problem. It sounds like bending lip could bandaid it for now but I really don't mind chasing the problem till we get it right. The tank in the "middle" of boat doesn't sound like a bad idea for the simple fact of being full to empty. Does anyone have pictures of jacks ?
 
12ftgso480 said:
I off load dry so much I'm trying to find a solution to the real problem. It sounds like bending lip could bandaid it for now but I really don't mind chasing the problem till we get it right. The tank in the "middle" of boat doesn't sound like a bad idea for the simple fact of being full to empty. Does anyone have pictures of jacks ?
Click to expand...
3,657.6 mm,

I would first try John's original recommendation. Hammer the lip down and see how she rides, it's a quick fix. If that solves it you know you don't need to go all drastic.
 
My hull is less than a year old and my trailer has never been in the water. That being said if you don’t get carried away when unloading and be picky about where you unload I get anywhere between 8-15 rides before I have to straighten the lip behind the transom. Engine valve covers perfectly parallel with t bars, fuel tank 1/3 from transom directly below rear seat as its front driver and I run a jack beneath radiator and one just in front of the fuel tank because that’s all that is in the water on plane.

Put your boat on the ground and pull it up over a 2” pipe and see where your balance point is. If it’s at the 1/3 from rear and your transom lip is parallel to ground with engine level it is probably in the hull
 
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