[CarpCommander]

I had an interesting conversation with a guy about my porpoising issue, and whether or not my prop may be contributing to the problem. During the discussion he mentioned the poly may be causing drag at higher speeds.

Naturally I told him how slick the material was, and how airboats use it, and how I didn't think that was a factor. BUT, without me mentioning it to him, he stated the poly is most likely 'gripping' the water surface. This was interesting to me because that's precisely the sensation I got when the hull smacks the water on the downward bounce of the porpoising thing. It actually feels like it IS grabbing my hull.

I tried a hydrofoil and having people move to the front of the boat, which helped some, but there's still a good bit of bounce.

Most importantly, I've noticed my prop is only turning 4500 RPM at WOT, which 'in theory' would mean I'd need a 17P prop instead of a 21P. However that literally would not make sense, and my prop guy totally agreed.

With airboats their hulls are designed differently obviously, and they are using full poly vs strips like me. They also have their thrust coming from a totally different angle, so maybe they aren't experiencing the same effects? I've seen mod-V jet boats with full poly bottoms, that obviously run fine. Maybe it's the mix of poly and aluminum on my boat? Different drag coefficients?

I'm grasping at straws here-the only difference between my last boat and this boat is the poly and the prop. Same hull, same weight motor, same jack plate, same setback, same mounting height. The bottom of my hull is perfectly flat, and I've got no flex in the transom or keel.

So....given the facts, what say the masses?

 

[Alinder]

Definitely no expert on this but I could see it doing that. To me since ariboats slide on the water more than cutting through it it wouldn't grab. But on a mod v it sits farther in the water which could cause it catch or drag. Just a thought but since it's strips could the edges of the poly be grabbing instead? You know way more about this than I do though

 

[afish85]

CC, the guy is right. The worst thing for a planing hull boat is a completely smooth bottom. Google search "surface tension". A rough surface is preferred so that it traps air bubbles, and the water rides on those instead of a smooth surface that it will try and "stick" to. Ever notice how a boat seems to have a bit more speed on slight choppy water? Same principle. Air under the hull-less drag by water. Hope this helps.

 

[awaldro7]

Just a thought but since it's strips could the edges of the poly be grabbing instead? You know way more about this than I do though

I would bet this is the answer. an airboat has one solid piece of poly all the way across the bottom of the hull so its smooth. on each side of your poly you have an edge no matter how tight you have it fitted. This could greatly increase the drag coefficient of the bottom of the hull even though the edges are in the direction of flow. Aluminum and poly have very similar manning's roughness coefficients which we use in hydraulic calculations to calculate head loss. For an experiment I would try to take some epoxy or hard forming silicon and smooth out all of the transitions between poly and aluminum down the full length of the hull.

 

[SaLtY dAwG]

lots of the jet boats out west have poly stripped like you have done... my boat has a large section of poly on the keel and on each turning stake... and proposing in inboard jet boats is pretty common... the big pump boats have trim nozzles and sport jet owners have a few different options such as nozzle degree rings or trim plates to adjust the proposing... I know a guy that has the same boat as mine without the poly option and he has had no proposing issue like I see when my deck is removed... I think you may be on to something with the poly... Gar God added some trim tabs to his boat and he said it fixed his problems... I was going to go this route but when i talked to my boat dealer they had me bring it in and adjusted the left over edge of the hull bottom that stuck out under the transom... I was told this was designed this way to correct proposing... i know guys running flat bottom mud boats also weld a bead along the transom line under the hull to dig into the water to make the bow drop at full speed...

 

[Payphone Jones]

CC, the guy is right. The worst thing for a planing hull boat is a completely smooth bottom. Google search "surface tension". A rough surface is preferred so that it traps air bubbles, and the water rides on those instead of a smooth surface that it will try and "stick" to. Ever notice how a boat seems to have a bit more speed on slight choppy water? Same principle. Air under the hull-less drag by water. Hope this helps.

I have the solution for this problem. Mount the effin kicker already and lets go rough her up a little! It's a good thing I went to a college every weekend for six years. :headbang:

 

[skyshark186]

If you want to test your theory tie a rope tightly under your hull, or a ratchet strap......something that can be held in place temporarily and "rough" up the water a bit. See if it helps. If so.....you've found a possible route to a cure. If not.....other options may come to light.

 

[Payphone Jones]

No disrespect skyshark, but we lost a couple good dudes this spring in a similar situation. What's the worst that can happen?

 

[LDHorn]

Interesting stuff…

 

[why3zx]

I know the surface tension thing is a very big deal in faster boats, its why the bass boat guys "blueprint" the pads on their boats. Would a possible solution be to dimple the poly similar to a golf ball?

 

[JPHolla]

https://en.wikipedia.org/wiki/Boundary_layer Slipperiness against solid objects and a fluid is different. It is true that boats that attempt to set speed records cannot do so on perfectly calm water, but only water with a slight ripple. The friction does not come between the hull and the water molecules; it comes between water molecules and other water molecules.

 

[CarpCommander]

I know the surface tension thing is a very big deal in faster boats, its why the bass boat guys "blueprint" the pads on their boats. Would a possible solution be to dimple the poly similar to a golf ball?

You know I was thinking this exact thing halfway down the page.

I remember the episode of Myth Busters where they covered the car in Bondo, and cut all kinds of dimples on the Bondo, to simulate the surface of a golf ball. Then they tested it against a car that was totally smooth, and found the dimpled car actually for better gas mileage, meaning it was more aerodynamic.

Since it's easier than pulling the poly off the boat completely and welding up all those holes, I think I'll take a half a day and drill around 38,567 dimples in the bottom of my poly and then see what that does. :headbang:

If it fixes the issue I should set a new speed record for a Jon boat, because despite the drag and porposing, I managed to hit 56mph GPS the other day, at 4500 RPM....and my Opti redlines around 6000 RPM

Y'all make some good points and confirm what I feared-that my boat is being held tight by surface tension/friction. I gotta work this weekend but I have a few days off next week. I'll update this thread with my findings.

It really sucks being the guinnie pig sometimes...:cen:

 

[CarpCommander]

lots of the jet boats out west have poly stripped like you have done... my boat has a large section of poly on the keel and on each turning stake... and proposing in inboard jet boats is pretty common... the big pump boats have trim nozzles and sport jet owners have a few different options such as nozzle degree rings or trim plates to adjust the proposing... I know a guy that has the same boat as mine without the poly option and he has had no proposing issue like I see when my deck is removed... I think you may be on to something with the poly... Gar God added some trim tabs to his boat and he said it fixed his problems... I was going to go this route but when i talked to my boat dealer they had me bring it in and adjusted the left over edge of the hull bottom that stuck out under the transom... I was told this was designed this way to correct proposing... i know guys running flat bottom mud boats also weld a bead along the transom line under the hull to dig into the water to make the bow drop at full speed...

I tried calling some builders today to ask this exact question-nobody answered of course. I was curious if the jet guys had porpoising, since they had poly on mod-V hulls. Very interesting to hear how you and another guy have the same boat except the poly, and that yours porpoises and his doesn't.

I may try some short tabs if the dimple experiment fails.

I would bet this is the answer. an airboat has one solid piece of poly all the way across the bottom of the hull so its smooth. on each side of your poly you have an edge no matter how tight you have it fitted. This could greatly increase the drag coefficient of the bottom of the hull even though the edges are in the direction of flow. Aluminum and poly have very similar manning's roughness coefficients which we use in hydraulic calculations to calculate head loss. For an experiment I would try to take some epoxy or hard forming silicon and smooth out all of the transitions between poly and aluminum down the full length of the hull.

This may be a contributing factor as well. There are a few sections where water can get under the poly strips, and it crossed my mind that the gap or maybe even water trapped under the poly is making it worse.

CC, the guy is right. The worst thing for a planing hull boat is a completely smooth bottom. Google search "surface tension". A rough surface is preferred so that it traps air bubbles, and the water rides on those instead of a smooth surface that it will try and "stick" to. Ever notice how a boat seems to have a bit more speed on slight choppy water? Same principle. Air under the hull-less drag by water. Hope this helps.

Makes total sense. We'll see if our collective redneck intelligence is enough to catapult me into warp drive

 

[volock]

So the bass boat/other boat guys use Wearlon SpeedCoat (or similar) to roughen the surface like we're talking about. There's a lot of threads of people deciding to wax the bottom of the boat and ending up with issues online.

My thought is drilling all those dimples would be hard and time consuming (and feel like a real waste if it doesn't work/give more places for Poly to be torn off from. I wonder if instead a pass with a wire brush or maybe a coat of truck bed liner spray (it's textured, cheap enough, and tough) might work out better, and be less of a loss if they don't if you're actually going to experiment. I know when I did model boats as a kid, to get some extra speed we'd sand the last 1/3 the length in rougher grit than the rest, and always sand in the direction front to rear. Good for maybe a 5% bump over a uniform sanding job for us (porpoising was easier to fight by putting weight... I mean a 10-20lb model doesn't need much added to shift it where we want).

Then again, I'd think you'd need to be over 50mph by a good amount to be an issue, but it's been a while since a fluid dynamics class, and I have less experience in real world boats ignoring small RC/weed whacker engine ones. Now let's ignore all I wrote, since I don't own a boat.

 

[dirtpoorsticker]

Try scuffing the poly to break the surface tension, this should help with drag if that is the root cause. Maybe a quick pass with some 60 or 80 grit?

 

[mudfish]

Airboat can porpoising like crazy, that is why they have trim tabs.
PS poly wil grow and contract with tempiture.

 

[CarpCommander]

I think I'll try drilling some shallow dimples this week. Not all the way through, but deep enough to break the surface up.

Poly is too slick for anything to really stick to it, so bedliner and things of that mature are out. Scuffing may work, but as time goes by its gonna smooth out again. I'll make a template, mark a few hundred thousand holes, then mod a 5/8" bit so it can only dish out a depression, and go to town....

If it works, Im in the chips. If not...Ill have some pretty blue trailer strips for sale

 

[GaryBriggs]

See if someone can lend you a Needle Scaler, would make quick work of roughing up that poly.

 

[Phillips_211]

I would get a 19 pitch prop before I got a 17 but thats just my .02's. Also Im going to question the poly actually being your problem. You have added a good bit of weight in different locations, I feel like a different pitch prop and adjusting the rise on the motor will smooth this out just fine.

 

[slowpoke]

Honestly man, a LOT of flat boats have that problem. Have you ever used real trim tabs? That would be my first move. They actually make your boat turn better and more stable in rough water on pad in addition to killing the porpoise. One of my boys had the same issue and he tried 5 different props with no real difference. He got the mechanical tabs and the boat rides like a dream now and I was very skeptical. The only negative we have discovered is trying to back out of a thick mat of cattails but we just don't get in that situation anymore. He did not have poly though. But I am looking at your issue thinking that you don't really want to abandon the poly (good reason to put it on in the first place) and 9 jillion holes to fill. It is a relatively inexpensive fix with tabs and they work.

Having had 3 different airboats, I can tell you even though your hull seems the same as the last it could have one slight difference in the hull and it can make all the difference, even if you can't see it. Maybe one area got hotter while welding or cooled faster or there was a slight deformity in some of the material. A 1/32 of an inch in rise or fall near the stern can totally change the ride. If that is the issue, tabs will fix it.

Good luck