Ideal hull shape to not hop or plow?

[TDezso]

I'm about to put a couple jacks in my boat this weekend. It porpoises like crazy with anything over 20mph. Just curious what kind of shape I should shoot for on the bottom. I'm thinking probably push down in the front and pull up in the rear would make sense.


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[mojoe]

Mine has the rear almost flat with 1/16 down on the middle skrew.

The center skrew to center skrew, my center jack pushes down 1/4" more than the rear jack and 3/16" farther down on its middle skrew than the sides.

Don't have a front jack... yet.

Rides really good until 29 mph, then it still wants to hop in deep water. A little angle with the rudders holds it flat. I had the rear flat and it felt a little too loose for my liking so I pushed the middle down a hair. I wouldn't say it's perfect, but feels real good.

[One Eyed Gator]

Before I did anything I would check the bottom with a straight edge or string line. I notice the bolts on the side If you're running poly make sure it does not have any issue. Mine started hopping on the way back to the ramp, after looking I found a 1' long cut in mine at a slight angle.

Many hull end up with a hook at the back will cause it to plow or Hop. Where is the motor it in relation to the bottom. Level, Up a little or down.

Jacks are for fine tuning in my opinion.

[TDezso]

I'd love to get 29 before it hops. Lol and I've messed with the motor, up down it doesn't matter. The hull ran smooth with the previous setup on it. But it was also a 4cyl and now I've got a 220 on it.


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[mojoe]

X2 with one eyed Gator. Jacks are last and just like motor angle, a little does a lot. With mine, I got to a point where I thought my old school hull was only gonna run so good. Then the slightest adjustment to the jacks and it "broke free". Literally felt like a different boat. Then a turn too far and it stopped in its tracks on the ground and hopped like a bunny In the water. The sweet spot is literally less than one turn on the jacks on mine.

that doesn't mean yours will be tha same, but a good set of jacks isn't likely to hurt anything. Just check the bottom first to make sure you are putting them where they need to go.

[glades cat]

Yea, the 'ole cyclic porpoising syndrome. One of the biggest reasons is too great a payload for the hull size, combined with an aft center of gravity and enough speed to climb out of the depression it displaces in the water. You mentioned it changed when you went to the GPU from a 4 cyl. It may be an unstable combination you have there. Even a slight hook can mess things up. Try straightening and jacking the bottom, leveling the engine and everything else you can try, but if all else fails...Remedy: Lenco trim tab. Adjustability may be your best friend now.
.02

[TDezso]

Yea, the 'ole cyclic porpoising syndrome. One of the biggest reasons is too great a payload for the hull size, combined with an aft center of gravity and enough speed to climb out of the depression it displaces in the water. You mentioned it changed when you went to the GPU from a 4 cyl. It may be an unstable combination you have there. Even a slight hook can mess things up. Try straightening and jacking the bottom, leveling the engine and everything else you can try, but if all else fails...Remedy: Lenco trim tab. Adjustability may be your best friend now.
.02

Yeah. The guy who owned the hull had a 4cyl ac motor. It ran nice. I'm gonna do the jacks this weekend I hope. If that doesn't work I'll do the tab or probably I little piece of angle aluminum. I'm trying everything I can to not drill holes in the transom though. Lol


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[mojoe]

Have you tried wedging different size blocks of wood under that bar to imitate a center jack?

[TDezso]

Have you tried wedging different size blocks of wood under that bar to imitate a center jack?

Those were all I could get under it. It's pretty stiff bushing it down. It runs across ground a little better I think like this, but it'll bounce you out of the seat in 1/4" of water. It's bad.


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[Rick McC.]

I had a boat years ago that would porpoise whenever you ran it just a little hard.

I moved the engine stand two inches forward (too much weight to the rear), and it never porpoised again.

[TDezso]

I had a boat years ago that would porpoise whenever you ran it just a little hard.

I moved the engine stand two inches forward (too much weight to the rear), and it never porpoised again.

I don't have that option. I'm at my limit with the rigging that I have.


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[Deano]

You can believe it now or believe it later, but with the number of variables involved, your chances of randomly changes things hoping to find the right combo to correct a porpoising problem are akin to winning the lottery.

To start in the beginning, just answer the question "Why does it porpoise?". Then you will know which direction to go and what method(s) will allow you to correct the issue at hand. Of the potential causes, you have:
the bottom is not straight and flat and/or
angle of thrust is not parallel with the bottom and/or
weight payload is not correctly positioned fore and aft.

Be aware that tuning a hull to run as best it is capable of, is like setting up a race car in that when you make one change it may effect the other variables and give a false indication of progress. Similarly, making more than one change at a time makes it impossible to confirm what change had what effect and will have you chasing your tail until you throw in the towel and install a trim tab.

The best way to avoid the inevitable heartburn that goes with playing that game is start out by defining what you are trying to fix. In the case that the bottom is not straight and flat, then you need to determine WHERE to fix it. Having misplaced jacks will not correct the problem, even if you get lucky and minimize it some.

One Eyed Gator has shared multiple and very valuable words of wisdom (STRING LINE being one) that can not be overstated. Running a string line on the bottom will show you if the bottom is straight and flat and perhaps more importantly, where it is not. By identifying where the maximum variance is from straight, you will know where said jack needs to be placed. You may also find that the bottom is straight and flat, in which case you will know that playing any kind of jack game will get you no further than pissing up a rope. In that case, the same time could be spent correcting the real cause of the problem instead of creating more variables that are moving you in the wrong direction.

The other thing that One Eyed Gator pointed out that bears emphasis, is that jacks could/should be viewed as fine tuning in that, if your angle of thrust and/or weight distribution is way out of whack, they simply will can not offset that issue. Jacks should not be perceived as the initial, all around cure all where porpoising is concerned for that reason.

Where angle of thrust is concerned, this is all over the board already and needn't be repeated here. Check this thread for that:
http://southernairboat.com/phpBB3/viewt ... 0&p=276852

That leaves the other possibility being that your weight is not positioned optimally, as Glades Cat eluded to. If it ran as it should, and then you added weight (bigger, heavier engine) and then the problem started, that sounds to me like a contributing factor. Naturally, because this is the more difficult variable to deal with, I would exhaust the other possibilities first and hope this wasn't necessary. Still, the way you've described things this would seem like the culprit if the bottom is straight and the angle of thrust has been already been corrected. One easy way to determine the center of gravity is to unload the boat on the ground with a piece of PVC under it and find the balance point. Knowing where that is could very well tell a story all by itself. When/If you get as far as needing to move the rigging, a couple inches will make a BIG difference. Posting some pictures at that point would help you get some educated opinions about that.

If you have not corrected your angle of thrust after the new engine install, I would start there. Being to high in the back could create all your problems and would likely be the easiest to fix. That thread I linked earlier explains all that. As big as your stringers are, I'd like to think it doesn't have a hook, although anything is possible.

[TDezso]

You can believe it now or believe it later, but with the number of variables involved, your chances of randomly changes things hoping to find the right combo to correct a porpoising problem are akin to winning the lottery.

To start in the beginning, just answer the question "Why does it porpoise?". Then you will know which direction to go and what method(s) will allow you to correct the issue at hand. Of the potential causes, you have:
the bottom is not straight and flat and/or
angle of thrust is not parallel with the bottom and/or
weight payload is not correctly positioned fore and aft.

Be aware that tuning a hull to run as best it is capable of, is like setting up a race car in that when you make one change it may effect the other variables and give a false indication of progress. Similarly, making more than one change at a time makes it impossible to confirm what change had what effect and will have you chasing your tail until you throw in the towel and install a trim tab.

The best way to avoid the inevitable heartburn that goes with playing that game is start out by defining what you are trying to fix. In the case that the bottom is not straight and flat, then you need to determine WHERE to fix it. Having misplaced jacks will not correct the problem, even if you get lucky and minimize it some.

One Eyed Gator has shared multiple and very valuable words of wisdom (STRING LINE being one) that can not be overstated. Running a string line on the bottom will show you if the bottom is straight and flat and perhaps more importantly, where it is not. By identifying where the maximum variance is from straight, you will know where said jack needs to be placed. You may also find that the bottom is straight and flat, in which case you will know that playing any kind of jack game will get you no further than pissing up a rope. In that case, the same time could be spent correcting the real cause of the problem instead of creating more variables that are moving you in the wrong direction.

The other thing that One Eyed Gator pointed out that bears emphasis, is that jacks could/should be viewed as fine tuning in that, if your angle of thrust and/or weight distribution is way out of whack, they simply will can not offset that issue. Jacks should not be perceived as the initial, all around cure all where porpoising is concerned for that reason.

Where angle of thrust is concerned, this is all over the board already and needn't be repeated here. Check this thread for that:
http://southernairboat.com/phpBB3/viewt ... 0&p=276852

That leaves the other possibility being that your weight is not positioned optimally, as Glades Cat eluded to. If it ran as it should, and then you added weight (bigger, heavier engine) and then the problem started, that sounds to me like a contributing factor. Naturally, because this is the more difficult variable to deal with, I would exhaust the other possibilities first and hope this wasn't necessary. Still, the way you've described things this would seem like the culprit if the bottom is straight and the angle of thrust has been already been corrected. One easy way to determine the center of gravity is to unload the boat on the ground with a piece of PVC under it and find the balance point. Knowing where that is could very well tell a story all by itself. When/If you get as far as needing to move the rigging, a couple inches will make a BIG difference. Posting some pictures at that point would help you get some educated opinions about that.

If you have not corrected your angle of thrust after the new engine install, I would start there. Being to high in the back could create all your problems and would likely be the easiest to fix. That thread I linked earlier explains all that. As big as your stringers are, I'd like to think it doesn't have a hook, although anything is possible.

I'm trying to get as many ideas as I can before I set the time aside to mess with it. I know my weight is a huge factor now. And new rigging will be in the future. For now. What is the easiest way to get a string line on the bottom without removing the rigging? I have a center bunk on the trailer. I appreciate all the good info.


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[kwanjangnihm]

Have you tried moving your fuel tank forward to the next section of rigging? What size hull are you working with?

My 13.5 cottonmouth hopped fiercely when I purchased it. I added a lenco trim tab and then started working on the hop!
I worked with washer's and leveling the motor and got a lot of hop out. I went to a 3 blade prop and that helped too!
I also stopped filling my 40 gallon tank. I now top it off at 30 and this helps as well.
I dont have any proof but often wonder if my hayden oil cooler mounted at the top of my cage adds to the hop?
Since making these changes I can now run most of the time not using the trim tab.

[mojoe]

1) Dump it on the ground.
2) Remove the center bunk.
3) Run it back up on the trailer.
4) Run string line or use straight edge

[mojoe]

Just to prove Deano right, and...

I ran a string line on the bottom of mine just now (it has been about 4 years since I straight edged it). Right down the middle stringer checked out perfect, as did ALMOST everywhere, but I saw a spot in the polymer in between 2 stringers from about 5-15" from the transom that didn't look as worn as the rest. String line showed a hook about 3/32" right there. Now I could have put jacks all over the inside on my boat pressing on stringers and none of them would have had an effect on that hook. I grabbed a rag, a 2x4 and a sledge and beat that hook down.

BTW: It is amazing how many times and how hard you have to hit aluminum with a 5lb mallet to move it less than 1/8"!!

Anybody want to gamble as to wether or not it rides better next time out?

[TDezso]

Just to prove Deano right, and...

I ran a string line on the bottom of mine just now (it has been about 4 years since I straight edged it). Right down the middle stringer checked out perfect, as did ALMOST everywhere, but I saw a spot in the polymer in between 2 stringers from about 5-15" from the transom that didn't look as worn as the rest. String line showed a hook about 3/32" right there. Now I could have put jacks all over the inside on my boat pressing on stringers and none of them would have had an effect on that hook. I grabbed a rag, a 2x4 and a sledge and beat that hook down.

BTW: It is amazing how many times and how hard you have to hit aluminum with a 5lb mallet to move it less than 1/8"!!

Anybody want to gamble as to wether or not it rides better next time out?

I'll try to pull the center bunk tomorrow and put a string on it. Moving weight doesn't change anything. Adding 220lbs on the deck has little change also.


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[jopete]

my 12' alum hull bucked like a bronco with my gpu. played with engine angle and strings and all that. had a welder build me a jack. I put it under the drivers seat in front of the fuel tank and put about 3/4'' of round in boat bottom. boat freed up and ran awesome. turned it down some more to almost a inch of round in the bottom. boat is twitchy at high speeds, but runs the ground like a raped ape. that was a couple years ago, never had another issue. I may of just got lucky, but i'll take it. good luck

[TDezso]

I got a buddy that was a builder for diamond back for 10 years. I'm gonna take it to him and have it sorted out. I'll post back with the results.


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[Deano]

If you have that option and preference, there is certainly nothing wrong with that. I hope my effort to help save you some grief, I didn't make things seem like it isn't something you could do for yourself. I was only meaning to convey that there were more variables involved than what adding jacks would be expected to fix.

Perhaps by using the link as I did to save some typing, I unintentionally took emphasis off the importance of the thrust angle being paramount. If you added that much weight to the bow and it made little difference, that could seem to generically imply that the rear of the engine is to high. That, or Glades Cat was correct and your weight is simply centered to far aft. Have you checked/adjusted the motor angle since the install ?

I would also generically suggest that, in any case, you heed Mojoe's advice and loose that center bunk. Generally air boat trailers have a bunk under each stringer that the rigging is mounted to. This puts the weight of the rigging/engine etc. directly on the trailer and relieves the hull from having to perpetually support all that weight. This is largely how hulls develop 'hooks' in the beginning. The other big trailer error that leads to hooks is not having the bunk extend from behind the transom to in front of the planning surface while it is loaded. Ideally, the hull shouldn't directly be supporting any weight beyond its own. An ill fitting trailer can do more damage to a hull over time than anything else.

In any case, good luck with however you proceed. If at any point going forward you want to PM me your number, I'll be happy to call you back and answer questions or offer any insight. I'm happy to help if I can.

[TDezso]

If you have that option and preference, there is certainly nothing wrong with that. I hope my effort to help save you some grief, I didn't make things seem like it isn't something you could do for yourself. I was only meaning to convey that there were more variables involved than what adding jacks would be expected to fix.

Perhaps by using the link as I did to save some typing, I unintentionally took emphasis off the importance of the thrust angle being paramount. If you added that much weight to the bow and it made little difference, that could seem to generically imply that the rear of the engine is to high. That, or Glades Cat was correct and your weight is simply centered to far aft. Have you checked/adjusted the motor angle since the install ?

I would also generically suggest that, in any case, you heed Mojoe's advice and loose that center bunk. Generally air boat trailers have a bunk under each stringer that the rigging is mounted to. This puts the weight of the rigging/engine etc. directly on the trailer and relieves the hull from having to perpetually support all that weight. This is largely how hulls develop 'hooks' in the beginning. The other big trailer error that leads to hooks is not having the bunk extend from behind the transom to in front of the planning surface while it is loaded. Ideally, the hull shouldn't directly be supporting any weight beyond its own. An ill fitting trailer can do more damage to a hull over time than anything else.

In any case, good luck with however you proceed. If at any point going forward you want to PM me your number, I'll be happy to call you back and answer questions or offer any insight. I'm happy to help if I can.

Not at all. I appreciate all the help. I'm new to air boating and try to soak up as much as I can. My biggest problem is it's hard to find the time to mess with it. It's worse when I need to mess with the motor. I can't do it at my house, I have to take it to the landing and when I get there I want to ride. Lol. I did get up under the boat yesterday and there is a pocket just about where the bottom flattens out. Then there is a slight belly under the engine stand. My buddy is making the jacks now and is going to help me place them and set it all up. My engine is down in the rear about 1/2" so I'll level that out first thing and go from there.


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[mojoe]

I can't do it at my house, I have to take it to the landing and when I get there I want to ride.

Why not? As long as the trailer is set to dry load, you can load and unload the boat without turning it on. Un-hooking the trailer makes it easy to start on, just make sure you crank slow and are clear when you are at the point where the front of the trailer is about to pivot and drop back down.

I like to pull it down right before the weight slams it down and just step on it till I crank the weight the rest of the way on.

[TDezso]

I can't do it at my house, I have to take it to the landing and when I get there I want to ride.

Why not? As long as the trailer is set to dry load, you can load and unload the boat without turning it on. Un-hooking the trailer makes it easy to start on, just make sure you crank slow and are clear when you are at the point where the front of the trailer is about to pivot and drop back down.

I like to pull it down right before the weight slams it down and just step on it till I crank the weight the rest of the way on.

It's not that. I can drop it anywhere. It's my neighbors. Got a few that would flip their shit. I just try to not piss off the neighborhood. They loved it when I started it up open headers after reringing it. Lol


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[mojoe]

If you can't drop a boat on your property, remove a bunk off a trailer, then silently winch the boat back on the trailer, you need to move!

[Gary S]

I would have some pissed off neighbors.