Ideal hull shape to not hop or plow?

A general, non-powerplant specific, discussion on airboat technology, ie., hulls, rigging, polymer, etc..
TDezso
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Re: Ideal hull shape to not hop or plow?

Post by TDezso » Tue Mar 28, 2017 1:11 pm

mojoe wrote: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!
I can drop it for a short time. But my luck it would be sitting in the driveway for a few days.


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TDezso
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Re: Ideal hull shape to not hop or plow?

Post by TDezso » Fri Mar 31, 2017 8:42 am

Well. I think I found my problem. Put a string under it and it is pretty straight. Finally got it on flat pavement and it rocks back and fourth from side to side like it's supposed to. The problem is that the center stringers stay flat on the ground and the outer ones rock.


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mojoe
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Re: Ideal hull shape to not hop or plow?

Post by mojoe » Fri Mar 31, 2017 9:22 am

What is "pretty straight"? :dontknow:

That doesn't sound straight to me. If I was buying a car and somebody told me the frame was "pretty straight", I would walk away!
Mark 7:9

TDezso
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Ideal hull shape to not hop or plow?

Post by TDezso » Fri Mar 31, 2017 1:01 pm

mojoe wrote:What is "pretty straight"? :dontknow:

That doesn't sound straight to me. If I was buying a car and somebody told me the frame was "pretty straight", I would walk away!

Within an 1/8" till the curve of the hull. It has a lot of flex side to side. The back is around a 1/16 below the string and about halfway up the hull there is a pocket about a 1/16".


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Deano
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Re: Ideal hull shape to not hop or plow?

Post by Deano » Sat Apr 01, 2017 10:16 am

OK. Not to split hairs or nit-pick, but to fully understand what you've actually identified, a couple of clarifications please.
TDezso wrote: . . . The back is around a 1/16 below the string and about halfway up the hull there is a pocket about a 1/16".
When I went back and reread the thread to better understand this statement, I see where you said it had a belly under the engine stand. Even after blowing up the picture I am unable to see what kind of bolt pattern you have that fastens your polymer to the hull. It isn't only bolted around the perimeter is it? If so, that's not good and needs to be looked at. First, you need to identify if the belly (called a rocker) is actually in the hull or is it just in the polymer and there is in fact dead airspace between the polymer and the hull. This can most easily be done by placing a hydraulic jack under the back of the boat on the trailer and pushing up where the rocker appears to be. If there is dead air space behind the polymer you should be able to see it move before the hull starts to come up off the trailer. If so, then adding some bolts to pull it flush with the hull may very well solve that problem. If not, how thick is the polymer? If it is thick enough, you may be able to take some of the crown off it with a real course belt sander. Hopefully one of those two options will address that. Let us know what you find out in that regard.

Either way, realize that with a rocker in the middle of the planning surface all bets are off. This is one thing that simply can not be worked around. It has to be addressed one way or the other. What happens with a rocker at the planning surface is that as the boat tries to plane, it is actually pivoting on and off that high point. Straight and flat is absolutely paramount right there.

Now, on to the other potential issue . . .

When you say the hull rocks side to side but the center stringers stay flat on the ground, would it be more accurate to say that the hull is flexing that much and not really rocking because it is not rigid enough laterally to hold its shape while you attempt to rock it? What do your stringers measure? They look shorter and wider than a 1x6 that is typically glassed over, but there isn't really a point of reference in the pic.

If I'm understanding your description correctly and your hull is flexing that much, that could be an additional part of your problem. If that is in fact the case, installing a couple jacks could be advantageous; not with the intent of putting a crown in the hull, but only enough down pressure to remove any gap (called a hook) in the low spot you identified amidships to prevent the center stringers from flexing upwards.

Understand that having a crown in the hull laterally so it will rock side to side is in absolutely no way your objective at this point. That is a different concern entirely, and should be perceived as nothing more than fine tuning for running dry AFTER you have gotten beyond the porpoise issue you are attempting to correct. Don't get hung up on that part at this point. What you are after at this point, is straight and flat, bow to stern. You additionally need the stringers/bottom to be rigid enough to maintain straight and flat, regardless of how and where pressure applied to the hull. If I perceive your description correctly, you may be lacking that prerequisite rigidity and your hull may be flexing enough to feed your 'bounce' even if not creating it all together.

It is very likely that with the rocker addressed, the flexing will become less of an issue, but like MoJoe said earler in the thread, a good set of jacks isn't likely to hurt anything. In this case it sounds like they may help if set up properly.
"The suppression of uncomfortable ideas may be common in religion and politics,
but it is not the path to knowledge; it has no place in the endeavor of science."
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TDezso
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Re: Ideal hull shape to not hop or plow?

Post by TDezso » Sun Apr 02, 2017 9:24 am

Deano wrote:OK. Not to split hairs or nit-pick, but to fully understand what you've actually identified, a couple of clarifications please.
TDezso wrote: . . . The back is around a 1/16 below the string and about halfway up the hull there is a pocket about a 1/16".
When I went back and reread the thread to better understand this statement, I see where you said it had a belly under the engine stand. Even after blowing up the picture I am unable to see what kind of bolt pattern you have that fastens your polymer to the hull. It isn't only bolted around the perimeter is it? If so, that's not good and needs to be looked at. First, you need to identify if the belly (called a rocker) is actually in the hull or is it just in the polymer and there is in fact dead airspace between the polymer and the hull. This can most easily be done by placing a hydraulic jack under the back of the boat on the trailer and pushing up where the rocker appears to be. If there is dead air space behind the polymer you should be able to see it move before the hull starts to come up off the trailer. If so, then adding some bolts to pull it flush with the hull may very well solve that problem. If not, how thick is the polymer? If it is thick enough, you may be able to take some of the crown off it with a real course belt sander. Hopefully one of those two options will address that. Let us know what you find out in that regard.

Either way, realize that with a rocker in the middle of the planning surface all bets are off. This is one thing that simply can not be worked around. It has to be addressed one way or the other. What happens with a rocker at the planning surface is that as the boat tries to plane, it is actually pivoting on and off that high point. Straight and flat is absolutely paramount right there.

Now, on to the other potential issue . . .

When you say the hull rocks side to side but the center stringers stay flat on the ground, would it be more accurate to say that the hull is flexing that much and not really rocking because it is not rigid enough laterally to hold its shape while you attempt to rock it? What do your stringers measure? They look shorter and wider than a 1x6 that is typically glassed over, but there isn't really a point of reference in the pic.

If I'm understanding your description correctly and your hull is flexing that much, that could be an additional part of your problem. If that is in fact the case, installing a couple jacks could be advantageous; not with the intent of putting a crown in the hull, but only enough down pressure to remove any gap (called a hook) in the low spot you identified amidships to prevent the center stringers from flexing upwards.

Understand that having a crown in the hull laterally so it will rock side to side is in absolutely no way your objective at this point. That is a different concern entirely, and should be perceived as nothing more than fine tuning for running dry AFTER you have gotten beyond the porpoise issue you are attempting to correct. Don't get hung up on that part at this point. What you are after at this point, is straight and flat, bow to stern. You additionally need the stringers/bottom to be rigid enough to maintain straight and flat, regardless of how and where pressure applied to the hull. If I perceive your description correctly, you are currently lacking that prerequisite rigidity. In other words, it sounds like your hull may be flexing enough to feed your 'bounce' even if not creating it all together.

It is very likely that with the rocker addressed, the flexing will become less of an issue, but like MoJoe said earler in the thread, a good set of jacks isn't likely to hurt anything. In this case it sounds like they may help if placed and used properly.
It is hard to see in the picture, but my outer edge is bolted about 1 1/2" around the edge. Centers are 6-8". You are correct on the bottom flex. The bottom of the boat stays flat when it rocks. The stringers are 2x4. But. The hull issue is put on hold for now. I blew my motor yesterday. So, gotta get something on it to get it back on the water. Thanks for all the help. I'll probably bring this thread back after it's on the water again. Lol


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