hull tuning

A general, non-powerplant specific, discussion on airboat technology, ie., hulls, rigging, polymer, etc..
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scott wheeler
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hull tuning

Post by scott wheeler » Fri Jun 25, 2010 11:35 am

I was told when I started this hobby (my wife said have you lost your mind) that if a hull didn't bow porpoise it would not run the ground as well as one would that did. Is there a breaking point to this theory. Too much or too little or is it b/s. I keep reading about people that have tuned their hull not to bow porpoise. Thoughts please. Scott

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Deano
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Re: hull tuning

Post by Deano » Fri Jun 25, 2010 3:39 pm

I think you are misunderstanding a little bit of the theory you're asking about.

Given that everything else is according to Hoyle . . .

A hull will generally run on the best in the water and on the hill when it is set up to run 'free'. That point would be when the hull is trimmed to the extent that it almost does, but does not porpoise. Raising the rear of the engine, will raise the bow. When the bow is raised to much, it will start to porpoise. That would be the point when you have to much of a good thing. It should be set up to run just short of porpoising. By setting a hull up this way, you minimize the amount of wetted surface, reduce drag, and consequently will attain a 'free-er' running hull. With the hull setup like this, it will generally do better on the ground as the bow is not pushing down creating more drag.

Consequently, to say it should be set to porpose is WRONG. If it does porpose the rear of the engine needs to be lowered. What happens then is that with the rear of the engine to high, it tries to lift the bow beyond the capability of the available power to hold it up there. Then the bow falls, bounces off the water, and tries to lift it again. An endless cycle . . .
When you are running dry, a hull setup like this will create more drag on the ground because as it is trying to lift the bow it is actually pushing the stern down, creating more drag.

In other words, wether in the water or on the ground, it will perform the best when the angle of thrust is parrallel to the surface the boat is on. If the thrust is angled up, it produces excessive drag on the stern (and the bow porpoises). If the thrust is angled down, it produces excessive drag on the bow (and the bow plows). The objective of tuning the hull is to reduce or eliminate drag.

This is why people who understand what is going on, tune a hull NOT to porpoise, but to be very near that limit. 8)

Deano
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flying fish
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Re: hull tuning

Post by flying fish » Fri Jun 25, 2010 5:17 pm

Good read Deano-
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Re: hull tuning

Post by boobidges » Fri Jun 25, 2010 6:18 pm

he's exactly right.you also need to check the straightness of the bottom.if you have uneven spots in the bottom they will cause suction and the boat won't free up.the most important is the area under the engine,usually called the pad or something like that.this is where the boat rides on a plane.if its not flat you might not be able to get to the point of the porpouse to tune the engine angle.to true this you'll either have to use filler or build jacks in the boat.

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Duane Scarborough
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Re: hull tuning

Post by Duane Scarborough » Fri Jun 25, 2010 6:49 pm

Deano,

That's the best explanation I've heard.

Thanks,

Duane
Hull: Airboat Pros MiniPro 116 / Frog Spit on the bottom
Rigging: Stainless Steel-side by side seating
Motor: Teledyne 4A084-4
Carb: Solex H30/31
Reduction: Arrow Prop 1.73:1
Prop: 67" Whirl Wind Mini Prop
Speed: unknown yet

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nailit
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Re: hull tuning

Post by nailit » Fri Jun 25, 2010 7:21 pm

how come every step hull i see has the motor up in the back? i think they are trying to keep the bow down to keep it on the step. my motor is about 1/4 bubble low in the rear and it has a little porpose but i can drive through it. if i raise my motor in the rear past level it will run on the nose when you mash the gas and it does not side on the ground good. we've done this test on alot of boats and on all the regular hulls they would ride the on the nose when raised up in the rear. not trying to start a big arguement but that is just what we've found. every boat is different though.

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JUNIOR480
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Re: hull tuning

Post by JUNIOR480 » Fri Jun 25, 2010 8:54 pm

nailit wrote:how come every step hull i see has the motor up in the back? i think they are trying to keep the bow down to keep it on the step.
NO WHEN ITS UP IN THE BACK IT GETS IT OFF THE NOSE ...THERE LOOKING FOR THAT SWEET SPOT WHERE ITS RIGHT ON THE EDGE OF DISASTER..... :mrgreen:
HAULIN A$$ SINCE 78......

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Re: hull tuning

Post by ffcracker » Fri Jun 25, 2010 9:01 pm

And tuning a step hull is a different game than a flat bottom
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nailit
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Re: hull tuning

Post by nailit » Fri Jun 25, 2010 9:47 pm

Well i know what it does on my boat and a bunch of other boats and it is a never ending battle of which way to go. And i know you have to find what works on your boat, there is a point of too much one way or the other.

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Re: hull tuning

Post by blown480 » Fri Jun 25, 2010 10:11 pm

put one of my electric stainless trim tabs on it n forget about trimmin the boat if your just ridin te boat, set mtr paralell with bottom of boat n fine tune rear mtr height, if its a step or race boat different game ,bottom flat , mtr height etc. complete stainless trim tabs with toggle $175.00 321-636-4897

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Deano
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Re: hull tuning

Post by Deano » Sat Jun 26, 2010 4:29 am

nailit wrote:. . . on my boat and a bunch of other boats it is a never ending battle of which way to go. And i know you have to find what works on your boat, there is a point of too much one way or the other.
That is because in those cases, 'Everything else' is NOT according to Hoyle . . .
"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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nailit
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Re: hull tuning

Post by nailit » Sat Jun 26, 2010 10:08 am

Hoyle.. must run a caddi also :D

brnard51
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Re: hull tuning

Post by brnard51 » Sat Jun 26, 2010 11:13 am

GREAT POST DEANO, you explained it so even a simple man like me understood what is happening

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Deano
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Re: hull tuning

Post by Deano » Sat Jun 26, 2010 11:21 am

nailit wrote:Hoyle.. must run a caddi also :D
Well . . . he did write 'the book'. :wink:

I wonder if Charles has traced his family tree back that far? :)
"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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CRACKERSTYLE
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Re: hull tuning

Post by CRACKERSTYLE » Sat Jun 26, 2010 11:39 am

Whats a HOYLE?

Thought you just mis spelled HULL :lol:
John 3:16

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Deano
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Re: hull tuning

Post by Deano » Sat Jun 26, 2010 11:49 am

Hey CrackerStyle . . .
We're havin' a big ol' 4th of July Card Tournament next weekend. You in ?? :lol:
:occasion5:
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but it is not the path to knowledge; it has no place in the endeavor of science."
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Re: hull tuning

Post by shallowminded23 » Wed Sep 18, 2013 1:12 pm

Deano, you are 100 % correct. Great explanation.

papajack
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Re: hull tuning

Post by papajack » Wed Sep 18, 2013 1:18 pm

Three year old thread...
Just sayin...

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akblackdawg
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Re: hull tuning

Post by akblackdawg » Wed Sep 18, 2013 6:32 pm

You also have to tune taking into consideration you number of passengers and weight distribution. With just myself or myself and one passenger sitting next to me, I get a little bit of porporse around 35 or so. If I have a couple more passengers sitting in the seat in front of me, there is no porporse. I'm happy with the way it runs ground, so it works for me the way it is. I have been told to hammer down the trailing lip past the transome to take out the porporse that is there, but I see that as putting a hook in the bottom, and held off on doing that. Bud
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kwanjangnihm
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Re: hull tuning

Post by kwanjangnihm » Wed Sep 18, 2013 8:19 pm

Great explanation Deano no matter how old it is!

Typically is this done by adding washers to either the front or rear of the engine depending on porpoise or plow?

How many washers is normal for testing and what size do you guys use? I am running a AV 540 on a 13' Cottonmouth.

Thanks!
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Re: hull tuning

Post by toddkyle » Thu Nov 27, 2014 4:40 am

Deano this is something i never ever read about you made it really simple. thumbs up!

midnight rider
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Re: hull tuning

Post by midnight rider » Thu Nov 27, 2014 9:20 am

In response to the step hull question, its really optical illusion
That the motor is up in the rear when actually it's just parallel
To the planing surface. A step rides on two points and the
Motor is parallel to that.

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CONTINENTAL CRACKER
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Re: hull tuning

Post by CONTINENTAL CRACKER » Sun Jun 05, 2016 8:20 pm

akblackdawg wrote:You also have to tune taking into consideration you number of passengers and weight distribution. With just myself or myself and one passenger sitting next to me, I get a little bit of porporse around 35 or so. If I have a couple more passengers sitting in the seat in front of me, there is no porporse. I'm happy with the way it runs ground, so it works for me the way it is. I have been told to hammer down the trailing lip past the transome to take out the porporse that is there, but I see that as putting a hook in the bottom, and held off on doing that. Bud
it is better to bend the 1 inch lip in the curve than in the flat bottom. When you bend the lip in the flat bottom, it will bend back flat from running dry. But if you bend the lip on the curve on each side it doesn't get effected from running the dry since it is not on the ground.
I WOULD RATHER PADDLE A CANOE THAN OWN A WATERPUMPER

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