Is an overall lower center of gravity more stable ?

Airboats with smaller powerplants.
preventec47
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Is an overall lower center of gravity more stable ?

Postby preventec47 » Sun Aug 14, 2016 8:21 am

Does a low seat height and a low motor mount increase the stability of an airboat ?
I will be running a belt reduction unit to the prop which will allow the motor to basically
sit in the boat. I just never could understand why airboat guys sometimes sit in the
hi chairs ?

I am looking at a 12 ft Jon boat that is 52 inch wide and wondering if a low mounted
25hp Kohler motor and low seat would make it safe.

happy harold
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Re: Is an overall lower center of gravity more stable ?

Postby happy harold » Sun Aug 14, 2016 8:39 am

yes. lower center is more stable. higher seats are a must if you are running grass or brush 14 foot tall. with the size of your boat, this may not be a factor for you, although if you run these areas, you might want a tall flag pole.

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Deano
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Re: Is an overall lower center of gravity more stable ?

Postby Deano » Sun Aug 14, 2016 8:55 am

Hell, No. It don't matter; neither does the beam (width). For instance . . .

I have a friend whose ex-wife's brother-in-law heard of a girl that used to date a guy that
ran a 502 Chevy big block on a 16' Gheenoe. He was told it only when down once, and that
was at the dock during a hurricane just because the float switch on the bilge pump failed.

And the reason for the hi chairs is to be able to see the expression on the people's faces
who are in a Jon boats while they are thinking 'Oh ****, I hope he sees me!'. :lol:
"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."
- Carl Sagan

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mojoe
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Re: Is an overall lower center of gravity more stable ?

Postby mojoe » Sun Aug 14, 2016 10:39 am

1/4 hp per lb is a good rule of thumb for an airboat to run well. 1/5 hp per lb will run ok, but will get stuck here and there.

A 25hp motor is just right to push itself.. No boat, no gear box, no rigging, no gas, no people in the boat..

What I'm getting at is: you can have a lot of fun in a little boat like that, as long as nobody else is running an actual airboat anywhere near you.

In the same way off-roading a moped can be fun, as long as no one around has a dirt bike!!


Spend some time reading on here. Then go somewhere people run airboats and watch for a while.
Airboating is like marriage: There are unlimited ways a person can be right, and somehow still be wrong at the same time!!

boatair
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Re: Is an overall lower center of gravity more stable ?

Postby boatair » Sun Aug 14, 2016 11:09 am

:lol:
Deano wrote:Hell, No. It don't matter; neither does the beam (width). For instance . . .

I have a friend whose ex-wife's brother-in-law heard of a girl that used to date a guy that
ran a 502 Chevy big block on a 16' Gheenoe. He was told it only when down once, and that
was at the dock during a hurricane just because the float switch on the bilge pump failed.

And the reason for the hi chairs is to be able to see the expression on the people's faces
who are in a Jon boats while they are thinking 'Oh ****, I hope he sees me!'. :lol:

preventec47
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Re: Is an overall lower center of gravity more stable ?

Postby preventec47 » Sun Aug 14, 2016 11:13 am

I am a bit confused. I am attaching some photos I have been holding on to for about
ten years of a guy I knew who used Briggs or Kohler VTwins and he said they were
great fun. 20 and 25 hp respectively. I've held on to them thinking I wanted to copy
them someday and now I have my engine and next I need to get a hull.
Attachments
B kohler 25hp 3blade 50.JPG
AB vanguard 20hp at 42mph.jpg
AB ICEBOAT2.JPG

SWAMPHUNTER45
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Re: Is an overall lower center of gravity more stable ?

Postby SWAMPHUNTER45 » Mon Aug 15, 2016 2:32 am

A general rule is that when planning a build you want to power it to have the capability to move itself loaded on dry ground.

Some feel it is not an airboat if it won't run dry.


That said if your after a jon boat with an alternate means of propulsion the Briggs may provide you hours of enjoyment.

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CactusJack
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Re: Is an overall lower center of gravity more stable ?

Postby CactusJack » Mon Aug 15, 2016 3:16 am

The high seat is also to do with where you sit in front of the prop. There is a low pressure area right in front of the prop where the drivers head would be in a big boat with a low seat stand. Having your head in the middle of the low pressure area of a medium to full size boat can make a driver feel sick or get a headache and would be generally uncomfortable.... plus as the guys say, if you are running in tall grass... being able to see over it helps alot.

When the seat is high on a bigger boat, the drivers head is above the worst of the low pressure area... and the only person feeling sick is the driver of the boat following, if the driver of the front boat has had too much Mexican food :shock: :lol:

On a mini or jon with a small motor such as yours, the low pressure area is smaller and not so noticeable but the guy in the boat behind will still know if you have been eating Mexican.

A mini with Hp like yours prolly wouldn't be able to run tall grass or dry, but would be fun in skinny water just having fun, but things like sand bars or sticky mud will be a show stopper more than likely.

When I had the gyro on the punt, it was a stessyl punt with quite deep chines either side, not a true flat bottom, so it would plane in deeper water, but couldn't slide sideways and anything other than reeds laying in the water would slow it down.... if I yelled out 'dirt' the thing would prolly stop.... it was no go on anything dry. Was still fun tho :thumbleft:

It really depends on whether you want the boat to be playful or just to get up shallow rivers (with no dry crossings) as to whether 25 Hp is enough. Imo, you need to try and go for a ride with folks that have boats like you are thinking of and decide if it is suitable for what you want a boat to do. I only did the one I did because I had the damaged gyro and had access to a punt.... so, I just had to do it :lol:

I sure had alot of fun, but it would never have been any good where real airboats excel

I wouldn't recommend having the beam narrower than the diameter of the prop with a powerful motor tho... I was using a 55" prop in a hull that was the same at the gunnels and a foot narrower at the bottom and it didn't give me alot of confidence.... if I was stationary and turned right and gave too much power,, if I didn't back off real quick, it would have just rolled over to the right because of the narrow hull... but I could do the same to the left and it was fine (Turbo EA81 Subaru DD, prop turning anticlockwise when you standing behind it in the blast)

What do you want to use your proposed boat for and what type of water? (skinny/deep/rivers/lakes/swamp/fishing/fun)? :dontknow:
I was told I was wrong, once (not sure I believe it though)
If you can't blind 'em with brilliance, baffle 'em with BS
Airboat experience so far, limited to having bolted a gyrocopter to a flat bottom punt.... but THAT.... is about to change, bigtime :-)


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