Airboat porpoising

A general, non-powerplant specific, discussion on airboat technology, ie., hulls, rigging, polymer, etc..
Post Reply
arbtmn
Southern Airboat Member
Posts: 49
Joined: Wed Jun 10, 2015 6:18 pm
Location: slidell louisiana

Airboat porpoising

Post by arbtmn »

Hello. Can someone give any advice on how to properly shape the hull with the 6 bolts (in my case) from the engine frame. I know you want a slight belly I think. So for me that means the center bolts slightly pushed downward and the outside bolts slightly sucked upwards. My boat is 7.5 by 12.5 with a lycoming 360 180 hp and a 35 gallon tank. All built by Alumitech. The rig is gorgeous but I cannot get it to plane once I get it on step. I tried many variations of these bolt movements and it still porpoises severely once I get on plane.
Marty

User avatar
Deano
Site Supporter - V
Site Supporter - V
Posts: 4567
Joined: Thu Jan 06, 2005 11:16 am
Location: Inverness, FL

Re: Airboat porpoising

Post by Deano »

The best way is to start at the beginning; then at least you know what you are working with.
Jumping right to 'putting a belly in the hull' will only complicate matters and give you a heartburn.

To radically oversimplify the scenario:

First, the bottom of the hull should be straight and flat.
. (This may or may not mean loosening all the jacks before you go to step two)
Second, the angle of thrust should be adjusted to attain the best results.
. (The crankshaft should be parallel to the hull stringers for starters)

Then, and only then, you would alter the bottom with the jacks to fine tune your previous results.
. (Obviously, you will need the results from steps one and two before you can do this.)
"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

User avatar
trailerdon
Site Supporter - IV
Site Supporter - IV
Posts: 314
Joined: Mon Aug 02, 2010 9:24 pm
Location: Port Charlotte, Fl

Re: Airboat porpoising

Post by trailerdon »

Try taking a hammer to the trailing edge of your hull. LOL That sounds drastic, but really. Take a small sledge hammer and lightly tap down the trailing edge of your hull, does not take much "maybe 1/16" - 1/8". " You will be amazed at the improvement. Also to get that slight belly in the hull you desire; floor jacks would be the way to go.

User avatar
jopete
Site Supporter - II
Site Supporter - II
Posts: 978
Joined: Mon Mar 30, 2009 8:07 am
Location: loreauville, la

Re: Airboat porpoising

Post by jopete »

take your boat to alumitec and drop it off. tell them to call you when they get it to act right of get your money back !!!!!

arbtmn
Southern Airboat Member
Posts: 49
Joined: Wed Jun 10, 2015 6:18 pm
Location: slidell louisiana

Re: Airboat porpoising

Post by arbtmn »

ok very good. Your right about the heartburn. I will start from scratch. I assume place the boat in the water, then snug the bolts, then check the motor thrust line. To shim a motor the rear needs to be shimmed upward a bit, correct? I will consider the trim tabs lastly perhaps.
Marty

User avatar
Deano
Site Supporter - V
Site Supporter - V
Posts: 4567
Joined: Thu Jan 06, 2005 11:16 am
Location: Inverness, FL

Re: Airboat porpoising

Post by Deano »

Jopete's comment reads as if you were talking about a new boat.
You didn't say that, so I did not presume that to be the case. Is it a new or nearly new boat?
If it is, following his advice and having them make needed corrections could be an exponentially better plan.

If it is new, you should be able to presume the bottom is straight and flat.
If that is the case, you would lower, not raise the prop end of the engine to diminish the porpoising.

There and volumes of information about this very subject here for the reading.
If you [Search] these boards for things such as 'porpoising', 'hull tuning', 'hook', 'plowing' and the like, you will find massive amounts of information that will help you understand the subject and fine tune your questions.

The best way to check the bottom is with a string line or a four foot level while the boat is on the trailer.
This could also be one of the ways to set/adjust the jacks.
"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

arbtmn
Southern Airboat Member
Posts: 49
Joined: Wed Jun 10, 2015 6:18 pm
Location: slidell louisiana

Re: Airboat porpoising

Post by arbtmn »

Sorry. the hull is new. I will work with the tech rep to figure it out and I will also read up on the info you provided thanks!
Marty

User avatar
John Fenner
Southern Airboat Member
Posts: 2864
Joined: Tue Feb 14, 2012 12:26 pm
Location: Miami(Cuba) Fl.

Re: Airboat porpoising

Post by John Fenner »

Every one of these welded boats do this due to the transom plate being welded in and it draws that lip up, can't count how many I've tweaked over the years.
I never finish anyth,,,.

arbtmn
Southern Airboat Member
Posts: 49
Joined: Wed Jun 10, 2015 6:18 pm
Location: slidell louisiana

Re: Airboat porpoising

Post by arbtmn »

i think I will put a center trim tab with nautilus actuator (40) pound. This should do the trick
Marty

User avatar
glades cat
Site Supporter - III
Site Supporter - III
Posts: 3042
Joined: Mon Jan 22, 2007 2:36 pm
Location: Miami, Fl

Re: Airboat porpoising

Post by glades cat »

If your installing a trim tab, consider the Lenco. You will want the adjustability and be able to trim it way up to clear obstructions.
"Leave a legacy...Preserve the irreplaceable"
12' x 7' sled built by Scorpion, Cont. O-470-K, 72" Whisper Tip
Rigged by Gladescat & sons in 2009

flying fish
Site Supporter - II
Site Supporter - II
Posts: 1877
Joined: Thu Jun 21, 2007 11:09 pm
Location: Elkhart, Kansas

Re: Airboat porpoising

Post by flying fish »

Deano wrote:The best way is to start at the beginning; then at least you know what you are working with.
Jumping right to 'putting a belly in the hull' will only complicate matters and give you a heartburn.

To radically oversimplify the scenario:

First, the bottom of the hull should be straight and flat.
. (This may or may not mean loosening all the jacks before you go to step two)
Second, the angle of thrust should be adjusted to attain the best results.
. (The crankshaft should be parallel to the hull stringers for starters)

Then, and only then, you would alter the bottom with the jacks to fine tune your previous results.
. (Obviously, you will need the results from steps one and two before you can do this.)

Dang, look at this advice. Costs you nothing and will make your friends envious of how free she runs without a trim tab. :)

Good advice Deano :thumbleft:
15' DB - 6.2 LSA - 4 blade 82" R - 2.52 CH-4

Post Reply

Return to “Airboat Tech”