Boat hop

A general, non-powerplant specific, discussion on airboat technology, ie., hulls, rigging, polymer, etc..
dieseldog863
Southern Airboat Member
Posts: 17
Joined: Mon Oct 22, 2012 11:56 am

Boat hop

Postby dieseldog863 » Mon Aug 06, 2018 6:32 pm

Well to start off with I have a 14x8 aluminum hull with a 383 EFI LT1, 2.37 rotator and 77” 3 blade whisper tip EX.
Now my problem.... I went from 2 blade 77” with a mild porpoise to a 3 blade 77”to get better dry push and now my hull porpoises/ hops so bad I can only do about 20mph at best. It does this in 6” of water to deep water. I have tried to bend the back lip on my hull down some to act as a trim tab, but this didn’t work. Any help would me appreciated.

User avatar
Deano
Site Supporter - V
Site Supporter - V
Posts: 4478
Joined: Thu Jan 06, 2005 11:16 am
Location: Inverness, FL

Re: Boat hop

Postby Deano » Mon Aug 06, 2018 7:22 pm

It sounds like there is a reasonable likelihood that the bottom is not straight, flat and true.

There are two means of making that determination:
--> The first would be to run a string line under the boat along the bottom and check for gaps between the string and the hull.
Placing a three or four foot level (used as a straight edge) under the boat can show the same gap if it is small enough and located where you can get a level on it. Should you find a gap between the string line and the bottom, this will show you were placing a jack will be most effective.
--> The other means (and potentially more complicated of the two), is to remove some height (ie. washers if they exist) from under your gearbox mounts, thereby lowering it a minimal amount which will change the attitude of the prop thrust. It should be noted that if the design allows, it may be easier to add height to the front mounts. Either way will attain the same end, which is to lower the angle of thrust produced by the prop wash.

IF the bottom IS straight and true this is the means of making that correction.
This thread has an expanded explanation about the effect such adjustments will make:
https://southernairboat.com/phpBB3/viewtopic.php?f=85&t=28600&p=276720
"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
Prototype
Southern Airboat Member
Posts: 680
Joined: Sat Jan 26, 2013 2:28 pm

Re: Boat hop

Postby Prototype » Mon Aug 06, 2018 8:58 pm

Rarely have time to re-post old shit, but it sounds like you amplified your push, which a 3 blade will do! It will also amplify your boats faults!

Not that more than "goobers" have upgraded to eventually learn they down graded, but like you said, " more push on ground is now costing you on water"
I tend to tune/tweek the other way around!

Rich Andrews
Site Supporter - IV
Site Supporter - IV
Posts: 6694
Joined: Wed Jun 15, 2005 9:20 pm

Re: Boat hop

Postby Rich Andrews » Mon Aug 06, 2018 9:23 pm

for sure blades will make a boat act way differently... good post
I'll have my Manatee burger medium please...just say NObama

14x8 deckover FELBER 397ci.. HP ???..just stay back!!

LOHA is AHOL backwards

User avatar
Slidin Gator
Site Supporter - II
Site Supporter - II
Posts: 267
Joined: Sat Feb 03, 2018 11:33 pm
Location: Jupiter Farms, Florida

Re: Boat hop

Postby Slidin Gator » Mon Aug 06, 2018 11:59 pm

I had not read Deano's post before, but definitely a good read and great explanation. I tend to explain it as identical to trim adjustments on an outboard motor, down to push the nose down and motor up to free it up, it does not matter if the prop is below the hull (outboard) or above (airboat).

The only thing that was not covered is consideration of weight changes and distribution. The perfect balance only occurs with a single set of weight variables, and those change over a day's operation.

The key point Deano made was that the boat should be on the verge of Hop for good dry ground running, and that is a good guide. But, that needs to be based on the expected load and distribution. The stated purpose was for better dry ground running. The places I run I prefer more bow lift to ride over obstacles, myrtles, logs, wire grass etc. More often than not, bow lift is the solution. Riding water is just to cross a river to the other side, the primary goal is to haul a hog and a deer or so on the grass rake, replacing the fuel weight in the back I burned getting there, changing the load equation. Add in the afternoon after the sun has turned the nettles into glue guns and a successful day is a motor that needs extra idle time on the trailer to cool down.

I'm not putting on a trim tab, I can guarantee that at some point I will forget I have the ice break engaged when hitting some steep bank to climb out of a river and end up stalled, partially sunk. So starting the morning with a full tank, no game, and 30-35 MPH porpoising on the way to the jump is just the price of admission.

That said, 20 MPH to porpoise is too much. The new prop increased bow lift. Follow Deano's instructions and check the hull lines then trim the motor. Probably just some trim adjustments needed.
I grew up thinking that I-10 was the Mason Dixon line.
1986 Airboat Engineering Inc., 14' Marsh Master. Refreshed narrow deck, SV O-540. A Bob Stossel original, still running strong.

Rich Andrews
Site Supporter - IV
Site Supporter - IV
Posts: 6694
Joined: Wed Jun 15, 2005 9:20 pm

Re: Boat hop

Postby Rich Andrews » Tue Aug 07, 2018 10:17 am

Some people I have seen use some angle metal on the outer chimes and they work just fine.. real small brackets maybe 3 inch wide and when you hit dry ground depending on your hull style they dont touch.
I'll have my Manatee burger medium please...just say NObama

14x8 deckover FELBER 397ci.. HP ???..just stay back!!

LOHA is AHOL backwards

User avatar
kwanjangnihm
Site Supporter - IV
Site Supporter - IV
Posts: 1704
Joined: Sat Mar 30, 2013 8:23 pm
Location: Bartow FL

Re: Boat hop

Postby kwanjangnihm » Tue Aug 07, 2018 10:59 am

My old cottonmouth actually did the opposite, it hopped less when I installed my 3 blade sig. It used to porpose around 1800-2000 rpms and I would adjust with trim tab. After installing 3 bade I could go up to 2200 rpm's before it would start hopping.

Where is your engine angle now? (level with bottom of hull, up or down)
" I don't care who you are back in the world, you give away our position one more time, I'll bleed ya, real quiet. Leave ya here. Got that? "

dieseldog863
Southern Airboat Member
Posts: 17
Joined: Mon Oct 22, 2012 11:56 am

Re: Boat hop

Postby dieseldog863 » Thu Aug 09, 2018 5:10 pm

Thanks for the info guys. I’m gonna see what I can figure out this weekend.


Return to “Airboat Tech”

Who is online

Users browsing this forum: No registered users and 10 guests