• If you log in, the ads disappear in the forum and gallery. If you need help logging in or getting registered, send request to: webmaster@southernairboat.com

Dry ground setup?


Active member
Have a 16.5x9 aluminum hull, 8ft across bottom 9ft across the top. Boat weighs about 3200- 3300lbs. Engine is a built sbc pushing around 450hp. Stinger 2.38 gearbox with 3 blade 80" whirlwind. Have prop pitched (c-17.5) to run around 4800rpm on the trailer 5000ish on the water. Boat will run across sand bars and mud with some momentum but trying to stop and start on a sand bar or unload dry in field it just won't get going. Have a hull jack behind rigging and 1 almost midship with the bottom very slightly bowed in the middle. Any suggestions on tips/tricks or setup changes to make?
pretty simple, lose weight or add horsepower! 3300# / 450HP = 7.33 lbs per HP - per OneBFC's formula you are over the ratio for running dry!

kwanjangnihm said:
The formula never changes :salute:

Jebb said:
my 16x8 alumitech weighs just shy of 3k @ 525hp , when running dry in some terrain i think i could use more hp

3000 / 525 = 5.71#'s per HP

OneBFC said:
The formula never changes. 6lbs of loaded boat weight per available hp and it will run dry acceptably.
The less weight per hp the better. Imo, 400hp on a 16x8 is going to under perform because you will
exceed 2400lbs easily. Big boat, Big power. No way around it. Good luck!

OneBFC said:
Formula for success never changes. 6lbs of boat to 1hp and you will have an adequate performance experience. move that to 4lbs per 1hp and it's a do all machine. The more you trend to less weight the better in most use cases for airboats. There are exceptions of course.
The Rx to make a big heavy boat move with respect on dry is a 500 inch plus engine such as a "purpose built" BBC or Branch Caddy with a 4 blade prop.

I have been down this road as has LBW and there is really no better path.

While there are many exceptions to rules or theory, the past experiences of others is wisdom shared.

The general SBC limitations for say the good machine shop 383 are a light 14/15. Add 3 big guys a cooler and 30 gallons of fuel and that can be a stop. Once you cross over the 15ft length and add weight the big engines with long strokes become your friend. Are there small block boats that exceed 14 and do well loaded, YES but they are not common place.

A BBC around 540 inches on a 2.55 with 4 wide blades will fix your problem.

As a caveat and from having been in that spot myself there is an other option. Someone in your situation could look at adding a Nitrous kit in the 100-125hp range just to be used to break dry ground. I would suggest you consult with your engine builder and discuss this and learn all about the system and it's needs prior to going in this direction. It worked well for me and several folks in my circle carry NOS as insurance to get out of the bad spots. The PowerShot system from NOS is a 125hp entry level "WET" system and would be a affordable option at $550 apx.

Just need to have enough power. Doesnt matter how you get there really.

As previously stated, 6lbs per hp or less. Lower always better.

What engine you use to make the number you come up with based on the above is totally up to you based on a bunch of other factors that have nothing to do with making the boat move on dry ground.

Everyone has their favored platform for their favored reasons. You will have to navigate that on your own.

Best of luck, 3300 lbs loaded needs a minimum of 550hp. I would recommend you go for as much as your wallet can handle.

Swamp is not wrong though with NOS, as long as it's built for it! I would argue big inch these days unless needed all the time! Big inch has to be fed constantly, where a boost factor could work just as good at half the cost?
3000 plus and 9 feet wide is a hefty girl to feed though. Why it ever had a sbc to begin with is maybe someone wanted a air pontoon?
Post kind of said it all though, can power over stuff with enough speed? That deserves a second thought at 3000 plus and 9 feet wide!
Thanks for all the replies I kinda figured it was just hp issue. The boat does run and perform great in the water and like I said it only takes a little forward momentum to be able to run sandbars and the sort can basically idle up to them right before the nose is gonna touch give it some gas get on top and put the pedal down and scoot across.

I guess my next question is with having an 8ft bottom if I park it on a sand bar or grass ground should I have enough belly in the bottom to be able to slightly rock it side to side? Would that help it break free from ground eaaier?
Update, put a little more belly in the bottom and adjusted engine angle up a little higher in the back and that's all it took. Can start dry no problem now. With 4 people in boat I can spin 360 dry but hard to get going straight
Have you tried pitching the prop to allow you to get 5300 rpm on the trailer?

My experience was those blades liked to spin a little faster.
I have not tired taking any more pitch out because I didn't want to have a high cruise rpm. When I got the boat it only spun 4300rpm on trailer but I could cruise deep water around 3,000rpm. Now at 4800rpm on trailer I'm around 3,700rpm deep water cruise
Typically a 383 of a generic high performance auto specification would make its peak HP in the 5400 rpm range and max torque around 4600 rpm.

My friends with very high quality small blocks like to run them set at 5200 to 5500 for ride boat use. They play around with gear ratios from 2.37 to 2.68 most typically making their choices in concert with prop choice.

My experience with Whirlwind props has been they like to spin up at the high end of their range. So do what works for your application but know in theory there may be a bit of performance left on the table. That said if your having good results with longevity, fuel use and ride quality go with what works.
I think swamphunter is right. I have a whirlwind on a 18x 8,mine is a 3 blade stump puller and I have had to rev it up to around 5200 or so to get it to break free from sand before.
Was just wondering new to airboats but I thought a prop was supposed to be good for 3000 rpm’s, so you guy are turning yours to over 5 grand on trailer ?
Car motors run reduction drives to slow prop speed down. My prop maxes out at 2450, my reduction is a 2.55/1 so my motor could spin 6250 rpm before the prop is maxed out. I don't spin it that hard but I could.

There are direct drive motors both car and aircraft, with them you are limited to the prop speed as your motor speed