Airboat wont plane

[homer46]

Hi there
I was wondering if anyone can help me I'm from Northern Ireland I bought a hovercraft a couple of years ago to use to take me out duck hunting.It worked OK but I new that an airboat would be better with having nobody over here that knows anything about them I had a go at converting my hover craft.I got a flat bottom aluminium hull made and fixed that craft into it done away with the lift part of the craft so all the thrust is going out to drive it forward instead of a third of the air going into the Hull to lift it when it was a hovercraft.It had 8 fan blades as a hovercraft I removed 4 and have them pitched to optimum engine RPM which is 3500 my engine is a 31hp Briggs and stratton vanguard as a hovercraft with 2 people it would do 30_35mph I wouldn't be getting the half of that now I have a small video of it on an early test run maybe someone can give some help many thanks

 

[Striker543]

A picture of your setup would help, as would knowing the size and weight of your hull. Generally speaking though, 35hp is not nearly enough horsepower to push a full size boat. Light boats (12 feet or less in length and weighing under 1,000lbs) usually require around 150hp to perform well. When you start talking about heavy hunt boats, that number goes much higher.

 

[John Fenner]

Well, displacement of weight to water "flotation" is very important, plus weight to horsepower, not being sure of your hull design, or rigging for that matter, it is hard to say, now, the hovercraft ran on a cushion of air, that engine and prop were fine for that application, a few pics of your setup will help.

 

[homer46]

Hi there
Thanks for the reply I tried to post a video earlier but wouldn't upload here is some photos I can get more at the weekend as I will be out hunting then I really appreciate your help

 

[Slidin Gator]

Homer,

I hate being the A$$ hole, but I'll give it to you straight.

The opening description had me thinking this won't work, the pictures say it all. Rest assured, all is not lost, but you do have a good bit of work left if you want to meet your goals.

First, the reason a hover craft moves so well with a 35 Hp briggs pushing is because the air cushion nets you close to zero friction on the go forward part. It's like a buddy of mine explained to me when he was driving me home from the airport somewhere north with an iced up highway. I said, "good thing you got 4wd" he replied "4 times nothing is still nothing".

In other words, a hover craft does not need squat in forward thrust to go, as long as the air cushion is working. Get rid of that air cushion, now you are fighting water or ground drag, the thrust and Hp equation changes dramatically. Below the 100 Hp mark, figure you need about 1 Hp per 12-15 lbs of boat, driver, fat buddy in front, beer cooler blah blah blah. So that 35 Hp brigs is good for maybe 400 -500 lbs (200 kg) all in.

The first thing I see is too much weight forward. Send your buddy off to the pub for a few pints, while he's gone, move everything you can to the stern. Does it even plane?

The new aluminum hull you built actually looks like a great start and just might work. First I'm ditching everything that came off the hovercraft, starting with all the fiberglass. Get it down to the bare hull (what's the dimensions and weight?).

You could improve the thrust with a larger prop, but that brigs will never be enough for the size of that hull. Start thinking turbo Audi 2.0 etc.

Get rid of all the fiberglass, 100+Hp motor and a bigger prop, it will run half decent. I got a 150 Hp O-320 that would probably be about perfect, but would suck the petrol.

Otherwise, back to Hovercraft.

Sorry to be the A hole, but now you know.

 

[UnoWho]

 

[homer46]

What about a 600 or 1000cc motor bike engine I've a few contacts over here for that some of them will make up to 200hp be a lot smaller than a car engine and lighter

 

[glades cat]

You’re gonna need a bigger thrust column to push the airboat. It looks like you have about a 36” ducted prop now. You’re gonna have use a much larger prop and more power to get the necessary thrust.
I used to race hovercraft, many years ago. I had a Rotax 503 with gear reduction on a 10’ Scat hovercraft.
The airboat is a different animal and needs much more power.
As suggested. Take the hovercraft off the boat and start over.

 

[Slidin Gator]

What about a 600 or 1000cc motor bike engine I've a few contacts over here for that some of them will make up to 200hp be a lot smaller than a car engine and lighter

Yes, you are on the right track here. Just note that you will want a prop that is just a bit smaller than the width of your boat (leaving room for the cage around it). You will also need some gear reduction to run the bike engine to it's rev limit vs. a roughly 3,000 rpm limit on the prop.

Give these two threads on motorcycle powered boats a read.

Honda CBR 600cc 10x6 Aluminum Airboat Build

Advice for buildin aluminium hull.