New Project...Plenty Of Questions

Airboats with smaller powerplants.
Arcticshooter
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New Project...Plenty Of Questions

Postby Arcticshooter » Sun Jan 17, 2016 3:30 pm

Hello everyone!
I just joined the forum. Been doing some research here for a mini project and I figured it was time to step up, say hello, and start asking questions. :D
I live in Alaska, am an A&P(although I don't do it for work anymore) and an experienced fabricator and I've wanted to build a mini since I got out of A&P school back in the early 90's.

That project never got off the ground back then, mostly due to being a broke student.
Now we have a nice Jet Boat it's a 20' North River; Marine Power-injected 350; Hamilton 212 jet.
Very quick and Very nimble...however, not so light. Now it will gladly go zipping through the water at 1' or less and we're fine. But there's a whole world of sloughs and creeks that connect with the Susitna River where we live, that are just too small for our family sized Jet Boat.

I was looking at building one of those little mini Jets running a PWC drivetrain...goes just about anywhere and nearly indestructible.

Fast fwd to last week. My buddy is trying to convince me to buy his old ultralight because he doesn't fly anymore and it's in his way. Finally, I tell him I don't really want an UL, but we might be able to use his parts to build up an airboat I've dreamed about in the past. He says take and use whatever pieces and parts are needed. So he's ponied up a Rotax 447UL-SCDI single carb/B-box 2.58:1(alt. mount=high mount); exhaust; fuel can rack; 2 UL seats; and any of the instruments, rigging and tubing that I have a use for. The motor is clean and not abused, just dusty.

My part of this is Hull construction and boat fabrication.
I have 2 sheets of alum at the shop 1/8"-4x10 that I believe is a weldable grade alum(I'll have to confirm that). Also need to confirm that I can weld his UL tubing to the sheet stock.
If that all works out, I'm only looking for some T-Bar, a new Prop, 1 sheet of 1/4" UHMW ( I think you guys call it polymer down there), wire and gas, and a couple hundred $ worth of pieces and parts.
I'm looking at 10'6" to 11' long X 5'6" to 6' wide. Gunwhales will be 14"to16" as we have some pretty serious river here.

For starters, the questions I'd like your help with are:
1. Where do you buy Aluminum T-Bar?
What is the preferred size of T-Bar for a mini project?
How many T-Bars would you recommend for a hull this wide?

2. How do you determine the proper height of the motor/seat structure/passengers above the waterline?
Clearly this is critical to setting up the boat so it will want to lay into the corner and turn. Too low and it would want to stay flat and not catch the chine for a good turn, too high and it would try to fall over towards the inside of a hard corner.
I have a clear idea of balance fore/aft and how the pitch of the hull needs to be.
Just not sure how high the center of balance should be for good, yet safe, turning.

My plans are a boat that will seat 2 Tandem(low passenger seat in front) and allow for some overnight camping or hunting gear at max load.
Our Jet Boat will hop on step with 4 people; 65 gal of fuel; and a 100lb dog, in about 3 boat lengths.
It tops out at about 40mph GPS and handles like a racecar.
What I'd like is for the Airboat to get on step with 2 people and 10 gal fuel and a little gear, and reach at least 35 to 40mph.
Planning on building a chined hull so I can get better steer and less slide. We don't have a whole lot of nice friendly grass to slide through, at least not along the river. Trees and rocks and sandbars abound here if you miss your mark!!

3. I'm open to suggestions for a new prop that will perform on this setup.

Thanks guys
Les

papajack
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Re: New Project...Plenty Of Questions

Postby papajack » Sun Jan 17, 2016 3:42 pm

My opinion, based on my mini, you're going to be VERY underpowered. My 583 rotax with a 2.62:1 c gearbox turns a 2 blade whirlwind razor x-core and pushes a 12 x 7 ultralight Alumitech hull real well. Your hull will be much heavier as well as your load. My hull wouldn't hold up to the conditions up there. Again, these opinions are based on my experience with mini airboats (I have had several and did years of research). Good luck whichever way you go.

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goldhunter_2
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Re: New Project...Plenty Of Questions

Postby goldhunter_2 » Sun Jan 17, 2016 4:02 pm

The load your accustom to carrying don't seem to match up with your "mini"ideas very under-powered IMHO

you said your a AP why not find or build a cont io-346 , lycoming o-200, cont pe-90, cont o-470 or 0-520 with enough power to steer the boat where you want to go

fallen trees, banks, sandbars aren't something to be stuck on it's better to have the power to drive right over them, rocks mean lots of heavier stringer, A chine hull will add strength an make it track better but much harder to steer
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Arcticshooter
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Re: New Project...Plenty Of Questions

Postby Arcticshooter » Sun Jan 17, 2016 4:44 pm

Goldhunter,
Why would the chine hull be harder to steer?

I'm aware that this is at the low end of the power ladder, but I'm figuring I can maybe get it going and add the passenger and some more amenities later, when and if I upgrade to one of the readily available and light Rotax snowmobile motors on the local market.
Sounds like a 670 hooked to a c-drive will make about 100 and still be under 100lbs.
My primary reason for avoiding a timed out AC engine is hp to weight. I want a more modern powerplant that weighs 100 or more lbs less and makes the same or better hp.
I figure that 100lbs is more gear or less drag, either of which is a good thing.
Thanks, your advice is greatly appreciated.
Papajack, I agree with the heavier hull issues. I'm trying to find a good median, where it's not too heavy, but not too flimsy. What is the hull thickness on yours? and stringers?

My typical approach to projects has always been to build to the highest level I can with the best parts I can afford.
Not a cheap approach to building things.
This will be the first time I have tried to put something together with as little initial investment as I possibly can.
If I can get it built and running, I should be able to increase power and amenities later when the piggy bank catches up.
...and my wife Loves the Jet Boat and is very excited to have this little toy in the stable. Once I get it running with her having fun, getting the Chief Financial Officer to sign off on the upgrades will be a cinch.

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goldhunter_2
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Re: New Project...Plenty Of Questions

Postby goldhunter_2 » Sun Jan 17, 2016 6:16 pm

a hard chine side ill chacth as you turn and stop some sliding this can be good or bad depending on operator. a deep chine hull digs in a tracks straight by nature with a prop motor you actual have force to turn the hull where a airboat gets it control by forced air over the rudders the chines grabbing the water will resist the control until you reach a massive over power source to compensate for the chines. Years ago there was a cat style airboat hull that basically had the same issues they went straight good but didn't like to turn at all, I had one of those :blackeye: to and would not want one again. Any type of hull you end up with you'll have to set up specifically for how & what you want to run.

your looking at Hp (common car motor guy mistake...lol) airboat like lots of torque. allot of the Ac motors weight to hp ratios aren't bad take a io-346 or pe-90 with 520 jugs can make 200hp easy with lots of torque (now compare to the rotax) so now your roughly in the ballpark of 1hp per pound you can run dry climb over logs etc etc

I understand use of local available snowmobile engines if you have one , but you need to build the smaller and lighter,plan to stay in the water or do lots of pushing , can be built for two in water but preform better with one. Build your hull to fit you chose of engines you mite look into a form core like H-80 or corecell with vacuum bagged epoxy an carbonfiber to save weight. remember if your wife isn't happy with it she isn't going to sign off when you want a bigger one and you will:lol: you could also try one of those john-boat hulls for your experiment
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Support our future , get a kid involved in the outdoors!!

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papajack
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Re: New Project...Plenty Of Questions

Postby papajack » Sun Jan 17, 2016 7:00 pm

Hull is .075 with 5 t-bars. I have steel-flex 2000. Way too light for your application. Hull weighs 200#. Your 670 with a e box is much better idea than the smaller Rotax. I would also recommend a clutch be added to the gearbox. This saves prop torque on startup. About a $500 add, but worth it in my opinion.
Last edited by papajack on Thu Feb 04, 2016 7:11 am, edited 1 time in total.

Screamin Seaman
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Re: New Project...Plenty Of Questions

Postby Screamin Seaman » Thu Jan 21, 2016 8:08 am

Rotax gear boxes are garbage. Especially the C box with the clutch.

... at least when there is any decent power put to them.
11'6"
800cc (48CI) 2-smoke
3.11:1 reduction
67" 5 blade Warp

96' dry - 3.386s at 34 MPH
Yea its a mini.... the worlds fastest min!

There is a replacement for displacement, it's called volumetric efficiency... a/c guys don't know about it!

papajack
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Re: New Project...Plenty Of Questions

Postby papajack » Thu Jan 21, 2016 6:06 pm

Screamin Seaman wrote:Rotax gear boxes are garbage. Especially the C box with the clutch.

... at least when there is any decent power put to them.

Please don't tell all those frogs out there that my c box with a clutch is garbage. So far it has held up just fine for a couple of hundred hours. When I want to run dry I take my ac boat.

Arcticshooter
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Re: New Project...Plenty Of Questions

Postby Arcticshooter » Thu Feb 04, 2016 2:00 am

No worries, the frogs will figure it out when they look up at the light...

Do you know how much HP your 583 is putting out?

papajack
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Re: New Project...Plenty Of Questions

Postby papajack » Thu Feb 04, 2016 7:07 am

Somewhere north of 65 hp...


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