flat bottom gas airboat build!

Well I have quite a collection of weedy motors so time to put them to use

I have started on a scratch build flat bottom airboat, I use to have a .40 nitro powered airboat which was loads of fun on water and on land.
I will be taking ideas from the likes of the "Cajun Gator " and similar flatties.

I have sort of started this build a bit backwards in that I have already welded up the cage for the motor before I've even started on the Hull itself
Anyhoo moving along...
The hull should be about 48" x 24" or thereabouts, kinda sticking to the 2:1 ratio I've read about.

I've chosen one of the 25cc weedies as the powerplant, I actually found an old ryobi 31cc Reed case weedy during the council verge side pickup I might even use.

Here is some pics of the cage and motor dummied up. From my Googling, Ive decided on either a 16x10 3 blade or 18x10ish 2 blade prop, seems to be the norm for weedy powered airboats

Your starting it right mate, trust me.
The cage and motor stand are the heaviest part of the construction, building it first will allow you to set it up on the hull to get your COG and thrust line right.
You will also be able to position your radio gear to help get balance right.
Personally a 16x10 3 blade will be too much for a 25-30cc engine, for airboats we prop different to what the plane guys do. Unless you really work the motor add pipe etc. you wont be able to spin props that big up enough to get good speed( as the motor is always under load in an airboat). I have been running Groupner 16x8 2 blades on my modded 30cc homelite and on my other test motors(I get them cheap from the hobby shop), still too much pitch to allow the motor to really sing. Tried a 15x6 APC 2 blade this morning on my MAC 32cc and the difference was very noticeable as the motor actually came into the power band quite nicely and sounded so sweet as it accelerated through it. On the bench I pulled out to 9,370 rpm and ran nice and smooth.
Sorry to ramble on at you, these are just my opinions and I have been doing a lot of testing lately for my hydro airboat project, going for the speed record.

I appreciate the ramblings the more info i can get the better.
Im just hoping the cage wont be too heavy as its made up of 6mm galv rod. Havent weighed it yet but seems quite weighty.
Ah well if its too heavy, ill just leave it off at the expense of breaking stuff when (and it will) tip over. Hehe
motor mount will be made of aluminium so there will be a weight saving there

do you solid mount the motor or use the rubber isolators like in a regular boat?

I havent bought props yet so im glad u recommended a smaller prop which will be more suited to my motor.

We solid mount the motors on these, isolators will allow too much movement of the engine changing thrust line etc.
However you are mounting your engine to the stand, make sure it can have the angle changed slightly. You may need to change your engine alignment to tune the hull to run properly.

Not much to update on. But I have changed powerplant.
Have decided to go with the ryobi 31cc Reed case, mainly due to the fact the Carb and exhaust are at the rear which will make for more streamline mounts etc...
Also decided to go with a puller design to simplify mounts and rudders as the motor will face forward.
Should still be able to use the steel cage I've already made, but will make it possible to remove it if its too heavy.

Got a bit done today, drew up the bulkheads and stringers for the main section of hull.
Bought some 6mm ply and started on the hull. Cut 4 bulkheads and the 2 stringers and pieced them together, still to draw up the front bulkheads for the curved part, probably get that done tommorow.

Ive decided to scrap the original steel cage i made up, waayy to heavy. Ended up buying some 12mm x 3mm aluminium bar which ill bend up for the cage. Was only about $7 for a 3m length! Plus it will be easier to work with than the 6mm steel rod.

I went to the local boat builders and they let me have a scrounge through their scrap bin. Unfortunately i was too late and the bin had been emptied earlier that day... Bugger. But i did find some bits of aluminium that missed the scrap bin and will prove to be quite handy. Got a nice peice of 4.5mm thick plate which ive used for the main part of the engine mount. Still to trim it up and cut out weight saving holes.
Gee i tell ya, bending 4.5mm thick ally takes a bit of work. Nothing short of a gympie amd block of wood soon sorted that out, thought i was going to break me vice off the bench in the process! :dry:
Also found some nice slabs of 20mm thick x ~100mm x ~60mm aluminium which i may one day use to whip up my own stinger drive for me mono

What would be the ideal c.o.g. for such a build? Would i be aiming for something like 30%?

Heres some pics of progress so far and the nice 20mm ally chunks i found

Alrighty, another day on the end of the jigsaw and heres progress now.
Have pretty much finished the skeleton of the hull. Before i went crazy with the holesaws the frame weighed in at 1.56kg.
After the holy treatment she weighed in at 1.3kg. A massive saving of 260g! Makes all the difference in the end i guess especially considering the motor weighs 1.8kg with the aluminium mounting bracket.

still yet to sort the engine mounts as i wanted to get the frame done first and work around that.

Oh yeah, if you notice in the pictures that some of the ply is warped, thats because it is...
Theres a lot of moisture in the air lately with this cold rainy weather so i turn my back and all my ply warps itself into spectacular shapes making it a real pest to work with :pinch:

Daaaaammmmnnnn....you went to town with the hole saw!!! :laugh: :laugh:

Looks nice though! What work did the dog do? :laugh:

All the best and keep it up,

Brandon.

youp! The whole hole saw thing does wonders. It saves weight without reducing strength too much. And, not to forget: It's easy to do and makes things good looking and professional made.
Actually I have done the same thing on my Cigarette in places that will never be seen once the front part of the hull is closed.

Cheers!
Woodie.

Your really getting along now!
30% will be good for your COG to start, once you see how it handles you can tilt the motor slightly. down to bring the nose up, up to settle it down.
One of my best purchases was a set of hole saws.