chrispoulton

hi all, i'm new to airboats so please excuse the total lack of knowledge here.
i've just got my hands on a new pair of 40-size aircraft floats by hobbyking. they are 83cm / 33inches long, and weigh around 650 grams / 1.5 lbs with the frames, so i would need to take into account motor, esc, battery and radio weight.
queston is just how much power do i need. this is going to be for fast fun and also rescue duty for our seaplanes. it will operate in salt water. i guess i would be happy with 35mph flat out. thanks

arcdude

Welcome chrispoulton, Interesting idea, I'm sure it would make a great rescue boat. It would be nice to see a pic of what you are contemplating on doing in regards to mounting the engine and so on. I'm certainly no expert but what I can see is the engine mounted in the rear and my guess would be an engine in the 40 size range for what your planning on doing. Your probably going to have to create some sort of a platform between the two floats and work from there. What your proposing to do certainly isn't going to be a speed demon, however should be a lot of fun.

chrispoulton

thanks arcdude. i want to go electric really. at a wild guess i'm thinking around 600 watts but i have no idea what prop rpm to look for. from my experience with planes a 40 should spin a 10x6 prop at about 12,000 rpm?

arcdude

The prop size and rpm sound about right however with no knowledge of electrics I have no idea what a comparable motor would be. I would like to see what you propose to do for a platform on which to attach everything, it may be helpful for anyone viewing to offer suggestions.

Got RPM

Here is my scale airboat using 33" airplane floats. I used a Scorpion 3026/1190 motor on 4S, and with a 9x6 speeds are mid-30s. One thing I found is that the CG is critical to keep the boat from porpoising. The video (will post later) shows what happens with a CG slightly too far forward - crossing a wave will upset the balance. The boat needs to run on the front steps and the rear transoms.

1QwkSport2.5r

A 10x6 APC at 12,000rpm requires .74hp, a 9x6 APC at 17,600rpm requires 1.5hp. Just for some perspective. My SI3 .20 series with a Jett .35 turning a 9x6APC at 17,000rpm on a 5lb RTR hull goes 50mph minimum. So if you can keep the hull slick, balanced, and well powered, you should see some good speeds from it.

Got RPM

He won't get anywhere near the speeds that a hydroplane can do. The floats are not flat bottomed. they are vees with a lot of wetted surface. Plus they have to run at a high angle of attack. They simply will not run too fast without becoming unstable. But mid-30s are doable, mine runs 33-34 mph with the 9x6 APC averaging 15,500 rpm unloaded.




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1QwkSport2.5r

For what it is, that seems awfully slow for some reason. Even being a V hull shape, the lighter weight and faster speed should get it riding higher reducing the wetted surface. Not like a hydro exactly but along those lines. I suppose the shape of the floats and how the AoA is set up will ultimately determine its capabilities as a fully assembled hull.

chrispoulton

thanky you guys. gotrpm, where is the cg in relation to the step? i guess it needs to be somewhere behind the step? would you measure this as a percentage between the step and the back of the floats? i would be happy with a mid 30 speed. based on what you've told me, if i get 600 watts (741 watts is 1hp), 9x6, 9x8 or 10x6 to experiment, and can swing the props at about 12 to 14k i should be good. i hope to use a 3s setup for this.

Got RPM

For those speeds on 3S you will need a motor with a Kv around 1600. The CG on mine works best about 1/2" to 1" ahead of the step, I can't get it further back. In the video shot yesterday I tried the CG about 2" ahead of the step and the boat was unstable, wanting to porpoise and flip. You can see me recovering, it takes some finesse with that CG. Moving it back makes the boat easier to drive. It is safer to run, although a bit slower, running an 8x4 prop. I almost never gave the boat full throttle in the video.

Mine averages between 35 and 45 amps on 4S depending on the prop. For similar speeds you will be pulling 45 to 60 amps on 3S. I would be careful with prop pitches over 6", you might stall the prop since the speed is not high enough for efficient use.

http://www.youtube.com/watch?v=aIFig...ature=youtu.be




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Got RPM

Airplane floats are not designed for speed, but for directional stability. They are very draggy and supply only limited lift - the airplane's wing is supposed to do that. You simply cannot compare a flat sponson with a vee hull, the former packs air under it to increase lift while the vee sheds the air out the sides. This means more vee in the water causing drag. But the floats have much less drag than a flatty, so speeds and acceleration are higher. Add in ease of building and the floats provide an excellent way to put an airboat together.

Jeremy_H

I thought the step was arranged to be under the CoG of the aeroplane, so that the elevator has maximum effect by pivoting around the step. The slim and low lift rear end permits a high AoA on the aeroplane's wing to get initial lift, so I see your boat as rocking on the step, not true porpoising.

Got RPM

An airboat is not an airplane. It cannot balance on the step like a plane because it does not get its lift from a wing. For stability it has to balance on the step and the transom, just like any stepped hydroplane...this one just has two hulls. Boats porpoise regardless of the bottom configuration - monoplane, hydroplane or catamaran.



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Jeremy_H

Yes I know it's not an aeroplane and that's not porpoising.

arcdude

Just a thought, maybe an aluminum track for instance two lengths of aluminum angle to be used as tracks to move the motor mount fore or aft would be beneficial to determine exact placement. To me I'd be trying the engine towards the rear and use the engine to force it up onto the front step, never tried it, but with tracks it's easy to find out.

Got RPM

Sorry I tried to help Jeremy, my 40 years of R/C boating experience apparently pales compared to yours. Have fun learning a thing or two....

Another one for the Ignore List.

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1QwkSport2.5r

Duly noted. The V hulls/floats should handle rougher water then, right? As well as a flat bottom? I totally understand the ease of building, especially for an easier first project.