cordless airboat propeller question

Hello all. I recently rebuilt Four Candles, my Cordless Canoe Challenge boat to have air propellers for this years race which took place June 2012. After the race I had so much cruising time available that it was clear that I wasn't pushing the drills very hard so I bought the parts to step up the prop speed relative to the drills. Trouble is, I'm now wondering if it would be more or less efficient to have both drills driving one existing propeller. (If pictures post o.k. all should make sense).
I tried using Javaprop but it seems that you cannot fit an existing propeller into it. Then I tried playing around with Javaprop to get results which geometrically matched my propellers but didn't get anywhere close.
So my question is: should I drive each prop separately via 1.5:1 step up to get about 3x power from the drills or should I drive one prop via about 2:1 step up also getting about 3x power from the drills?
The drills are Bosch lithium and put about 200w into each prop at 1550rpm. The props are Xoar 32" x 10" wood.
Thanks in advance. I've been reading this forum for about 6 months now, a great place for boat nuts.

 

Why the rims around the props?
Planes do not have them, they cause drag. Looks great though....

 

I don't know much about airboats, but generally a larger prop is more efficient. Mount it directly behind you, and with belt-drive, you can mount the drills down on the hull to lower your CoG. Chain drive would work, also, and make it easier to adjust your ratios (add/subtract links). If you got really fancy, you could add a gearset and a derailleur (sp?) to spin the props up faster.

X2 on losing the ring-around-the-prop(s).

 

That looks pretty freakin awesome, how fast did it go?

 

Thanks for the early replies folks. The prop shrouds are very necessary for safety. The race is run quite close to the public at start and finish, also there is nothing between the propeller tips and other competitors without them.
I don't want to use a bigger propeller because I don't want to make one, or pay for what would probably be a microlight propeller.

plusses - only two blades instead of four, only one shroud and prop so probably less overall weight.
minuses - smaller mass of air being moved, I'll be sitting in the inward flow of air.
Maybe I should just toss a coin?

Some people on this forum go out of their way to help others so I should add that I may just make a whole new boat as some competitors think that it is now possible to make a planing boat powered by a couple of cordless drills.

 

what you are really saying is that you want to raise the load on the motor as its capable of giving more ...so why not use two props to give you a 4 blade prop on each motor ...easy to try just move one prop to join the other ...gears chains etc ..waste of time only add load to the motor that does not produce anything ...

 

Alan:

As far as the boat design itself, here is a proven efficient design which may help:

http://lancet.mit.edu/decavitator/

Resistance is cut back with the fairing, single prop, 2 hulls, and lower weight it would seem...

For a propulsion motor, would it be possible to use something that is already geared to the proper range, so that gear train losses are minimized from the down then up gearing? Seems like a single cordless saw would be less complex and cut overall weight... Maybe could be tweaked with with a proper and efficient prop pitch. An intricate and redundant dead man switch that makes it impossible to start the prop could be used so you could eliminate the ring?

Hope this helps.

Porta

 

Fantastic!

One large prop.

Javaprop design.

No shroud.

Simple.

You may evolve to a hydrofoil, but don't let me rush you.

 

Personally, I'm liking this suggestion. Raid a few old mountain bikes for thier shifting parts and gearsets, have an adapter or two made so you can mount the gearsets in the drill(s), fab a mount for the deraileurs, and away you go.

Roger

 

probably about 6 knots, as the winner apparently did about 7kt powered by cordless tools.

so why not use two props to give you a 4 blade prop on each motor (quote)
Not sure about this, but for a start I would have to buy two more expensive props if I went ahead, also they would not turn any faster. Would definitely give more thrust at the same speed though.

Portacruise and others, thanks for your suggestions. I had a great time before starting the build, daydreaming about all the different layouts I could use. I even thought that an electric airboat, in the style of Decavitator but without foils would make a great eco-cruiser! However I now have two nice propellers and two shrouds. Before the race I had some discussions regarding safety and we came to the conclusion that the shrouds would be necessary to get through scrutineering, bearing in mind the proximity of the public and the general aversion to being sued.
FWIW the air rudders and amas in the pictures were ditched in favour of a water rudder (Jeremy harris suggestion - it was much better) and steam bent PVC pipe amas to save about 6 to 8 Kg.
At the moment I am inclined to use two separate props simply because I have bought all the parts necessary to do it this way.

 

Is your vessel length limited by regulations? If not, a longer hull will be faster, all else being equal. As mentioned earlier, a catamaran style may be even more efficient in the displacement range. However, if you're going for a planing hull, you'll want a wide flat bottom...

Counter-rotating inline props will also give you more thrust, if you can devise or find a drive system that would work. Btw, are your props counter-rotating as they are now? If not, that may be something simple to try - it would reduce yaw and possibly increase speed/stability.

Variable-pitch props would also be an option, but I'm sure too expensive for your purposes.

Perhaps with a catamaran, you can use a large single prop mounted between the hulls with no ring, if the rules allow it. Speaking of the ring, I've often wondered if a shroud wouldn't be more efficient for concentrating/directing thrust, if required.

 

Hope my ideas above are of some use. Good Luck!

Pora

 

Village_idiot, it sounds like you're having as much fun as i had, designing a cordless boat! All your comments make sense, I'm just trying to push the hull I have as far as possible for my own interest really; I'm thinking of a flat bottomed planing boat for next year.

I've answered my own question, by doing what I should have done before starting this thread. I used javaprop to get some thrust figures for one prop driven at my best estimate of rpm, velocity and power, times by two as I have two props; then put in double the power (two drills) at my best guess for the new rpm. Two props gave 22% more thrust than a single prop driven by two drills. Plenty of room for errors but a big enough number for me to make my decision - two propellers.

When it's done I'll post some pics and possibly results here.

 

Just having a little fun

I'm still trying to figure out an elegant way to mount a cordless drill motor to my snowblower chute...

 

mmm sorry im not much help about the design side,
but gives me many of idears haha, looks like fun! and a race :O!