enjoymentboy

Hi,

I'm finally getting around to building an airboat...a project I "started" about 10 years ago.

I have a 45cc chainsaw engine that I have saved and modified to work with a prop. I'm running it with an 18x10 master airscrew and it holds a nice 1800rpm idle and runs up into the mid 7k. It has enough thrust to pull my workbench a bit. I had to add a cinderblock onto the bottom shelf to help keep it in place.

The airboat is very much like a gatorbait but it have resized some parts of it to suit my tastes a bit better. It is 46" long, 6" thick and 21" wide. I have used fibreglass and polyester marine resin inside to waterproof and hold all the inner bulkheads in place. Thsi thing is a rock. I have also filled each compartment with styrofoam for floatation and this also helped stiffen it up. It weighs just over 7lb and floats with the waterline at the 2" mark. Thats' 2" from the bottom with 4" still above the water. This is just the hull though without the engine, radio, tail or anyother parts that would make it work.

I figure that it will drop another 1.5" once it's done.

So here are my questions:

1. Where should I try to hit the CG? How far back from the leading egde?
2. Should I use any up/down/right/left thrust when I mount the engine?
3. What is a good size fuel tank? I'd like to get 20 - 30 minute runtimes.
4. How big should the rear fins(rudders) be? I'm going to use dual fins and they will stop just above the hull and not go into the water.
5. Just how much torque should the steering servo(s) have? Ovbiously a M\G high torque servo will be used here.
6. Should I glass the bottom of the hull for extra protection? I have a sheet of kevlar that I can use for this but I don't want to waste it if it really isn't needed.

Thanks for the help and I welcome any suggestions.

Enjoymentboy

TERBObob

My Feedback: (21)

1) figure your CG as you would a regular boat .
2) no .... a 90 degree prop angle will give your boat a good ride without porpoising
3) 16 to 20 ouncer
4) all depends on how fast you want the boat to react . The larger the fins , the faster the reaction to turning will be . This applies to BOTH - length AND height . If you build planes , same theory is used . As a matter of fact , I used a tail from an old 40 that I totalled for the rudders ( cut it in half and made twins ) and works SUPER on a 42 incher powered by a 65 KB Sportster .
5) I would suggest a minium of around 120 ouncer servo
6) you could , but , if your planning on water use only , then its not necessary , but a good idea . ( for added support and in case you want to run her on the ground , grass , what-ever )

enjoymentboy

Great, thanks for the help.

I'll take this all and put it to use once I get back to this project. It's a wee bit too cold to work in the garage right now. (-15°C).

I typically make my own battery packs for my cars, trucks, & boats. I'm thinking that with the higher current drain of a high torque servo and the longer run times the gas engines get that I should go with sub-c cells rather than AA. I do have 1 servo that is capable of over 300oz at 0.11 sec transit time IF it is powered at 7.2V. It can take up to 9.6V so I'm not worrried about hurting it. It's made for robots. So what I was thinking is making a 6 cell pack with a common negaitive lead second positive lead at the 4th cell. 4.8v for the RX and and throttle servo 7.2v for the steering servo.

Does this sound llike a good idea or ashould I just go with 2 packs and splice in a harness for the signal lead ont he steering servo?

I did a little retinking on th kevlar and since I don't plan on running on gravel or metal shavings () I think this might be a bit of overkill. Maybe I'll just stick with the 4oz glass cloth.

Enjoymentboy