java230

Newbie here, trying to figure out hull sizes.


I have an old hull I build as a kid, with no idea what I was doing, its 8x17, balsa covered with epoxy. It was powered with a .061 that I never could get running. Its all square edges with a rounded front, single rudder.

Should I use this hull with my new motor or build a new one? I was thinking foam covered with glass 12x24, but I think it may be too big and heavy for the .15.

What will end up lighter, balsa and epoxy or foam and a single layer of glass?

Should I make an even smaller hull for the .15? 7x12 maybe? I dont have the motor and prop yet, but I believe its a 8"x4 pitch.

Thanks!

java230

I had a few mins at work. Knocked out a 7x14 hull blank, and cut a 12.25 wide one I can make another out of if its too small. I am going to try to make a tower that could be swapped around if needed. I have more foam too if I need another larger one. I cut a 10 degree bevel on the sides.

Still looking for opinions as to size to motor.

1QwkSport2.5r

For a .15 using like a 7x5 or 8x4 prop, plan to keep your weight to about 1.5-2# MAX. You'll be lucky to get 3-3.5# thrust from a .15 so getting anything over 2# to move with any authority whatsoever will require a bigger engine. I'd be looking at 10x16 or so for a hull size. 1.5 X prop dia. for width and 2.5-3 X prop dia. for length is a good place to start.

java230

Thanks! Think the 7x14 will be too tippy?

1QwkSport2.5r

It might. One way to find out!

java230

I'll have to find out

The only thing I'm worried about is my electronics box is pretty heavy.... Its thick walled plastic with a gasket lid. I'll have to weigh it.

I put a coat of epoxy on the bottom and sides this evening.

1QwkSport2.5r

Are you planning to run on dry ground or on water?

java230

Probably mostly water, but I would like to be able to do ground also.

1QwkSport2.5r

Ground running requires a bit more power than water running, so keep your build weight as minimal as possible. What engine and prop are you going to use?

java230

Its a Chinese special 15 Nitro car motor with extension so the prop clears the carb. 8"x4 pitch plastic prop.

1QwkSport2.5r

Where'd you get the extension and what brand? I have a few car engines I'd like to put props on but no means of making a prop adapter..

java230

Found them here

http://m.aliexpress.com/item/8859429...bilesitedetail

1QwkSport2.5r

Cool! Thanks for the link.

java230

Thoughts on gas tank placement? Will placing it under a upside down U shaped motor mount block too much thrust? It's a 2oz tank

1QwkSport2.5r

The fuel tank should be mounted so the centerline of the tank be mounted in line with the spraybar of the carb. That is ideal. You can have the fuel tank centerline be mounted slightly below the spraybar centerline but you may encounter fuel draw problems when the tank gets low. Car engines tend to have larger carb chokes so this can cause further fuel draw issues. Be sure to use muffler pressure to the tank.

java230

Hmm so the tank should up mounted up with the motor? That seems troublesome on such a small hull.... I can't put it lower with the exhaust pressure line?

1QwkSport2.5r

You can go lower, but not much. These engines (especially the small ones) do not draw fuel well. Putting the tank farther away only makes the problem worse. All of my airboats have the fuel tank near the engine. This is necessary to get a proper needle setting and good throttling.

Heres a link to my twin .15 engine wood/foam airboat that is too heavy. Note where my fuel tanks are. http://youtu.be/uHB3Nh2lbgY

java230

Good to know thanks! I'll have to figure that out now.... My tower isn't really built for that. May have to RE work it...

1QwkSport2.5r

Can you post a picture of it?

java230

Its just a piece of 2"x1/8" aluminum bent into a upside down U. 3.25" wide by 4.5" tall. Ill take a pic tonight, its not finished yet...

java230

Made some tower mount blocks. Notched the foam and epoxied them on.




Then did a light coat of epoxy on the bottom and sides. Should add a little toughness to the foam.

1QwkSport2.5r

Looks pretty good so far. I'd suggest incorporating some adjustability in your engine mount to allow you to move the engine back and forth to adjust the Center of Gravity. If the engine is too far back, it will want to dig in and plow water. It's balanced properly when the hull hangs level or slightly bow heavy when you hold it up by the prop.

java230

Good call, Ill slot the mounting holes. Motor shipped yesterday so hopefully I will have it by next week.

Wayne Lester

Hi here is link to my new airboat which runs an OS15 engine.
https://www.youtube.com/watch?v=f_x0sO6hGfo

Have had several better runs with this as I found that I had fitted prop on back to front so speed has now much improved with it the right way round.
Length is 650mm, beam is 250mm, engine is running 8'x6" prop.

Awesome fun, look forward to seeing yours running!

1QwkSport2.5r

You're using an 8x6 on a .15? Wow. Usually guys use like a 7x4/7x5 or 8x3/8x4 on .15s. If it works it works.