Well, I'm bored to death now that classes are over, and I started thinking about building an airboat to drive around the pond in my backyard. I've built a few RC airplanes, but this will be my first attempt at designing and building from scratch. However, I know very little about boats.

What I've been thinking about is having a crude aluminum frame, with 2 pontoons, a rudder on each pontoon, a spoiler ontop of the rudders, and box inbetween the pontoons with the radio, servo's, and battery.

Here's an incredible poor MS paint, minus the engine, mount (which will be apart of the frame), and any sense of true dimensions:


Now for the questions:

First of all, is there anything inherently wrong with my idea?

Second, I have no idea how to glass things, or how I could waterproof the pontoons. Could someone point me towards a tutorial on doing this?

Is a functional spoiler necessary, or do people usually do it for cosmetic reasons? All that's needed is slightly negative angle of attack, correct?

How do you balance the boat? Where should the center of gravity be?

Do you make any modifications to the engines?

Do people usually put lips on the bottoms of their boats, or are they usually smooth? What are the upsides and downsides? I take it lips give more control but there's a possibility of flipping?

I plan on using a .46 engine, I use 11x6 2 blades for use on my airplane, but I want a 3 blade for the smaller diameter. What size 3 blade prop should I get?

Finally, as you see in my painting, I have the rudder going into the water. Does this have much of an effect?

Also, if you know of any common mistakes that beginners make, I'd like to hear them

Thanks

looks good, most of what your proposing will work. the rudders might need to move more to the center where the prop wash is.
In my trial and error I put lips on the bottom of my air boat and it made stop slipping in the water, it also makes it turn better.
I would leave you rudder out of the water,,if you position your rudders correctly and they are the right size, it will turn on a dime. what is the overall size?? W X L ??

Yeah, if you want to use air rudders, they'll work best if they are behind the prop, especially at low speeds. If you want to avoid that, use water rudders only. On a pontoon style boat like that, they can be a bit more difficult to get to turn, so the water rudder would work better anyway.

Looks good! Pontoons will make her a great straight runner, and tough to turn. DO NOT use wood prop, as yours will get a lot of spray. Spoilers may be used, probably should be used with a puller ( a pusher prop can more easily adjusted for angle of thrust ). For glassing, look on the net under 'surfboard making' -great stuff on shaping and glassing, layering etc. COG should be around 2/5 40%) of total length from transom (rear), and as low as possible. Your design should not have any problems with lateral stability, however, your drawing looks rather square, boxy. Most boats have about a 2-1 ratio, length vs. width. More length will increase your fore/aft stability, and keep you from porpoising. For prop conversion, Zingali web site has a chart, I believe the rule of thumb, add one blade, same pitch, decrease diam 1 inch. Add 2 blades ( four blade prop) decrease both pitch and diam by 1. Terminology check...Lip?? 'round boats a lip is usually an around the rim affair, such as around the table, to keep the stuff from rolling off. A Strake is a runner or ridge going lengthwise with the boat. Yes, these can increase stability, but your boat is basically two big strakes, ( so is mine- http://community.webshots.com/user/dogmoat ) so should not need any. A lip or ridge on the rear or transom, used to adjust the angle of attack of the hull to water, is called a trim tab ( just like planes, I believe) and could be very usefull in lieu of lengthing the boat. Be patient, there will be trial and error-so try to make things adjustable, moveable, and easily accessed. I hope some of this helps, Most importantly, HAVE FUN!

Very cool. I think I have everything set.

The boxyness should be fixed with what was said in the first reply. I'll have the engine forward from what I drew in those pictures and will move the pontoons much closer to each other so the rudders are in the propwash. Not sure why I didn't think of that to begin with

I'll draw up some detailed plans and post them here in a few days.

Thanks

you dont really need to move the pontoons,,, run a flat plate across from one side to the other to catch two round pins running down from the rudders.

Might work, but 'flat plate'=wing/airfoil, watch your angle of attack. Personally, I'd go with the narrower pontoons.

Is there a way to hinge rudders on a rod?

Like this?