Well guys, here it is, the new boat. I WAS going to vacuum form the hull, I had everything made...molds, clamping devise and vacuum box. I did not have enough electricity coming into the house to make the heater hot enough to melt a piece of plastic 14" x 50" OH WELL!![&o] Anyway...the new boat has the same problem as the old boat...prop walk. The only difference is that the new one does it much WORSE[:@]. On the maiden voyage... everything was going and looking real good until I put her in the water, Got on plane real fast, and took a left turn with no help from me. I got her to go in a straight line by having the rudders turned all the way to the right. ( at full throttle ) ARGGGH !! WTH? I also tried a new prop, an 18 x 12 competition propeller. for some reason it has less thrust than the 18 x 10 prop. ????

Is it possible for the engine stand to be flexing?

the engine is solid to the "a" frame but the connection between the hull and mounting plate are in rubber,
the engine does flex forward max 1/4 " under heavy throttle.
never mind the pointing finger in the right side of the pic....it's not pointing to anything

That a nice set up and looks good . You said it turned on its own ? MAYBE you have a metal to metal contact causing a radio glitch affecting just your rudder servo . You might not have enough rudder blade in the air flow making it not turn . MAYBE move the blades apart a couple or three inches and add a little to the height . Just some things you might look at .

Try a lower pitched prop for more thrust. Higher pitch will give you higher top speed with less low end thrust.

Thank you for the compliment. Picture this: I throttle up slowly, get her on plane,but still going slow. It will go straight at this point.
While moving slowly on plane...hammer the throttle and viola'.... left turn, all by itself. Glitching is non-existent, one of the first things I thought of. Mr.Cajun Gator: I will remove rubber mounts and see what that does. After going over the differences and similarities between the old hull and new hull, one thing stuck in my head.......could the problem stem from the prop swinging below the level of the gunnel? ( see pic ) That is the only thing that is constant between the two at the stern anyway. anybody have any thoughts on this?

i dont know much about air boats but looking at that pic,
the engine head is to the right behind the prop and that may be acting
like a rudder, becuase the wind cant get around it

a few things, first i always make sure that the engine is high enough that when the prop is as far down as it will go that it clears the back edge of the hull slightly that way no thrust is wasted and blowing into the hull and comming back forward(dont think its causeing the prop though) i doubt that the engine head being over makes a whole lot of difference either, is it strait with the hull, also you can turn it one way or another to counter prop walk, hows it point vertical wise, if its lifting the back to much it could be causing a spin id imagine. also higher pitch equals more prop walk, find something that the engine likes and go from there, this is also why you had less thrust it just didnt have the stuff to turn that bigger prop, i have a 16-8 on my boat but its smaller than your engine a little i believe maybe an 18-10 like you had or and 18-8 to help the walking. hope that helps a little, mine needs a bigger motor or smaller hull as its 40-20 and to much for a motor that swings a 16 incher, but it runs great on water(id like nore speed though) there are picks of it under a thread titled something like pics of my bigboat, i like it for water and snow though but it wil be upgraded when i get a chance(twaeking in my nitro vee at the moment)....

My guess is that yes, the prop going below the gunwale is causing the problems. Look at it from the beam, with one tip hidden, (as like in a wheel well) it does not push sideways on the downstroke, while the other end does, while on the upstroke. I am pretty sure the high speed water screw boats that use 'surface drive' props...where only half of the prop is in the water... have to account for the one way prop walk. Can this be adjusted??
Sean

Makes sense to me, with the prop being that for below the gunnel. One way to check if this is the problem, get a small diameter prop, (I'm not saying to keep a small prop, just to test with).
I would not completely ruel out radio glitching, since the boat has a different harmonic, as the speed of the boat changes.
If you can install a water thight floor cut the transom down.

Everyone has been great in providing possible solutions: Thanks guys!
Here are some of the things I've tried {in order}(1). cut transom to allow full air flow,see pic. result: nada. (2) cut down an 18 x 10 prop to 16". result: not enough thrust to move boat on land, still walks on water. (3) mounted a 16 x 10 3 - blade prop, result: same as #2. (4) re-mounted cut down prop and turned engine on axis 12 deg. result: nada (5) remounted 3-blade prop and left engine turned on axis, result: snake eyes. (6) mounted the new 18 x 12 prop, returned engine to original position, played with the carb to gain some rpm, engine now runs up to full rpm with no bogging out, result: it goes really fast, but did not cure problem. (7) took out rubber engine mounts and made it solid, result: no change. I weighed her on a very accurate digital scale, she is 28.45 lbs. Is that too heavy? could the weight be an issue? It is a big hull, 48" long x 26" wide. It's heavy...ergo, I need a lot of thrust, lots of thrust leading to prop walk on water because of the torque needed to push such a monster?

Does ANYONE else have such a problem? Do you guys even have just a little bit of a problem with prop walk, or am I alone here?

Maybe a longer (not taller) rudder on the right??? I'm fishing here people, grasping at straws,, etc,etc,etc.

Looks like you have been trying everything. I have a little prop walk on one hull, but the reason of it was the engine had too much torque for the small boat. The only other thing that I can think of at the moment, just for the fun of it, try closing in the top of the hull. I doubt that this would make much difference.

I agree with Hal... raising the prop would clear the gunwales, but would not keep the air from still being trapped there. clear the prop and cover the hull see what happens. Sean

Thanks guys. I did extend the right rudder by 2" today, It had a profound effect on performance: It will now turn a little more than it did before...but it also makes the left side dig in more during the turn. I came across this pic on a website, I was thinking of trying to copy it. Although it doesn't look like it has the displacement to float.

I have no idea, mine doesnt do that, and its 40-20 inch and wieghs around 15-20 pounds, needs more power though, just short of moving in grass, one question, your last boat had abs wear strips on the bottom, does this one, are they removed for water use, that could easily have a huge affect on water handling, if so, take them off once, seal holse where they where and try that......hope that helped...im quite puzzled here....

Crispyspa,

If I remember correctly, I believe that boat belongs to a drag racer here in Florida. That boat is a designated ground dragger.

I bet it'd be fun though to hit the water moving though, so long as it didnt stop, I bet it'd fly on water.

yea, there are wear strips, but before they were permanently glued on, i beveled them back 5/8 " . I'm afraid if I try to remove them it will damage the hull......I think I might try it anyway, what do I have to lose? probably be a few days before I get to try it again......the engine mount threw a screw right into the new prop....when I was taking the prop off, I noticed that it had also thrown an aluminum fin off of the flywheel * I wondered where the vibration was coming from* which is probably why it threw the screw.[] must repair before i go into airboat withdrawal.

so I assume that you guys don't use wear strips? How do you keep the bottom from wearing out? or don't you run on land much?

I might just go ahead a build a ground dragger. would seem I have the power.

HEY, what kind of epoxy is best for gluing birch ply??? Yes....I orderd a 5' x 5' x 1/8" piece of birch today....gonna build a deck-over with wood.

On some of my hulls I use bottom runners, as long as they are true to the center line of the boat it will not give any problems. Bob Smith Inds. makes a good 2 part epoxy(the only brand I use).
Tip for building with plywood, make sure that your work bench is completly flat. If your work bench is twisted so will be your boat.

You have a pretty big boat. A lot of the boats around here are much smaller and lighter and a layer of fiberglass on the bottom is sufficient protection from abrasion when ground running.

You might try removing the wear strips. If that solves the handling problems on water, try adding temporary skid pads to the bottom for ground running. I did this a while back to run my coro boat in the street (no fiberglass on the bottom). I just duct taped a strip of coroplast to each side of the bottom of the hull. Worked great!

Thanks for the reply. I'm reading all the comments and problems and learning more each day.

As many problems as this thing has.............It's darn fun. got the flywheel re-balanced, replaced screw in engine mount, and shot some video today, All in all, a good day! I talked to my neighbor, (who builds real airboats). he said " The large ones have your problem too, but it is usually the totally insane people who put 500 hp engines on 12 foot boats" he said my problem is that I'm just overpowered. I'll learn to live with it i guess here is the vids I shot today.[link]http://www.rc-airboats.com/gallery/view_album.php?set_albumName=new-quarter-scale[/link]

Very simple solution ... the engine is shoving that piston at aprox 11K and to the side to boot ...
either moutnt the engine up-right , or spark plug facing down ... Your "prop' walk , is being created by the R's from the piston .
( have you ever ran a weedeater engine in your hand and tried turning it ... you get that "prop" walk thrust at cha ...... same here . )

I would have seen that the engine is set sideways until Terbo said something about it. He is very much correct. Ican already hear someone saying the piston travels back and forth that it equals itself out, just one thing to keep in mind, there is an explosion on one side, giving push making the boat turn left.
I watch your vid over and over, after repostioning your engine, you will have a very good running boat. The only other thing that you may need to do is tweek the engine to the boat.

Right Hal,
There is an explosion, the thing to keep in mind is that the explosion pushes on the cylender head just as much as it does on the piston. This is the equalizing force within the engine. this is the other thing - because the piston and the explosion are enclosed within the engine casing, it is a closed system. Bobs analogy with the weedwacker is incorrect. turning the weedwacker at high rpms is difficult, this is due to gyroscopic forces. If what Bob had said was true(and what you are saying is true with this boat), the weed wacker would try to twist out of your hand when it's throttle is gunned(you say it would twist perpendicular to the driveshaft - it will torque in your hand, but along the same axis as the driveshaft), It does not; it only tries to stay in the same orientation (like a gyroscope). An engine, unmounted, and sitting on the ground, will vibrate and 'walk' due to the vibration, but not due to the force of the piston being 'blown' one way and comming back the other(you've seen generators sitting on the ground, haven't you?).
I believe this problem is from prop walk(minor), and from engine torque. With a large engine, and a large prop, the torque will drive one side of the boat deeper into the water. with a flatbottom boat, this means the boat banks into a turn. The harder you push the engine, the harder the turn. You might try air flaps, on the cage of your boat, to counter the torque. Otherwise, the hull would need to be modified, perhaps to a rounded nose/rake so the leaning caused by the engine torque does not affect the boat as much.
Sean