I am building a twin engine airboat using 2 .40ci engines, swinging 11x7" props. Normally my design called for the engine drivshafts to be spaced 12" apart to allow 1" clearance between props, as you can see this makes for a possibility to have a very nstable boat with that much weight spread out about 6 inches above the hull. What i am wondering is if i can recess on engine in it's nacelle enough to allow me to space the enignes closer to actually allow the prosp to over lap by about 2.5" to bring them closer together. I am wondering if the thrust developed in that overlap area will create an imbalance in that same area for the second prop due to the air allready being accelerated by the first. ie will this cause mass vibration??

Josh,

RCM did a construction article a number of years back on a giant twin with two fusealages (ala P-82) and twin G38's. The engines were staggered as you suggest and there were no reports of adverse effect. My only concern would be making sure you can safely start the second engine without getting into the first.

Good luck.

Bedford

Depending on the amount of overlap, speed, and a lot of other factors (which are usually found by testing and saying, whoops, look at that!) it is possible to develop a situation that overly flexes the tips of the props. It can lead to material fatigue in the prop.

If there are sounds other than the sound of the two engines running they are due to the overlap. If the sound is large it indicates a lot of energy being changed into sound and the higher possibility of prop damage.

Position the props so that they are at 90 degrees to each other to minimize the possible tip aero interference.

Why not just drop down to a three blade prop? The diameter could let you set the engines closer, make starting less hazardless and not have any worries about interference. A push pull set up with them inline is also a good solution.

A question about the push pull setup?. If i were to do this, wouldthe second prop(pusher) need to be a larger pitch due to the incoming air allready being accelerated? ie. Say the air is accelerated to 100m/s by the first prop and it enters the second prop with a velocity of 80m/s should i use a larger pitch prop to accelerate it another 100m/s? Would using the same pitch prop let the second engine over rev?

I honestly don't know the answer to this one. It seems that the aft motor would benefit from the higher pitch but I don't know of anywhere I have seen a study performed on the requirements.

As a rough approach I would prop the first motor as you plan to do and take a tach reading. Then with the first motor running I would prop the second motor to give the same tach reading.

With the various models that the members of the forum have built (they after all old and seasoned modelers) I would think at least one would have first hand data.

Do you have a pict or drawing of what the boat will look like? I was spell bound by the first airboat that I saw as it flew over a mixture of mud and wet grass in Florida. I made one many years ago but at the speeds it went it was more of a barge than an airboat

here are some pics of my previous boat the fell victim to wood rot, i guess i missed a spot when i sealed the hull. and a rough drawing of my plan. PICS of BOAT

as you can see this is not a small boat by any means. However one thing it did do well was haul @$$. The haul design was not your typical one in that it was a catamaran style, the idea came straight from my Hobie cat 17 in the background of one of the pics. The hauls also feature steps so that when it planes the surface friction decreases. The only downfall of this boat is that it did not turn too well at low speeds, you had to be going full speed to cut a turn with a radius smaller than 10feet at which point the lean in the boat was pretty wicked. Also as mentioned above, when starting this thing you become much more aware of the spinning props, i have had my knuckles hit more than once. I think on my new model i will paint the tips white so i can see the tips better.

It is an airplane that you forgot to put wings on :-)

There are a couple of things you could do to stop the lean in turn.

Lower the CG. I would go with 3 or 4 blade props and lower the CG that way. The prop efficiency would go down a little but the lack of lean in high speed turns would be much better.
Little stub wings on either side of the outer sides of the fuse that would deflect at high speeds to keep the fuselage level would help also.

For low speed turning you might mix in throttle control with rudder to get variable thrust.

I have to comment about the lake, the water looks crystal clear, beautiful. If I had access to it Iwould have a lot of floaters also. I guess living in Canada has it's benefits.

I am accoustomed to looking at the Missouri and Mississippi Rivers as I drive over them. One inch visibility on a good day and I really wouldn't want to put a finger in either of them.

thanks about the lake. That is Lake Cowichan on Vancouver Island in British Columbia. It is about 20 miles long and about 2 miles wide. There is a huge float fly at the end of the lake every yr. I was running my boat at my parent's cabin up there and in the morning the lake is dead calm. Great place to run trials where things might go wrong because there are not a lot of ppl up there. As for the turning, on my radio i have the second engine on the elevator channel and the first on the engine channel. When starting i run them individually, so that i can rev/tune each engine seperately, but then i can have both controlled by the single engine channel when i am running it out on the water, for low speed i was experimenting with the variable thrust bit, it's a bit tricky though. And actually as you can see in one of the pics it will run pretty well slowly on one engine and full rudder over.