I am designing a type of boat for speed that uses a tail wing and fin to lift the rear of the boat off the water. Would a symetrical airfoil be best for speed and also what is a good width of the stab for a 40 size plane wing?

There are a lot more things to think about other that what aerofoil to use. You will need to think about what angle the wing will be relative to the airflow when stopped and at speed when the wing has taken effect and to get any lift on a symmetrical aerofoil you will need angle of attack. It will probably be quite hard to get it properly balanced.

We could probably give you a pretty exact answer if you could describe...
1. Amount of lift required at the location of the wing.
2. Speed at which you want the wing optimized.
3. Limitations on size.

Is this a tunnel hull boat? My understanding is that a certain amount of lift is achieved by the positive pressure under the hull. Be sure to factor that in too.

It is a type of rigger boat. I am going for a speed record with it so I will send you guys a pic of something simular in a pm.

The size won't be as important as making sure you have the ability to trim the angle it sits at. Too little and it won't do anything. Too much and it'll lift the tail out of the water so the prop doesn't get enough bite.

At around 40 mph you'll probably want to have about a 15 to 20 square inch area. And set it on hinge pins at the 25% point and make some way to adjust the angle and lock it. Then do lots of testing at various angles.

I will make it pivot for adjustability, seems that 15 to 20sq inch is kind of small though.

Well, yeah, it is. But I figured that a 2 by 10 inch wing would be just about all you would want. After all it's supposed to be a helper and not a flying boat.

Besides doesn't your prop supply a lot of lift to get to the point where it's operating in a surface piercing mode?

And also if you locate the steps of the sponsons in the right relationship with where the boat balances you should not have much weight at the back of the boat. This last bit being just an educated guess since I'm not a hyroplane sort of guy.

Ok, now I see where your coming from. I do want to fly the rear of the boat, sorry about that I should have made it a lil more clear what im trying to do. Ill pm you a pic of something simular of what Im after.

hooty...don't mind me stepping in here, but you oughta tell them (also ) that it's an air boat...(you're gonna use that O.S. .91FX you were talking about, right?)

Check out this thread guys [8D]
http://www.rcuniverse.com/forum/m_5150339/tm.htm

Yes I failed to mention it was airpropped. Im going to use the 91fx then once everthing is worked out a 91VR-DF with a prop.

Thanks proptop

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For the trim tab function that you want you don't need a large moving surface. I'd go for a fixed wing on fixed mounts such that the wing angle is around 0 degrees to the table when the sponsons and stern steps are level on the table. That will give you zero lift when the boat is planing on those three points.

To provide the adjustment in lift you want for trimming the stern I'd go for a more or less full span conventional flap with an adjustment range of about 15 degrees either side of neutral. The flap being about 20 to 25% of the chord (width) of the wing.

To adjust your boat's ride at the stern it'll be done with a combination of altering the balance point back and forth and adding in some aerodynamic trim with the flap. Finding the right combination that results in a stable but stern light ride will be the trick. I can't help you there at all.