I've got a newbie question, and will appreciate any suggestions.

I've got a very small sailboat that I want to put my Icom706MkIIG into.
I've got the Icom AH-4 tuner, too. What I need is a small inverted "V"
dipole antenna that I can hoist up the mast, which would work with both 20
and 40 meters. I'm thinking that the poles would be about about 8 feet each
on a 1/4 wave.

So...
1. What should the poles actually be, lengthwise?
2. Can I get 20 and 40 out of a single antenna (with tuner)?
3. Do I need to create a copper ground plane in the hull if I use a dipole?
4. Will a 1/4 wave antenna let me use the maximum power of the
radio?(100watts).

Thanks,
Bill