I have never understood why it is neccessary to place these large amp draw motors with a gear reduction setup in boats like this.
In combat warship clubs we are running boats as large as 8' with direct drive. All of my warships, from Schnellboote to HIJMS Shimakaze are direct drive.
The geared motor will draw more power than a smaller direct drive system. If you take a high RPM motor and gear it down you are looseing amps from your battery. I find that it is far more efficient to utilize a smaller motor and do a direct drive.
A motor like that which I have linked to All Electronics would be more of what we would look for use in driving a boat of the size you are building. Just in case it is item number: CAT# DCM-165.

[link]http://www.allelectronics.com/cgi-bin/category.cgi?category=400200&item=DCM-165&type=store[/link]

I have a Dumas Typhon on hold ( one of many projects) when it comes down to out fitting the drive for this boat, I will most likely use something very similar to what I have linked you to. I see no reason to spend your money on a system that is designed to reduce your RPM's to make a large motor fit.