You could obtain one of these motors from All Electronics and run the boat anywhere from 6V-12V and get what you want speed wish. These are the same type of motors that we use to run combat warships with twin screws and two motors for ships that are over 50". Check through the rest of the motors on the page. Look for high RPM with a low amp draw. This motor that I selected should work well for a boat that size with a 1" to 1 1/4" three bladed prop.
CAT# DCM-165


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