These are the motors that I used to replace the motors in my Schnellboote. The previous motors were from All Electronics and were a motor that they listed as coming from the film rewind of a camera. They worked for almost 2 years, but were not very efficient for what I was asking them to do, and required me to use a two bladed prop that bugged the hell out of me because they were so unscale[:'(]
The new motors are stock HPI Micro RS4 motors. They are far more efficient with a lot more torque. They do weigh more, but because of the increase torque and increased efficiency, I was able to reduce the battery packs by one cell to 4 cells. I have also made three bladed props that are 1.5cm in diameter. This configuration now allows me to drive the 21" 1/72nd scale Schnellboot at our club rules speed of about 39k for two to three sorties before having to change out the battery. I am very happy.
These motors are on Ebay, item number #5909028922. I think that these will work well for any boat at speed up to 30".