well after putting in my immense sum of $5, it can reach about 50cm/sec
or 0.5 metres/sec, 0.03km/h or 0.019mph
it was built from scrap, car had buggered diff, so it became direct drive, to a propellor (the driveshaft is a mix of the old rear axle, and a shrink wrap plug connector).
the hull is plywood and gutter seal
the packing box (thing that stops the driveshaft leaking water in) is a penlid with rubber in it
rudder is a coke can that i smashed until it was flat.
and the prop is scratch built by me. (coffee tin flattened, cut into circle, hole drilled in centre, cut slits about 2/3 in, bend with pliers)
it's actually pretty fun, i'll get pics of it in "action"
oh yeah: it's got a two speed engine!!!!!! (go, and go faster)