Hello zmajcek

I see the picture of the shaft. This is Graupner and has a M4 (4mm diameter) thread for the propellor so a Graupner M4 propellor will fit directly on - no need for an extra bolt.
Graupner makes lots of different propellors with M4 screw threads (see my earlier post). I suggest you start with something about 40mm diameter and see how the boat runs - the Graupner props are not expensive so you can try bigger ones later if everything is OK to see whether the boat runs better.

The motor you showed me is designed for an airplane not a boat, and may not have enough cooling - normally the air from the propellor cools this motor.

Boat motors should have at least a built-in cooling fan and ideally water-cooling, depending on the power level. This one is in stock at Hobbyking at the moment and would be better suited for your boat:

http://www.hobbyking.com/hobbyking/s...er_1100kv.html

However motors are not very expensive so you can always experiment.

Here are links to some boat prop calculators:

http://www.radiocontrolinfo.com/RCca....php#Brushless
http://www.props4u.com/prop_chart.htm

-yellow-bird