Well according to my motor data, a decaperm winds 6000 rpm at 6 volts. With 2.75:1 gearing, that gives an unloaded shaft speed of 2182 rpm. Assuming the motor loads up a bit we can say the shaft speed is likely to be around 2000 rpm if well matched to the prop. Assuming the prop pitch is around 1 to 1.2 times diameter, then the boat should motor along at around a brisk walking pace.