This speed issues are a stock bug and remain unfixed.
When you click on a speed (10knots) and you are in a type VIIa with untrained crew - you will go 10 knots.
If you set speed to 10 knots in a type VIIc (heavy boat) you will only go 8 knots.
Trainingg your crew (motor engineer) can also effect the speed set.
So to get your boat to go 10 knots - click 10 knots - when only 8 is reached - click 12 knots
