Thanks Derek I should have checked there in the first place I checked my locals but they are mainley river boats on the Thames so they never get much above 4 knots.They have different sizes at force 4,( 30 ,40,60 ,and 80,) I think the wing parts are all the same,>> and the sizes may be the pressure in the piston .On the web sight there isnt any info on boat lenght etc.Ill have to give them a ring.... Thanks again Johnnyb.