Thanks for your comment on this issue. In reply I can tell you that the knotmeter is calibrated on a average speed of 5.5 knots according to the indicated speed on GPS. This was done on a lake with no current what so ever. It is true that the knotmeter will give a different indication on lower and higher speeds but this is a difference of only 0.2 knots. As far as I know my GPS doesn't have a filter wich allows me to give speed indications over a longer period. However its possible to give an average over a certain track, for instance 1 NM. I noticed that on higher speeds the speed indication is more sustain and therefore probably more accurate. In the situation where I reached this 8.5 kn my GPS indicated 10.4 kn. The boat was more or less upright and on a halfwind coarse. Therefor the waterline wasn’t lenghtened for that matter and was probably shortened due to the very salt water. In this area (North Sea) we meet currants up to 6 nots so this can give substantial differances on speedreading. In the U.K. there are currants measured up to 13(!) knots. There might be a substantial calibration error when these speed/currant differances occur and I can't give a modus to equalize this. I think its a matter of multiplying an dividing to achieve an average speed. Maybe you or anyone have a suggestion? Thanks Oliver