Hi Marchi,
Regarding your p.s., comment about a speed "mismatch" between the boat speed and GPS speed, this could be down to tidal current, and a simple check is to turn your boat about onto a reciprocal course where the speed difference should be the same but in the opposite sense. So if your log is reading low in one direction, it should read high in the opposite direction and by the same amount. If not then you need adjust your log, but before you do so you should also check to see that your log impeller is free from fouling as marine organisms enjoy being in tight cosy little spaces and the consequence is that the impeller gets slowed down giving you a lower speed reading. Most speed logs can be withdrawn into the boat and temporarily the hole that is left can be plugged while any fouling or marine organisms inside the Impellor housing are cleaned out. On my boat the whole operation can be carried out very quickly, even with the boat afloat, such that there have been times where I've not even bothered to plug the hole before cleaning and re-inserting the speed transducer. The amount of water that comes in on my Bav 36 during this operation rarely exceeds a bucket full. But it's best to have the speed log manufacturers plug to hand just in case !! Do be aware that the speed log transducer usually only fits one way round, and so make a note of any marking on the top of the transducer to ensure that it is refitted exactly the way it was before you started.
