as far as I know there is no calibration adjustment available to the user. Check the condition of the inner cable and the radius of the bends of the cable both can cause problems. If the boats out of the water check the impellor and the final bearing condition.
Problem is that the instrument is past its use by date, possibly has worn bearings, weak magnets etc which are not worth repairing, and cables??? I presume this is not an electrical cable. Time to invest in a modern electronic paddlewheel model with electric cables. Even so, these do not always give perfect results nor can they always be adjusted accurately. On my boat I have tried all the adjustments and have resorted to regular checks against a measured mile and known points and voyaging logs. I have thus calculated a correction factor which I apply when I need it. But then GPS technology has made many of the traditional methods rather obsolete in themselves. Happy sailing.
Dont go relying on this 'modern technology' as you call it, too much. After all would you go to sea with all this 'modern technology' and no magnetic steering compass as back up??
if you need any spare parts for your sumlog get in touch!!!
Well, my GPS (and before that DECCA) have been 100% reliable.......which is a lot more than I can say about my paddle-wheel log which never meant a thing and is certainly not going to be replaced. OK, I'll keep the fixed and hand-bearing compass's on board but in reality hardly ever use them. (Heresey!).
I have the same problems but will replace the cable and try it again next season,sorry but just dont trust electronics 100% and would rather have something with "known" errors .30 Boat, thanks for the offer and will send you a PM with details.