How inaccurate are speedometers in boats? Mine seems to be about 2 mph. off. I dont think thats too bad. How bad do they get? Assumeing they are in working condition.
Boat speedometers are notoriously inaccurate. If you want the true speed, use a GPS- it'll give you precise data. Even a cheap eTrex has that capability. Measure twice (in opposite directions) to cancel any effect of current or wind.
How do you know it is inaccurate? Depending on current, if any, you WILL have different speeds readings. Either way, yes, they can be inaccurate but 2 mph is not really bad.
A speedometer on a boat will only give you the speed through the water that the hull experiences. Current that the boat is in will not have any affect on the speedometer reading. A GPS which takes it's cues from stations that are outside of the boat will give you readings that are different depending upon the current and wind.
Pitot speedometers are nortorious liers as has been mentioned. Trafflail logs and paddelwheel indicators are much bettter, but are normally limited to slow speed use. All speedometers must be cleaned regularly to be relied upon.
For grins and giggles I still toss a chip log occasionally, but then again I have a sextant on board in addition to the GPS. I have found the old chip log amazingly accurate. I just wish I could reel enough line to make it usable above fifteen knots.
