Speedometer Question

Discussion in 'Boats and Motors' started by Salmonid, Apr 21, 2008.

  1. I now have a GPS fishfinder unit on the boat and I noticed my boat speedometer shows 39 MPH while the GPS shows 34, 5 MPH seems a lot to be off on 35 mph. can someone explain to me how the boat speedometer actually calculates its speed since Im waaaay more apt to believe the GPS unit. Im not complaining but was wondering how the boat calibrates its speed, is it on RPM's???
    Thanks, Im sure I am not the only idiot out there who doesnt know how this works,

  2. fishingguy

    fishingguy The Inferior Fisherman

    Look on the transom, there is probably a wheel or another type of speed sensing device. Mine doesn't have a wheel but it just as silly. They are not very accurate. Your gps is the better choice for accuracy.

  3. I do not have any wheel like devices except the one I just put on for sensing current drift speeds. so again, Im back where I started???
    Thanks though, yes'm, I trust the GPS more then the regular speedometer.
  4. the boat speedometer is probably a water pressure gauge . Not very accurate a novelty at best, but neat to put on the dash.
  5. UFM82

    UFM82 The one others want to be

    Either a plastic pitot on the transom or a port in the lower unit for the speedo line to hook to. If it's a Merc, look about an inch or two above the bullet and you'll see a small hole in the leading edge of the L/U. A small hose will run out of the L/U and go into the boat where the steering and throttle cables go. That's the water line for the speedo. It works off a diaphragm and spring deal- the harder the water pushed the more the needle moves. As stated above, very inaccurate but better than nothing. Mine is actually very close until I get above 40mph or so. At 50 GPS my speedo reads 55-57. That's as fast as my boat goes right now so it may get worse the faster I get.

    Many, many bass boat guys get the shock of their life when they GPS their boats and find out they're going 10-15 mph slower than what their speedos were telling them...

  6. Thanks guys, see, I actually learned something today!!:p

  7. Orlando

    Orlando Set The Hook!

    Yes boat speedos are usually always alittle on the high side, and are inaccurate. The GPS is more accurate
  8. Mark from what i know about boat speedometors if you have it within 5-6mph you are doing great