Speaking of "Need for Speed" How to MEASURE speed?

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  • paulfromyork


    Replies: 42
    Topics: 26
    #7298

    I now have a ’54 12 hp Gale, (only had it out twice so far) for my slightly less than 12 foot SEARS Sea Nymph clone. (rated for ten!) I like how it feels to open up the throttle. And I don’t have anything like a smart phone yet. Last time out, I @ 220# and friend @ 260# Got up on plane anyhow. Not real fast to get up on plane, but you can hear the motor gaining rpm, and as my friend Scott says, "It just sounds happy when that happens." I agree. But to the point.

    Is there a better way to measure speed on the water besides timing your distance and scaling the map?? How do they work on a boat so equipped? I guess it’s similar to an airplane. Don’t they work off of pressure of head on flow?
    How can a speedo be added and how accurate are they?


    garry-in-michigan

    Lifetime Member
    Replies: 3583
    Topics: 31
    #59452

    The water speed indicator is simply a water pressure gauge calibrated in miles per hour. The water pressure pick up tube fastens to the transom so it picks up pressure 2 or 3 inches below the bottom of the boat. One of our members brought part of a case of them to our January swap meet. I was given one he had left at the end of the day. Accuracy varies. The ones we used testing props were calibrated over a measured mile. . . 😕


    westwind

    Canada Member - 1 Year (includes $3 online payment fee)
    Replies: 374
    Topics: 10
    #59453

    I have an old Android cell phone that is no longer used and have a downloaded app for GPS speed. Works awesome, has huge numbers so it’s easy to read and it was next to free and very accurate.


    frankr

    US Member - 1 Year (includes $3 online payment fee)
    Replies: 6715
    Topics: 51
    #59454

    Boat speedometers are worthless, in my not so humble opinion. We live in a better day. We have GPS. I haven’t gone shopping recently, but I’m sure there must be lots to choose from. I have an old Garmin E trex that I bought years ago. It measures speed, as well as drops (virtual) bread crumbs so I can find my way home when I’m out in the woods or on a big lake full of islands, which is what I bought it for.


    Pete

    US Member - 2 Years
    Replies: 154
    Topics: 6
    #59456

    If you accurately calibrate the water pressure pitot type speedos against a standard like a dead weight test they are accurate. The water pickup must be in the free stream with no air being picked up. Keller pitot speedos are good units used on race rigs especially since you could easily observe the speed changes in turns and accelerating. I do speed checks over a distance and time that is how GPS does it but they also have their accuracy issues with satellite signals. GPS units are certainly the most convenient to use. If you are traveling on water GPS measures speed relative to fixed earth regardless of currents, water pressure pitot measures relative to the water with currents accounted for.


    outboard315


    Replies: 197
    Topics: 16
    #59466

    I would use a smartphone or a car GPS or find someone with one you could use. Or I picked up a used GPS for like $20.


    olcah

    US Member - 1 Year (includes $3 online payment fee)
    Replies: 566
    Topics: 94
    #59548

    I have an old hand held automobile GPS that has a battery. It has a feature called "dashboard" that shows mph. That is the best way. No pitot tube to attach to the transom and no tubing to route/step on inside the boat. I still do like the old speedometers for the looks and they do give a comparison with different motors, trim etc. If you do get an old type one, be sure to get an appropriate range for your boat and motor. (for example if your boat goes 15 mph, a speedometer with 80 mph range will hardly move the needle.)

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