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Thread: Prop slip and speed

  1. #1
    Carbon Dave
    I have an older 25 foot Eliminator. Here is the boat (http://www.dcperformancecarbon.com/images/boat3). Mild BBC powered making maybe 375hp.
    The drive is a Stern Powr (http://www.dcperformancecarbon.com/images/drive1) and the prop is a 14 1/2" x 25.5 pitch.
    Using this prop calculator (http://www.rbbi.com/folders/prop/propcalc.htm) it shows I have 38% slip.
    I have not had the boat on GPS yet but my speedo indicates 50mph @ 4400RPM. Could my speedo be that off? Maybe my tach? The calculator says at 20% slip I would be doing 65.
    Next thing is where the hell do I find a prop for this boat if indeed all the numbers are correct?
    The prop shaft is strange from what I can find.
    19 splines, 1 3/8" dia shaft but the splined part is smaller. Then on the outdrive side there is a steep taper to about 1 11/16" . Also the splined portion is welded to the actual prop. Below are some pictures. Any help would be great.
    Prop 1 (http://www.dcperformancecarbon.com/images/prop1)
    Prop 2 (http://www.dcperformancecarbon.com/images/prop2)
    Prop 3 (http://www.dcperformancecarbon.com/images/prop3)

  2. #2
    Havasu Hangin'
    I'd first throw a tach/dwell meter on it (to make sure the tach is correct) and GPS it (to make sure the speedo is correct).
    Also, did you verify the gear ratio?
    I bet Hill could make those choppers.

  3. #3
    Carbon Dave
    Also, did you verify the gear ratio?
    Good question. I was just going off what the serial numbers on the drive. Then again the numbers say it is a LH prop but unless I am wrong it sure looks like a RH to me. I guess they could have changed it. Maybe for better prop availablity?? I will check the drive ratio.
    I am hoping it is the speedo being way off. It would be nice to have a faster boat for the price of a speedometer.

  4. #4
    Havasu Hangin'
    In my (non-expert) opinion, 30% slip is very tough to do without overtrimming the boat to the point where the prop surfaces (and you'd definately know it ).
    50 MPH does sound a little slow for a boat that size...so you may be pleasantly suprised if you carried a hand-held GPS next time.

  5. #5
    Carbon Dave
    I will be bringing the GPS next time. I just couldn't borrow one in time for our last trip.

  6. #6
    Trash
    Without accurate data used to calculate speed/slip all these numbers are useless. You MUST get an accurate speed via GPS/Radar before calculating slip.

  7. #7
    THOR
    What HH and Trash were saying is right. Check the gear ratio and very both the tach and the speed. Also, the basic prop/speed calculators are good estimates but they pay little or no attention to hull type or weight. Both of which have serious influences on speed. They are a good estimate but were off with my boat too until I got a handheld GPS in the boat.

  8. #8
    Tahiti350
    Actually they do factor in hull type and weight, that's what the "prop slip" correction is for. Heavier boat with more hull drag = more slip, pretty simple. :boxed:
    As they said above, GPS it and verify the tach, use those with a known gear ratio and prop, and get an accurate slip factor, then use the calculator to fine tune the combination.
    BTW, my '80 Tahiti 18ST has a 12.02% slip: 59.3 mph GPS, 1.36 gears, 14.25x21+ cupped prop (proggram says to call it a 22), running at 4400 rpm. Turned a non-GPS pass later the same day at 4600 rpm, using the now known 12.02% slip puts me at 62.0 MPH. :rollside:

  9. #9
    spectras only
    Carbon Dave , download the prop calculator from this website >
    http://www.castlemarine.co.uk/pitch.htm
    Much more accurate , using critical data like waterline length and weight as well .

  10. #10
    Carbon Dave
    Well, guess I have eliminated one of my variables.
    I had it on GPS today. This was with 1 full and another almost full tank of fuel and 4 people.
    It turned 4200 rpm @ 50.7mph. I think when it was turning more RPM I had it trimmed up too high.
    Tomorrow I am going to check the gear ratios in the drive.
    I just think I have a slow boat...LOL

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