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Speed: Improving pontoon drag

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  • Speed: Improving pontoon drag

    I just bought an older 18' Spectrum pontoon with a very nice Suzuki 70 4stroke on back. Max speed 17 knts and appears to be really dragging the motor mount support and putting out a large wake from the mount. The motor mount is a similar to one in the photo, structural, and cannot be modified as far as I know.

    We moved the engine up as high as it could go with no improvement. I suspect the engine is a little heavy.for the boat.

    Any ideas for improving efficiency/speed and reducing drag with the current engine?


  • #2
    add a 3rd log and lifting strakes....

    However you may want to check the capacity tag. from what I found, 60hp was the max back in the 90's for a 18 foot spectrum

    BTW, welcome aboard.

    you may want to do some reading. most pontoons act like a semi-displacement hull. a member a few years back switched from a 50hp to a 90hp on his boat. other than more nose and more fuel consumption, there was no more speed

    your 4-stroke is much heavier than a 2-stroke.
    Cheesehead boating the Gulf Coast of FLA 27.51° N, 82.53° W

    1988 Cruisers Rogue 2420 -VP 290 DP now powered by custom 468 - https://forums.iboats.com/forum/owner...988-rogue-2420

    Past Boats
    1970 Wooster Hellion - Merc 9.8
    2002 SeaRay 190BR - 5.0 - A1G2 - "Cheeseheads in Paradise"
    1984 Avanti 170DLI -3.0 stringer- "Ship Happens"

    What’s behind you doesn’t matter.Enzo Ferrari


    • #3
      The three major points for speed improvement are propeller, water drag and air drag.
      1. To be sure you are running the optimum propeller at the optimum engine speed you will need actual speed across the water (no wind, no current), actual RPM and the recommended max RPM for your engine. There is another whole forum here on iboats devoted to getting the right propeller.
      2. To minimize water drag you have to consider the depth of the motor/propeller, the weight of the boat (i.e., empty weight, known load, unknown load (e.g., water in logs)), and how that weight is distributed. Running flatter will reduce the cross section of the logs in the water and may help.
      3. Air drag is not insignificant above 10 MPH. If you have a large top or other structure it can slow you down.
      My WAG is that you are likely close to optimum for your current setup. I'd be surprised if you could gain more than 2 MPH without some of the suggestions mentioned earlier (e.g, strakes).


      • #4
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        My advice would be to make sure the engine is running in the proper rpm range at wide open throttle, then enjoy the boat for what it is. You can mess around further possibly, and maybe crack 20 mph, but it's a safe bet that going to be about it.