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    Kicker motor prop selection-gctid827388

    Bayliner 2556. Yamaha T8 (8hp) high-thrust long shaft outboard, 11 3/4" dia x 5 3/4" pitch (the lowest pitch they make) aluminum 3-blade prop.

    The kicker was originally a downrigger trolling motor, which most fishermen use at 1-3 knots. It pushes the boat at about 3 knots at half throttle. Just under 4 knots at 2/3 throttle. Here's the part that confuses me. If I open it up to full throttle, the outboard revs up and kicks up more water, but the boat doesn't go any faster. As I understand it, if it were overpropped, it should stop increasing RPM before I reach full throttle. If it were underpropped, it should rev up continuously all the way to full throttle, but also gain speed continuously.

    But it's not doing either of these things. It's revving up all the way to full throttle like it's underpropped, but stops gaining speed after about 2/3 throttle like it's overpropped. So what gives? Is it overpropped? Underpropped? I'd like to get at least 5 knots out of it if possible, and don't really care about trolling at 1-2 knots. Should I try a bigger pitch prop?
    1994 2556, 350 MAG MPI Horizon, Bravo 2

    #2
    You could try a bigger pitch prop and maybe achieve a slight increase in speed. Keep in mind that a small engine like that will only push a heavy boat so fast. I have a Merc 9.9 on my 2859. I dont know thew prop sizing off hand. Its a 4 blade prop made for power to get a large boat moving, but not travel fast. The same thing happens with mine. Once you get up to a certain speed more throttle doesn't make it go faster.I top out at about 4 mph. I would assume that a larger pitch would increase your speed slightly, however if you encountered any wind having the power of the lower pitch may be favorable to have. I've had to putt putt home 30 miles at 4 mph before and it sucks. However we made it home.
    "REEL WILD"
    2001 2859 FNM 300 Diesel-Bravo 2
    Anchorage, Alaska
    If you don't like the weather, wait 15 minutes.......

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      #3
      It sounds like you've found your hull speed. Without doing the math, 4MPH seems about right for your boat. To move faster than hull speed takes exponentially more power, which your kicker can't provide. This is why runabouts are plaining hulls. Once they jump up on plain, they are free of the restraints of hull speed.

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        #4
        "1990 2556" post=827443 wrote:
        It sounds like you've found your hull speed. Without doing the math, 4MPH seems about right for your boat. To move faster than hull speed takes exponentially more power, which your kicker can't provide. This is why runabouts are plaining hulls. Once they jump up on plain, they are free of the restraints of hull speed.
        Hull speed for the OP's boat is around 6kts, so it's not hull speed. This has to do with the resistance of the shape of the bow to the wave it is creating versus the power being used. Simply put, the power he is using will only create a certain size wave before it can't push the boat any faster against that wave. If it had more power, it could create a bigger bow wave and eventually get to hull speed.
        "B on D C", is a 1989 2459 Trophy Offshore HT, OMC 5.7L, Cobra OD, Yamaha 15hp kicker. Lots of toys! I'm no mechanic, just a blue water sailer and woodworker who loves deep sea fishing.
        MMSI: 367637220
        HAM: KE7TTR
        TDI tech diver
        BoD Puget Sound Anglers North Olympic Peninsula Chapter
        Kevin

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