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Outboard prop coming out of water as throttle / speed increases?

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  • Outboard prop coming out of water as throttle / speed increases?

    Hello iBoats! This is my first post, so if it's in the wrong place please let me know.

    I recently purchased a 15hp short shaft outboard for my 14' soft bottomed inflatable. The motor has been working well, but I've found that as I increased the throttle (and speed of the motor/boat), the boat rises ('on a plane'?) and the prop begins to come out of the water. Presumably without any water resistance, the motor revs high and isn't pushing against anything to move the boat forward.

    Here is a short video showing what I mean: https://vid.me/G2TKb
    And here are some photos of the boat transom and outboard: http://imgur.com/a/B3G6Q

    I'm new to boating and outboards -- I tried to do some research and thought that a short shaft would be right for this transom (~15.5 inches high), and I know the outboard angle can be adjusted (though I think I have it set to the 'lowest' setting, as close to perpendicular to the transom as possible; the problem seemed worse when it was one 'notch' higher).

    Looking for advice: this isn't normal, right? Are there any other adjustments I can make to the outboard? Do I need a long shaft motor for this transom / boat?

    Thanks for the help!

  • #2
    welcome aboard.

    SIBS respond a bit different than RIBS or other boats. if you dont have enough weight, the prop can blow-out like your experiencing.

    tuck the motor in a bit more, or move some of the stuff around, or if you dont have much weight most of the time, back off the throttle.
    1988 Cruisers Rogue 2420 -VP 290 DP now powered by custom 468 - http://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

      Seems water flow at speed is passing right over lower plate which is called the Antiventilation Plate, right ? ideal is for water flow to skim right unde small upper water defector plate, that's the Tip Top OB/transom height spot, the only way to achieve that cond is to chop transom down at least 1", answer that didn't wanted to hear. You could play trimming OB up/down to compensate some prop aeration, but will lose top prop thrust.

      This prime example illustrates a typical OB/transom height mismatch. Can you post a pic of lower leg but shot sideways to have a look.

      Happy Boating
      Last edited by Sea Rider; July 29th, 2017, 10:19 AM.

      Sea Rider 320, 380 Sibs, 450 Rib, 2 Strokes Tohatsu 5,18 & 30 HP Proud Smokers


      • #4
        Thanks for the replies. Here's a shot from the side: http://imgur.com/a/oy6ys

        I'm definitely not ready to cut into the transom -- would a long shaft motor be a better fit, or would that have problems of its own?

        So far, have not gone out with much weight (and it's a "7 person boat") so I will try to see how that affects it.


        • #5
          A L size OB will worsen things. Huge water splashes out/over transom along excessive water drag on lower leg due to 5" height difference. Lower leg will sit too deep on water.

          If adding more weight or full passengers, Sib & OB will sit slight deeper on water, but with a very under powered OB probably won't be possible to plane that 14 footer out. To plane max 7 allowed passengers will need at least a 30 HP-OB for that size Sib in order to have fun.

          Happy Boating

          Sea Rider 320, 380 Sibs, 450 Rib, 2 Strokes Tohatsu 5,18 & 30 HP Proud Smokers


          • #6
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            I recently switched from a regular boat to a SIB and had the same problem. First I thought that the gear was shifting into neutral when it happened. After researching, as other guys mentioned above, I put more weight near the transom, and that kind of fixed the problem. It still happens once in a while.