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OB trim

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  • OB trim

    tried my used 4hp ob by myself and the boat
    tilts up a bit high, i thought.
    boat is 9.6 soft bottom.
    and the trim was on the first hole.
    there are 5/6 holes to chose from.
    THANKS in advance.

  • #2
    Welcome to Iboats,

    Firstly need to inflate tubes and keel to their factory working parameters, min air pressure should be 3.0 PSI to achieve a firm inflatable boat. A 4 HP-OB is a very underpowered OB powering a realtive short size inflatable. probably just good for fast displacement speed depending on your current weight.

    Which OB brand is it ? Is it an old model or a new one, 2 or 4 strokes OB ?

    Try sittting inside middle floor towards bow, with extender arm go full throttle to see if planes. If can't, go for one less pitch prop to pull wot revs higher as to increase hole shot, with an induction tach it's much easier to dial a top prop for that application.

    Click image for larger version

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    Trim must be set to a hole that achieves for OB to sit completely dead vertical at 90, same as illustration, usually second-third hole out transom. Report your findings....

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


    • #3
      thank you.
      actually, i find okay for my liking(speed-wise).
      not expecting much from a 4hp.
      it's a merc 2-stroke, 90's.
      did notice that at 3/4 throttle is faster than wot.
      a degree lesser on prop. gotta google that.
      again, THANK YOU!


      • #4
        At 3/4 throttle it's posible to plane the combo, right ? If so, once on plane while sitting aft towards middle transom pull head out and check at which lower leg height is water flow passing at speed, check if passing through upper or lower plate and if with back or over transom splashes.If passing right under lower upper plate it's perfect, from there a less pitch prop maximization is the way to go.

        If wanting better performance, probably can upgrade that 4 to a 5 HP by just changing carb along a less pitch prop, a matter to check tech specs for both models which should be same CC & weight.

        My 5 HP OB had same issue, could not plane a 3.80 Mtr Sib with one up with an 8 pitch prop no matter if at 3/4 or full throttle, changed to a 7 pitch prop and made a night/day difference. Now planes at full throttle and remains on plane at 3/4 throttle while lightly loaded.

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


        • #5
          very insightful. THANK YOU!
          so is my prop an 8 pitch? i would not know.
          so 7pitch it is.
          and also, carb change leads to hp upgrade?
          that i didn't know. i should go change my
          1/5scale gas rc's carbs. haha.
          it seems to plane decent on 3/4throttle. have to check that water flow thing next outing.
          but i'll start scouting for prop and carb in the mean time.
          again, THANKS.


          • #6
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            Don't know if a 2 strokes Merc from that era is a propietary or a rebadged 4 HP Tohatsu same model, an internal pic would help to determine so if plan going for a 5 HP later upgrade. Which number is stamped on prop body. Is it a throught or non through hub exhaust prop ?

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