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  • #16
    Qboy, if you happen to boat with a IN-board engine, sorry, we're discussing Outboards in which you can vary OB/transom different heights.

    This issue was started moons back and there's no light at the end of the tunnel besides what each boater wants to try on its own. it's a personal choice. Some important issues that everybody is bypassing and are important to consider when sitting any OB on transom on a dry installation.

    (1) It's NOT MENTIONED in any OB's Owners Manual basic Installation procedures that AV plate must set even with rear hull whatsoever, have personally read all OB brands researching 20 to 350 HP-OB's and all states heights between 20 to as much as 50 MM with AV plate to lower rear hull. This height depend entirely on the model sitting on transom. Smaller gaps for portables ones, much larger gaps for larger HP-OB's

    (2) In real world wet boating these parameters heights means nothing, when boat has undergone the Hydrodynamic Lift term for Hole Shot won't know if water flow passing at speed is passing right were you guys like or not.

    (3) Not all boats brands transoms are built with same height, varies from 40 to 43 cm on large ones, the ones we're discussing as of right now.

    (4) Not all OB's brands have same length no matter if all are L sizes, can vary a bit between brands.

    Going back to the posted pic example showing the vents located in middle of both rear plates. Made a water test to that combo on a flat straight, no wind rowing water course with 2 up. Water flow was skimming right over AV plate, this 140 HP-OB just released from full maintenance achieved 5960 wot revs.

    As OB was sitting at the upper side of the swivel bracket holes was impossible to lower the height for water flow to skim right under upper plate, my fiberglass techie made a spot on transom height modification.

    On exact same second test, OB lost only 60 wot revs as there is a higher water bed for prop to grip much better than if passing by around AV plate. What was achieved : (1) Noise reduction, exhaust noise when vents are air exposed is now dumped and canceled into the water flow. (2) Combo performs much better on tight close turns, on choppy, wavy water cond as there's O prop aeration in comparison to as before transom height modification.

    This combo doesn't need a prop maximization, current Yam prop is revving fully towards the upper wot rpm range with 2 up and will lose slight wot rpm with 3 up which is intended to be the max boaters that will be on board for this coming summer.

    Hey folks, to get the idea of what I'm trying to discuss here, forget that all of you have electric trim OB's, are shot at 90 deg or running manual trim OB's set at that given angle which is a very basic manual trim installation.



    If you like for water flow to skim right over AV plate once on plane with deck weight evenly distributed, New Question: how will these pair of doel fins perform : Excellent, SO-SO, Awful and why...

    Happy Boating
    Last edited by Sea Rider; December 1st, 2018, 05:13 PM.


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

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    • #17
      Since you don't understand dynamic's you opened a new can of worms. You wasted your time,fiber tech time and customer time and money.That prop in picture is designed to run with prop tips surfacing that's why they are pointed. You buried the prop reducing its performance.
      (1) Noise reduction...nope you misunderstand function of vents as more water than exhaust exit thru these at speed/RPM.
      (2) In real world wet boating these parameters heights means nothing, when boat has undergone the Hydrodynamic Lift term for Hole Shot …Need you to understand proper terms..Hole shot is time to get boat from dead stop to plane, nothing to do with AV height at this time
      (3) Not all boats brands transoms are built with same height, varies from 40 to 43 cm on large ones, the ones we're discussing as of right now.... No as only boats with 40-43CM transoms are johnboats 30Hp and below or race hulls in this part of world. . That 140 you did work on is not a 15" shaft as would like to see pic's of performed work.


      When in doubt,Trim it out!!!
      Business Owner & Certifed Technician
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      • #18
        Originally posted by Faztbullet View Post
        Qboy… since you cant adjust your mounting height all you can due is play with props till you find one with best lift and reset your trim limit to raise drive a tad higher. When we sold Checkmates at dealer I(service mgr) would take each I/O to lake with several dozen props to find best overall performance. When trimming I used the trailer button to override the limit switch. When found best speed and angle would note gauge reading and adjust. All our boats would outperform the other dealers selling same brand and we was nations largest Mariner and Hydra-Sports,Lotus,Ebbtide,Checkmate dealer at the time. I was questioned several times at boat shows how we made our rigs outperformed other dealers.
        Yeah. That’s what I’ve been doing really. Almost there with props using the enertia just now. Maybe one more adjustment to try.
        At full chat I have the trim slightly higher than the max trim setting and just into the trailer mode. Same button, just needs pressed further in. Anymore and she will drop speed and rooster.

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        • #19
          Searider, I think you have to take trim and it’s effects into this. It is directly proportional almost to the level of the water slow on on the gear case surely ?

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          • #20
            Sign up today
            Will end this post right here, Fazt being the ultra supreme boater and his posts being law will keep on demonizing, bashing, ignoring, discrediting my posts done under full practice, not plain theory. It's useless to continue posting wasting my precious boating time. So.....

            -If you like boating by the parameters set when Ole Evinrude fabricated his first outboard, keep doing so.

            -If you think that will have an outstanding OB/transom height match by lowering the OB when prop aerates or rising the OB when back splashes are produced at back of transom, keep doing so.

            -If you like going playing with other brands props and not test OEM ones in other pitches is better, keep doing so.

            -If you like boating at speed with 90% of the hull out of the water and all boaters must follow those insane and dangerous boating parameters, keep doing so.

            Since Ole's times to 2019, outboards, hulls has evolved for better boating than the old boating days, but the prop forum remains boating in other planet...

            Bye, bye...water is calling...


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

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