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Should i raise to 41/4 ptp

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  • #31
    The jurassic period started 201 million years ago and ended 145 million years ago. If you find an out made during that period, please advise of make and model. The above photo pretty much shows all that is needed for outboards made in the last 60 years.

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    • #32
      Sea is just uninformed on hull design and HP set-ups. Rubber boats totally different than FG hulls. Johnson/Evinrudes V-6 lower have always been 8" from center of propshaft to AV plate..from 1976 to 2015. The V-4's always been 7-1/4 except for batwing unit and same(8") for the Merc V-6 units 1976-2018. Don't see a Jurassic year in there.
      When in doubt,Trim it out!!!
      Business Owner & Certifed Technician
      Boats I own & use
      20ft Javlin w/250 Junkrude
      17ft Bullet w/200 Merc (SOLD!!)
      13ft Allison w/150 I-6
      17ft Hydra-Sport 175GT
      15ft Allison w/75 Stinger(project)
      14 x 48 Drifter with 1100SS Merc

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      • #33
        This is how it should run or close to it...…
        https://www.youtube.com/watch?v=bTFuihWZ__A
        When in doubt,Trim it out!!!
        Business Owner & Certifed Technician
        Boats I own & use
        20ft Javlin w/250 Junkrude
        17ft Bullet w/200 Merc (SOLD!!)
        13ft Allison w/150 I-6
        17ft Hydra-Sport 175GT
        15ft Allison w/75 Stinger(project)
        14 x 48 Drifter with 1100SS Merc

        Comment


        • #34
          WN, you always like flying over the cukoo's nest when don't have an appropriate tech answer to give or even evade about what posted.. Congrats, now with very few post you've turned into the new Guru of the of the OB-Transom Height Optimization and Prop Maximization for all type of OB's.

          Fazt, stop pigeonholed me in that only worked with inflatable boats, been doing OB-transom modifications, optimizations along prop maximizations on many type of boats for the last 10 years, not with Jurassic ones thought which you guys love boating with.

          What find really amazing is that you guys always give for granted what's been posted for years now about lower leg heights, which are complete different installations between Old and New OB's which you like throwing in same sack.

          As we will never will be in plain accordance about this issue, keep rigging your OB's with flow passing by AV plate and I will keep mines rigged with flow passing right under Upper Plate....

          Happy Boating


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

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          • #35
            Just catching up on this....
            searider are we thinking that the water flow should be in line with the upper fin ? Surely not ? Should be right on the cavitation lower plate at best position, with the lower cav plate being in line with the most lower line of the hull.
            Is there a difference with older engines ? Is that what we mean ? Sorry...my iPad is very slow and takes an age to read through all the posts.

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            • #36
              I hate typing because I'm not a typist, but here goes. It's a simple matter of hydrodynamics. An outboard mounted too low induces drag from the lower unit. This causes the need for more power to achieve speed. And since the engine is at max, the rpm is also reduced as a result of the extra resistance. Now if you try and trim the engine up to reduce the drag, it causes the trust of the engine to also push downward on the stern of the boat thus raising the bow. Once the bow has lifted enough to get the trust of the prop in parallel with the water, the bow will then drop back down and this cycle continues giving you a porpoising effect. And now if the engine is mounted too low, the shape of the av plateb eing wider at back exacerbates the problem by forcing the stern of the boat down faster as it is below the waterline. And if you have a jackplate, this just makes it worse with the engine mounted to low since it acts like a short lever putting more downward force on the stern. There's only one way to do away with this problem and that's to mount the gine so the AV plate rides above the waterline at planing speeds. Even then you may experience some porpoising if you trim the ngine too high, but it will bee much less severe than with the engine mounted too low. There are a lot of hull designs from almost flat across to deep v's, On flat and modified V's the AV plate in line with the bottom of the boat should give you a good smooth with little loss in speed or fuel economy. For a deep v hull you will probably want it an inch or more higher. And even higher if you have a jackplate. And if you have an aluminum boat with a stringer about an inch wide and deep down the center like mine, you'll wnat the av plate evn with the top of that V or maybe a little higher.

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              • #37
                My boat runs along both upper and lower lines. Cruising it looks as if it on the upper and trimmed out at WOT with the boat running up on its pad (on bottom and rear of hull) it’s on the bottom AV plate with prop surfacing a little. As we know some props are designed to have a little surface on boats like this. See link below. Feel sorry for the wee Alpha one running likely a wee bit over 300hp through it !

                https://youtu.be/MF-v2LYlogw

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                • #38
                  which are complete different installations between Old and New OB's which you like throwing in same sack.
                  What is meant by this(above) as 1976 to 2018 lower units haven't changed AV height. Now Tohatsu may have but Johnson/Evinrude/Yamaha/Force/BRP have not....Explain with numbers. I only offer insite to some of your posts as the information you posted is not totally accurate.
                  When in doubt,Trim it out!!!
                  Business Owner & Certifed Technician
                  Boats I own & use
                  20ft Javlin w/250 Junkrude
                  17ft Bullet w/200 Merc (SOLD!!)
                  13ft Allison w/150 I-6
                  17ft Hydra-Sport 175GT
                  15ft Allison w/75 Stinger(project)
                  14 x 48 Drifter with 1100SS Merc

                  Comment


                  • #39
                    Hey guys, lets stop high lacking the OP thread. Give me some days and will open a new post in which will deeply discuss what has been a long time quarreling about best lower leg height in tech terms.

                    Happy Boating


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

                    Comment


                    • #40
                      Sign up today
                      Faztbullet, WesNewell, Qbhoy, Scott and any other member are invited to join the "Lower Leg Plates" conversation...

                      Happy Bpating


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

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