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Lower Leg Plates.-

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  • Lower Leg Plates.-

    As there is too much upper and lower plates controversy going on the forums as which lower plate dials top water performance have made a fond research at my boating Club about different outboards living there. 3 types were found : Old and New ones being bolt on transom OB’s in which the height can be varied by simply playing with different holes height and the Clamp OB’s which are not going to be discussed here.

    As to follow a proper hydrodynamic tech conversation between parts will add real world pics and examples accordingly as the post evolves as you dislike following a conversation with simple flat drawings.

    Please answer in simple terms the basic, what's there : Don't write an essay though..


    Click image for larger version

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    (1) What are the rear openings found between middle upper and lower plates for ?

    (2) What’s the front lip protruding at front of upper plate, why is it there for, what happens if It’s chopped even with lower edge located under it ?

    PD : Don't quote my posts, simply write an answer on a new post as to follow properly..

    Happy Boating
    Attached Files


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

  • #2
    1 - idle relief ports
    2 - leading edge of anti-splash plate. if you removed it, you will get a lot of splashing of water into the boat
    Cheesehead boating the Gulf Coast of FLA 27.51° N, 82.53° W

    1988 Cruisers Rogue 2420 -VP 290 DP now powered by custom 468 - https://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

    Comment


    • #3
      1) Vent for exhaust and exhaust cooling water..it is exit only.
      2) Splash deflector.....If you mean chopping it even with leading edge it will result in spray being deflected upward.
      3) Add that motor in pic is a good set-up...
      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


      • #4
        (1) On my 60hp 4s merc this is where my sacrificial anode is also. Doesn't appear to one on that engine. Perhaps that's why the prop i so pitted.

        And there's also the fact that neither 1 or 2 has anything to do with how high/low you should mount your engine. The lower plate and also the water intake below that is where one should be looking for proper engine height. For max performance, the lower plate should be just above the water line at wot max speed on plane. The only other concern is that the water intake should always be below the water line. On most boats, that means the lower AV plate should be even or slightly above the bottom pf the boat when av plate is inline with the hull bottom. It should never be below it. With a jack plate, add ~3/8" higher for every inch of setback.

        Comment


        • #5
          With this 2018 lower unit, which color line would be even with bottom of boat?????( I know the answer)

          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


          • #6
            What about this one? Which color???
            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


            • #7
              Impossible to answer since one can't see what they are mounted to in relationship to the hull.

              Comment


              • #8
                OK guys, let some days to pass by to post your answers, so will continue with our conversation...

                We all agreed that those rear leg vents are relief ports, part of the exhaust system spillway that OB's needs to count with to decrease the amount of total exhaust exiting through prop at say max wot rpm range. Those vents are present in modern 40´s bolt OB's and higher HP ones. Have also seen modern 60 HP ones without them, it's entirely up to each fabricator if adds those vents not.

                One big favor, Mr.Trim was not invited to the water party, remains trimmed to 90 deg as to have hull, prop shaft and both plates riding parallel to water level at plane along a perfectly distributed balanced deck weight.

                Question : when at full wot rpm range would you say that water flow at speed must cover or not those vents ? if not what's going to happen ?

                Will not answer Fazt's posted pic as it's irrelevant as we will find latter when all the practice data is posted and confronted with the theoretical data and elucidate which plate achieves the best "Overall "water performance.

                WN, I'm curious about (1) where did you get the information that AV plate must be set even or close to rear middle hull shape, read it somewhere, other forum, elder boaters recommendation, marina pier talk ? (2) what's your next move if you've played with all possible holes while raising, lowering the OB on transom and that weird parameter is not dialed ?

                BTW, the prop shown on first pic has only paint pitting, overall alum prop material is spot on, that props needs be fully sanded, primed and re painted. Lower leg anode needs to be removed and sanded. Next month will take care of that as new owner has inherited that combo from his father in that condition.

                Happy Boating


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

                Comment


                • #9
                  Gee, the idle relief ports are usually much higher on the midsection, as they need to be out of the water to allow the exhaust to exit, when engine is idling (hence the name). Om my Mercs, they are just under the bottom cowl.

                  The gratings above the A-V plate allow water to exit whenever the motor is running.

                  On my speedboat, I want the A-V plate about 1 inch above the water when on plane at 1/2 throttle or higher. Any higher and she will go a bit faster, but she requires more attention to exact trim, than I want to do.

                  Comment


                  • #10
                    Originally posted by Sea Rider View Post
                    WN, I'm curious about (1) where did you get the information that AV plate must be set even or close to rear middle hull shape, read it somewhere, other forum, elder boaters recommendation, marina pier talk ? (2) what's your next move if you've played with all possible holes while raising, lowering the OB on transom and that weird parameter is not dialed ?
                    (1) That only applies to engines mounted directly to the hull. If you have a jackplate or setback it would need to be higher as I've explained before. Where did I get this info? A lot of research and from personal experience with different engine mount heights. Mainly with the last new boat I bought. The procedure is documented here.
                    https://www.thehulltruth.com/boating...on-advice.html
                    (2) There is no next move for my boat. I've raised the engine as high as it would go mounted directly to the transom and it's in an ideal position now. If I ever repower it, I will have an adjustable jackplate added at that time so height will be easy to change.

                    Comment


                    • #11
                      Originally posted by Faztbullet View Post
                      What about this one? Which color???
                      Green

                      Comment


                      • #12
                        Originally posted by Faztbullet View Post
                        With this 2018 lower unit, which color line would be even with bottom of boat?????( I know the answer)
                        Red

                        Comment


                        • #13
                          Hi. Glad you started this searider. Finding it interesting. I always presumed in the average case, the cav plate or AV plate (green and red in pictures above respectively) would follow the line of the lower hull of your average boat.
                          After looking closer, I’m sure my boat is a bit different. The lowest point of the V at the very rear of mine is almost in line with the upper plate with trim at level....but, the exception is that this is not the lowest part of the boat from memeory. I have a flat running pad a few inches forward of the transom. The boat runs on this when at high speeds. This is in line with the lower AV plate, if not perhaps lower and just in line with the water intake ports. She will go through an impeller each season easily. See the video below. You can see it running at the upper plate cruising speed then at higher speed it runs on or below the AF plate with the prop surfacing.

                          https://youtu.be/MF-v2LYlogw

                          Comment


                          • #14
                            Question : when at full wot rpm range would you say that water flow at speed must cover or not those vents ? if not what's going to happen ?
                            Water DOES not cover those at speed.....and if not covered will have no effect on engine or performance. Also some 4 strokes don't have these due to exhaust system design of 4 stroke.


                            Will not answer Fazt's posted pic as it's irrelevant as we will find latter when all the practice data is posted and confronted with the theoretical data and elucidate which plate achieves the best "Overall "water performance.
                            Why is it irrelevant ??? It shows different motors/units use different set-ups and heights depending on hull design.... and the correct answer is blue line. There is a lot more data, pictures,videos, websites,that debunk your ideas and suggestions.

                            Mr.Trim was not invited to the water party,
                            And why not as 90% of the motors have trim and that effects attitude of boat dynamics.Trim is part of engine height set ups as when trimmed correctly you get best ride, less steering torque more efficient running angle . This leads to less hull and lower unit in water which decreases drag which improves mileage and speed....hint parallel is not a perfect running angle.
                            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


                            • #15
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                              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.
                              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

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