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    How far below skag/pad?

    Hey guys n gals got a quick question. I have a 26-ft rendezvous 2659. I'm trying to get the correct measurements of how far below pad should my prop be or my Scag be does anybody have any formulas that Bayliner recommended to get maximum performance, right now I have a really long shaft (25in) for my boat I have gotten a Jack plate ordered but I need to make sure that I'm going to be in the right vicinity. Obviously I could just go blind and play with it but I was wondering if anybody had any formula or any recommendation on the style of boat I have to where I'm supposed to actually be! Thanks in advance!

    #2
    The anti cavitation plate should be about an inch below the bottom plane of the hull... the prop should swing no less than 1/2" from the bottom of the anti-cav plate...

    the skeg is the fin that hangs down verticle from the bottom of the lower gear housing that gets ground off when you forget to raise the drive when trailering the boat..


    NU LIBERTE'
    Salem, OR

    1989 Bayliner 2556 Convertible
    5.7 OMC Cobra - 15.5x11 prop
    N2K equipped throughout..
    2014 Ram 3500 crew cab, 6.7 Cummins
    2007 M-3705 SLC weekend warrior, 5th wheel
    '04 Polaris Sportsman 700 -- '05 Polaris Sportsman 500 HO
    Heavy Equipment Repair and Specialty Welding

    Comment


      #3
      This is what the back of my boat looks like .. of course my motor sits right in the middle . I assume my cavitation plate sits 1in below the bottom of my transom right ? My set up is almost like a cat hull or tri toon.

      Pic
      https://photos.app.goo.gl/4djEa9Koe6M7GZ5W9

      Comment


        #4
        Originally posted by Amott1989 View Post
        This is what the back of my boat looks like .. of course my motor sits right in the middle . I assume my cavitation plate sits 1in below the bottom of my transom right ? My set up is almost like a cat hull or tri toon.

        Pic
        https://photos.app.goo.gl/4djEa9Koe6M7GZ5W9
        The faster the boat is the higher you are going to want to mount the engine. We did not have this boat but we did have a CAT that was rather fast and the mounting height was well above the keel height.
        Our jack plates were hydraulic so we could change height in real time - we easily saw that speed like height and slower boats like to have lower mountings.
        Northport NY

        Comment


          #5
          Gotcha .. awesome thanks so much .. I'll be installing a Jack plate next week

          Comment


            #6
            Click image for larger version

Name:	IMG_5237.JPG
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ID:	602651 In our case the anticav plates were mounted well above the keel (3.5"+) as you can see.....
            Northport NY

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              #7
              Here is a business angle view ..... Click image for larger version

Name:	IMG_5209.JPG
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Size:	84.7 KB
ID:	602654
              Northport NY

              Comment


                #8
                That's kind of how my setup is other than I'm way too low and need to be up a lot more. You almost have that tri haul set up to ...

                Comment


                  #9
                  So basically this is what I need to do move this cavitation plate all the way up to the bottom of this transom

                  Comment


                    #10
                    Originally posted by Amott1989 View Post
                    So basically this is what I need to do move this cavitation plate all the way up to the bottom of this transom
                    I am sorry I do not know your boat.
                    In general terms the faster the boat the higher the engine, the further back from the hull step the higher the engine.

                    If this were a typical "V" type boat the guidelines for starting woudl look like this:
                    Cavplate 1" below hull - workboat , slowboat
                    Cavplate = Hull - rec boat , slower hull
                    Cavplate 3/4'+ = faster rec boat, lighter rec boat
                    Cavplate 2+ = very fast rec boat , very light boat

                    Note to above --- these starting points are for engines about 1/2' back off the rear hull step. For every 6" added behind that add about 1/2" rise in engine height.
                    Last edited by smitty477; 10-16-2020, 12:11 PM. Reason: Add note
                    Northport NY

                    Comment


                      #11
                      Originally posted by smitty477 View Post
                      Click image for larger version  Name:	IMG_5237.JPG Views:	8 Size:	175.7 KB ID:	602651 In our case the anticav plates were mounted well above the keel (3.5"+) as you can see.....
                      well above the keel but not so far above the tunnels where the water flow hit the drive units...

                      most sport boats arent considered fast boats, but some do run faster than others.... and in a fast boat that is running in a straight line, the anti-cav plate can be higher, but on the tighter turns, the prop will ventilate unless the drive is lowered.. outboards can utilize a jack plate for this, and usually is on a race boat because they like to keep the drive trimmed at its ultimate setting for the speeds they want to run at, without ever changing it during a run....

                      on sport boats, the anti-cav plate does more than just keep the prop from ventilating, but it also helps lift the transom when the drives are trimmed in... at high, straight line speeds where there is no interruption in water flow, there is enough water getting to the prop at a high enough pressure that cavitation is rare.... most of us who arent primarily racers, but sport boaters, dont operate our boats that way.

                      the anti-cav plate should be about 1 inch below the plane of the hull when the anti-cav plate is level with the plane of the hull.... for most sport boat applications.

                      those who know what they want and need for their particular boat and usage, have the knowledge to change from the standard set up...


                      NU LIBERTE'
                      Salem, OR

                      1989 Bayliner 2556 Convertible
                      5.7 OMC Cobra - 15.5x11 prop
                      N2K equipped throughout..
                      2014 Ram 3500 crew cab, 6.7 Cummins
                      2007 M-3705 SLC weekend warrior, 5th wheel
                      '04 Polaris Sportsman 700 -- '05 Polaris Sportsman 500 HO
                      Heavy Equipment Repair and Specialty Welding

                      Comment


                        #12
                        Well I definitely don't have a race boat lol I have a big 26-ft deck boat and I'm trying to understand where my motor should be adjusted because as of right now I have a 20-in transom with a 25-in shaft so I know I'm too low. Basically it seems like I'm going to have to just trial and error it

                        Comment


                        • Centerline2
                          Centerline2 commented
                          Editing a comment
                          Im not sure how you are measuring, but the shaft has to be longer than the transom is tall, so the prop is in the water below the transom....

                          the shaft length needs to measured from the seat of the tilt bracket that sets on the top edge of the transom, to the centerline of the prop shaft.... but its where the anti-cav plate sets is the determining factor.... if it has no hull (like some pontoon boats) that does not cause a slip stream of water to reach the drive unit, then it need to set even deeper to keep it from ventilating in rougher water.

                        #13
                        Originally posted by Centerline2 View Post

                        well above the keel https://www.boats.com/how-to/the-out...djustments/but not so far above the tunnels where the water flow hit the drive units...

                        most sport boats arent considered fast boats, but some do run faster than others.... and in a fast boat that is running in a straight line, the anti-cav plate can be higher, but on the tighter turns, the prop will ventilate unless the drive is lowered.. outboards can utilize a jack plate for this, and usually is on a race boat because they like to keep the drive trimmed at its ultimate setting for the speeds they want to run at, without ever changing it during a run....

                        on sport boats, the anti-cav plate does more than just keep the prop from ventilating, but it also helps lift the transom when the drives are trimmed in... at high, straight line speeds where there is no interruption in water flow, there is enough water getting to the prop at a high enough pressure that cavitation is rare.... most of us who arent primarily racers, but sport boaters, dont operate our boats that way.

                        the anti-cav plate should be about 1 inch below the plane of the hull when the anti-cav plate is level with the plane of the hull.... for most sport boat applications.

                        those who know what they want and need for their particular boat and usage, have the knowledge to change from the standard set up...

                        "the anti-cav plate should be about 1 inch below the plane of the hull when the anti-cav plate is level with the plane of the hull.... for most sport boat applications."
                        Standard recreation mounting heights for outboards are about 1" above the lowest hull plane - general good starting point.
                        Refer to any maunfacturer maybe like Mercury here:
                        https://www.boats.com/how-to/the-out...t-adjustments/
                        Northport NY

                        Comment


                        • Centerline2
                          Centerline2 commented
                          Editing a comment
                          its a good read, but still doesnt give us any firm number... like i said, it depends on the usage of the boat, but the faster the boat, the higher the probability that the anti-cav plate can be raised...
                          and like you said, on fast boats, it may be fully acceptable to have the anti-cav plate above the plane of the hull....

                          NO ONE knows for sure where the best place for it to be mounted until AFTER they mount it and do some experimentation....

                          it also says different manufactures set them up differently, because they dont know how the boat is going to be used.... and for the bit of extra rpm or a mile per hour as said in the article, its my opinion that unless you race your boat, it really doesnt matter much and not worth the time and trouble unless you are always going to have the boat loaded in the same way and the same weight all the time and want the very optimum top speed.... which most of us go at cruising speed rather than top speed anyway....

                        • smitty477
                          smitty477 commented
                          Editing a comment
                          I only know from experience from numerous "V" style boats as well as the pictured CAT that the height has always been above the hull plane. The benifits do include higher top speed but i never really cared about that it was the better fuel economy and lower rpm operation that was the real key. With the outboards that we had (numerous Merc and Yamaha mostly) the handbooks always directed one to start about 3/4" above the hull and continue to raise the motor untill you get blowout in a turn and then go back one mounting hole. Never raced any boat at all and that article points out that most cruising boats perform all around better at 1-3" above the hull as well. Race boats woud be above those numbers...

                        #14
                        Yeah I agree especially in my situation where I have a kind of unique try-haul catamaran boat I'm just going to have to put it on raise my motor a little bit and bring it down in trials

                        Comment


                          #15
                          Originally posted by Amott1989 View Post
                          Yeah I agree especially in my situation where I have a kind of unique try-haul catamaran boat I'm just going to have to put it on raise my motor a little bit and bring it down in trials
                          I do not know about how this will work out on your tri hull boat. Perhaps there are forums where these topics come up specifically for these hulls - maybe a search will yield something.
                          Northport NY

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