twin cat prop/strut distance. How close is too close?????

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  • millzee
    Ozzie Bloke
    • Nov 2010
    • 1092

    #1

    twin cat prop/strut distance. How close is too close?????

    After playing around with a few different hardware mounting options, I found that the transom on the 42" SC I have is not symmetrical, I mean the small v in the tunnel is not a true center between the sponsons it's peak is about 4-5mm off to the left, so after a quick phone call to a wiseman I found the true center and worked off that. Another small issue I've come across is that the strut heights I have are a tad short, I've drilled them out as much as I think is safe but they still leave my mounting options limited. I have however come up with two options but I have come to a stand still, I'm a little undecided on which way to go I've attached a few pics and will try to explain what I'm thinking.

    Option A
    I measured the distance between the center of the couplers with the motors fitted in the twin motor mount, it was 64.5mm. Thinking that it would be easier for me to have one gradual sweeping bend in the stuffing tube, I figured I'd see how the struts would go at the same distance (center of the flex shaft in the strut would be 64.5mm). The biggest props I think I'd use are x450/3 l&r, thus leaving approx. 12-15mm distance between the prop tips. Mounting the struts like this made me have to come up with a more secure mounting bracket as I can only use one side so I made up the opposing side with some 50x50x3mm angle aluminum and drilled a few holes to suit the other side. With the strut level with the bottom of the sponsons, the bottom screw in the mounting bracket only just has enough clearance to put the nut on inside the hull above the floor of the tunnel and the inside of the transom. After fitting the hardware the props looked a bit close, they have approx 15mm gap between tips but I'm a little worried about propwash and how well they will work. Please take a look at the pic's and offer me some advice.





    Option B
    After looking closely at option A, I thought I'd better find an alternative. The easiest being that I remove the inside brackets that I made, move the strut over and use the original bracket using the drilled holes for the inside strut mount. This has eased my issue with propwash but has raised a few others, the bottom hole on the outside strut mount will not be able to be drilled, it'd almost be inline with the tunnel floor so I would simply remove the bottom corner (including the bolt hole) of the mount and just drill and bolt the top hole. Doing this would also remove the overhang of the bottom corner of the mount that could cause tunnel air issues in the future. The other issue that I'd have if I go this way is bending the stuffing tube, it'd be in a bit of an s-bend, I haven't done one b4 and would be more confident with a single bend. I'm sure I'll soon learn.





    Which of the above ways should I choose???????????????????????????????????????????? ????????????????????????
    and is this rudder set back too far?????????????
    http://www.youtube.com/user/1millzee?feature=mhum
    "Gimmy Toyz for FUN"
  • Boaterguy
    Fast Electric Addict!
    • Apr 2011
    • 1760

    #2
    The rudder looks to be pretty good.
    I've never done twins before but I know this, the closer they are the easier the turning.
    one thing to think about the close ones is that if one prop/shaft fails the other will most likely fail also. lifting props may have issues with prop wash but nothing significant.

    Comment

    • Rumdog
      Fast Electric Addict!
      • Mar 2009
      • 6453

      #3
      Option B. Closer together does not mean better turning.
      Option A will have the rudder in prop wash.. especially while turning. You're probably going to end up running the struts a bit higher than even with the sponsons as well. I would have mounted the brackets higher.
      Not sure what boaterguy is talking about with the propshaft failing... that isnt a true statement.

      Comment

      • carlcisneros
        Fast Electric Addict!
        • Jan 2010
        • 1218

        #4
        to me, option B is a better set up.
        a bit high on the strut mounting is what I would have done, BUT, with your set up right now,
        you can always run the bottom outer tips of the strut brackets ona belt sander to remove the
        portion of Ali. sticking into the tunnel area. jmo.

        Comment

        • Boaterguy
          Fast Electric Addict!
          • Apr 2011
          • 1760

          #5
          Originally posted by Rumdog
          Option B. Closer together does not mean better turning.
          Option A will have the rudder in prop wash.. especially while turning. You're probably going to end up running the struts a bit higher than even with the sponsons as well. I would have mounted the brackets higher.
          Not sure what boaterguy is talking about with the propshaft failing... that isnt a true statement.
          I'm jut saying if one shaft were to fail it would most likely hit the other shaft since the props are close.

          Comment

          • LarrysDrifter
            Big Booty Daddy
            • May 2010
            • 3278

            #6
            The other would not fail. When part of the driveline fails and the flex and or wire, stub shaft and prop comes out, it does not make a turn and head for the other prop. The boat "pulls away" because its moving forward and the piece sinks to the bottom. Really simple if you think about it.

            Comment

            • Fluid
              Fast and Furious
              • Apr 2007
              • 8012

              #7
              I would chose option B. Prop wash will be hitting the rudder in A, upsetting handling. I doubt you'll see much difference in ease of turning between the two options, but Option A risks wandering in the straights and squirrely handling in the turns.

              You have much simpler options than this guy did:



              ERROR 403 - This is not the page you are looking for

              Comment

              • LarrysDrifter
                Big Booty Daddy
                • May 2010
                • 3278

                #8
                Maybe getting a shorter rudder extention would help with option B to get the leading edge of the rudder even as possible with the drive dogs? Dag gum, thats a lot of overkill in that hull. (Jays pics).

                Comment

                • Boaterguy
                  Fast Electric Addict!
                  • Apr 2011
                  • 1760

                  #9
                  need to put two motors in the sponsons and add stingers, make it a septuple drive.

                  Comment

                  • millzee
                    Ozzie Bloke
                    • Nov 2010
                    • 1092

                    #10
                    thanks for all the feedback guy's. Option B it is then, I was starting to favor this one also as it'll give me a bit more room for the pushrod going to the rudder. I'm starting to find it very handy having good size photo's to look at, as much as I study things in the shed, I always seem to see things better on the computer. Back to the shed for me today to get this thing closer to seeing water.
                    thanks again
                    http://www.youtube.com/user/1millzee?feature=mhum
                    "Gimmy Toyz for FUN"

                    Comment

                    • millzee
                      Ozzie Bloke
                      • Nov 2010
                      • 1092

                      #11
                      Here what we come up with
                      thanks again




                      http://www.youtube.com/user/1millzee?feature=mhum
                      "Gimmy Toyz for FUN"

                      Comment

                      • carlcisneros
                        Fast Electric Addict!
                        • Jan 2010
                        • 1218

                        #12
                        good one.

                        now take 1 strut off, head down to your hobby shop and hit the tubing section and find 2 pieces
                        that fit snuggly into the strut and you are set.

                        then when you get back home, you can fine sand the tubing so they slide into the struts so that they
                        will be easy enough to slide when it comes time for strut angle adjustment.

                        don't forget to put silicon on the bolts going thru the transom for sealing.

                        this is what you are shooting for.
                        it is rumdogs twin cat:



                        IT HAULS FREIGHT!!!!!!!!!!!!!!!!!!!

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