Weight distribution vs. CoG

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  • kevinpratt823
    Fast Electric Addict!
    • Aug 2012
    • 1361

    #1

    Weight distribution vs. CoG

    In light of some of the good discussion going around, I'd like to pose a topic that I've been pondering for quite a while. Mind you, I am just opening a discussion, and much of it is purely hypothetical for other's feedback.
    We constantly refer to CoG, but I don't seem to hear any focus on the effect of weight distribution, and I wanted to open that up to see what others think. For example, If I have 2 see-saw's, and I balance a cinder block on the very ends of one, and 1' from the middle of the other, they both have the same CoG, but very different pivoting characteristics. I can disrupt the one with the blocks close to the middle with a flick of my little finger at the end of the plank, the one with the blocks at the ends takes some more deliberate force to disrupt it as quickly. As such, with two identical boats with identical CoG, but one has the packs on the sides of the motor all located right at the CoG, the other has them spread out away from the CoG, say motor in back, packs spread out in front, they will behave differently. Or say in a Cat, the difference between single 6s packs on either side, as opposed to (2) 3s end to end on each side. One will pivot on the CoG more freely, while the other will be a bit more inclined to stay flat. I'm sure both boats have their strengths and weaknesses, and one may do better in certain conditions than others, or be better for one hull type but not another. When I think about my boats, and the spot I run at is almost always a bit choppy, I'm inclined to think that a bit of distribution helps keep the boat flat and allows it to be less affected by chop, where as the one with the weight concentrated at the CoG oscillates more, and could get out of control quicker. It applies to balance from side to side as well, but maybe in a different way, say a Mono with a single 6s in the middle, as opposed to a 3s on each side, or a cat with packs in the sponsons, or over the tunnel. I know both ways are successful, just wondering what effects people see from these differences.

    Or maybe I just think too much
    My private off road rc track
    https://www.youtube.com/watch?v=XC3H...yaNZNA&index=8
  • ray schrauwen
    Fast Electric Addict!
    • Apr 2007
    • 9472

    #2
    Two books, "The Basics of Fast Electric R/C Boats" By Jay Turner and "Advanced R/C Boat Modelling" By John Finch go over this topic. I haven't read them in a while (need to again!) but, I believe what you are trying to understand is the difference between CoG and CoB (Center of Balance).

    Sorry, must get back to sanding my Whip 40...

    P.S. You are thinking perfectly.
    Nortavlag Bulc

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    • kevinpratt823
      Fast Electric Addict!
      • Aug 2012
      • 1361

      #3
      Originally posted by ray schrauwen
      Two books, "The Basics of Fast Electric R/C Boats" By Jay Turner and "Advanced R/C Boat Modelling" By John Finch go over this topic. I haven't read them in a while (need to again!) but, I believe what you are trying to understand is the difference between CoG and CoB (Center of Balance).

      Sorry, must get back to sanding my Whip 40...

      P.S. You are thinking perfectly.
      Nothing in Jay's book, got it the other day.....I am interested to hear some of the experienced people's perspective of this today though, especially considering the technology that has changed a lot with much higher speeds being achieved. Those 2 books are 20 years ago now.
      Good luck with the new boat, Ray! can't wait to see some pics/video!
      My private off road rc track
      https://www.youtube.com/watch?v=XC3H...yaNZNA&index=8

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      • 785boats
        Wet Track Racing
        • Nov 2008
        • 3169

        #4
        Lots of links here.


        In large scale boats (like freighters, ocean liners or battleships etc) we spread out the ballast. Even put the ballast up the sides to slow down the rolling motion, so the boat doesn't look like a cork bobbing on the water. It gives the boat a more realistic motion. That would be your blocks at the ends of the see saw. These are displacement hulls but I think that the same effects happen on surface boats. Maybe not. I'm not real sure.
        See the danger. THEN DO IT ANYWAY!!!
        http://forums.offshoreelectrics.com/...hp?albumid=319
        http://forums.offshoreelectrics.com/...hp?albumid=320

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        • NativePaul
          Greased Weasel
          • Feb 2008
          • 2760

          #5
          The year before last I gutted my JAE.21FE to fit a wire drive, to minimise the bend in the wire I used 20" of wire and moved my motor right up front as far as it would go, to counterbalance the motor and keep the CoG in the right place I moved the battery to the back.

          It had the same motor and mount, same ESC, same battery, same sponsons and sponson angle, same CoG, same prop, same strut position and angle, same lack of ski, same weight (more or less (I suspect it may have lost an ounce, loosing the stuffing tube))

          It is more stable in rough water now and I can only put that down to the wire setup having a higher polar moment of inertia with the weight at each end of the tub, rather than the cells right next to the motor in the middle as they were with the flex.
          Paul Upton-Taylor, Greased Weasel Racing.

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          • kevinpratt823
            Fast Electric Addict!
            • Aug 2012
            • 1361

            #6
            Thank you Paul, this is exactly the kind of real world experience I was hoping to hear to validate my thoughts. I'm surprised this topic doesn't come up in planning layout in builds.
            My private off road rc track
            https://www.youtube.com/watch?v=XC3H...yaNZNA&index=8

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            • steveo
              Fast Electric Addict!
              • Apr 2007
              • 1454

              #7
              I to have given this a thought and i feel the boat would cut through a wave better or be less disturbed by it is my take on it.. Thats how i set my boats up.

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              • Shooter
                Team Mojo
                • Jun 2009
                • 2558

                #8
                I played with weight distribution last year to see if I could eliminate some chine walk. I thought it would have a similar effect as the balance pole that a tight rope walker uses. Figured moving batts to the extreme sides of the boat would slow down the overall roll response. It certainly works, but it also slows down the recovery back to neutral. So I guess it's a 'balance act'. Ha!

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                • iridebikes247
                  Fast Electric Addict!
                  • Dec 2011
                  • 1449

                  #9
                  I haven't really given this one too much thought but I keep hearing about Jay's book

                  http://www.amazon.com/Basics-Electri.../dp/0911295356

                  I'll be ordering one too figured I would post a link if anyone else is interested! Sorry slightly off topic.
                  Youtube: https://www.youtube.com/channel/UCSr...6EH3l3zT6mWHsw

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                  • NativePaul
                    Greased Weasel
                    • Feb 2008
                    • 2760

                    #10
                    It is not really an option for most boats I see built here, which are monos or cats, if you put the motor in the nose you couldn't get the CoG back far enough without mounting the battery on a pole sticking out behind the boat or adding a ridiculous amount of lead. I mount my motor about half way down my cats and the battery goes right at the transom in good conditions, that is about as spread out as you can practically get it.
                    Paul Upton-Taylor, Greased Weasel Racing.

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                    • kevinpratt823
                      Fast Electric Addict!
                      • Aug 2012
                      • 1361

                      #11
                      Originally posted by NativePaul
                      It is not really an option for most boats I see built here, which are monos or cats, if you put the motor in the nose you couldn't get the CoG back far enough without mounting the battery on a pole sticking out behind the boat or adding a ridiculous amount of lead. I mount my motor about half way down my cats and the battery goes right at the transom in good conditions, that is about as spread out as you can practically get it.
                      Depends on how you want to build I guess, but of course sometimes you are limited. My 45" Sniper was built with this somewhat in mind, motor way up front and batts lined up behind it from the transom. My Cyberstorm has the motor all the way in back, with the packs towards the front in the center. I'm pretty sure there are monos out there with the packs placed more alongside the motor centralizing the mass. It is certainly something worth considering at least when laying out options for a build IMO.
                      My private off road rc track
                      https://www.youtube.com/watch?v=XC3H...yaNZNA&index=8

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