Announcement

Collapse
No announcement yet.

OT: Dock Flotation Calculations

Collapse
X
 
  • Filter
  • Time
  • Show
Clear All
new posts

  • OT: Dock Flotation Calculations

    I am building a 6'X16' floating boat dock. I will use the blue plastic 55 gallon barrels to float this thing (yes, it's legal here). I think I have my calculations correct, but I REALLY want to be sure before shoving it into the river. So here is how I figured it:

    A 55 gallon drum (24" dia x 36" long) should float at water level using 55 (gal) X 8.3 (water per gallon)= 456.5 lb flotation.

    I want the dock to set about 12" (+,-) above waterline. So, 1/2 of 456.5=228.25 lbs. flotation per barrel. Yes?

    Pressure treated wood weight as taken from a table found on the Internet:

    4- 2x6x16 @ 51lb ea.= 204
    4- 2x6x12 @ 38lb ea.= 152
    4- 2x6x8 @ 26lb ea.= 104
    18- [email protected] 19lb ea= 342

    Total= 802lb in lumber.

    Does not include corner stiffening plates and hardware. I can only guess about 50-75 lb of steel and hardware.

    I have calculated six equally spaced floats (each corner, [email protected] mid-span). So, 6x228.25=1369.5 lb (flotation) -802 lb (lumber) - 67.5 lb (hardware) = 500lb excess flotation. Three or four people on the dock getting into the boat or swimming= about 500 lb= dead nuts 12" off the water.

    What might I be overlooking? Is OK, close, or is the whole shootin match a bust?

  • #2
    Originally posted by garagemark
    What might I be overlooking?
    The weight of the empty drums?

    But that is probably not enough to upset your calculations, which seem okay to me.
    Allan Ostling

    Comment


    • #3
      I haven't check the numbers but the theory looks OK provided the people all stand in the middle of the dock or are evenly distributed. If they all stand close to one corner/end then you could have a problem. The math also gets complicated.

      Phil

      Comment


      • #4
        When I do the math that 24" dia. x 36" length comes to over 70 gal. for 584 # displacement. Maybe the drums aren't that size.

        Comment


        • #5
          Looks OK except for:
          "Three or four people on the dock getting into the boat or swimming= about 500 lb= dead nuts 12" off the water. "
          May be OK in West Virginia but here in IN I'd say closer to 1000# with a big cooler of Beer.

          Larry S
          Fort Wayne, IN
          Larry Swearingen
          Fort Wayne, IN
          New Hoosier

          Comment


          • #6
            I assume your timber weights are for dry wood. Get wood wet it will easily double its dry weight (depending on timber species).

            Comment


            • #7
              To help keep it stable and to keep the lines out of the way of the boat(s) tie the dock to anchors at the far end so that they cross under the dock from opposite corners on the end of the dock. Engine blocks make pretty good anchors (minus oil).
              Free software for calculating bolt circles and similar: Click Here

              Comment


              • #8
                The narrow width will make it very easy to rock from side to side. I have a 6x20 on the side of a fixed dock.. and that's built with Merco Marine floats and topped with Trex (heavy). Merco sells great hardware for dock building.

                If I did it again, I'd double up on all the floats and suspend concrete beams/blocks (or maybe old engine blocks) a few feet below the dock on cables.

                Comment


                • #9
                  Foam

                  Originally posted by garagemark
                  I am building a 6'X16' floating boat dock. I will use the blue plastic 55 gallon barrels to float this thing (yes, it's legal here).
                  I found it amusing you were calculating to a tenth of a pound.

                  Several decades ago my uncle built a float for us kids to swim out to and back from, sunbathe on, whatever. Anyway he filled it with foam to make life a lot simpler for him when the inevitable happened. You may wish to consider this.

                  You don't need to fill it with enough foam to support 500 extra pounds, just enough to make recovery simpler when it sinks.

                  Also back in ye olden days we suspended numerous tires along the side so as to not destroy the docked boat(s), had a collection of eyebolts at varying distances for different sized boats... Also we painted our floats and piers. That was the cause of my amusement at the tenth of a pound.

                  I will warn you that a 55 gallon drum is not terribly stable in the water unless there's a fair amount of space between the drums. Think of how a pontoon boat floats and its roughly 55 gallon drum cross-section floats are like 10 feet apart. Close together you'll find it's dynamics to be ... spritely

                  Comment


                  • #10
                    Originally posted by garagemark
                    Three or four people on the dock getting into the boat or swimming= about 500 lb= dead nuts 12" off the water.
                    No clue about your other math, but this one is dead wrong.
                    People in the US weigh a lot more than 125/133# on average.
                    Just recently, the USCG changed the rules for evaluating commercial
                    ferry boat stability. They increased the average weight per person
                    to use when calculating boat stability from 160# to 185# per person.

                    see
                    https://homeport.uscg.mil/mycg/porta...e=EDITORIAL&BV

                    I'd assume that they still are behind the times, and did this
                    increase only to satisfy the growth of US waistlines in the 1990s.
                    I would not be surprised if 190 or 195# is a better number to use
                    now.

                    And then there is cargo.... I don't know what your intended use is,
                    but it takes almost no effort to have 100# or more cargo these days.
                    Possibly 100-200# per person, depending on the use.

                    Finally, you want to be careful using something like barrels; if they
                    get a hole in them, they lose all their buoyancy --- unlike foam blocks
                    where each and every little bubble of air in the foam has to fill with
                    water - a lot harder to do.

                    Frank

                    p.s. I may not know much about machining --- but I have been involved in
                    some small governmental committees overseeing the operation of a
                    ferry boat and public dock and these were issues that arose -- esp
                    the weight/person one.

                    Comment


                    • #11
                      Agree on the mass per person issue, and the "baggage" issue.

                      Also agree on foam filling, since the barrels WILL get holes..... packing foam material (the 2 part stuff) is closed cell (at least the ones I know about), and might be practical, but I'd do the filling in more than one step..... it's tough to stop that expansion, don't try.

                      Another issue you may not have thought of..... barrels are ROUND.

                      What THAT means is that as you load weight onto one floating, for half the displacement the depth of submersion per "unit" of added weight gets smaller.... But after you have submerged it halfway, the depth of submersion per unit added weight INCREASES.

                      This is because for half the submersion, the displacement per inch of depth keeps INCREASING, but after half, it starts DECREASING.

                      In practical terms, it means that what looks like plenty of freeboard left can disappear in a hurry.

                      So you probably should plan for the max design "cargo" weight to get to about half of the total available displacement. When you get past that it's time to stop adding load.
                      1601

                      Keep eye on ball.
                      Hashim Khan

                      Comment


                      • #12
                        Foam

                        If you decide to go with foam. There are at least two types, open cell and closed cell. The open cell will quickly become water logged, closed cell will not. Believe it or not most boat builders use open cell foam. That why so many boats have rotten stringers and transoms.

                        Ken

                        Comment


                        • #13
                          Originally posted by vincemulhollon
                          I will warn you that a 55 gallon drum is not terribly stable in the water unless there's a fair amount of space between the drums. Think of how a pontoon boat floats and its roughly 55 gallon drum cross-section floats are like 10 feet apart. Close together you'll find it's dynamics to be ... spritely
                          This would be my biggest concern as well. If your dock is only 6 feet wide and the barrels are basically "under" it the whole thing is going to be pretty unstable. I'd try and get the width out to 8 or even 10 feet with a row of barrels down either side. A pontoon boat is so stable because the centre of gravity is near the centre of the the boat but the flotation is all on the outsides...
                          Keith
                          __________________________
                          Just one project too many--that's what finally got him...

                          Comment


                          • #14
                            Flotation

                            Originally posted by LKeithR
                            This would be my biggest concern as well. If your dock is only 6 feet wide and the barrels are basically "under" it the whole thing is going to be pretty unstable. I'd try and get the width out to 8 or even 10 feet with a row of barrels down either side. A pontoon boat is so stable because the centre of gravity is near the centre of the the boat but the flotation is all on the outsides...
                            Keith, et al have very good points. The center of gravity for your barrels will only 4 feet apart if placed parallel with the dock. I learned about closed cell styrafoam tonight, points well made.

                            One way to keep your load centered is to build the deck 4' wide with a couple of walkways out to the 6' width, yes it will reduce convience but may help the economics.

                            Good luck

                            Ray

                            Comment


                            • #15
                              Hm, how about making two 3 ft wide floating walkways coupled together about a third of the way from shore with a cross piece. The boat would moor between the walkways, and other boats could still tie up outside of them. Add suitable bumper protection of course. The cross piece would be another, probably wider section, and maybe even have a Vee to bring the front of the boat into. The whole thing would look kind of like an H. Would be a lot more stable, and you could even have a motor mount on the cross piece and take it out like a pontoon boat. There's a swimming platform and a place to moor a boat and some canoes or kayaks.

                              I'm on the same opinion as far as the barrels go- fill them with foam. I was notified today about some styrofoam being removed from the outside of a building- 20,000 sq ft of it, 2 inches thick. I would not be adverse to opening the barrels up in a suitable way, then filling them with close-fitting pieces of scrap styrofoam. The barrel protects the foam, the foam gives the barrel lots of impact resistance and gives the flotation. Just some ideas.
                              Last edited by darryl; 05-08-2012, 01:47 AM.
                              I seldom do anything within the scope of logical reason and calculated cost/benefit, etc- I'm following my passion-

                              Comment

                              Working...
                              X