Floating bridge

Discussion in 'Boat Design' started by Engineerium, Mar 12, 2023.

  1. Engineerium
    Joined: Mar 2023
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    Engineerium New Member

    I know, not a boat. But I keep getting this forum in my search results, and you guys seem pretty smart about the points I'm trying to figure out.

    Basically, I have spent years trying to figure out an economical way to access a 16 acre field on my farm through the winter. A normal bridge of any permanent design is out because of environmental red tape. There is only 1 access point, and it is across a river that floods beyond fording ability Nov through Apr. I can (physically and legally) drive across when it is low enough May to Oct, and it can go dry in late August, but that is rare.

    It occurred to me that a floating bridge would work, since it isn't "disturbing the stream bed" (legal phrase), and the anchors could be outside the high water marks (overhead anchors described here: 0223-2812P-MTDC: Floating Trail Bridges and Docks https://www.fs.usda.gov/t-d/pubs/htmlpubs/htm02232812/page07.htm)

    So I've considered and discarded lots of floatation options, and I'm thinking 8 runs of 100ft 6in drainage pipe, with boards bolted across them (6ft width) should work. It would give the bridge flexibility to rise and fall with the river level, including laying across the contour of the land for any or all portions when the water is low. I don't even have to cap the ends, just extend them vertical and hook over to keep rain out.

    The math is a bit harder to figure out if it would work. A 6in diameter pipe 1in long has about 28.2 cubic inches of volume, which has a boyancy of about 1lb. So 8 would be 8lbs of boyancy per linear inch of the bridge. I'd probably use 6ft 2x6s, which would weigh about 11.7lbs each. Leave half an inch gap for drainage and flexibility anyway, and we are looking at 6 inches of linear bridge per board, so 48 lbs of boyancy, minus 11.7 board weight, let's call it 12 with the pipe hangers. 36lbs of capacity per board, 72 per linear foot.

    It seems low, but since they are all tied together though, flanking distance of the bridge matters and will add capacity, which is where I start struggling. Even just 1 foot ahead and behind me, for 3 linear feet, brings the capacity to 216lbs, plenty to carry me (about 170lbs wet). I figure the actual capacity would have to be higher than that, as the more weight is on the bridge, the more distance of the piping will come into play.

    Can anyone see any glaring holes in this? Am I missing something? Any way to calculate better? Different material recommendations?

    (I can get into all the legal reasons for "why not <just do this instead>" if you all want. We have been pursuing a solution for years, and the restrictions are a bit nutty.)
  2. bajansailor
    Joined: Oct 2007
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    bajansailor Marine Surveyor

    Welcome to the Forum Engineerium.

    Does this floating bridge have to be capable of carrying a maximum weight of just one person?
    And is the total length required about 100'?
    Will you have the eight pipes (each 6" diameter) side by side, with a 6' long 2" x 6" board top and bottom forming a (sort of) sandwich, with the pipes being the filling?
    Can you perhaps post a sketch showing your proposed cross section if it is different to what I am trying to describe above?
    It seems to me that the system as described above will use an awful lot of timber - the timber on the underside of the pipes will contribute 'some' buoyancy, but the timber on top of the pipes will be just deadweight.

    Have you considered using something like Dock Blocks instead?
    There is a floating jetty here made with Dock Blocks, and it works very well.
    Dock Blocks | Modular Floating Docks & Boat Lifts https://dock-blocks.com/

    Or maybe something like a Rolling Barge kit?
  3. Engineerium
    Joined: Mar 2023
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    Location: PNW

    Engineerium New Member

    Thank you for replying.

    At a minimum it would need to be able to support a single person walking across. If expanding it is reasonably affordable then taking more weight could provide more opportunity for me in the long run. I don't have a requirement currently of needing to get a tractor across, for example. I can just leave a tractor over there during the winter months.

    Yes, total length about 100 ft.

    I was picturing a design that was having boards only on top, not underneath. In addition to the weight, as you note, the wood on the bottom would also provide a significant amount of drag on the river current. I was envisioning using pipe brackets to secure the pipes to the bottom of the boards. It's an interesting thought that you have though, if I just put them on the bottom and had the pipes on top, I could pretty much guarantee always walking through water, but would probably only ever be a couple inches deep, easily manageable by boots.

    Both of those links you provided are definitely intriguing. I'll have to inquire more in that direction too.
  4. Rumars
    Joined: Mar 2013
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    Rumars Senior Member

    A floating bridge with anchors only on the river banks would have to use the elevated anchor cable design, wich isn't cheap at all. If you do towers it's a lot simpler to hang a basket from the cable and winch yourself across (manually or with a cordless drill). The pontoons would also have to be aligned with the flow, the continuous pipes you envision will offer much greater resistance and become dangerous if any debris is floating.
    The cheapest possible way is a cable ferry, two short stout pillars on the banks, a lenght of chain between them and a basic punt with a winch on it.
  5. bajansailor
    Joined: Oct 2007
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    bajansailor Marine Surveyor

    I think that Rumars has nailed it very well here - ANY continuous floating bridge structure is going to have enormous resistance to the flow of the river, as well as catch all manner of debris that comes floating down the river.
    A simple cable or chain ferry, consisting of a very simple punt that you pull yourself across on, will have very little resistance to the flow of the river, and there is not much to go wrong really with it.
    There are some very neat human power small cable ferries mentioned in this link -
  6. clmanges
    Joined: Jul 2008
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    clmanges Senior Member

    You can even design a cable ferry that uses the river's current to power it. I'm fuzzy on the details--it might need two parallel cables--but it holds the boat hull at an angle to the current such as to push the boat across. Of course, you have to change the boat's angle for the return trip, but I think these are pretty simple.
  7. wet feet
    Joined: Nov 2004
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    wet feet Senior Member

    What Rumars describes is exactly what serves as a floating bridge in Cowes,England.The current version is far from the first as an earlier example served as Uffa Fox's workshop in the 1920's-30's.All you need to do is scale the various parts to fit the location.

  8. mitchgrunes
    Joined: Jul 2020
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    mitchgrunes Senior Member

    There are also many types of floating "walk on water" shoes, if the current isn't fast. You could build your own pretty cheap.

    Could you legally set up a zip line? Perhaps one in each direction?

    If you don't own the river, putting a cable or bridge across, that blocks other peoples' access, should be investigated from a legal point of view. E.g., in the U.S.A., you mostly can't block a navigable river.

    I'm completely unclear why you don't just use a small boat. Unless it sometimes doesn't flood deep enough.
    Barry likes this.
  9. Blueknarr
    Joined: Aug 2017
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    Blueknarr Senior Member

    Yes, current propelled ferries are possible.
    The angle of attack can be controlled by rudder or by a bridle adjustment.
    However, some additional power is often needed at the end of the crossing when the vessel has to climb up the slack in the suspension cable.

    The simplest way is to anchor a long floating line to one shore. One way is free the other minimal effort.
  10. gonzo
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    gonzo Senior Member

    A single cable or chain would work fine. It is wrapped around a drum that reverses rotation. At each end of the ferry there are usually hawse pipes or rollers. However, a small jon boat with an outboard would be cheaper and easier.
  11. Engineerium
    Joined: Mar 2023
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    Engineerium New Member

    Current has lots of variability in speed, but if it is high enough that I can't ford it, it will be too fast for water walking.

    Zip line "gets me to and from", but I would likely have enough reasons for tools, bags of feed/seed/other that the complications climb to manage all the options. Also completely precludes the possibility of animals (80lb sheep, 150lb pigs). Any solution at best would be 1 animal at a time, and that added effort of wrangling, harnessing, moving, in harnessing, etc... makes it a non starter except for purely recreational usage. And I got a farm to run.

    I technically own the river, but a lot of legal stuff for environmental reasons is most of the challenge. It isn't navigatable. A bridge is legal, but requires permitting from 11 different agencies and the restrictions of not disturbing the stream bed, can't touch ground within 25ft of the high water mark, and has to be 10ft over the highest water mark with our gradient means it would be a 600-1000ft suspension bridge. I'd be in if for hundreds of thousands at best. A million or more is possible. Just no go on that.

    I do have a rowboat that I use now. But again, animals are out and current makes it really tough. I end up 100+ yards downstream on the other side, have to drag the boat out into the field around the blackberries and back up 200 yards to put in 100 yards upstream, to make the landing point.

    It just doesn't work for being able to properly farm that field at will.

    A ferry would work with a somewhat stable river flow, but again, it varies so considerably in flow over the course of a year, I would constantly have to keep managing where it is. Mooring it to the bank one day could mean that it is 20ft out in the river the next, or on land 20ft from the river.

    Like I said, I've been working on this problem for years. :)

    A floating bridge solves the bank variability, without crossing the legal restrictions, and allows for at will continuous flow of me/animals. Debris isn't a concern in the river. Basically even at its peak, the river sits in a 500+ acre marsh upstream. It is wide and slow in the winter, grass choked in the summer. The only debris is silt, gravel, and grasses. My land creates the pinch point for it to drain through.

    I totally agree that the drag on the bridge by the current would be significant. But I think there are ways to solve that too. For example, I imagine it would be possible to craft the upstream side to "surf" the current, helping the bridge ride high and light.

    The current is something that the earlier floating dock modules will probably not be able to handle without modifications. But still possible.

    Overhead anchors are also doable without significant expense.

    Reference pictures. This is the same spot. A theoretical bridge doesn't have to be here. I have about a quarter mile of river length to work with.

    Low flow, https://photos.app.goo.gl/fyDcmrUu5qgxntYBA

    Record flow: https://photos.app.goo.gl/2QVnhEW1ss6YnQGJ7

    Normal high flow: https://photos.app.goo.gl/xVkVQR5JFRUVjS4g8

    The tree in the normal high flow is the same tree out in the river in the record flow picture. It is just to the left off camera in the low flow picture.

    And the field is bordered by train tracks I have explicitly been forbidden to enter/cross (plus fencing, a long way around to get there, owners over there to deal with, etc), and a marsh upstream and another downstream that spans to the tracks. There really is only 1 access direction, and historically grandfathered in to ford over at this one spot.

    That you for the responses. It is helping to refine.
    bajansailor likes this.
  12. bajansailor
    Joined: Oct 2007
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    bajansailor Marine Surveyor

    Here are Engineerium's three photos all together, to allow for easier comparison :

    Low Flow :
    Low Flow .jpg

    Normal Flow :
    Normal Flow .jpg

    Record Flow :
    Record Flow .jpg
  13. Rumars
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    Rumars Senior Member

    Looking at the pictures and considering sheeps and pigs I would buy myself an amphibious ATV and call it a day.
    Will Gilmore and Blueknarr like this.
  14. Blueknarr
    Joined: Aug 2017
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    Blueknarr Senior Member

    A floating bridge won't be legal.
    It will touch the ground within 25 feet of the high water mark and won't provide ten feet of clearance.

  15. latestarter
    Joined: Jul 2010
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    latestarter Senior Member

    I suggest a small hovercraft.
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