1. Limited time only! Sign up for a free 30min personal tutor trial with Chegg Tutors
    Dismiss Notice
Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

I need to anchor a raft

  1. Jun 6, 2015 #1
    My son and I built a swim platform (raft) on a half dozen 55 gallon drums. I then made some 1 cf concrete anchors, which do nothing to hold the raft in a moderate wind. I have since learned that I need to account for the buoyancy of the concrete in water, but now I need to calculate the required weight of anchor to hold the raft in place in a 35 mph wind on an open lake at 6000 ft elevation.

    What is the force that the wind exerts on a surface? I'll just assume a flat-sided raft and not worry about all the spaces between drums under the platform unless that would significantly change the results.

    Simple units would be preferred (instead of things like newtons, joules, pascals, etc.) because I'm old and my brain doesn't take to new units too well. Note: I'm still good with math, just not new units that I don't know how to convert.
  2. jcsd
  3. Jun 6, 2015 #2


    User Avatar
    2017 Award

    Staff: Mentor

    While it is possible to make a model of this system, getting a proper estimate would be a lot of work (and a much more detailed description of the raft and the lake). Testing it is much easier.

    The SI units are much simpler than the US-units. All conversion factors are 1, 10, 1000 or similar. They are also older than the current US-units.
  4. Jun 6, 2015 #3


    User Avatar
    Science Advisor

    What kind of angle are you using for the anchor line? If it is a steep angle and a short rope then wave action can exert a great deal of force and could cause the anchor to "drag". A shallower angle would reduce the vertical component of that force and would allow for the cable to sag so that it would not then snap taut as easily.

    One reference I found for this mentions three to one or seven to one ratios. http://www.ftkltd.com/How%20much%20Anchor%20Rode%20do%20I%20need.htm [Broken]

    That said, six fifty-five gallon drums riding high in the water in a 35 mph wind -- sounds like a handful.
    Last edited by a moderator: May 7, 2017
  5. Jun 6, 2015 #4


    User Avatar
    Staff Emeritus
    Science Advisor
    Homework Helper

    An anchor is more than just a heavy weight. An anchor has flukes and whatnot to help it set into the bottom, which increases the amount of force it takes to drag once it is set. You haven't said what shape your concrete anchors are, but if they are smooth on the outside, they won't be able to hold very well.

    Concrete is a fairly light material, unless you add something to it, like scrap steel, to make the anchor heavier.

    Concrete - density 150 lbs. per cubic foot
    Iron or steel - density 490 lbs. per cubic foot
    fresh water - density 62.4 lbs. per cubic foot

    As you have found, the buoyant force acting on a block of concrete affects its net weight quite significantly. About 40% of the weight of the concrete anchor in air is negated by the buoyant force of the water it displaces.

    This article discusses some important points to consider when choosing an anchor:


    You should find out what sort of lake bottom in which you are trying to anchor (muddy, rocky, or something else) and how deep the water is. Even on a lake at altitude, sustained winds of 35 mph are going to generate some waves, so the amount of anchor line you use will have to take this into account. If you expect a storm or rough conditions might develop which will last for an extended period, it might be prudent to move your platform to a location which offers some shelter from any wave action.

    Vessels, whether a simple swim platform or an ocean liner, have to have their moorings tended to periodically to keep them in position. You can't just heave an anchor over the side and forget about it.
  6. Jun 6, 2015 #5


    Staff: Mentor

    If you have a muddy bottom, give the concrete anchors a month or so to sink in the mud before attaching the raft.

    See How to install a permanent mooring.

    You will need sufficient chain, and about 200 pounds for a sheltered location, 800 pounds for an exposed location. A single large block is better than multiple small ones. You may need a barge and a crane to get it out there. A mushroom anchor is lighter and better than any block. All the alternatives and tables listing sizes needed are in the linked article.
    Last edited by a moderator: May 7, 2017
  7. Jun 7, 2015 #6
    Use what the ancients used for thousands of years. Stone doughnuts.
    Or concrete ones with metal for weight. Make several maybe 20kg each. If your need is a platform then tie it to a rope and throw it some distance away from a corner. Do the same at the other corners. Let them settle a bit.
    If you still require more then slide new rings into the ropes.
    Caution: this is a heavy job and never underestimate the power of water
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook