Weight rating on toons -- Question for you

Discussion in 'Watercraft' started by David Dalan, Nov 11, 2012.

  1. David Dalan

    David Dalan 69°19'15.35" N 18°44'22.74" E

    OK, so I have a small pontoon boat. Weight rating 375# according to the manufacturer. So as I am patching a hole in one of the tubes, it occurs to me to wonder where this rating comes from. I think "this pontoon has to displace WAY more than 375#'s of water." So I break out my calculator.

    Tube is roughly 18" in diameter (9" radius) and, while it is 11' long, there is an 6' section that makes a nice cylinder. So the volume of a 72" x 9" (radius) cylinder, is 18,312 cubic inches. There are 231 cubic inches in a gallon, so this is 79 gallons. At room temperature, water is about 8.3# per gallon. So the weight of water this tube could displace is 657#...per pontoon. Now to reach the maximum, the tube would have to be submerged entirely (hard to row that way).

    But, if 50% submerged, I'm still displacing 675# (50% of the displacement of both pontoons). Or is this relationship non-linear? Some ship design sites I found, seem to indicate planning displacement is a linear activity. I assume at some point the strength of the pontoon shell must come into play (frame pressing on the pontoon, with a relatively small contact area could result in very high forces on the pontoon).

    But I cannot figure out how to get from displacement capacity to anything like the rating on the pontoon boat. With a total displacement of about 1300#, assuming less than 50% submerged, I still come up with almost double the advertised weight rating.

    Any smarty smart pants out there capable of explaining to me how to reconcile this? More of an engineering question than one of physics? In the long run it does not really matter, but it tickles my curiosity.

    I need to go fishing...(tomorrow!)
  2. PT

    PT Physhicist

    Put the calculator away because you're putting way too much thought into it. If the boat is a Scadden and floats you slap at least a 1000# capacity sticker on it regardless. If it's a Buck's you slap a 500# sticker, Outcast 500+ and so on.

    I think each company just has a 300# person jump up and down on the frame to see if it breaks. If it breaks, you slap a 250# sticker on it. If it doesn't, they have a 2nd 300#er jump up and down.... and so on.

    Same concept as this.....

    David Dalan likes this.
  3. mbowers

    mbowers Active Member

    I'm not a naval architect but I am an engineer so I think my comments are accurate but no guarantees! :)
    With a general hull shape when you are 50% submerged you won't be displacing 50% of the water and a cylinder is very non-linear hull shape but because a cylinder happens to be symmetrical about the 50% level you would be 50% submerged when you used up 50% of your buoyancy.
    Just because the vessel has not sunk at that point doesn't mean it's seaworthy. There is also the possibility of rolling the boat over. If the 670ish# are strapped between the pontoons at the waterline they can't do much to roll the boat but if that 670ish# is 3 or 4 feet off the floor and the boat is already rolled to 45deg by a wave, those 670# might well be enough to roll the boat over.
    Jerry Daschofsky likes this.
  4. David Dalan

    David Dalan 69°19'15.35" N 18°44'22.74" E

    That's interesting. It seems that larger weight rating pontoon boats use longer pontoons and not wider frames. I wonder if the net rating is exactly what you're pointing at...perhaps the draft that the boat is considered safe/useful is well less than 50% submerged. At 25% submerged, the rating and the apparent displacement of the tubes comes closer together.

    BTW I did go fishing and while I took a skunking, my buddy caught his first steel on a fly. Green Butt Skunk on a full floater no less.
  5. Ed Call

    Ed Call Mumbling Moderator Staff Member

    I believe most companies weight rate their pontoons to a 40% submersion point. I think the tubes dictate the rating, not frame width. The width may add stability as it spreads. The width may add maneuverability as it narrows. Neither, in my opinion, adds to the flotation capacity. When 40% submerged a craft rows like a PIG. At 50% I can only imagine it being more HIPPO like.
    David Dalan likes this.
  6. BDD

    BDD Active Member

    I hear that question all the time...what is the weight capacity of a particular pontoon boat? I usually respond with something like, "how much performance do you want to get out of it?" You can load it up past the manufacture recommendations but it won't perform nearly as well compared to reducing the weight load in half.
    Jerry Daschofsky and Blue like this.
  7. Blue

    Blue Active Member

    I was going to say "The industry standard is to apply certified weight to the boat until it reaches 1/2 of the draft of the boat"
    But I like BDD's answer.

    That's like seeing a 300+ lber riding a moped....that just ain't right, although it could work.
  8. Olive bugger

    Olive bugger Active Member

    You guys make my head hurt. Great thread. Thanks for twanging my thinking twanger this morning. I have nothing to base my opinion on, but I believe the gentleman with the engineering degree is correct in his apprasisal of the matter. Not that my opinion matters all that much.
  9. mtskibum16

    mtskibum16 Active Member

    Also remember that many of these crafts are rated for moving water (whitewater ratings even) which puts dynamic and impact loading on the whole system. I'm sure the annalysis they (the manufacturers) do is much more complex than just a pontoon water displacement formula. Even if it's not, you're forgetting a "safety factor" from your calculations. There's a good chance they have some sort of "safety factor" or "reduction constant" figured out that allows them to use a displacement type formula knowing that their "reduction" accounts for things like frame strength, moving water, rolling, waves, fisherman reaching out to land a fish, etc. A safety factor of 2-3 is very common in design for many things. If you like "living on the edge" you can usually get away with about doube a rated capacity (do this at your own risk)! ;)
  10. David Dalan

    David Dalan 69°19'15.35" N 18°44'22.74" E

    Oh no worries, more an academic curiosity :)