Bruce’s Pontoons

We were planning to use Bruce’s pontoon boat that he scored somewhere around the Lake Shasta area as the base for our shantyboat.  In fact, it was Bruce’s pontoon score that gave us the green-light for this project.  Not so sure about this now.  Every boat has a buoyancy, a certain weight that it can support safely.  Bruce writes:

Sorry, I don’t know why I had a block on doing this for awhile. The pontoons are sort of irregular shaped, they are flat on the top and sort of u-shaped on the bottom. The top to bottom dimension is 17 1/2″. They are 18 1/2″ wide.  As I’ve mentioned before, the platform is 16′ long. The pontoons extend another 18″ to the rear of the platform. They also extend about 24″ to the front of the platform. However, in the front they are tapering down to be much more narrow, expecially in that last foot.

I don’t know how easy it is to calculate this. During high school geometry I had a crush on the Castellucio twin sisters (Karyn and Kathleen) and most of my mental energies were taken up by trying to flirt with the two of them rather than on the substance of the coursework. I haven’t really revisited the subject of geometry (nor, alas, the Castellucios) since that time.

Okay, I’m gonna make some assumptions about Bruce’s pontoon boat.  Doing some crazy math that requires me to get out my geometry book again, I guesstimate the total cross-sectional area of the pontoon is about 2 sq ft.  To get the volume, we multiply that times the length. We’ll say the length is about 18.5 feet. So the volume of one pontoon is perhaps 37 cu ft.

We just look at one pontoon, because we want to make sure that the capacity we work with never exceeds the floatation of one pontoon, lest a shift in weight drive it under the water and flip the craft. This situation even has an ominous-sounding name:  Pontoon Effect.  Plus, if you think about two pontoons, you want them both no more than half in the water.

To get the floatation, we multiply the volume times the weight of the same amount of water. Thanks, Archimedes.  One cu ft of fresh water weighs 62.4 lbs. So the flotation of one pontoon is 37 cu ft  * 62.4 lbs/cu ft = 2308 lbs. So the weight of the boat, plus the passengers, plus their gear and stuff, plus some margin of safety 10 to 25% should not exceed 2308 lbs.

Probably too tiny for our heavy shantyboat. However, we can build Bruce’s pontoon out to be a little hillbilly sun porch to accompany our shantyboat.  Something like this…

Originally posted 2012-04-16 10:04:44.

Wes Modes' irrepressible sense of adventure has lead him to decades of train hopping and DIY rafting on a half dozen major American rivers. In various lives, he is a sculptor, writer, performer, adventurer, comic artist, and most recently a shanty boat maker. The chronicles of his ongoing personal journey to build a shanty boat can be found at


  1. I recently went through the “how much pontoon do I need to convert a 22′ travel trailer to a shanty boat” question.

    One commercial pontoon manufacturer talks on their site of a pontoon draft of 1/2 of the diameter (50%) of the aluminum pontoons they manufacture.

    Another site which sells plans for plywood pontoons (essentially rectangular) speaks of a draft of 1/3 (33%) the height of the pontoon, the “recommended draft according to U.S. Coast Guard 33%.”

    (I have not verified that Coast Guard recommendation.)

    The 1/2 of the diameter draft rule seems to correspond to this article’s “lets calculate it for one pontoon” rule and then toss on some unspecified safety margin.

    The 1/3 height rule seems to add a specified safety margin to that.

    Using that plywood pontoon site’s formula, but modifying it to 50% draft, the hypothetical gross supportable weight would be about 2330 pounds – close to the article’s calculated 2308 pounds.

    Using the 1/3 draft rule (33%) the estimated load for two 18.5′ pontoons at 2 square feet cross section would be reduced to about 1,480 pounds.

    That site runs calculations for a boat with 2 – 12″ x 12″ x 8′ pontoons.

    Their calculations:
    ((length x (width x 1/3 draft))
    96″ x (12″ x 4″)
    96″ x 48″ = 4608 cubic inches displacement per pontoon.

    4608 cu. in. x 2 Pontoons = 9216 cu in.

    So at a 4 inch draft, the two pontoons at 33% draft will displace
    9216 cu. in. of water.

    1 Gallon of fresh water = 231 cu. in.

    So 9216 cu. in. / 231 cu. in. = 39.89 gallons

    39.89 gallons X 8 lbs per gallon = 319.16 lbs.
    (The actual weight of water is 8.34 lb/gal.)

    So the gross supported weight of the 8′ x 1′ x1′ pontoon boat, with passenger and supplies would be approx 319 lbs.

    Taking that calculation, then, for a rectangular pontoon, each linear foot of 1 square foot of pontoon will support about 20 pounds at 33% draft.

    Using that figure and the estimated cross section of pontoons at 2 square feet, those 37 feet of pontoons would support about 1,480.

    Note: At a 50% draft and a 2 sq foot cross section, that calculates to about 63 lbs per linear foot, which calculates to about 2330 pounds – close to the article’s 2308 pounds.)

  2. FWIW, my current Shantyboat dream involves a 22′ travel trailer floated on some pontoons.

    The trailer I’m looking at has a gross vehicle weight of 4,070. Adding 1,000 lbs for 5 people at 200 each, and 6 gal hot water+40 gal fresh +32 gal waste +32 gal gray +40 gallons fuel = about 1,080 lbs, plus 200 lbs misc (optimistic?) plus 120 pounds for 2 small motors — giving me a net load of 6,470 lbs.

    Using the plywood pontoon calculations, I’d need 3 pontoons of 22″ x 22″ x 30′ feet long (to support the load. (Givien me 8′ of deck space to be distributed fore and aft.)

    (The source of the plans says not to increase their 1′ x 1′ cross section….)

    My idea is that I can spend most my time as a pontoon shanty boat, but when needed, take the trailer off the pontoons, throw the pontoons and “platform” cross pieces on top and trailer it (or park it) as needed.

    A lot of the trailer’s interior is constructed with composition board, probably MDF with printed “wood” veneer.

    I’m thinking of replacing it with alternative light weight (cedar?) cabinetry and getting the gross weight down a bit.

    Dreamin’ my life away….

  3. When we calculate our gross weight, we take the weight of the materials that make up the boat plus 8 people at 140 pounds (the Canadian Coast Guard standard, apparently), plus 25% excess capacity.

    The pontoons you are considering building are 1’x1′? Nowadays, commercial pontoons are 24″ diameter, as much as 26″. It makes a HUGE difference in terms of flotation to have bigger pontoons. Travel trailers are already made just about as light as they can be. With bigger pontoons you would not have to worry about shaving off a half ton or so.

    We started thinking about a full hulled boat when the advice on the boat design forums was that a full hull gave us more buoyancy, more room, higher freeboard, and more stability.

    • >>The pontoons you are considering building are 1′x1′?

      Sorry I wan’t clear – the website I was using to get the formula to base my calculations on sells plans for a 1′ x 1′ x 8′ and gives an illustration using those dimensions.

      I figured for the weights I am anticipating I’d need 3 22″ x 22″ x 30′ pontoons.

      >>We started thinking about a full hulled boat when the advice on the boat design forums was that a full hull gave us more buoyancy, more room, higher freeboard, and more stability.

      Yes, indeed. I’ve also been dreaming about a Friendship by Theil.

      The things I like about the trailer idea are (i) much less construction work and (ii) it could be used as a shanty boat or, throwing the pontoons on top, hauled from place to place on the roads.

      >>Travel trailers are already made just about as light as they can be.

      I don’t know about that – it sure looks like they use a fair amount of heavy MDF throughout.

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