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Thread: Some simple raft engineering

  1. #1
    New member
    Join Date
    Apr 2006

    Default Some simple raft engineering

    Hi Folks,

    I hope you enjoy this, I wasn't sure where to post but this seems as good as any. I'm a professional engineeer, been lurking a while (years) and have Mike's great book. Got interested after floating the Gulkana with the scouts and taking my family down the Lil Su. Unfortunately I'm interested in the hunting part and it's an expensive hobby.

    So I was looking to build a frame myself and use the maxxon tubes. As an engineer I had to calculate the actual load carrying capactiy as best I could since the specs seem to be inconsistent. So I found the tedious equations and beat them out.

    Then I ran across this site

    you don't have to put in name or date and I just use 1 and 1 for reference and case.

    It has a pretty handy user interface to simplify figuring out the actual water displacement for at least a cat. Once you get the gallons of displacement you just multiply by 8.345 (lb/gal). If you choose gallons as your unit.

    As an example, for a leopard, I used a 10 ft waterline, I assume that is conservative since as you displace (add weight) your going to bring the tapered ends into play. And assuming that your max displacement is 30% (very close to 8 inches on a 26.5 inch tube), then you would get about 73 gal of displacement per tube resulting in a load of about 1218 lbs (includes boat and load), at 10 inches your almost 100 gals of displacement or about 1670 lbs of load.

    A round boat would be more complicated and an inflatable floor more difficult still.

    I'm guessing and hoping Mike will have fun with this in small return for the great book!


  2. #2

    Default Cataraft VS Raft Load capasities

    Ryne, thatís a great website link for calculating displacement of a cylinder! Thank you for bring up this topic and using the approach that is so straight forward.

    At first glance the calculations in your posts show the load capacities of an 18í cataraft tube to be quite impressive.

    However there is one assumption that is made in your post that I feel would be a good point for discussion. That Assumption is the handling characteristics of a river craft thatís drawing 8 inches of water are not greatly compromised when itís used floating a river.

    The moving dynamics of a river tells us that the surface currents of a river mover slower than the currents just 4-5 inches below the surface. Often being able to catch an eddy or being able to ferry or surf your craft into a more desired part of the river depends on being able to ride or surf those slower surface currents. A craft that draws more water will often get blowing downstream. This is one of the reasons that the same craft will handle better with a light load than it does with a heaver load.

    In short all river crafts start to handle like a real dog when it starts to draw more than 5 inches of water. The deeper the draw the more doggy the performance of the river craft

    When it comes to rafting over shallows and riffles often a craft has to glide over 2-3 inches of water. When a river craft gets hung up on the shallows (depending on the current speed) the water will pillow up on the upstream side of the craft. This pillowing effect can add another 1-3 inches of water to carry the craft downstream (if there is enough surface area on the bottom of the craft to take advantage of this pillowing effect). The more cross current width impeding the current, the wider and deeper of a pillow the craft will create.

    This is why a raft will often be pushed through the shallows where a cataraft often has to be lightened and or dragged.

    Iíve listed on the attached PDF file the displacement comparisons between a cylinder (cataraft tubes) and a flatter surface (river raft floor)

    The bottom footprint of the 420ST. (14í bucket boat) is 7íx7í this is before the kick on each end. Keep in mind that as the raft draws more water, the water line will lengthen. And the increase of cubic inches surface area on the water of a river raft is far greater than the increase that the cataraft tube kick will create. The difference between a bucket boat and a self-bailer river raft, for the first 6-7 inches of draft, makes very little difference in displacement.

    You can see in the PDF file chart that at 5Ē of draft the 14í river raft (1273 pounds) has twice the displacement of the 18í cataraft (626 pounds).

    The shallower the draw of the craft the more dynamic the load capacity difference between a river raft and a cataraft.

    You can also see how a cataraft draws substantially more water with every pound loaded on the craft and that the reserve capacity of a cataraft doesnít really kick in until it starts drawing 8 inches and greater. Yet the river raft has much more displacement at every level.

    This is all just food for thought. I was thinking that this thread wasnít getting nearly the feedback it deserves.

    Iíll see you all at next monthís outdoor and sportsman shows.

    Jim King

    Alaska Series Inflatable Boats,
    The Toughest Bottoms in the Business,
    River Rafts, Catarafts, Inflatable Kayaks,
    Inflatable Canoes, Inflatable Sport Boats,
    Inflatable Jet Boats, Tenders and Dinghies.

    Attached Images Attached Images
    Attached Files Attached Files

  3. #3
    New member
    Join Date
    Apr 2006


    Hi Jim,

    That is great information. I used the example I did because I had read somewhere that a cat turns into a dog at about 30% of the tube displacement.

    Those round boats carry a pretty good load! And the shallow water issue and "pillowing" are very positive advantages for the round boats. One big issue for me has always been the real estate available for loads. When I see those round boats loaded like Santa's sleigh I start researching cats. Then I lean back to round boats for convenience and the folks here aren't helping me make a decision because of all the good info supporting both. I think I need one of each

    Of course the engineer in me wants to know where your round boat numbers come from. Is it a software program? If so, can it handle a super leopard? That comparison would be interesting.

    take care

  4. #4
    Join Date
    Aug 2006


    "I think I need one of each." ----- Now you're thinking.

    I like rowing cats better than round boats, especially in technical water, 'cause they go where you point them. Round boats are always sliding sideways in less than predictable ways, but for tripping I prefer them because they are easier to set up and draft less water. Best to have both.

  5. #5

    Default Round Boat With inflatable floor?

    So, What does this mean for my round boat that has an inflatable floor? Does that mean it is like a round boat untill I sink the inflatable floor part, and then it is like a cat?

    Also, what does this mean for my 14' Kenai Drifter that is kind of like a cat and kind of like a round boat? All I know was that I really like rowing it with 1-2 people with gear, but when I put 4 grown men and gear in it for 6 day float down Lake Creek... it felt slightly better than a barge.

    I always seem to overload the boat, like last summer when I put two adults and three teenage girls and gear in it and floated the upper gulkana. So what can this enginering tell me?

  6. #6
    webmaster Michael Strahan's Avatar
    Join Date
    Apr 1999
    Anchorage, Alaska


    Quote Originally Posted by AK flyster View Post
    So, What does this mean for my round boat that has an inflatable floor? Does that mean it is like a round boat untill I sink the inflatable floor part, and then it is like a cat?...
    No, it's like a bucket full of water! It's just a heavy slug at that point.

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