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Thread: New Tri Hulls for the Grumman 17

  1. #1
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    Default New Tri Hulls for the Grumman 17

    A busy couple of days has seen progress on my oft threatened new hulls for my Grumman 17 tricanoe. The former set will go to a gentleman in Ely, MN., USA. by way of a friend from Canada who will swing through here to pick them up. Lots of great memories will go with them.

    https://www.youtube.com/watch?v=koblflGI5nQ&t=6s

  2. #2
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    Can you explain why you have blunt bow and stern?
    looks like it will just cause more drag. IMHO

    How much buoyancy?

  3. #3
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    If you mean the 3 inch flat portion at the tips, that is really just for strength. Having sharp points is fragile. These hulls will get banged up, and it usually happens out there at the ends.

    If you are referring to the areas below the waterlines, they're really not that blunt. Rather, they have an ogive, similar to a full jacketed 9mm bullet. That is, the the shape fades in angularity til they come to a flat along the keel line that is about an inch wide. Again, I didn't want a fragile "edge" with no plywood under the glass like my old design. I will be running up onto gravel beaches and rough shorelines around here quite a lot.

    Buoyancy? I have only a guess, based upon a calculation someone on the forum did on my other set. SInce the amount of foam is a bit more on these than the others, I would hazard a guess at around 230lbs. each. Please, if any one has a better calc, I'm very interested. The basic dimensions are 10ft x 1ftx x 6 inches. I can only guess at how much material was removed in the shaping process.
    Last edited by OutnBacker; 13th-July-2017 at 01:24 AM.

  4. #4
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    Okay, buoyancy.

    The foam panels are 2.7cu ft each. There are two panels in each hull, minus the amount shaved off. Water weighs about 64lb per cu ft. So, each panel displaces 168lbs of water. Guessing that the second panel lost about 30% of its volume in the shaping, I'd estimate the total buoyancy at around 286lbs per hull. Reduce that for the weigh of the hulls, which I will estimate will be 20lbs and you have 266. If I'm even close, that's not too shabby for bouyancy.

    Let the violent rebuttal begin...

  5. #5
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    Okay, buoyancy.

    The foam panels are 2.7cu ft each. There are two panels in each hull, minus the amount shaved off. Water weighs about 64lb per cu ft. So, each panel displaces 168lbs of water. Guessing that the second panel lost about 30% of its volume in the shaping, I'd estimate the total buoyancy at around 286lbs per hull. Reduce that for the weigh of the hulls, which I will estimate will be 20lbs and you have 266. If I'm even close, that's not too shabby for bouyancy.

    Let the violent rebuttal begin...

  6. #6
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    A way of estimating the buoyancy would be by using weight and density to calculate the volume.
    Assuming the float is all foam, weigh the float prior to glassing and subtract any extras e.g. the weight of the studs that protrude for fixing to the cross beams also weight of glue etc. if significant.
    Find the density of the foam by weighing an undamaged sheet and divide by its volume or check Dow specification.
    Dividing the nett weight by the density will give the volume, then multiply the volume by density of water to give buoyancy.
    This should give the minimum figure, any voids will be a bonus.

  7. #7
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    Thank you Watermark. That's kind of what I did, using an on-line calculator. The panels are spec'd at 2.7 cu ft from Dow. The amount of foam that was cut off at assembly and shaping is about 30% of one panel - best guess by weight of waste mat'l. The hulls used four panels going in.

    Add the glass, resin and the steel studs, based on my last set of hulls - I'm going with 266lbs displacement.

  8. #8
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    Looking fwd to the onthe water report.

  9. #9
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    Me, too. It'll be a couple of weeks I'm afraid. Lots to do around the place with the good weather now here at last. I'll glass starting Saturday morning and marathon it all weekend. If I get it done, painting can be done after work the second week. Ready by the 22nd, I hope.

    If possible, I'll video some of the glassing process if it's not too inconvenient. My hands will be a sticky mess, alternating between dipping into a denatured alcohol bath and handling the cloth, the roller, and the squeegie. Perhaps I can get my wife to be the videographer if I promise to watch my language.

  10. #10
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    Get some gloves - don't put your hands in that stuff. Epoxy or alcohol.
    Gloves are cheap - do you have a Harbor Freight near you?

    I knew a guy who kept doing that with a lot of different chemicals - he now has little growths all over his fingers - sensitive too.

    I'm not overly concerned with epoxy exposure - but its easy to avoid.

  11. #11
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    I've been wearing good nitrile gloves, be assured. I have at this point most of one pontoon done. Having a lunch break. I keep getting interrupted by neighbors out for a chat. They like to wander into my shop when the bay doors are open. I should probably not play Led Zeppelin so loud.

  12. #12
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    Step by step pics of the pontoon build for those who may be interested:

    EDIT: On another forum, I was correctly advised that these albums were not visible and were most likely in Private. Here are the Shared albums...sorry...

    Part 1
    https://photos.google.com/share/AF1Q...R1akFtVzE2WDZn

    Part2
    https://photos.google.com/share/AF1Q...JOY184WjB5RmJR
    Last edited by OutnBacker; 27th-July-2017 at 03:50 AM.

  13. #13
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    Why not just build them out of plywood?

  14. #14
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    Hello Stu,
    I've built two stitch and glue fishing boats and was happy with the results. Plywood is a good option, but I like the free hand aspects of shaping the foam. Perhaps my next set will be plywood.

  15. #15
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  16. #16
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    A good day at Baker lake, testing the new pontoons. Not much wind til a bit later, but enough to check the tacking ability and basic stability. Tacking was flawless at near calm speeds and at 5.2mph, very little submersion of the lee tri hull was evident. I had hoped to see winds that would result in speeds of about 6.5 to 8mph, in order to observe the turbulence. Based upon what I saw, I don't expect much, even in large waves.

    https://photos.google.com/photo/AF1Q...FDALcP3lhQTZAd

  17. #17
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    Quote Originally Posted by OutnBacker View Post
    A busy couple of days has seen progress on my oft threatened new hulls for my Grumman 17 tricanoe. The former set will go to a gentleman in Ely, MN., USA. by way of a friend from Canada who will swing through here to pick them up. Lots of great memories will go with them.

    https://www.youtube.com/watch?v=koblflGI5nQ&t=6s
    Outn'Backer,

    Just had a wild thought on covering a set of outriggers/floats out of styrofoam.

    If you got some spray on (or roller) bedliner that was safe on styrofoam, would that work about as well as the fiberglass?

    It is tough, waterproof, strong, easier to put on than fiberglass, maybe even cheaper considering the costs of fiberglass cloth, resin and hardener. Plus it is not brittle. The only downside I can see is that it might be a little tricky to get a color other than black.

    https://www.youtube.com/watch?v=2GNCV6lf_6A

    It would also cut the build time on the floats down considerably. Don't know if a guy could control the surface texture 100%

    There are many suppliers of this stuff too. I think it would stand up to the sun exposure better than fiberglass and you don't need to paint it.

    What do you think?

  18. #18
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    Eagles Nest Lakes, Ely, Northeastern Minnesota, USA
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    I think it looks promising and worth experimenting. Why not give it a try?
    SB

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