Flotation foam or not?

Discussion in 'Boat Design' started by captainstick, Nov 12, 2021.

  1. captainstick
    Joined: Nov 2013
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    captainstick Greg

    I am building new fiberglass hulls for my small houseboat. The old hulls are foam filled plastic. They have not faired well over the last 7 years, are leaky, and stretched and cracked mostly due a quite heavy load bouncing around on the trailer.

    So everything with the new build is going pretty well and I'm going to start producing hulls in the next month or so. My plan is to use the existing plastic hulls as molds. It's convenient because the resin won't stick to the polyethylene and I know all the specs for the existing hulls. The new hulls are going to be partitioned into 8 sealed segments, and then glassed together as 2 large 30ft hulls. I've run a lot of tests with various fabrics and layups and the overall weight of just the fiberglass will be similar to the weight of the polyethylene. But it should be quite a bit stronger, especially with the 7 vertical bulkheads planned into them where the segments join together. The old hulls only had 2 bulkheads/ 3 segments.

    So my question is should I add the flotation foam back in for safety? The foam is about 300 lbs per hull, 600 lbs total. It would be nice to drop the weight. Another thought I had was to cut the foam in half and just glass back in the bottom half only. If I don't add back the foam, I thought I might add a drain through the bottom of all the bulkheads and use a bilge pump like a regular boat, but then I'm betting on the bilge pump being stronger than any potential leak(s).
     
  2. Ike
    Joined: Apr 2006
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    Ike Senior Member

    If the compartments are sealed then flotation foam is not required. It would take a catastrophic failure to sink it. Pontoon boats are not required to have added flotation anyway. Although, separating each pontoon into multiple sealed compartments is a good idea.
     
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  3. fallguy
    Joined: Dec 2016
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    fallguy Senior Member

    Sealing the compartments is not without issues.

    You need to make sure the operating temperature range of the hulls is not too severe if you intend to seal them.

    Sometimes, people use surg tube to equalize the pressures, but to me that defeats separate wstertights.

    My hulls will be subject to a range of temperatures from -30F to 130F which is a 160 degree swing.

    I holed 11 watertights and have purchased these:

    McMaster-Carr https://www.mcmaster.com/breather-vents/
     
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  4. fallguy
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    fallguy Senior Member

    Mine are not really subject to splash; fyi.
     
  5. bajansailor
    Joined: Oct 2007
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    bajansailor Marine Surveyor

    I presume that this is a follow on from your previous thread -
    riverbus hull rebuild https://www.boatdesign.net/threads/riverbus-hull-rebuild.65980/

    With this boat -

    Pontoon boat - Captain Stick.png

    Re using your Plastic Pontoons as moulds for your new hulls, are you going to keep the circular cross-section of the hulls, or will you make the sides vertical, to give extra buoyancy, and get away from the danger of having rapidly decreasing buoyancy once you pass the half way point of a circular section pontoon?

    If the hulls will be of circular cross-section, then you will have to build each hull in two halves.

    I am pleased to see that Ike has confirmed that you do not need to have foam if the compartments are sealed.
    Note Fallguy's very valid comments about the effect of a huge range of temperature - I am thinking that you could also experience quite a large range on your river / lake in Colorado?
    @fallguy which type of vent did you purchase, re your link above?
     
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  6. fallguy
    Joined: Dec 2016
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    fallguy Senior Member

    Submersible threaded breather..nylon plastic

    I tested them and they don't leak water in, but they allow air to move..

    @DogCavalry gets some credit and I forget who recommended them to him
     
    Last edited: Nov 13, 2021
  7. captainstick
    Joined: Nov 2013
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    captainstick Greg


    That’s very helpful thanks. I thought about this issue in terms of altitude. I think I calculated that there is a 2.5 psi difference between where I live in Colorado and sea level. 2.5 psi can wreak havoc on cans, jarred food, pontoons? But that device looks like a great idea. Thanks
     
  8. captainstick
    Joined: Nov 2013
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    captainstick Greg

    My plan was to keep the same shape for the hulls. I had no issue with the shape, just the material was too soft for the abuse I was putting them through (mostly on the trailer). It’s hard to tell in the picture, but it’s a complex shape and the sides are vertical at the water line. They radius back in pretty close to the top. The hulls have more buoyancy than I could ever need at 20k lbs. The boat weighs about 8k full of fuel. Since the new hulls are being formed on the outside of the old, I was pleased to discover that extra .25” will net me another 800lbs of buoyancy.

    I’ve run some tests and I can glass the entire hull, upside down, except for the top. My plan is to make a plywood mold for the top of the hulls using the aluminum angle where they attach to the boat. Then the top will be sure to fit when it’s time to reattach the hulls. Then I will fit each section into the top as i assemble them.
     
  9. hoytedow
    Joined: Sep 2009
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    hoytedow Carbon Based Life Form

    Maybe redo the trailer bunks.
     
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  10. fallguy
    Joined: Dec 2016
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    fallguy Senior Member

    Wouldn't it be easier to epoxy repair the old ones and skin them with another layer of glass using epoxy and call it good?
     
  11. captainstick
    Joined: Nov 2013
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    captainstick Greg

    i thought about that. It’s pretty difficult to get anything to stick to polyethylene. I could weld the plastic to the point where it was perfectly sealed and didn’t leak. But then a few more trips down the road and new cracks would open up. I found that the plastic got a little brittle wherever I had welded. Maybe my technique was bad, but welding plastic is a strange art. Not many people out there doing much of it other than pipe welding with equipment perfectly calibrated to the task.
     
  12. captainstick
    Joined: Nov 2013
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    captainstick Greg


    I ordered a new trailer up to the task. The old trailer was definitely a part of the problem.
     
  13. bajansailor
    Joined: Oct 2007
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    bajansailor Marine Surveyor

    Captain Stick river bus.jpeg

    Is there distortion in the photo, due perhaps to a wide-angle lense, or are the hulls hogged slightly (the port hull appears to 'droop' a bit towards the stern)?

    In view of how the hulls 'radius back in pretty close to the top', how are you going to separate the new fibreglass hull from the plastic mould - or will you build the hull in two sections, and then glass them together?
    Note the cross sections shown here re your plastic pontoons - I think you said that yours are 36" diameter?
    36-inch Diameter Pontoons – Plastic Pontoon https://plasticpontoon.com/36-inch-diameter-pontoons/

    Re your estimated extra 800 lbs of buoyancy from a 0.25" increase in hull thickness - I think it might be worthwhile checking your sums again here. This sounds a bit optimistic to me.
     
  14. captainstick
    Joined: Nov 2013
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    captainstick Greg

    Yes, you spotted the distortion where the hulls are bending down towards the stern. That got worse over a few years. They also eventually distorted down at the bow as well. Both issues are a big part of the reason I am starting over with new hulls.

    Yea, I have the 36” plastic pontoons. I am only glassing 3/4 of the hull over the plastic. The top will be a separate mold. So I can just pry the glass off pretty easily. I’ve tested it and it works well even without mold release.

    Math is not my strength, but this is how I came up with the extra buoyancy…
    (.25” / 12” per ft) x 30ft Len x 11ft dia = 6.875 cu ft x 64 lbs displacement per cu ft = 440lbs x 2 hulls = 880lbs. I realize I will only be able to functionally access less than 1/2 of that, but I was surprised how much that 1/4” helped the cause. Unless my math is bad, which it very well might be. And let’s hope the highway patrol doesn’t bring out a measuring tape to discover I’m over width by 1/2”.
     

  15. fallguy
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    fallguy Senior Member

    I am sorry; I read poorly and assumed polyester/glass hulls.
     
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