Adding flotation

Discussion in 'Fiberglass and Composite Boat Building' started by catsketcher, Apr 26, 2020.

  1. catsketcher
    Joined: Mar 2006
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    catsketcher Senior Member

    I designed and built a sweet little sailing trailer sailer. She is a folding catamaran of my own design. I got the weight study and the hull lines a little wrong and she sits too heavy down aft. I would like to foam up the stern sections by adding foam up to 40mm thick in the aft sections, tapering to zero at 40% from the bow. I was thinking of using XPS or polyurethane foam. Has anyone done this and is there any advice on pitfalls to avoid and glues to adhere the foam to the hull skin?

    cheers

    Phil
     

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  2. bajansailor
    Joined: Oct 2007
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    bajansailor Marine Surveyor

    That sounds like a lot of work, to get some extra buoyancy aft.
    She does look very nice 'as is', and to my eye at least, she does not appear to be sitting too low in the water at the stern.
    How deeply are your transoms immersed?
    If she sails well, and you are otherwise happy with her, it might be easier to just paint a nice waterline (with a boot top as well?) an inch or two above where she is currently floating?
     
  3. Mr Efficiency
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    Mr Efficiency Senior Member

    That sounds like a splendid idea, bajansailor, if the boat sails well, just take the painted waterline down to where it actually is ! It reminds me of the song about always taking the (good) weather with you, anywhere you go. Adding belts of buoyancy, will end up looking like a battleship with belts of armour at the waterline. It would be a nightmare to try and fair in.
     
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  4. Ike
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    Ike Senior Member

    I agree with bajan. Looks fine to me. Nice looking boat. Adding flotation to the stern would probably not help a lot. If it's sitting low in the stern I would say it needs to have a change in weight distribution. Reduce weight in the stern if that's possible.
     
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  5. Mr Efficiency
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    Mr Efficiency Senior Member

    Certainly adding weight right forward in a cat can alter trim, with little mass, but there is the snag of it affecting pitching motions adversely.
     
  6. bajansailor
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    bajansailor Marine Surveyor

    And since you have started this thread, and hopefully you have now been convinced that most folk on here think that your waterline looks fine, can we see some more photos of your splendid cat please?
    The more the better!

    Re Mr E's comment above - hence you should look hard to see how much weight you can take out of the back end, if you are still worried about it.
     
  7. catsketcher
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    catsketcher Senior Member

    Thanks for the nice comments about the boat, as her designer and builder I appreciate it.

    One of the problems with a cat is when the boat is sailing to windward the leeward hull gets pushed downwards. So cats have to be designed to have the hulls get lighter (windward hull) and deeper (leeward hull). With the boat's wineglass hull shape I really need to get some extra flotation in her stern sections when sailing to windward in a big breeze. She is a trailer sailer so I can pull her out on the trailer (she is beside the house right now) and turn her upsidedown. I already have put weight forward a bit but weight in the ends is a killer for multis. I need to foam her up.

    As for boot stripes - she has none. I like the plain white dinghy look.

    cheers

    Phil
     
  8. Mr Efficiency
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    Mr Efficiency Senior Member

    I guess if its out of sight, underwater, it doesn't matter that much with appearances. Can you add a short extension to the hull ?
     
  9. Mr Efficiency
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    Mr Efficiency Senior Member

    Looking at it, a little extra added length would not offend the eye, even if it was more a bustle, than a "solid" extension.
     
  10. trip the light fandango
    Joined: Apr 2018
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    Location: Rhyll Phillip Island Victoria Australia

    trip the light fandango Senior Member

    I have done something similar to a tremolino,, I added about 200mm of polyurethane to increase freeboard and build in floatation in case of hitting something. I plan to solo my boat and poke around basalt coast line. The daggerboard case took a lot of time, in my case I should have added beam more so than depth . . 2 layers of Polyester with chop mat then 3 coats of epoxy. Fairing the bow was time consuming.
    I have experience with fibreglass but far less experience with boats than most people here.
    My next perhaps slightly hair brained idea..
    I plan to add more to the stern and have a plumb bow, but I wont use foam, making a form by bending on cheap 3mm melamine laminate screwing it on then laying a light glass . Then take it off, glue the glass on to the prepped surface of the original hull, re use screw holes ,alternately removing then filling with resin mixed with silica. then build up layers of glass to finish
    I will remove some rocker so a mini keel will be glassed in to take loads underneath the form.
    The steps would add the buoyancy.

    The polyurethane became slightly damp although not adding much weight after being on the mooring for a 5 years , possibly absorbed from the inside ..all polyester..
    I'm not an expert, but doing it this way means inspection holes can be made in the original rocker void created which works for me ,, although I can cut through my original hull ,remove foam and do the same. I'll need to move the mast aft if and when I go through with this.
    I slowed my trem down a bit[no significant extra length] but I feel more at ease on my own, now I just have to not fall off... Hopefully this is of some use to you.
     
    Last edited: Apr 30, 2020
  11. Mr Efficiency
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    Mr Efficiency Senior Member

    trippy, I got tired just reading about all that work, I think I'd rather build another boat and be done with it ! :eek:
     
  12. trip the light fandango
    Joined: Apr 2018
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    Location: Rhyll Phillip Island Victoria Australia

    trip the light fandango Senior Member

    Yes I think I may have some issues.. The time blowout has been amazing for my alterations, as a result the cost has been eaked out and not really that noticeable.
    Edit.
    Plus there is no way I could justify the money for a fibreglass 25 ft + trimaran, and tremolino's are very simple and built to last,
     
    Last edited: Apr 28, 2020
  13. kapnD
    Joined: Jan 2003
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    kapnD Senior Member

    Both the OP and Trippy need to get handy with their camera skills!
    Boats are a very visual thing, and words often don’t do justice without graphic illustration.
    Let’s see it!
     
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  14. BlueBell
    Joined: May 2017
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    Location: Victoria BC Canada

    BlueBell Ahhhhh...

    Phil,

    CatSketcher, as you've become known around here.
    Nice design, nice build, nice going. Wow.

    You want to add up to 40mm of foam over the aft 60% of the hulls.
    I'm really surprised you want to do this.
    How much weight will this add, at what cost and over what period of time?

    I have to assume you've already moved everything you can forward, even if only a couple of inches or feet.
    I would relocate a tank before I started what you're proposing.

    Have you found your error in calculating your initial weight and balance?
    It would be interesting to know the numbers on that error.

    There has to be a better way on this.
    How many hours do you have sailing her?

    We need more info including pictures, sketches, drawings, doodles, video... anything.

    Cheers
     

  15. trip the light fandango
    Joined: Apr 2018
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    Location: Rhyll Phillip Island Victoria Australia

    trip the light fandango Senior Member

    I've shown my progress on recycling beach cat hulls, the best glue to use is adding fumed silica [mix it inside a large clear plastic bag] with what ever resin the hull is made out of.
    You pretty much have to tie yourself to the mast blind folded so as not to look at the beautiful singing sirens on the isle of lesbos,[wanting more length] which I struggle with. Touche with the trippy moniker..ha .
    Also I would like to be able to lower my mast without sinking the transom.
    That is a really nice looking cat, very impressive.
     
    Last edited: Apr 28, 2020
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