Bulbous bow addition

Discussion in 'Fiberglass and Composite Boat Building' started by Frosty, Aug 9, 2011.

  1. Frosty

    Frosty Previous Member

    My Bulbous bows are not a very good design. I have modified them to see if I was on the right track and I was, increasing speed and control.

    Now I know this,-- it is obvious the design is needing some modification making it lift and not dive causing steering probs over 22Kts.

    I have been advised to use foam from the air con man A & b mix, I have a design in mind but I don't see how I can just form a shape in mid air being much bigger and longer and even higher.

    A man once told me he could do it but wouldnt tell me how. 3 years later it came to me -yesterday in fact while I was sitting with my feet up.

    What if I get a sheet of ply and rip it up into 1 inch strips and placed one on the bottom of the hull held by a piece of wood jammed in and two more forming the sides, some more strips (ribs) stapled in place and fill the void with plastic bag and dump the A&b in the temp ribbed up mould.

    This would give me time to measure up and make them in line with hull with correct height etc and in line with each other.

    If that was made a little larger to cut it down and sand it then glass it over.

    Is that how you would do it?

    I hav'nt used foam before, can you push it about with your hands and make it do what you want,--with gloves on of course, or is it iether pourable or not.
     
  2. PAR
    Joined: Nov 2003
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    PAR Yacht Designer/Builder

    Build it in "lifts" like a loaf of bread. You can buy sheet foam or make your own in a simple plywood mold. Shaping it as it expands with your hands isn't practical at all. Foam is very easy to shape once cured and several tools make quick work of it.

    You can strip plank a bulb mold from plywood too, though I think this is more work then just carving up the foam until it fits shape templates you've made.

    I just made sugar scoop extension for a sailboat, that used a foam mold for the 'glass work. I used 2" thick foam, glued into a stack about 24" tall. I made 1/4" plywood templates for checking the shape as I whittled it down with regular wood working tools. Sand a little, check for fit, sand a little more, check for fit, until it's there. Once I was there, I cover the foam in plastic packaging tape and applied 3 layers of 12 ounce biax. When this cured, I removed the foam and completed the layup on the inside.
     
  3. Frosty

    Frosty Previous Member

    I cant buy sheet foam here --its A&B mix or nothing. How do you attach the foam to the hull and each other. Your saying once you got the mold and glassed you then take it away, how do you glass inside something as small as a sugar scoop?

    I was intending to leave the foam there.
     
  4. masalai
    Joined: Oct 2007
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    masalai masalai

    Easiest is to make the foam in sheets, glue together with epoxy (or the SAME system used in the manufacture of your boat) and do the final shaping that is your mould...

    The brown-tape is to release the glass skin from the foam, so only one mould is made for the two hulls, then with a bit of care in marking and aligning, both bulbs will be identical and hopefully retain equalised balance...

    Leave the bottom open to do the glass-taping inside... then place a bit of ply (nicely covered with GRP on both sides) inside with a couple of cup-hooks to hold the weights whilst the epoxy tape on the inside cures... Then glass over the external side of the ply and fair that nicely... Then tape, fair and fully seal with GRP - do the final fairing, then paint and antifoul...

    The engineering (for strength is your affair)....
     
  5. Frosty

    Frosty Previous Member

    Its not that big mass. It might only be inches bigger than original in places. I have no design or drawings or template,--I don't do that I just shape and look at it with my head cocked to one side with one eye closed till im happy with it.

    As long as I destroy the down force of the top of the bulb that is not letting the boat ride up at high speed.

    The factory is aware of the problem but wont talk about it but I got some one to tell me they did the same as me but in foam and a bit more than what I did.

    I would not know how to cut the foam until I saw it on the boat, hence my idea that I would be able to 'tweak" the ribbed mold before pouring foam.

    Its not far off now and will be the center of the mould, at a guess it will be foot longer and turn upwards instead of down. At the moment the bow looks like its upside down.

    It might be possible to just cut it off at the end and invert it, but if im gonna do it ille make a better job of it and maybe a speed increase, and hope fully that silly spray that comes right up to the bridge at 22KTS. So I want to beak the water further forward of the bow. I'lle know it when I see it.

    Its not a very good picture for the bulbs but you can see the additional modification to the top but lots of upper surface causing a dip down still left to cure.

    Any one care to think I should remove them completely?
     

    Attached Files:

  6. daiquiri
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    daiquiri Engineering and Design

  7. masalai
    Joined: Oct 2007
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    masalai masalai

    I dunnow, there is foam and there is foam and some is really useless to shape or glass - are there any shops that make surfboards of those "surfboards that they use to sail the surf with a kite? - they should have access to the right stuff... I still suggest make it into layers/sheets glued together ... even if you leave it inside...
     
  8. Landlubber
    Joined: Jun 2007
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    Landlubber Senior Member

    You can make them from styrene foam (Koolite), paint with water based paints, glass over that, then dissolve the foam with acetone, quick and easy way to make hollow glass jobs.
     
  9. Frosty

    Frosty Previous Member

    Cant get Koolite, cant get water based paint, don't want to remove the foam.

    The foam will act as a cushion if I hit something. The bulbs there have foam in them.

    The only foam I can get gentlemen is A&B from the air conditioning shop, which I am told by a guy that just completed a cat is the stuff to use.

    I probably need about 2 square foot each side.

    Just had a thought while I was typing --what about wire mesh?

    Its not the materials it the method.
     
  10. masalai
    Joined: Oct 2007
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    masalai masalai

    Nah, wire mesh will rust/corrode or suffer electrolysis etc... Find the stuff, shape it and before doing the final fixing grind off the gelcoat and paint and whatever, so the epoxy has a key onto which it will happily adhere.... and then play with high build and orbital and other means of sanding a fair shape... Show regular pictures of progress...

    Ask Leo he is into that sort of stuff - reducing the wave generated by a hull shape... He may be able to show you some pictures of shapes that may be helpful...
     
  11. Stumble
    Joined: Oct 2008
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    Stumble Senior Member

    Frosty why not build a box large enough for the part, and pour a&b foam to fill it. This way you get a block that can be shaped to fit what you need. It is a little wasteful, but it shouldn't be that expensive.
     
  12. Frosty

    Frosty Previous Member

    Thanks Stumble thats another way. I saw a lovely plastic salad bowl in the cheap shop and I thought it would be a wonderful nose to the bow, bit like a nuclear submarine.

    I bought 2.

    I can think the job thru in my head but still a little dark area on how the stuff handles.

    I mean how long does it take before it starts to expand, how long in time does it expand, how long does it take to stop expanding, how long does it take to solidify and work on, if the outside seems hard will the inside be too.

    Is it toxic , do I need gloves, does it give fumes, does it damage paint, will it come of my hands.

    Being A&B can you mix differently to adjust activation times or is A&B non adjustable. How critical is 50 50, can it be guessed near enough or has to to be measured accurately.

    Can the mixing bowl be cleaned out and re used, is a solvent needed to clean up --what is it?

    When its gone off will it come away from plastic sheet or polythene salad bowls or does it stick like chocolate to a blanket.
     
  13. masalai
    Joined: Oct 2007
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    masalai masalai

    Some stuff I used, almost instantaneous, expands into foam as it mixes... do a small test... say a shot glass of each... I would guess that heat hastens the reaction...
     
  14. Stumble
    Joined: Oct 2008
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    Stumble Senior Member

    Frosty,

    Check out http://www.uscomposites.com/faq_foam.html

    I haven't worked with it much, just some screwing around with 2pm stuff, but I would guess that an 8lb foam would work for you. The FAQ answers most of your questions, and I am sure the manufacturers would be happy to answer anything it doesn't cover. Though it also appears there are different types of foams, about which I know nothing.
     

  15. brian eiland
    Joined: Jun 2002
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    brian eiland Senior Member

    Are you sure it is the bulb bow that is forcing your bows down? If I remember correctly you have an unusual drive arrangement that has your prop shafts more inline with the CB, thus creating very little moment arm to rotate vessel's bow up or down. And that CB can move with your fwd motion. So I would look at this first, before blaming the bulbs totally.

    Does the factory use the same drive system as yourself?

    Can't really tell the shape from the photo you've supplied

    Really? Not even at a home supply store? I would have to think someone in that country uses sheet foam of some type. Gluing it together to make certain size blocks, the carving those to your desired shape seems the easiest for this sort of experimentation.

    Just another thought, how about those 'foam blocks' they use in packaging computers and big screen TV's ?? Should be some scrap of that around? Or if they build refrigerators over there those guys often use sheet foam, not pour in place.

    Does the vessel EVER have any tendancy to go bow down in seas such that the wingdeck would pound....then I wouldn't remove them...maybe lenghten like you are thinking. Bulb Bows are special for each case.

    I don't think your modification of the bulb bow is going to help with you 'climbing spray' problem at the speeds you are talking. Rather you may have to resort to some sort of spray rail
     
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