Built up bow question

Discussion in 'Fiberglass and Composite Boat Building' started by Scuff, Nov 19, 2021.

  1. Rumars
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    Rumars Senior Member

    I think my mention if G10 might have confused you a little. The G10 is just a high strength core, saving you having to laminate over foam. The composite chainplate looks nothing like the metal ones from your plans. Look at the pictures on this site, Owners / Builders Fusion Kit Catamarans http://www.fusioncats.com/members/mcg/chain_plates.php that's how your finished fitting should look. You adjust the angle of the metal pin to match the angle of your forestay. Carbon is not necessary, just use unidirectional fiberglass, layup thickness is until it looks right. You fabricate that thing out of the boat on a core (G10 or glassed foam), then you tab it into the bow with lots of tape. Over this you add some foam to shape into a bow.

    There are also other options like a G10 backing plate under the deck, drill holes and insert a dyneema loop with a dogbone.
     
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  2. Scuff
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    Scuff Senior Member

  3. Rumars
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    Rumars Senior Member

    If you go for the classic composite chainplate just build one with the required number of slots (number of holes from the plan), it's not any more work, the glass is laid in wide enough pieces, then the slots are cut into it.

    The loop method is conceptually the same thing as your linked ropeye, the difference is that it's not glued in. Instead of a deck thimble, you just make the hole larger and round it's edges so there is no chafe (drill at an angle for the forestay). The toggle underneath the deck (piece of metal bar) is housed in a miniature selfdraining locker. Very simple and cheap to make, looks very sleek and modern.
     
  4. Scuff
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    Scuff Senior Member

    Rumars, you've really set the wheels spinning!
    I really like the loop with the dog bone approach. Would you have any details on how the area is reinforced? The forward section is the anchor locker so self draining already cared for.
    Could the g10 approach also be used for a bow eye .. anchor snubber/sprit stay? It seems ideal for that as well.
     
  5. Rumars
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    Rumars Senior Member

    It's not complicated, the same as a normal composite chainplate, just flat. The deck gets a suitable high density core insert where the holes will be (G10 is best). When you laminate the outside of the deck you add a few unidirectional plies doing transversly (hullside, deck, hullside). Then you glue in a backing plate on the underside (G10). The toggle minimum diameter is governed by the the dyneema thickness, but it's best to go bigger, since it doesn't add significant weight.
    Yes you can use it for bow eyes, etc. same priciple, non crushable core insert for the hole, backing plate, some uni on the outside. For some load orientations you can also use another method, drill a hole in the stem from hullside to hullside, suitably reinforce and insert a loop.
    If you google pictures of "dyneema bobstay" you will find both methods.
     
  6. Scuff
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    Scuff Senior Member

    For my own misery and keeping in mind I'm an amateur builder how would I go about doing the math to get an idea of the laminate schedule?
     
  7. fallguy
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    fallguy Senior Member

    For trading the G10 for the Locust the schedule is the same as by designer. Probably the hull laminates plus the tabbing, but make the tabbing 2" wider per layer. Inside same.

    Something on the order of 3 layers of 1708 each side seems about right..that'd be strong enough to tow the boat by..

    For the sailing bits; not me..
     
  8. Rumars
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    Rumars Senior Member

    You just keep adding glas until it looks like it will hold, at wich point you are ridiculously overbuildt. Properly engineered ones are really thin actually.
    The hole trough the deck or side functions the same as a metal chainplate, the deck laminate takes the actual load. You need reinforcement in order to not crush the core and distribute the pressure from the toggle. The toggle needs to be thick enough to resist the bending force in the middle, and not to short. Adding another layer of glass to the laminate on the outside in the bow area is just a precaution.
     
  9. Scuff
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    Scuff Senior Member

    I'm going to use the g10 plate, the dog bone under the deck is really neat but this is a cruising boat and I think chafe would be a real issue.
    The plans call for 7/32" 1x19 with a breaking strength of 6300 lbs at 20% of that I get 1260 lbs. I'm going to use dyneema instead and sizing that for stretch at less than 10% of breaking strength (16000 lbs) 6mm is acceptable at 7%. Going up to 9mm gives just 4% stretch with a whopping 26000 lbs breaking.
    Since the bond of the uni to the plate is a shearing load I used 2200 psi for the epoxy .. the pro set rep agreed that was a good conservative value.
    Using 26000 lbs and a safety factor of 3 .. 26000 x 3/2200 I get 35.45 square inches of bonding area. Did I do that correctly?
     
  10. leaky
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    leaky Senior Member

    G10 has been mentioned a lot here, I don't know how the $$ works out but it's nothing special really is just fiberglass.

    When I run into these needs I make my own using basic polyester resin, a flat sheet is very easy (but I make angles, T, various things) - wax a table, layup a few layers (ie up to say 5/32 thick), let cure, layup a few more - any size or thickness you want. For fast build up something like a 24 oz woven roving between layers of 1 ounce csm, is an easy way to do it.

    G10 is known to be harder to bond to, no idea why, but if you make the same with a polyester resin hand layup you can bond with any resin system.

    If I want a full surface cure, say I'm gluing onto it later with epoxy or its going to sit around a long time before use I'll generously roll waxed gel or resin on the top - flattens it out and makes it sand easy too. Bottom side of course always requires cleaning and prep/sanding as it contacts the mold release.
     
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  11. Rumars
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    Rumars Senior Member

    Do you want to use a "conventional" external composite chainplate? If yes just make the uni bands go as low on the hull as possible, that will take care of your dilemma. For example, 2" wide band, 20" long is 40sqin, and that's just one band going 10" on each side. You will certainly use more then one band and fan them out, so by the time you put three or four bands, the safety factor exceeds 10.
     
  12. Scuff
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    Scuff Senior Member

    forestay chainplate.jpg forestay overlay.jpg Ok dragging this back up. The main hull is laminated and I'm ready to work this out. I attached a pic of the fullsize pattern provided for the chainplate. The second pic is a simple plywood template for a composite plate. All three relief's for the stay and tack are 1/2" deep. The forestay slot is 5/8" and the tacks are 1/2" widths. I measured a turnbuckle for the forestay slot but guessed on the tack openings. There's 12" of length for the uni and I made the landings wide enough for about 10* off angle. Please let me know if I'm on the right track here. I was thinking 3/8" g10 plate and 1" 11oz carbon uni laid up first then installed/tabbed into the hull/bow bulkhead.
     

  13. Rumars
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    Rumars Senior Member

    Looks good from here. The angle depends on mast position, if you have a scale drawing of the rigging you can take an approximate measure Wich is close enough.
     
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