Composite bulbous bows

Discussion in 'Fiberglass and Composite Boat Building' started by fallguy, Apr 5, 2023.

  1. fallguy
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    fallguy Senior Member

    I am seeking advice and input on how to build a composite set of bulbous bows (BB) for the Skoota 32.

    DETAILS
    I've got professional assistance on the sizing and need for the BB. The boat has about a 10" difference between CoG and CoB. I did all the calculations and can only modify the existing boat and pickup 4" of change through major efforts inside. This is not to become a thread for others to bash the need or the fact it is needed. The person assisting me is not a composite builder and has asked me to seek guidance from forum members.

    I will have drawings at some point, but would prefer a more theoretical discussion early on.

    Some of you may understand I posted another thread about how to takeoff and build a hull replica. That effort is to support this one so that I can build in the shop, if possible.

    The existing hull is built in the front with a combination of solid frp and a transistion to 12mm foam core. The core is Corecell M. The solid frp is 5 layers of 17 oz biaxial or 600g/sqm. This was done because the bottom of the hull is rather narrow forward and has some curvature that would have been hard to fab in foam. The first 3 layers of solid glass were hand laminated up onto vac bagged core panels. When I flipped the boat, I vac bagged two additional layers on in situ over a relief in the exterior core that was planned to simplify fairing and this work was exceptional quality, all epoxy.

    The existing foam core has a light triaxial both sides which varied a bit during the build from two different vendors, but is about 900g/sqm evenly spread between the layers ftmp. The core was all vac bagged per panel and the finished panels set in a jig the next day.

    There is not a ton of structural crossmember forward. The hulls have 7 main bulkheads and really the BB is only crossing two of them. One of the decisions is whether to tie the BB to the existing crossmembers or not. This would require cutting access holes and unfortunately both are inside watertights and only accessible from outside and the hole cut. I am leaning toward not holing the hull. If people think it necessary; we would sacrifice existing hull integrity, so need to really establish why so, if so.

    The other decisions are all general construction issues. Since the BB is rather small, we need to determine how to build and connect it to the hulls.

    @Eric Sponberg had an article in Professional Boatbuilder that @bajansailor shared with me, but I'd rather not share that just yet or without permission from Eric.

    For me the major challenge is building something super small. And also determining point of failure. If we lose the BB at sea, but the hulls have integrity, the boat loses about 30% of her bdeck clearance but would make it back to port. So, an obvious question is do we remove all the paint and bond this thing with epoxy? Or do we take our chances and bond it with something like 5200 which will take weeks to cure over such a wide surface?

    Anyhow, if you made it this far, please let me know how you might build this thing. I expect this discussion will evolve as ideas are accepted.

    My basic thought is to build the BB and then bring it to the boat completed and use epoxy paste to bond it on. This would mean the BB is perhaps solid frp on the bonding surface and probably a set of webs inside tabbed to the solid frp and then skinned with foam core panels epoxy bonded to the webs without tabbing. Then the forces of the sea slamming this thing have some potential to break the webbing from the skin unless perhaps the bottom of the BB is skinned; then the webs tabbed and then the top of the BB is bonded without tabbing.

    Years ago I build Dierking inspired amas. They are solid foam (xps). That foam is not good enough for this project and would delam.

    Thanks.
     
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  2. gonzo
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    gonzo Senior Member

    I think you should call Woods Designs. It will be the cheapest route since they designed it. Backwards engineering your boat will be expensive and the results not as reliable.
     
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  3. Barry
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    Barry Senior Member

    Reprinted from the Woods Design Website, wrt to the first Skoota 32 built
    "Easy to reach 20 knots even though the trim wasn't quite right."
    I am curious as to how they solved this problem and if it was a bow down trim

    There were some earlier threads on CG and CB and I am under the impression that the center of buoyancy acts through the center of gravity for any object. Otherwise there would be movement. If the design CG and the design CB at 0 trim is 10 inches off, then the hull position in the water will change to get the CB lined up.

    There is no doubt that the BB will add more buoyancy to the front of the hulls but will it
    increase drag? Your hulls have an extremely fine entry to minimize drag. Certainly in a boat with blunt bows, the gain with BB's is significant but I am not sure if I have seen them on fine entry hulls.
    I am not criticizing your decision to do this, just wondering.

    ( different for planing hulls when moving of course)
     
    Last edited: Apr 5, 2023
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  4. fallguy
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    fallguy Senior Member

    I was quite clear about what I asked for. If you have contributions related to composite construction; please let me know.

    I was actually looking forward to your contributions Barry as I believe they have been the best thus far. Thanks.

    The business of beating the design up can be done elsewhere, but best avoided here. The bows need to be fuller for amateur builders and American fitouts. Richard is a minimalist and that does not always fit all builders. The business of a snappy reply and reference to Woods is not of much value. I have 90 days to build the BBs. The issue is bow down trim.

    I got 2.2nm/gallon on 35 hours, but am tempted to add a flow meter to one engine to find a sweet spot. We rarely ran on plane with break in 10 hours, a small narrow river and no wake zones and the beaching keel needs some modifications, yet.
     
  5. fallguy
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    fallguy Senior Member

    As for the drag questions Barry, my boat is about 6.5" down in front which means there is approximately 14" of additional drag already. This means 14" width of bulbous bow could be added without much penalty. The hulls have rocker, so bow down trim means a lot of water is plowing against the rounded bottom as well. So, the drag added may net to zero..

    But, here we digress.

    Still hoping to hear your thoughts on building these..
     
  6. fallguy
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    fallguy Senior Member

    Here is a shot or two of the bow.
    99098F8A-E319-4F2C-8817-255B0C2CDBF5.jpeg B2E1D120-7990-4F88-B6F9-ED4BFC1C0C35.jpeg 2EB1B012-DB0C-46AA-BEE5-AD6C78C4DA7D.jpeg 2A0A8DF0-8221-4694-B88D-EAB51179940C.jpeg E15CDD8F-9A02-44BC-AC75-880642CA584D.jpeg
     
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  7. fallguy
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    fallguy Senior Member

    Here is an image of what is planned; more or less.

    79DF0906-6985-4AEA-9184-D95A75A0B6E4.png
     
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  8. bajansailor
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    bajansailor Marine Surveyor

    That scum line waterline in your photos does speak volumes!
    If you can build a fairly accurate moulding that can be literally just glued on with epoxy, that could be the way to go - if you then hit some floating debris while underway at speed then in a worst case scenario the bow extension would simply 'break off' hopefully leaving the original bow intact.
    Even if it does not break off (eg the front just gets crumpled) it might be easier in the long run to simply build a new extension and bond it on, rather than trying to repair the crumpled area?
    Especially if you have a mould for it, and building a mould might be worthwhile as you need two extensions - or are the bows slightly assymmetric?

    Edit - re your other thread about fairing the front of the beaching keels, it looks like the bulbous bow in the drawings above will come back as far as the beaching keel, so maybe the beaching keel could be just 'faired in' to the bulbous bow?
    Modifyng the beaching keel https://www.boatdesign.net/threads/modifyng-the-beaching-keel.67970/
     
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  9. fallguy
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    fallguy Senior Member

    The hulls are drawn symmetricly. Any differences would be in fairing. I used a chain jig during fairing to match the two hulls, but some potential exists for minor differences that would be within grinding or paste differences I'd say.

    That scum line is with no weights forward and solar removed. Even a light aluminum anchor was kept back and no bow pulpit added.
     
  10. wet feet
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    wet feet Senior Member

    This isn't a modest undertaking on any level,but will be a great deal less trouble if the hulls are symmetrical,as the one mould will have a chance of working for both hulls.It is slightly more difficult for a catamaran with a fine entry than for a bluff bowed monohull.I totally support the idea from bajansailor to leave the existing hulls intact and bond on the new sections.An amount of fairing may hide a few minor discrepancies if there is a hefty dollop of bonding paste over a large enough area to provide lots of strength.

    What may be the most challenging aspect will be creating the flanges for the mould sections to join together and to leave access for laminating.It may be simplest to split the bulbous section down the centre and the section from the hull trimmed to match.You could them use a a vertical central flange to bond the two halves together.It would be extra insurance to have a large enough hole to get into on the aft face so that the joint could be reinforced with additional glass.
    Holding the new bows in contact with the hull while the bonding paste cures will call for some creativity.Maybe ratchet straps tucked aft of a mooring cleat and a few shores from the ground would be a start.

    For assistance with understanding the project,what total length and width will the new sections be?
     
  11. gonzo
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    gonzo Senior Member

    If you build the bulb as a separate section, there is no need to laminate. Several adhesives would work. For example, epoxy, 5200, Sikaflex, etc.
     
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  12. Rumars
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    Rumars Senior Member

    I would tack xps with hot glue to the hull, then carve the shape I want into it with saw and sandpaper. This plug can then be used on the bench to make a mold, into wich you can laminate. Alternatively you can copy the plug into a high density PU foam block and cover that with fiberglass.

    To attach I would glue it with thickened epoxy. While other glues work, epoxy is simplest to use since it requires the least pressure for a good bond.
     
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  13. fallguy
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    fallguy Senior Member

    Thanks to all contributors.

    My current plan is to

    1. Make a female plug with expanding foam based on Barry's tub idea. Will tape the hull and wax the tape and spray the hull with partall. Need to go up as high as the top of the BB. Will make a double tape line at dwl.

    2. Build a hull replica by filling the female plug with epoxy and then wax and such and foam.

    3. Build the bb with fiberglass and core using the hull replica in a controlled environment,

    4. The contact area of the BB will be solid glass. The bottom keel will be laminated core. There will be some structural web inside. The BB is basically a vee hull with a vee top to shed water. I will tab all the webs to the keel and solid frp and the bottom vee. Then I will cap the top section without tabbing.

    5. Then fair and paint the BB. I am tempted to go all antifouling.

    6. Then epoxy bond it to a sanded hull.


    The hull replica is needed because my shop is away from the boat and we still have lots of snow and rainy weather. And I need 2! Maybe I can use some xps to use less expanding foam.

    Anyone willing to help spec the glass and core?

    I happen to have quite a bit of 6mm corecell M on hand. Would that be thick enough?
     
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  14. wet feet
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    wet feet Senior Member

    I would have some concerns about persuading corecell to follow the shape of a bulbous bow,even if applied in sections.I'd also be concerned about a sandwich laminate with a 6mm core being exposed to impact with bodies while moving at speed.It will inevitably be the point of impact and needs to be able to shrug off a minor contact,but it may come into it's own as a form of crumple zone if something really substantial gets walloped.I would tend to use a reasonably heavy woven cloth with the warp aligned fore and aft for as much of the structure as can easily be reached as an impact will take advantage of the weave to pull each strand against the fibres running around the bow which is a bit more encouraging than depending solely on the peel resistance of the resin.It might be worth making a sacrificial section,perhaps 6-8 inches long at the tip over a foam core and keeping a mould on the shelf with a replacement in it.The actual section that provides the buoyancy might as well be a similar layup to the original hull as it will be assuming much the same duty.

    I admit to a preference for a female mould for this type of thing,split down the centre and with an aft section that duplicates the hull shape.Which means you can work through the open aft end-with some difficulty.I'd use a 3/16 radius around the aft end,that could be faired to the existing hull with an epoxy/microballoon mix before painting.I would expect a bonding flange width of 1 1/2 to 2 inches would be more than adequate and subject to a dry fit proving successful,I would probably use Plexus to bond the piece in place.
     
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  15. fallguy
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    fallguy Senior Member

    plan is to make the hull contact area solid frp and then a frame/keel web and core on developable surfaces

    the original hull is solid frp on the bottom and transistions to 12mm corecell at about the waterline

    not sure I understand your idea well
     
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