Building a Bruce Roberts Longboat 21

Discussion in 'Fiberglass and Composite Boat Building' started by alaskarog, Feb 22, 2009.

  1. alaskarog
    Joined: Feb 2009
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    Location: Eagle River, Alaska

    alaskarog Junior Member

    I'm new to this forum and will soon start building a fiberglass Bruce Roberts designed boat called the Longboat 21. Below is the link to the designers web page for this boat.

    http://www.bruceroberts.com/public/HTML/descriptions/LongBoat21_description.htm

    The designer is currently redrawing the plans for me to increase the overall length to 25'. I'll use the 4' of extra length as additional cabin space and will keep the cockpit the same as on the original 21' version. I'll have an enclosed head, decent galley and reasonable berths for several people for overnight journeys in the waters of south central Alaska. This is an inboard diesel powered displacement hull vessel. I'm probably going to build this with a foam core but might also consider doing it in C-flex if I can come up with some information on insulating a C-flex hull. Here in Alaska cold water , cool air temperatures, and cabin heat on are the norm, even during much of the summer. If anyone has any experience with building this particular boat or on effectively insulating a C-flex hull I'd sure enjoy hearing from you.

    Regards, Roger
     
  2. apex1

    apex1 Guest

    Welcome here,
    what is C - flex please?

    Regards
    Richard
     
  3. alaskarog
    Joined: Feb 2009
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    Location: Eagle River, Alaska

    alaskarog Junior Member

    C-flex is a fiberglass planking system that is laid over forms and saturated with resin to become the initial layer in a built up fiberglass hull. Additional layers of fiberglass are then applied on top of the C-flex to complete the hull lay up. See the link below for more info.

    http://seemanncomposites.com/cflex.html
     
  4. marshmat
    Joined: Apr 2005
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    Location: Ontario

    marshmat Senior Member

    C-Flex is essentially bundles of stiff but flexible fibreglass rods held together with dry fibreglass; it looks a bit like reinforced packing tape but is about a quarter-inch thick by maybe 6 to 18 inches wide. You tack long strips of it over a rough male plug skeleton, then laminate your fibreglass; the C-flex supposedly holds a reasonably fair shape and becomes part of the structure.

    It's a Bruce Roberts favourite; I've heard that it can be fairly easy to use but I've never been able to find it around here.

    Roger, I like the look of the Longboat 21; you'll be especially happy when you can sit around the fuel docks and gloat the next time gas hits a buck-twenty a litre ;) Above-waterline insulation shouldn't be too tricky, any of the flexible sheet insulations (Armaflex, etc.) made for steel/alu hulls ought to work just fine with solid fibreglass. Do be careful if you decide to do foam core.... it can work very well, but there are so many little details that can trip you up and cause trouble ten years later.
     
  5. apex1

    apex1 Guest

    Thanks Matt and Alaska. If B.R. prefers it i would have some doubts. Looks not too bad that stuff (resin to glass 1:1) but I dont buy their claims so easily. And seems to be a busy job with filler and fairing!? But if it´s proven.............
    If the boat (nice design) could be built in wood or plywood strips it would be much more rigid and better suited for the environment intended. A good foam is much easier to handle, but I agree very much with Matt, foam is tricky if one does´nt know howto!
    Armaflex, as mentioned is the ONLY one material of choice if it comes to insulation. It can be used on every boatbuilding surface, has never a risk of intrusion by humidity, is easy to work and costs a lot of money. But replacing the wrong stuff will cost five times as much! And everything else is the wrong stuff. We use it under, as above waterline even on wood Epoxy built Yachts!!! Look my Gallery.

    Regards
    Richard
     
  6. Kaptin-Jer
    Joined: Mar 2004
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    Location: South Florida

    Kaptin-Jer Semi-Pro

    I have built 2 Bruce Roberts designs one 'glass and one wood. If you are interested in an R value look into the many types of foam core. You will kill two birds at the same time. There are various types; 2" squares on a backing, strips--planks, Do some research see what is available in your area and if the reb will talk to you. Think about which core will give you the best finish, because you will be building over a male mold and if you are not careful the final finish can be brutal.
     
  7. Commuter Boats
    Joined: Oct 2006
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    Location: Southeast Alaska

    Commuter Boats Commuter Boats

    alaskarog, although I haven't used it I've heard of some difficult cases of faring a "C flex" hull as a result of shrinking between stations.
    marshmat & apex1 expressed caution in the consideration of foam core, and I agree, professional results are difficult for a part timer to obtain in female molding with foam cores. But I believe one-of male molding over foam presents the amateur builder with fewer challenges and addresses your requirements well.
    I'm a few hundred miles south of you ( southeast Alaska) and have built a few custom boats for the same operating conditions that you're in.
    I have a 40 and a 32 that were built with foam over a bird cage ( a structure only slightly more complex than what would be required for "C flex"), which have proven to be, attractive, stiff, light, low maintenance, and durable as well as being warm with minimal condensation.
    Considering the design challenges that I face constructing the type of vessels that I'm involved with I think this is a very good way to build a one-of hull and would prefer it to "C-flex" for your project.
    As Kaptin-Jer put it " You will kill two birds at the same time."
     
    Last edited: Feb 25, 2009
  8. daiquiri
    Joined: May 2004
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    Location: Italy (Garda Lake) and Croatia (Istria)

    daiquiri Engineering and Design

    Hello,
    It is probably a matter of my inability to translate the term "foam over a bird cage" - can you please give me a link to some text about that technique?
    Thanks. :)
     
  9. Commuter Boats
    Joined: Oct 2006
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    Location: Southeast Alaska

    Commuter Boats Commuter Boats

    Good morning, I don't have a quick link available and am a little shy on time for lengthy post so I'll try a quick description... A "C-flex" hull would be constructed by first erecting a male representation of the hull consisting of transverse stations, in order to support the foam of a one-of foam corded hull I set up similar transfers stations and fasten closely spaced, longitudinal battens over the transfers stations creating a male representation of the inside surface of the core. This form with its transfers and longitudinal members resembles a cage, hence its name " birdcage". The birdcage is then covered with a thin layer of plastic to prevent incidental bonding of foam core to the birdcage, and the foam is fastened to the longitudinal members with either screws from the inside or sewn on with cotton string. After the foam is fared, the outside skin is laminated, fared, and finished. A cradle is constructed to support the hull and the cradle, hull, and birdcage are turned right side up, the bird cage is removed, and construction continues with the inside skin and grid ( bulkheads and stringers).
     
  10. Commuter Boats
    Joined: Oct 2006
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    Location: Southeast Alaska

    Commuter Boats Commuter Boats

    I just took a few minutes searching for a link but as of yet have not found one, the form or birdcage for one-of foam construction, is very similar to the form a cold molded hull might be laid up on.
     
  11. alaskarog
    Joined: Feb 2009
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    Location: Eagle River, Alaska

    alaskarog Junior Member

    Commuter Boats, I'm curious about several things. First of all which foam core product have you used for the boats that you have built? Secondly, what product did you use for fairing the foam prior to beginning the laminates? Is it pretty easy to sand the fairing compound without sanding into the foam? It seems like the foam would be quite a bit softer than most fairing compounds?
     
  12. apex1

    apex1 Guest

    The foam is impressively rigid (pvc foam) and you have to sand the foam in shape!!! not a fairing compound!
    This might help a bit:
    http://boatbuildercentral.com/howto/foam1.php
    I any way would go for strip planking instead of foam (easier to work, cheaper, more forgiving, no delamination, no risk of sag during building etc etc.).

    Regards
    Richard
     
  13. Commuter Boats
    Joined: Oct 2006
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    Location: Southeast Alaska

    Commuter Boats Commuter Boats

    Hello alaskarog, What he said, kind-of...
    With all the different densities of foam as well as, cross-linked, linear, and blended PVC foams, a discussion of which foam to use where gets pretty technical. Your designer / engineer, Bruce Roberts should recommend the densities and properties for each part of the boat. It wouldn't be unusual to use five different foams ( thicknesses, densities, and different physical properties) on the same boat.
    As to fairing the foam, if while planking the birdcage, you detect a low area, you would shim the foam away from the birdcage and high spots are handled with a long board. Gaps, butts, and divots are handled with core bonding putty which will work easily if you can catch it early in the cure cycle with a cheese-grater ( Sureform File).

    apex1, thank you for providing that link http://boatbuildercentral.com/howto/foam1.php
    I was having trouble finding a website that illustrated that.
     
  14. alaskarog
    Joined: Feb 2009
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    Location: Eagle River, Alaska

    alaskarog Junior Member

    Thanks for the information guys! I've been on the boatbuildercentral site quite a few times and looked seriously at several of their boats before deciding on the Longboat. I should actually have the plans on hand within the next week and then can start seriously studying the details and starting to source materials. Any more information from those of you have done this stuff before and would like to share your experiences will be greatly appreciated. I've built a canoe and a kayak with plywood stitch and glue construction but have never worked with any of the foam core materials. I've also never worked with any resins other than epoxies so working with polyesters will also be a new task to master. Richard (Apex), your idea of strip planking sounds very appealing to me. I'd be much more comfortable working in wood and epoxy but would not want to do it without having a set of plans that were intended for that type of construction. I guess I could ask Bruce Roberts his thoughts on building it that way but I don't want to spend any more money on custom design work.

    Thanks again for all the advice, Roger
     

  15. apex1

    apex1 Guest

    Welcome both... Commuter Boats and alaskarog...
    Alaska, you might be happy to find, that the design will fit the strip planking method already with just minor modification, cos in principle the way to build is equal.
    Stick to epoxy !!! it is the more forgiving, easier to handle resin, and has the big advantage to stick better to any material than poly (lower delam risk). And you have your experience already in the pocket.:!:
    Have some very deeep thoughts about the material!!! You are familiar with wood and Epoxy, the wooden strip is much, much cheaper than foam, the hull is not as weak as a foam cored one while you have to turn the junk (read, stays in shape with less effort). Maybe the savings on your foam bill can balance some extra $ you have to transfer to B. Roberts for modding the design. :?:
    Lay back, have a beer and weigh out point per point......................
    During your construction lookout for bargain prices on Armaflex. AF Armaflex is the quality you need! Neither the wood core, nor the foam is a sufficient insulation in your Latitude! And 3 - 5 people below deck import a massive load of humidity below deck, on top of the natural developing moisture on your resin or painted surface! But believe me, it´s painful to pay that amount of money for the stuff, but worth every penny.

    Kindest regards, success and good luck
    Richard
     
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