12 ft boat foam core sandwich building questions

Discussion in 'Sailboats' started by yenice, Jan 8, 2020.

  1. yenice
    Joined: Oct 2015
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    yenice Junior Member

    We are a team of 2, thinking about building a sailboat similar to farr 3.7.

    The main parameters of this design are:
    Single handed trapeze sailing,
    Length 3.7 m (12')
    Beam 1.52 m
    Weight about 48 kg
    Mast: 6 m
    Sail area 8.8 meter square

    Our wights are 65 and 75kg. We are planning to sail the boat single handed, by using trapeze. But
    occasionally, we also think of sailing together, without expecting much speed, but more of a fun togethernin whatever wind and wave conditions we get.

    We want to construct the boat as light as possible, without compromising seaworthiness in final result.
    Having a light boat target will enable us to easily transport, easily reach planning speeds in lighter winds, building in lower cost and also construct with less work. Since there is no sailing class constraint, we can enjoy light construction.

    The sailing will be done mainly in the Sea of Marmara, but also in open areas of Black Sea, Aegean Sea,Mediterranean Sea and one or two lakes inland. The wind blows up to about 25 knots in those areas with waves of suitable sizes.
    I come from windsurfing, so, planing conditions will be welcomed, but we also want to make use of light breezes, of course with displacement sailing, in order to make our free time more on sailing.

    We are not interested in racing, but sailing fast or planning sometimes would be fun. We have no Farr 3.7 boat or class here, we also do not intend to participate in any class event officially with the boat, we just want to sail whenever we can, wherever we can.

    Since we are not limited by class rules and conditions, we can tweak the design as we want if necessary.

    The main objective is to build the boat as light as possible, without using carbon or expensive materials.

    ıf we want to use carbon we can, but at the moment we want to see what can be done by using justnfiberglass foam core sandwich method without exotics.

    I have never used Fiberglass before, but I have about 50 years of experience doing DIY projects, and enjoy building things. My friend has done construction building designs by using Google Sketch-up, but he has not done 3D free forms yet. We have access to empty ground level closed hall, with the front door sliding to bring large pieces like a boat in or out. A relative of my friend is also working as a carpenter nearby. The family of this carpenter has built wooden boats before, but he builds and sells furniture at the moment. Some wooden structures of our boat building will probably will need to be done by the machines of his shop.

    As we check the different home DIY building websites of Farr 3.7, they are all doing wood construction. As we can read from different internet sources, fiberglass foam core sandwich construction of these boats seems to be only done by professional companies, but not DIY at home.

    Blogs https://www.uk3-7class.org/blogs.html
    Monkeys Farr Story : Farr's new home! http://monkeyfarrstory.blogspot.com/2013/06/farrs-new-home.html
    Building a Farr 3.7 https://farr37build.wordpress.com/
    UK Farr 3.7 http://ukfarr37.blogspot.com/
    Dave's Farr3.7 Build Blog https://davesfarr37project.weebly.com/
    Farr 3.7 Building in Southern California https://socalfarr37.blogspot.com/
    Building a Farr 3.7 https://www.peterkovesi.com/home/farr37/

    I have followed the book "The Elements of Boat Strength" by Dave Gerr and my calculations resulted at the thicknesses of the sandwich for the areas of the boat sections as in the sketches.
    dimensions.jpg areas.jpg

    Strongback and keel foam thicknesses is assumed to be 8.5 mm, total fiberglass thicknesses for strongback and keel are also assumed to be 3.7 mm (both sides of core together). Geometry of these parts are assumed to be the same as in wood construction.

    When I use these thicknesses with areas, I end up the following weights for the fiberglass-foam-core sandwich building:

    Fiberglass density: 1500 kg per meter cube,
    Foam core density: 90 kg per meter cube,

    Deck Weight: 20.4 kg
    Hull Weight: 27.7 kg
    Bulkheads plus keel weight (no stringers): 8.6 kg
    Total sandwich construction boat weight: 57 kg


    1. Why amateurs do not construct this boat by using fiberglass and foam-core sandwich?
    2. Which method would give us a lighter boat (seaworthy) wood in fiberglass protection or fiberglass-foam-core-sandwich construction?
    3.How light can we build the hull in kilograms?
    4. If we use vacuum, how many total kg can we reach for this particular form?
    5.Do we need to change the number and size of the keel and strongback parts if we construct fiberglass-foam-core sandwich?
  2. TeddyDiver
    Joined: Dec 2007
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    TeddyDiver Gollywobbler

    To consider in this size the fiberglass skins required for structural strength aren't necessarily thick enough for point loadings. In that regard might strip planked core be eventually lighter..
  3. gonzo
    Joined: Aug 2002
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    Location: Milwaukee, WI

    gonzo Senior Member

    The Farr is a plywood hull. To get the same weight and stiffness you will need to use exotic materials and very precise laminates. I don't think you can do that with fiberglass. Is there a reason you don't want to use plywood?
  4. yenice
    Joined: Oct 2015
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    yenice Junior Member

    Class Rules https://www.3-7class.org.nz/class-rules.html

    Section 8. Weight, paragraph d states : " Extra-lightweight composite boats with provision for detachable prodder and gennaker shall weigh at least 36.5kg with no weight correctors."

    Therefore, I thought that we could go at least below 50 kg with fiberglass-foam-core-sandwich with vacuum, without using carbon. But if my calculations above are correct or not, I do not know. That is one of the reasons I summarized my results above for the crosscheck of experienced forum members.

    The book of Dave Gerr mentioned above requires at least 500 kg/m3 wood to be used in wood construction with at least 3 layers of fiberglass epoxy resin protection. When I compare these figures with 90 kg/m3 foam core, how will I reduce weight, I do not quite understand.

    We have a lot of sunny days per year and most of the wood boats need frequent painting and repairing jobs. Fiberglass boats need much less repairs. That is why I am inclined to foam-epoxy rather than wood-paint. If I used wood-epoxy to stop wood rotting and deterioration, how it will result in light-weight, is my question now.
  5. TeddyDiver
    Joined: Dec 2007
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    TeddyDiver Gollywobbler

    Three layers doesn't apply for boat only occasionally on water. Think about strip epoxy gf canoes..
  6. Mr Efficiency
    Joined: Oct 2010
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    Mr Efficiency Senior Member

    One-off GRP sandwich build would be far more work than desirable, unless you have access to an existing female mould(s).
  7. messabout
    Joined: Jan 2006
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    Location: Lakeland Fl USA

    messabout Senior Member

    You can build the boat of ocumee ply that will probably weigh less than the material combination you are anticipating. One of our members here is in the process of building a 4 meter boat that will perform in ways similar to your aims. Laukejas has done a lot of work on the design and has done the hydrostatic computations carefully. His bare boat will weigh 45KG. He will use a 9 meter sail. The boat has a double floor and open transom as is the current fashion for go fast boats. ( that is to assure that the boat is self bailing) The boat will use 4mm and 6 mm ply and have a fiberglass skin over the exposed ply.
  8. fallguy
    Joined: Dec 2016
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    fallguy Senior Member

    To determine the savings by vac bagging is rather easy. You just take the weight of the glass cloth and assume the resin at 100% of the cloth weight. Then you reduce the weight of the resin by say 30% and this is the savings; a better vac job might get you to a 35-40% reduction....but the reduction is not a lot for a small boat like the Farr.

    The reason people don't build these small boats in foam is it takes longer to do, costs are higher, and the gains are incremental and it is obviously harder and requires a jig or mould. But there is nothing wrong with building it in foam at all. However, you will need a female jig to form the hulls correctly and this jig would be best cut by a cnc operator with dxf files at the needed locations. Building in a female jig can be a little tricky and you will have to see if the bagged laminates would cooperate with the curve of the Farr. You might end up glassing the inside on a table and then bonding the entire thing on the jig and flopping it over to glass the exterior or some such. The boat is small enough to build on a male form I suppose as well. You could perhaps laminate the panel insides and then screw then to cleats on the frames, but you really need to think through all the steps. I am realizing I am trying to and don't want to!

    A foam build would be fun, but the deck might require some extra support (I don't know how it is supported or the dimensions, but ftmp foam might flex a bit more if you need to stand on the decks. This is easy to do.

    Most any boat done in tortured ply is capable to be done in foam.

    The weight assumptions people have been giving you are not valid. A sheet of okume 1/4" plywood is 21 pounds, hand laminating both sides in 12 oz biax (overkill) is 10.7 pounds; you can reduce by 25% for lighter inside glass, or 31.7 pounds or say 29 pounds going lighter. A sheet of 12mm corecell 5# foam with the same glass is 17 pounds hand laid 100% resin, 14.8 pounds vac bagged at 60% which is about max. So, you save lots. Now, this is going to cause all sorts of consternation on boatdesign.net because the comparison is not apples to apples, but glassing at 6 oz on a ply bottom is really thin.

    And, you can save more weight if you use a bit thinner core as you indicated. The hull weight won't end up at 17# vs 29# as a strict percentage, but these are the numbers I would use unless you need to go to 18 oz biax for the foam hull, in which case, you will be comparing 22.3# vs 29#. But a decent number for a rough estimate is 80% for the heavier laminate and 60% for the lighter laminate in hand laids and you will save about 25% of the resin or say 75% for the heavier and 55% for the lighter (vac bagging). (all vs 1/4" ply)

    Clear as mud? Foam will be somewhere between 55-80% of the weight of the ply boat. Some of that will depend upon how you tape the seams as well. Mat backed tapes are easier to use, but weigh more..etc. The ply boat will always be a stiffer boat, but more than needed.
    Last edited: Jan 9, 2020
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  9. fallguy
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    fallguy Senior Member

    As an addendum to my rather broken up commentary (dinner got in the middle of it), I would add, the bottom of the boat or stress areas need to be considered in at least the 12mm core. And, anywhere you intend for loads to be involved, you'll need to install a minimum of a 12 pound core. I don't know how the loading works on the Farr, but the mast foot, for example cannot be made with 8mm 5# density core, so this will add some more weight to your hull and must be considered and located before you laminate the hull which can be a little tricky.
  10. yenice
    Joined: Oct 2015
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    yenice Junior Member

    If I can make substantial weight difference, One-off GRP sandwich build even without a female mold could be interesting, very interesting for us.

    I have great respect for his work, especially when I consider that boat building supplies are not to be found in his country, he has reached such a distance with his boat. But, on the other hand, I like the shape and curves of Farr 3.7 and would like to stick for a while on the possibilities of this design.

    I asked one of the plywood builders of the Farr 3.7 for the weight of the bare hull and the answer I got was 53 kg. According to your above estimation, 55-80% will result in 29 kg to 42 kg for the hull. The average of these are about 35.5 kg which is near the NZ class rule for weight that I quoted above, that also makes sense to me. Me and my friend would accept to work a lot more than the plywood option and would also accept the extra cost associated, because the weight reduction would mean a lot.

    Alternative A:
    I was thinking of going the following direction:
    Free Forming in Fiberglass https://www.boatdesign.net/articles/free-forming-fiberglass/

    My friend can make a 3D data of the boat and then we can use that data to make any curved area be flatted on a table, glass it on table and then bring back on the female jig to put the pieces together. Vacuum on table and/or on the female jig-supported-planes.

    Alternative B:
    We could first build a boat, maybe the plywood boat. Use it for a season or two and see if we like it or not. Then we could use this boat as a plug to make the female mold for a second boat build. Then we could go to fiberglass-foam-core sandwich building path with vacuum.

    I am not experienced with fiberglass building. Just learning the basics by reading. The details of your above statements I can at the moment not grasp, my lack of knowledge, but I hope I will be able to get the feeling as time goes on. Everyday I learn new things from books and from this forum.

    My calculation for deck was 11 mm core where the mast foot will be placed. But your proposals are noted to be followed. Dave Gerr recommends real wood to be placed in high load areas. The details of incorporating these wood sections are also mentioned in Geugeon Brothers book on Boat Construction. These special details will be followed. I know these will increase the weight too.
  11. fallguy
    Joined: Dec 2016
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    fallguy Senior Member

    A female jig is not so complicated to build. I did one for a 32 foot waterline vessel with iirc 11 stations plus bow and transom. For the 12 foot far, this would be about 4 stations plus 2.

    The stations are not much different than male stations and the hull mucj easier to remove (generally).

    Here is a pic of the female jig I used for a much larger boat. This is a really fun jig because we could walk into it for the bulkhead placements.

    We had to screw the panels to the stations here and there to hold shapes and we used wooden washers 1-2" square to keep from crushing laminate. BF3664BF-9133-4871-931D-B02EC5440063.jpeg 06BC5F22-2F5B-4979-92F0-1E9895CF2C07.jpeg
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  12. fallguy
    Joined: Dec 2016
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    fallguy Senior Member

    Using plywood for the stress areas is wise. High density cores are very expensive and you won't use the whole sheet and the weight savings very small in say a square foot.

    I see some guys standing on the gunwhale sailing the Farr. This area, for example, you may wish to relief the cores on each side and then you could run double glass tapes there or you may consider the penalties for using plywood on the decks, for example.

    Remember the stiffness rule about thickness cubed. You will most certainly want a 12mm core on bottom and cockpit.

    Personally, I would build in foam in a female jig and I would hand lay all the glass. The gains for vacuum are just so small they could be made up by say single skin glassing one side and avoiding lotsa seam tapes. For example, you would glass all the panels on the inside. Then fit all the panels to the jig (or so) and then bond the panels and tape all the seams with say a 12 oz tape. Then you would put some placeholders inside the boat and hotglue them in to keep the shape.

    Then flip her over and glass the outside skin in a single go; perhaps overlapping on the centerline. Or you could relief and tape the exterior chines and then glass, etc.

    once you glassed the exterior skin; you flip again for the interior sidewalls and deck fitments.

    I would probably use 12 oz biax glass for the entire boat. It wets nicely and you will have time to squeegee excess resin a bit.

    It would be a great project and very fast boat I am sure.

    There is a new foam sandwich book coming out soon you might want to read.
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  13. gggGuest
    Joined: Feb 2005
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    gggGuest ...

    People home build in foam sandwich regularly. Its not a big deal, indeed easier than plywood.
    Have a read of this.
    Building in Foam Sandwich [UK-Cherub Class] http://www.uk-cherub.org/doku.php/tech/foam_sandwich
    Vacuum Bagging [UK-Cherub Class] http://www.uk-cherub.org/doku.php/tech/vacuum_bagging
    An interesting option I've seen done is to use prelaminated panels to get something approaching stitch and glue.
    Everest 1 Cherub https://www.uk-cherub.org/forum/index.php/topic,1487.0.html

    [Later]I suspect that the main reason so many Farrs are done in plywood is quite simply the existence of the superb CAM file to produce an exceptionally accurate and easy to use set of panels. If it weren't for that I imagine there would be more foam sandwich boats like other home build performance classes.
    Last edited: Jan 11, 2020
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  14. yenice
    Joined: Oct 2015
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    yenice Junior Member

    Thank you for your guidance. May I ask for name and writer of the new book you mention?

    Thank you for the valuable links. I checked them. The last link which leads to UK Cherub forum is closed to membership because of spammers. I posted a request to their facebook pages. If it comes, I will be able to see the related picture attachments of the building steps.
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