Building a 12' cold-moulded dinghy

Discussion in 'Wooden Boat Building and Restoration' started by Graham Tapper, Mar 29, 2019.

  1. Graham Tapper
    Joined: Mar 2019
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    Location: Devon UK

    Graham Tapper Junior Member

    Doug Lord and fallguy like this.
  2. fallguy
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    fallguy Senior Member

    I enjoyed your video Graham, a bit dark and a tickity sound wil not a Pulitzer bring, but I did enjoy it.

    I also do like the boat. The longitudinals seem like they would look nicer as a last layer, but I gather you will paint the boat at the end.

    The boat has a good bit of freeboard amd would make a heluva life raft.
     
  3. Graham Tapper
    Joined: Mar 2019
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    Location: Devon UK

    Graham Tapper Junior Member

    Fallguy,
    Many thanks for your responce and glad you liked the video. A frank opinion and feedback is quite good to have.
    I havn't decided yet on the final finish but I hope to leave at least part brightwork to reveal the fact its wood and not plastic!
    The freeboard is a little high and follows the Firefly design in this respect, appart from the transon area. The transom shape adopts the section of the original design but is taken from the section 16" forward of the original with the idea of beefing up the area and giving more support to the outboard.
    Graham.
     
  4. Dejay
    Joined: Mar 2018
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    Dejay Senior Newbie

    Very beautiful! I'd be curious what the difficulty is with clear coating. I've seen some builds that were painted white afterwards which seems like a shame to hide the beautiful woodwork.
     
  5. Rumars
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    Rumars Senior Member

    Nice work. Cascamite resin is really old school, most would have used epoxy. Remove the staples as you go, only removing under the plank to be installed next. Please keep the build balanced, if you plank one side completley then do the other side you could warp the mold.
    Read the Gougeon book, it's available free for download, and is probably the best resource for cold molding: https://www.westsystem.com/wp-content/uploads/GougeonBook-061205-1.pdf
    Good luck and keep us posted.

    Dejay, the problem with clear coating is twofold: most people use epoxy and epoxy is not UV stable so you need to keep on top of the varnish and second is wood veneer species, for varnish only without epoxy you need good ply from a rot resistant species wich in turn is heavy and expensive.
     
  6. fallguy
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    fallguy Senior Member

    Although he has not told us which ply he is using, okume is not heavy as a rot resistant marine ply. If it were me, I'd be inclined to run the longitudinals last for the look of it. Of course, those have butt joints, so perhaps this is unwise (function first, unless you scarf them). I have a cold moulded 1960 Carver Commander with the final layers sideways and I finished it brightsides and tops. We put 8 layers of varnish on the sides and 20 layers or so on the tops. I also redid a Windward 15 and we put okume decks and gunwales on it and the sides are painted and the decks are bright. This is a lot less brightwork, but it still shouts wood boat. I am assuming some sort of gunwale here?

    I do believe, GT is planning on moving the boat side to side while he builds, but your advice is rock solid on messing up his mould.
     
  7. Rumars
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    Rumars Senior Member

    Okoume is not a rot resistant species, and ply made from it is also not rot resistant. Meranti is in theory, the problem is what actually goes into the ply. Mahagony has the same problem.

    I mentioned the mould wraping because from his post #16. To me it seems he wants to continue planking on the starboard side. IMHO the longitudinal layer is unnecessary. This is a 12' dinghy with a seagull after all. 6mm is enough, no need to go main battle tank on it.
     
  8. fallguy
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    fallguy Senior Member

    I concur about the thickness and the approach. My Carver Commander runabout is 16' long and there are 10-11 plies on bottom and 5 on the sides, but they plywood plies, not layers of plywood! So more like 3/8" and 3/4" dims. So, comparitively, he would be building 9 plies (assuming 3 in 3'm) on the entire craft which is a much smaller and much lower horsepower. However, to be a little fair, there is something lost in plywood when you cut it narrow. That is to say he doesn't have 9 real layers.

    It might be best to do a weight calc. The thing should be fairly light when done and each successive layer is not adding much to strength vs weight.
     
  9. Rumars
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    Rumars Senior Member

    Yes he does not have 9 layers contribuing to the load sharing even if he does 3 layers of 3 ply. The inner layer of the three ply is basicly a core. But this is not a powerboat slamming into waves. The increased thickness might be desirable from a scratch&dent point of view. Structurally it's dead weight. Even if it's installation is desired I would not do it longitudinally but transverse. Longitudinal stiffness is provided by the keel and gunwales. This beeing such a short fat hull there will be no problems longitudinally. Even in long slim hulls (think multihull) the solution is to add one or several stringers. The three main longitudinal members are kept in column by the skin. The diagonal veneers act like braces. If more layers are required the best way to use them is transversal at 90°.
     
  10. fallguy
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    fallguy Senior Member

    The only real way to know is to do a deflection test, but I have some experience working with wood and agree the 9mm thickness seems excessive based on my limited experience with various plywoods and a strip built canoe. 6mm makes for a very stiff boat and I inclined as much in an early post. My experience is limited to with glass. I would be inclined to want to build it with 6mm of ply and a layer of 6 oz glass. Not sure if glass is needed here with 9mm.
     

  11. Graham Tapper
    Joined: Mar 2019
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    Location: Devon UK

    Graham Tapper Junior Member

    Conclusion and Observations: This was a big effort for me; the most ambitious woodworking task I have undertaken. Most enjoyable of course was the fitting out of the interior and varnishing, seeing the result of all that labour. The most tedious was the fitting of all those strips of ply and pulling out thousands of staples. Epiphanes was used on the interior. Epoxy on the outside. The but joints of all the strips were scarphed on a diagonal line to give ample glueing area without loss of any strength. The three layers of ply give me a hull thickness of 12mm or 0.5in which may sound too much, but I am delighted with the result. She doesn't feel overweight at all and is quite robust. The freeboard is perhaps a little too high and the buoyancy results in the bow riding out of the water when under power with one up. The tiller extension allows me to sit forward enough to get the balance right, but with two/three/four up I am confident she will be ideal. Today I shall be taking my first passenger with me up the river Teign Estuary.
    Weight is fine, certainly no heavier than any of my previous wooden boats of similar proportions. The Cascamite glue may seem inferior to Epoxy, but I find it easier to use. Simply mix the powder with water and ready to go. Much easier to wash off places it's not supposed to reach, fully waterproof and strong. For more data please refer to my Blog : Link is Head for the Hills https://moggo-blog.blogspot.com/
     
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