[Webmaster edit 16.3.2013: Jonathan Snodgrass began a plywood boats thread in the
Yacht Club Bar. It's started getting nicely technical, so I've moved Gerry's description of the build here.]
Gerald O'Brien 6.3.2013
Stage one of the build of 'Ivory Gull's' new tender is complete. The hulls are assembled - though far from complete as there's fitting out and finishing to come yet. I've posted a photo of an
assembled hull in the photo gallery.
Techie details: beam 1', height 1', length 7' 11". Max buoyancy 440kg, working load up to 250kg. The hulls are sealed units with six internal bulkheads creating seven watertight compartments. Each compartment is filled with fitted buoyancy in the form of plastic milk bottles with the lids glued on to prevent them shaking off and fixed in place with expanding foam. The deck is supported on the bulkheads and an additional beam fitted between each bulkhead.
The box hull form maximises buoyancy for a given beam/hull height and gives an economical cutting pattern for the plywood. Fine, slender hulls with good high-speed performance were not requirements for this project.
All hull material material is 3.6mm WBP plywood except for the bow and stern plates which are 8mm. Internal surfaces are unfinished. External surfaces to be finished with epoxy seal, primer and paint. Hull weight at this stage is 12.5kg each, which is heavier than I would have liked, but not too bad. My target is to bring the build in at less than my Walker Bay RID which is 40kg.
All joints are epoxy fillets. I accidentally broke one of the joints where a deck beam connects to the side of the hull. The failure mode was delamination of the plywood, which means that the fillet joints are stronger than the wood. Comforting to know, even if the test-to-destruction was unintended. The repair was straightforward.
The free-draining cuddy will be 4' wide and positioned above the hulls, which will stow away underneath it. This gives the vessel a minimum, stowed, beam of 4' and a maximum, operational, beam of 6'. Positioning all the load between the hulls increases stability.
Gerry O'Brien 14.3.2013
I cut and assembled the first composite beam for "Ivory Gull's" tender today. Thought I'd post a
couple of pictures in the photo gallery to show what I've been talking about.
The first photo shows the components and the second the assembled beam. It's just a small thing, 2' x 4", the stub beam that attaches to a hull and which will slide home into a housing on the cuddy. The stiffeners and plywood are stuck together in a sandwich with waterproof glue.
The idea of beams like this is that they should carry loads disproportionate to their weight, as compared with solid beams of other materials. Material costs are also reduced, though labour costs are higher.
The stiffeners are 10x20mm redwood. Not ideal but what I had to hand and OK for experimental purposes. The average density for the finished beam is 0.4kg/m3. This is 11% lighter than redwood and significantly lighter than that compared to most hardwoods. I'm sure that the average density will be substantially reduced in the bigger beams that will form the main structure of the cuddy.
Power will be be oars or up to 4hp outboard. Though eventually I'd like to try a split rig junk sail on each hull as seen on Pete Hill's 'Oryx' since I see this as the first in a series of development vessels for 'Ivory Gull's' eventual replacement.