DD by David Webb

09 May 2021 09:06 | Anonymous member (Administrator)

This is a variant of the dinghy I posted a couple of years ago in my designs on the website. I recently built a proof of concept prototype and have attached some photos of the build, more available if wanted.

The design as shown on the submitted plans is a one-piece dinghy, but it can be made into a nesting two-part dinghy, which reduces the size for storage onboard. I re-used a 45 sq ft balanced lugsail rig from another dinghy design and have included in the design for that rig or a four-panel junk rig of approximately the same area.

The dinghy rows and sails nicely and is good in choppy conditions with its fine bow, deadrise, flare, and rocker.

Construction of the hull is very simple, the bottom is a single sheet of 1/4" plywood split diagonally from corner to corner and a 3'-10" curve is cut into the panels as shown on the drawings. These two pieces are then joined at the forward and aft end. The parts are not joined along the centerline at this stage. The small transom is constructed as shown on the drawings. The second sheet of 1/4" plywood is cut into three 1'-4" wide pieces, one of which is cut in half. The full-length pieces are then attached to the half-length pieces with an epoxy glass scarf using a 4" wide piece of 12 oz double bias tape overlaid by a 6 inch wide piece of 6 oz glass cloth. The sides are then screwed and glued to the transom, the square end of the side panel being aligned with the start of the taper on the edge of the transom piece. The assembly is then attached to the bottom. The transom sits on top of the bottom piece and the sides lap down over the bottom and are fastened at about 6" centers by cable ties, working from the stern forward. The holes in the side panels need to be drilled 1/4" clear above the bottom to allow the bottom panel to fit inside and be drawn up tight and level when everything is assembled. The holes in the bottom panel should be clear at least 3/8" from the edge and should be drilled to align with the side panel holes as the sides are installed. Once the side panel has been attached up to the bow mark a vertical line on the side panel, then measure 7 1/2" forward from this point, draw a line from there back to the bottom panel, and cut off the excess. The stem post should then be installed to hold the sides together. Leave about 3" above the sides to allow for a purchase to be attached from the transom to get the correct rocker and flare to the hull.

Clamp a straight timber to the top of the side planks to keep the side straight for the first four and a half feet from the bow, this helps ensure that the hull has the correct flare. Attach a rope in a loop around the extended stem and through the sculling port to a short cross timber on the aft face of the transom. This can then be rotated to tension the rope and hold the hull at the designed flare and rocker. This will create a narrow tapered gap between the two halves of the bottom panel along the centerline. This should be filled by tapered slivers cut from the offcuts of the bottom panel. Once these have been glued in place then the bottom joint can be glassed and epoxied with two layers 12 oz biaxial and one of 6 oz cloth inside and out.

At this stage the cable ties connecting the sides to the bottom can be finally tightened and aligned. The joint on the inside can then be filled with an epoxy cove and glassed with a layer of 12 oz biaxial and topped with a layer of 6 oz cloth. Once this has set the cable ties can be cut off on the outside, the joint filled, and when set rounded to a radius of at least 1/2". The joint is then glassed with the same layup as the inside.

At this point, the hull is basically complete and all of the finishing etc can proceed from there as it would for any other dinghy.






 


Comments

  • 22 May 2021 01:23 | Anonymous member
    This competition has produced some surprises. Since the job of a tender is fairly universal, and since literally hundreds have been designed, one might have predicted that some kind of optimum dinghy might have evolved by now, to suit the size range we are looking at here (approximately the length of a sheet of plywood). If so, then I suppose the pram-bowed dinghy in its various forms is probably it.

    But no – this competition has produced some “out of left field” solutions. Oyster is one example -and this creation is another. This tender has been built and trialled, so no point in speculating about its characteristics. I have seen it – didn’t really like it – too extreme for me. But – horses for courses – lets keep an open mind and look at this remarkable concept.

    Firstly, for the bottom panels, by a clever stroke (taking the diagonal of the 8x4 sheet instead of the length) a larger dinghy has been created from two sheets of plywood. The other sheet has been used entirely (no offcuts) to make the side panels. There is almost nothing left over from the two sheets (so offcuts must be found from elsewhere to make the tombstone transom, the thwarts and other necessary bits). And the lofting is, if anything, even simpler than that of Slieve’s KISS.

    Not only is the dinghy longer than we would expect from a two-sheet build – it has considerably more displacement (I mean, load-carrying capacity). It would be interesting to know just how much weight could be loaded into the huge after-body of this dinghy, without affecting the trim very much at all. The seating right round the stern would easily accommodate a couple of adult passengers, and the floorboards there would carry enough groceries to keep them for a month. The third adult (rowing) would hardly need to shift his/her position. Once again, its horses for courses – if you want the biggest load-carrier from the least amount of plywood, this exercise in geometric ingenuity will be the dinghy of choice. And its not a scow – its virtually a double-ender, for goodness sake!

    I’ll take Dave’s word for it that it sails and rows OK. I like the towing attachment (and the painter) to be on the inside of a stem, not the outside, but to make this fine-bowed wedge tow well, perhaps the outside placement is best in this case. (That remark applies also to Alex’s entry, Youyou.)

    As an extra feature, this dinghy comes in half and can be nested, though apart from mentioning the fact, Dave gives no details as to how this is achieved. I presume its is a conventional double bulkhead bolted together – with the transverse thwart perhaps locking the top of the bulkheads. It might be almost as difficult to stow nested as it would be whole, since so much of bulk is contained in one half of it.

    Maybe one of the most imaginative and surprising contributions, and if it does the job required of it, then it’s a good one. Dead easy to loft – and I’ll take Dave’s word for it that it is easier to build than it looks. It represents “bang for your bucks” from two sheets of plywood, but its just not my cup of tea.

    (Complaint: I don’t understand any dinghy having a fixed rudder, and I hope a comment from Dave will explain the reason for the forward-raking transom – which does not jar the eye so much here, as it does in the lovely little pram which is one of Dave’s other contributions. I am eager to be educated on this oddly-placed centreboard.)
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    • 22 May 2021 11:42 | Anonymous member
      Graeme, part of the reason for the design of this dinghy was to get a tender that was easier to row in to a chop than the normal pram bowed dinghies. The flare and rocker that result from the construction create a very seaworthy dinghy. I have rowed her into a 20 knot breeze with about a two foot chop with no problem and with just a tiny bit of light spray coming over the bow. I made the rudder fixed to avoid the complexity of building one that kicks up, also the balance area locks the rudder in place. It can only be installed or removed with the rudder almost at right angles to the centerline of the boat.
      The two part dinghy has a double bulkhead below the center thwart as you suspected and the slots in the end of the center thwart provide location automatically so that the four bolts with neoprene washers can be installed. I have not installed a neoprene gasket to the joining face, but this would provide additional security against leaks at the join above thwart height.
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