The George Blog – 68\' Design Progress & Comments

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68' Design Progress

The frames, floors and backbone of the boat are now designed and drawn out full size ready to be exported to DXF files. 1/20th scale versions of each drawing have also been prepared ready to export to PDF files so that the builder and the CNC operator can see what all the bits should look like!

All the parts (and there are over 700 of them) can be cut from a standard 2440 x 1220 sheet of plywood. We are utilizing either 12mm or 18mm ply.

Where the frames incorporate interior bulkheads and furniture partitions, we use six layers of 12mm ply to make up the 72mm siding (thickness) of the frames. Where we have a major watertight bulkhead and for all frames that don't have any furniture components we are using four layers of 18mm ply to make up the 72mm siding. The mast frame is six layers of 18mm ply and is also deeper. Stainless steel shroudplates are incorporated directly in the frames where appropriate.

The floors are generally four 18mm layers, with additional parts in way of the rudder stock and bow thruster.

The deep backbone is 4 layers of 18mm, with a further five layers each side to form a hog. In way of the keel, there are additional layers to accommodate the keel bolts. The engine beds also form part of the backbone structure.

The current task is to work through each frame, super-imposing the images of all the layer components of a particular frame on top of each other to make sure that everything matches correctly. The floors and backbone receive the same treatment. With so many parts, drawn out in such detail, inevitably a few errors creep in and this is the time to correct them.

At the same time, we check to see if we can lighten any of the components, which is often the case once you start to look at the thing as a whole rather than a lot of individual bits. Once this is completed we calculate the weight and centre of gravity (CoG) of each component and enter these in the weight estimate calculation.

The next job is to write the instructions to go with this section of the construction. Fortunately this is very easy – the components really do almost self-assemble. There are 15mm and 8mm locating holes in all the components, through which a short length of 15mm or 8mm dowel rod is pushed and this locates the components in the correct relationship to each other. Usually the components are all pre-WESTed (epoxy coated), so they simply have to be bonded together on a good flat surface.

The backbone/hog/engine bed structure is a bit more challenging as it is the full length of the boat, so it is quite a large item when fully assembled. The design of the components does allow for it to be assembled in sections which will then step-scarph together – not exactly a true scarph of course: more a series of well-spaced butt joins. But effective and strong.

The backbone and frames/floors are cross jointed (egg crate style) so, once they are all bonded together and epoxy filleted, the whole structure becomes very strong indeed. Which is of course what we are aiming for!

When designing this part of the boats structure we are not only thinking of the normal stresses and strains imposed by the rig and the keel and the boat generally in a seaway; we are also considering the possibility of running aground – even in quite a light swell, touching a hard sandy bottom can be pretty tough on the centreline structures. That is an East Coast of England sailor talking now, where, as our South Coast compatriots put it, we tend to have to sail "on a damp sponge"!

Exciting to see the structures coming together like this.


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