The George Blog – Designing 044/07 & Comments

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Designing 044/07

It is several years since I designed a big boat pretty much from scratch. Admittedly the new 68' is a development of an existing design. But being a little bigger, the hull shape is slightly different – so nothing from the existing boat fits. And this is the first of these boats to be built in wood/epoxy, so the structures are all entirely new.

What you do discover, working through the design and structures, is that some of the structures are deeper than their equivalent in aluminium alloy or steel. And this is specially so because we want to be able to CNC cut all the structural components in order to speed up and simplify the build. With the centreline structure this results in vertical plywood layers, rather than a solid timber horizontally laminated structure, which inevitably produces a deeper backbone.

This extra depth in its turn provides challenges when it comes to fitting in systems like the generator, air conditioning and so on. It is always surprising how far you need to go into the design before the structures can be finalized.

For example, we have already selected the main engine and the bow thruster. We have done a fairly accurate air conditioning calculation and selected a suitable chilled water plant. In its turn this gives us a handle on the generating capacity required – even with soft start options, an air conditioning plant's starting load (known as LRA = Lock Rotor Amperage) is many times greater than its running load. So a cooler that, say, takes 10amps at 230 volts could take 50 amps or so to start. And the A/c has to be able to start while at least some other loads are connected, or else the boat is a misery to run. Which all goes to mean that we need a much bigger generator that one would suppose!

On this boat all these things – and lots more have to fit mostly under the deck saloon sole – which in its turn dictates some of the geometry of the saloon and the structures. Not all the decisions are pure naval architecture – in fact now I come to think of it, man of them aren't.

Another early area to get sorted is the spar and rig. Shroudplates (in this structural design) need to be incorporated early on in the frames, so we need to know the rig sizes pretty accurately, in order to get the shroudplates the right size, with the correct size holes in them, the right number of correct size fastenings and so on. This is, I have to say, much easier to do when you are working from an existing design, quite a few of which have already been built.

Right now we are about two-thirds through the basic structures and most of the key decisions have now been made, so it is more a matter of drawing work – and there is a lot of that! We still have the steering system and boat stowage to consider and I will be talking more about that when we get to it.

George

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