Thanks for the replies chaps.
I'll answer in backwards order to the questions:
Eric:
Carbon was my first choice for this - I'm pretty happy working in Carbon fibre, but I was put off for a few reasons. The first is that this boat will be designed to go as fast as I can possibly make it - 120mph+ is the end target, but will be based partly on a wave-piercing design ( with an Axe-bow and a 3 point hydroplane all thrown into the mix ). Because I'll be racing this at sea - it's almost impossible to use I.C or jet power on inland waters in the UK for boats - it could be hitting some fairly sizable waves at very high speed. I CAN just keep upping the carbon thickness, but I reckon I will end up with 1/2"+ thick of carbon all around the hull, and sadly Mrs Steve knows full well how much carbon is at the moment as she saw my last bill!
Also the boat will be between 5.5 foot and 6 foot long, and I have no access to an auto-clave that big without spending thousands to hire one ( and lets say 2x de-bulks and 1x bake x 3layers is at least 4 days that I would be hiring the clave out for. I reckon it would end up being £2,000 for the carbon, resins, waxs etc, and another £3,000 for the autoclave. - about $8,000 for a toy boat.)
My thinking is that a space frame will be good practise for working with metal, and will allow for a stressed Ali skin to go over the top of it. This should keep stregth up at a much lower cost. Weight I am less concerned about as it the boat is too light it will need a load of ballast in the nose anyway for the design to work.
Lastly, this may end up being a flash steam driven boat ( flash steam model boats were doing 150mph+ in the 1950's ) and I'm a bit wary about using carbon around lots of open flames from a flash steam plant.
Scott and Eric
I think the question I'm struggling with most is how I deal with the detailed design. It would be fairly simple to build an empty hull, and then fit a space frame into it - no real planning needed, just juggle the metal in based on a back of a fag packet sketch. That makes the design easy, but the build itself I would expect to be fairly tricky - how do you weld the frame inside the hull for example?
Designing the frame first, and then working a hull around it would seem to take a LOT more effort at the design stage, but I suspect it would make for a much easier build.
So I think that's my basic question - Do I go with "difficult design, simple build" or "simple design, complex build"?
Many thanks for all the responses so far.
Steve