Saturday 23 May 2009

3. Grainger MTB920 Hull mould frames




The prospect of building this Grainger MTB 920 Trimaran was exciting, and waiting for the boat plans to arrive had me prancing around like a caged lion.
The plans arrived, and I spent a few days figuring out where to start, and what to do next.
These days some home builders would probably purchase mould frames that have been cut out by computer aided laser cutting methods, which ensures greater accuracy for each mould frame or as known on the plans as station mould frames. They were available when I started but my budget was too small to employ others to do what I could learn and gain experience in doing.
Tony provided an A4 page of offsets which I had to mark out on the particle board, effectively marking out in full size the shape of each section of the main hull. An offset is a measurement from the centre line. It will become clear as we continue.
All of these terms such as offsets did not mean too much to me, until I decided before hand to read and learn about boat building. The terms sound scary but certainly are not and I will simply explain some of these throughout this blog.
Each of the stations has a vertical centre line, and a horizontal datum water line.
The datum water line or DWL is where the hull will float, it is where the designer has calculated the hull to float with all its designed equipment and number of people loaded on board etc.
Both of these lines had to be marked onto the particle board and must also be drawn at 90 degrees to each other, accurately. In fact every line must be drawn accurately.
From the vertical centre line along each water line I measured out towards where the outside of the hull was going to be with the measurement known as the offsets.
For example Tony Grainger’s offsets would show the waterline WL200, which has been marked 200mm above the Datum water line DWL, to have offsets each side of the centre line at say 350mm. I marked WL200 at 350mm with a pencil mark forming a cross.
Parallel to the datum water line were other parallel waterlines spaced at 200mm intervals above the DWL. As an example WL400, WL600 etc. The offsets were applied to all of these as well.
What I ended up with was a lot of crosses which could then be joined up together forming the outside shape of the hull.
Then from the intersection of some of these waterlines and the vertical centre line, was several extra lines were drawn at specified angles towards where the outside of the hull would be. These lines are named diagonals. Their purpose is to provide more location points, filling up the space between the waterlines measurements with lots more little crosses, therefore providing more accuracy in drawing the outside of the hull.
After drawing the outside edge of the hull shape, I then had to subtract 13mm for the thickness of the duracore which was the core of the hull sandwiched between layers of fibreglass.
After all the offsets were marked up, then all that was left to do was join up the crosses , and then cut the hull shape out using a jig saw. 13 station frame later and I was ready to mount these onto the strong back.