Some progress today: removed mould (including support bricks) from base plate and mounted on a stand (made from LEGO blocks). Covered the mould with cling wrap and laminated with some glass fiber (2 layers) and epoxy (pictures below).
Not a perfect lay up (scrap glass, old resin, lack of care) but it should be sufficient to test the method.
With the epoxy set I removed the plastic film and roughly trimmed away excess glass. The support bricks (Blue layer) were removed and the plasticine cut away to be level with the deck line. This nicely showed the line that the glass had to be cut to (obviously this won’t work for a carbon boat). The glass was then trimmed to the deck line using a Dremel with an abrasive disk (thanks Marc). Pictures below.
Next: layup of the deck.
The hull was covered with tape and then additional LEGO plates added for deck profile. The deck was then moulded with plasticine. This was a bit free-form as there was no guide. Deck then covered in cling wrap ready for lamination. Pictures below.
Next: lamination of deck.
Lamination with two layers of glass fiber/epoxy:
Deck sufficiently firm to disassemble (for testing purposes):
Pictures below:
The whole procedure was remarkably easy and clean. I got better at smoothing the plasticine with practice, and by the time I got to the deck was able to get a very smooth, even finish using fingers alone. I used a lot more plasticine than I had anticipated, but it appeared to be in good condition, and suitable for re-use at the end. The things that worried me prior to starting turned out not to be problematic. The LEGO and plasticine were easily removed from the reversed bow. The plasticine was easily cleaned off the LEGO (simply by pressing bricks with a larger ball of plasticine).
The cost of LEGO remains an issue (not in this case as I borrowed it from my kids), and I will provide an estimate of the cost of blocks and plasticine for this test when I get a chance. I’m hoping to find a source of second-hand blocks.
I promised a costing for the LEGO used, but having now done some research I find myself reluctant to give one. It turns out (probably not unexpected given the volume of LEGO purchased) that there is a huge market in second-hand bricks, and the prices vary considerably. For example, I looked up the price of the standard brick I used (1x8) on a popular second-hand brick site and found that prices started from $0.01/brick, whereas the same brick new from another popular site was $0.25/brick. I used ca. 90 bricks for the test section, so assuming all bricks cost the same, this gives a price range from $0.90-$22.5. Clearly the amount spent on bricks will depend upon whether they are new or used, and where they are purchased from. I think that I would estimate (on the basis of my limited search) that I would expect to spend ca. $0.05/brick for second-hand bricks. Hence the section tested would have cost ca. $4.5 for the bricks. Given that this section was ca. 8% of the volume of the whole hull, I estimate that it would have cost over $50 in bricks for the complete hull (this estimate is a worst case scenario, as the remainder of the hull would have been mostly hollow). This does look fairly expensive, and I wouldn’t be planning to use the method to make a 2m tri, but because the bricks can be re-used it seems to me to be an affordable solution for smaller hulls.
I estimate that I would have used ca. 3.5 kg of plasticine for the whole hull (once again, a worst case scenario). A typical price for cheap plasticine seems to be ca. $4/500g, so I estimate that the total cost of the plasticine would have been ca. $28. As for the LEGO the plasticine can be re-used, but probably not so many times. In the long-term the plasticine may turn out to be more expensive than the LEGO.
In the coming months I intend to use this method for a whole hull, so hopefully I will be able to update these estimates with actual values.