8. DuraKore strip planking the Grainger MTB920 Trimaran bilges

Laying the DuraKore onto the temporary hull mould frames progressed fairly quickly. I had both sides done in less than 2 months of spare time around a full time job.
I was racing against the seasons, as I needed to be at the fibre glassing stage during summer, so that the west system resin could cure properly in suitable temperatures. Fibre glassing was to be another first for me, apart from a few small repair jobs on previous yachts.
You can see by the photograph that I had deliberately left the bottom of the hull until both sides were completed.
This was because that area had the most curvature and in my mind it was not going to be easy. So while working on the sides I was mentally working out how I was going to attack the bottom of the hull.
Most of the sides were done with 50mm width strips, and the flatter areas were planked with 75mm strips.
I found that I had to cut various sized strips for the bottom, but predominately they were approximately 25mm strips.
This was a lot more work, as there was more cutting, edge gluing and screws to be screwed in, but too me the best way to produce a nicely curved hull shape.

A tip
In hind sight I should have investigated using a staple gun to hold the strips on. The staples could have been fired in through a piece of plastic placed over the top of the DuraKore strip, which then could be used to pull the staple out after the resin had cured.

All the strips were cut with a skill saw set up to follow a fixed stationary straight edge 10M long.

6. West system resin to edge glue DuraKore strip planking

Now that I had a stock of 50mm x 13mm x 10m plus planks prepared, I was ready to screw the first plank onto the temporary male mould frames. A power screw driver was real handy for this job, as there were thousands of screws to be screwed in, temporarily holding the plank in position while the west system, brand of slow curing epoxy resin cured.
I need to use slow curing west system epoxy so that I could get several planks edge glued and installed onto the hull mould frames, as well as temporary plywood battens screwed across the planks to support the planks in the unsupported areas between the mould frames. You can also see the battens in the photos.
I did all this by myself with no volunteers; it is possible when you set up correctly.
The first plank placement is the most important, as this positioning will be the difference in how all the other planks lay up around the hull frames. Basically you lay the plank onto the hull into the lowest point in the curve along the entire hull or the highest point along the whole hull, and visualise how the next planks will look and sit before selecting the position to screw the first plank on. Some planks will be expected to lay up with a slight twist like a propeller blade, following the hull shape and they can only take a limited amount of flex.
It is not wise to make them lay across the mould frames unnaturally.
The photo demonstrates where I started with my first plank.
Epoxy resin and west system filler powders are mixed to a peanut butter like consistency and then applied to one edge of the fixed plank.
Then the 2nd plank is lifted up and placed against the glued edge and then screwed in place. This job goes on and on until the entire main hull is fully planked up.
After a few planks are edge glued and fitted, I made sure to scrap off all excess resin to save myself a lot of sanding later on.
Remember that DuraKore is only the core, just as foam or divinycell etc are an alternative core material for hulls.

5. Grainger MTB920 Trimaran built with DuraKore planking

Now that the temporary frames were all set up and secured in place, Paula and I held a long board over several frames at a time to see how a Durakore strip plank would look when screwed to the frames, just as a final check to make sure that there were going to be no humps or hollows in the finished hull due to poor temporary frame measurements or cut out.
The 9.2 Meter hull was now ready for the DuraKore planks, but the Durakore was only 2.4 meters long and too wide at 300mm.
DuraKore is a marine grade composite of Baltek end grain balsa bonded in between 2 veneers of marine Gaboon hardwood, providing strength to weight ratio far superior to any traditional method of one off construction.

My next job was to splice those 2.4M planks together to at least 10 meters or longer, and then cut those planks up into suitable strips so that as they could be edge glued and screwed to the male mould frames to form a round bilge hull, and not an obvious multi chined hull shape.
The spliced joint made (male to female) was a 1 in 12 taper cut across each of the hardwood veneers.
Effectively you end up with a tapered male edge, with the marine hardwood veneers tapered down to the edge of the balsa core.
The female joint is obviously the opposite, as the balsa core is machined out a distance equal to a 1 in 12 taper of the hardwood veneers.
I’ll explain, if the hardwood veneer is 1.5mm thick then a 1 in 12 taper will cause a tapered edge to be 1.5mm x 12 = 18mm long. Therefore the balsa removed from between the hardwood veneers will be 18mm deep.
You can see the female joint in the attached photo.
And you will now see that the matching male edge is tapered out to 18mm to accept the female edge.
This 1 in 12 taper splice produces a joint equal if not stronger than the uncut hardwood. West System sells a scarfing tool that can be attached to any Skill saw to do this machining job very easily.
I made a flat “table” area along side one side of the hull, which I could glue and join the planks together, and that they would be perfectly flat long planks (at least 10M or 11M) once the west system epoxy resin was cured. I did not want to make strips which are bent at the joints. That would also have resulted in a hump or hollow to fix up later.
The reason why I made these strips much longer than the hull, was that it is important to stagger the spliced joints. It would be a potentially weaker core if all the strip joints were aligned together.
Once I had glued up a lot of 9.2 meter x 300mm planks, I then sliced them up into approximately 50mm width strips, with a skill saw.
The photos show a pile of those durakore strips ready to be edge glued and fitted to the hull mould.