So what worked and what didn't
This is the part of the process I've really been looking forward to. To have a chance to put down everything that I’ve learned from the process so far.
Rocker table design
This proved to be a really big one because the mistakes made here caused the rocker and concave to be much less than designed for and so what was going to be a high wind, chop eating board turned out much flatter than expected and so more suited to flat water.
What went right?
The overall dimensions although on the large size were good for having plenty of room around the board to stick the plastic, get the vac connector in place and stretch film to get wrinkles out. However, this was mostly a benefit because it was my first time and I needed the room to fix mistakes. I would aim at next time leaving only about 10-15cm extra beyond the board width for reasons I’ll outline below.
What went wrong?
The biggest issue was that the concave and rocker actually realized was much less that desired and this was due to 3 things:
i) As the rocker table was about twice as wide as the board I had to make and estimate on how much lower the edges of the jigs should be than the middle line to get the desired 10mm of rocker at a point that was about half way to the edge of the rocker table. I estimated that this was going to be about ½ height difference and so to get 10mm of rocker of made the jigs 20mm lower that the centerline at the midpoint of the board. In hindsight this was a really poor approximation because if this were correct it would mean that the table surface (the mold) would have to form a straight line from the centre of the mold to the jigs at the edge of the mold. Instead (after a bit of googling) it turns out that this estimate was 50% off and in fact when you put the actual numbers in for my board you end up with the amount of concave at the edges of the board being just 25% of the height difference between the center and the jigs. So what was to be 10mm can out to 5mm which is actually pretty bloody close to what happened? Also it meant that at the center edge of the board it was 5mm higher relative to the ends and so 5mm of rocker disappeared as well.
The equations governing this is really useful and comes directly from beam bending theory. Assuming that your mold surface is made of material that has a uniform modulus of elasticity, the half width of the board is L and the height difference between the center of the board and the point at the edge where the jigs clamp down is H, then the equation describing the shape the mold surface takes is
Y(x) = -3H/L^3 (L.x^2/2 – x^3/6) - (profile)
So it’s a bit like a flattened out parabola.
If you are aiming for a concave of say c and for my board the edge of board was at x=L/2 (roughly) then you would determine the height difference H that you need to factor into the design by setting Y(L/2)=c and calculating H.
From my calc this number turned out to be about H=4cm instead of the H=2 that I had assumed. One important implication for the design process is that you would first design the centerline of the rocker profile and then use this H to determine the required radius of the rocker line at the edge of the mold. This is necessary as if you do in the other order then you would end up with a hump in the middle of you board.
ii) Second, it turned out to be hard to get the 4mm plywood mold that I used to conform to the jigs exactly as the it ended up causing the plywood to crack if I have put enough pressure on it to get it to conform exactly. This meant that the jig rocker line was flatter that the actual curve of the jig. Again this contributed to the reduction in rocker from the desired 3cm down to 2 cm.
iii) Thirdly, I cut the jig with square edges where the mold surface was going to sit. However, the surface comes in at an angle and so it rested on the inside edge which was higher than mid point where the actual rocker line was designed around. The lesson here is to use a router of the like to shave off the inside edge of the jig.
Together these issues account for most of the loss of curvature. It had been suggested to me that it may have been that the fibreglassed laminate will ‘relax’ after it comes of the jig and I understand that this can be up to 20% of the curve that flattens out. However, I checked the finished board against the rocker table there wasn’t any perceivable relaxation at that stage. Maybe this comes later as the lamination ages.
The rocker table surface
All in all the use of plywood for the surface of the mold was a poor choice. Despite about 6 layers of acrylic paint on the surface to seal it and 4 layers of release agent, when the board came off after laminating the bottom it took acrylic paint and in some cases a bit of the wood with it. Either the release agent was insufficient or the epoxy reacted with the paint. Also, the uneven surface of the wood means the board surface was uneven and didn’t recover that well with sanding.
While I still think that there is something to the issue of using stiffer material, especially if you’re going to put stringer in the board or use wood cores, I think that using wood alone is not the right move. For the next try I think that I’ll lay either acrylic, laminex or maybe a sheet tin over the surface of the wood.
Rails. Trying to cut the rails to the exact curve of the board edge was a real pain in the arse and turned out crooked. The best tip I’ve come across for this is to just rectangular strips which you can use a straight edge to guide you and then heat the plastic and bend it to the shape you need.
The other alternative which I’m keen to try is to pour the rails using resin with chopped fibres in it for strength. For the current board this would have used around 190gms material (160gm resin +30gm hardener + couple of clumps of chopped fibres and bit of qcell to harden it).
Fill the t-nuts with wax before you put them in the resin. This will stop the resin filling up from the bottom and causing the available depth to be short. This can be a real problem as when you tighten the bolts in to your foot straps, excess resin in the inserts will cause the fiberglass to be pushed off the underside of the t-nut and possibly split the underside laminate. 5 minute epoxy is good stuff to use here to speed the process up.
Vacuum set up
The compress worked like an absolute champion. Because of a leak in the setup when I laminated the top (which I was never able to find) the compress ran continuously for about 7 hours and didn’t blow up.
The vacuum connector made life very easy, the ripstop nylon (with extra holes pushed into it) worked perfectly in place of peel ply and the polyester wadding worker well as breather material. However, the surface of the board came out a bit bumpy and I am trying to work through whether the uneven density of the wadding might have caused this. However, I stuffed up the top layer laminating ( forgot to just the glass to size before I mixed the resin and so the resin had started to go off before I finally clamped it down. This is more likely the reason)
The tacky tape was a really pain in the arse to work with. It was way too tacky and so trying to reuse it (and not waste it) ended up being responsible for damage to the vac film and the subsequent unfindable hole. If I end up using a more uniform mold surface next time I think that I will try just using packing tape around the outside or maybe use plasticine in place of the tape. It might help to form a first line of defense to assist the tape.
PVC tube greater than 8mm doesn’t seem to be up to the job of holding pressures above 15inHg without deforming and running the risk of pinching off. Putting wire strands inside the tube stopped any problems.
QCell is great stuff. I used to little initially because I added an amount equal to the volume increase I was after (e.g. 10% of volume of the resin) but overlooked the fact that a lot of that is air and so when mixed in the volume increase was much less. It seems like the best approach is to mark the volume increase on the side and check it when it’s mixed through. Also, as qcell is often filler, you can just do it by feel. ‘Peanut butter’ seems to be the going description of the right consistency for using it as filler.
When you are putting the resin on the glass, really work it through to get all the air bubbles that will be created in the holes between the weave out. I didn’t work it hard enough and so ended up with a lot of bubbles which will most likely be responsible for the board ultimately cracking.
Get as much excess resin off before clamp it. This resin will flow everywhere on the board you don’t want it to even if you have plenty of breather material.
Don’t let wrinkles appear in the vac film as this will create weaknesses.
Let it cure under pressure for longer than you need to and let it finish the process with the vac film removed before you take it off the mold. Even after 8 hours under pressure the resin still had a slightly waxy feel to it which took another couple of hours to disappear.