I was supposed to write this earlier on this spring, however, it did not happen and now a long time due, let us view at one of the performance optimization carried on during the past winter: sharpening the trailing edges of underwater foils.
The keel trailing edge comes from the manufacturer 9 mm thick with a very uneven edge. The rudder edge is even, but about 7 mm thick.
According to different sources, 10% reduction in cord length amounts to 50% increase in drag. Furthermore, turbulence behind the keel is encountered by the leading edge of the rudder blade, decreasing its effectiveness. I decided then to extend the trailing edge of both foils and sharpen the rudder end.
I elongated the cord of both rudder and keel of 11%, a length which is significant and is practical in its feasibility as it creates a thickness of the rudder edge around 1.5mm and about 2.5 for the keel.
The could both be sharper, however 1) the lateral pressure would require some form of structural support other than blended fiberglass in the epoxy putty 2) it would be impractical as the edge would probably become too thin and fragile.
The first step was to rough up the edges and a few millimiters up the cord of the foils for the epoxy to have good mechanical grip.
Then, I taped very carefully a flat plank about a meter long with brown packaging tape, against which epoxy does not hold. At this point I have glued the plank in place along the keel edge on the starboard side so to be able to apply the putty from the port side.
Not that the same procedure applied for both keel and rudder blade, except that the rudder was taken home as I wanted to have a finer finish and needed to work at higher temperature.
To fill quickly mass and provide some structural support, I first filled most of the gap with a putty from Pattex Plastic Padding called Glasfiberspackel. It was the first time I used Plastic Padding and loved it. It worked well in spite of the low temperature, although a little difficult to apply. It was very easy to sand, extremely strong and the long glass fibers provide great strength. Most of all, it hardens real fast (within 20 mins) at 5 degrees Celsius, which allowed to apply, sand and reapply several times within a very short time. It fills up volume quickly. I used a warm air gun to speed up to make it easier to apply and to start the first hardening phase (5 minutes were enough). Proportions of the blend were also very easy to reach with no scale or other instrument.
While the putty from Plastic Padding, it is for larger repairs and very rough. Their epoxy for fine work requires much higher temperatures and was not an option. As I had good experience from fairing the hull the previous year with West System 205 epoxy fast hardener at low temperature, I decided to go for it again, adding microbaloons from West System as well to increase consistency and ease the sanding process.
The entire foil was then sanded with increasing fine sandpaper down to a P600 grit which is around 25µm particles.
Fine puttying and sanding was repeated a few times until satisfied with the result.
After a few months in the water, I cannot say whether this has improved speed and pointing in any way, however it is very clear that we can bear a spinnaker at about 5 degrees AWA higher than before with a lot of heeling before the rudder lets go and we initiate a broach. This is by no means more effective in terms of performance, however it allows a little wider range to avoid obstacles inshore, like rocks or shallows. As we do not carry an overlapping genoa and have introduced an heavy reacher, I am convinced that the finer trailing edges contribute to being able to carry the reacher at very steep angles in lighter winds chipping away the gap due to the missing genoa.