Sunday, January 27, 2019

40 Knots Or Not

It was wicked windy a few days ago when a big storm system pulled through. We followed the local experts who drove an hour south to Australind, looking for the strongest wind and the best spot for westerlies. Once we got there, the rain and chop made it quite uninviting. After driving up and down the bay looking in vain for a spot that was both flat deep enough, the final straw came when Hardie sent a screen shot from his top speed at Mandurah: he had gotten a 40-knot reading on his GT-31!

Back we drove, and sailed in Mandurah Bay at Coodanup. For once, it did not deserve the name "Fangy's Weed Farm", since a high storm surge covered the weed completely. The there actually was some chop - although it should more correctly be labeled "mini-chop", since it was just maybe 5 or 10 centimeters high. To keep things interesting, the weed had ganged up at some spots, building "weedbergs" (the weedy equivalent of icebergs) which were big enough for birds to stand on. Sailing into these at speed created interesting "sudden drag" situations, but the high-rake fins we used parted the weedbergs without catapults.

I ended up with a top 2-second speed of 33.6 knots and 32.6 knots for 5x10 seconds, which is the 3rd-best I ever did. Nina got two new personal bests for 2 seconds (32.6 knots) and 5x10 (31.5 knots). However, we both ended up disappointed, since our top speeds were about 5 knots slower than the speeds many others got, and almost 10 knots slower that the 42.3 knots that Stroppo, speed king of the day, posted!

Here's a summary of the local speed surfers' top speeds (2 seconds):
  • Fastest speed: 42.288 knots (Stroppo)
  • Above 40 knots: 5 sailors
  • 37-39.9 knots: 8 sailors
  • 31-34 knots: 9 sailors
  • Spots with 40+ knot results: Coodanup (3), Australind (1), Melville (1)
The numbers may change a bit since not everyone has posted yet; I think we'll see at least one 40+ knot posting.

Seeing these numbers made me feel a bit better, because we're at least in the largest group. The spread of 10 knots is quite amazing - what causes it? There is probably a small contribution that the gear makes. For example, whenever a heavy gust hit, Nina was very overpowered on her 5.0 meter sail and 54 cm speed board. Stroppo, who is close to twice her weight, used a 6.0 and a 47 speed board - no surprise he was able to stay in control! Sail sizes should (in first approximation) increase of decrease in proportion to weight, so Nina's sail should have been somewhere in the 3.x meter range, or at least much closer to 4 square meters. I was on a 5.6 m sail, which is close to what most guys my weight used. My board was an 89 l slalom board, and about 10 cm wider than the speed boards used by most others, which might have slowed me down by a couple of knots. Indeed, when I got hold of Nina's speed board for a run, I did immediately got my best speed of the day. But looking at the faster windsurfers, there's still a 3 to 5 knot difference between the fastest guys and the next group.

Our host Mike, who did 37.7 knots on a 4.7/43 combo, gave me his GPS tracks to compare to ours. Here is a polar diagram which shows the maximum speed relative to the wind direction for Nina (in red) and Mike (in blue):
The left half of the diagram shows the starboard tack runs, the right half the port tack runs. On starboard, Nina's and Mike's speed were about the same for most angles; but Mike went deeper downwind, and reached a slightly higher top speed at about 137 degrees. On the port tack (right side), Mike's speeds were about 2-3 knots faster than Nina's over a wide range. Again, he went deeper downwind, and got his top speed at about 145 degrees. Going about 15 degrees deeper increased his speed by about 3 knots.
After the session, Nina said that she simple could not go deeper because she was so overpowered. She was on a 5.0 m sail and a 54 cm wide board; Mike was on a 4.7 and a 43 cm wide board. Seeing how deep Mike's top speed angles were, he was certainly fully powered even on the deepest angles, where the apparent wind is significantly lower than when sailing on a beam reach (90 degrees to the wind).
The difference between the starboard and port tracks points to another very important factor: being familiar with the conditions. There was enough chop to make both Nina and me "put on the handbrakes" - we were definitely not going all-out. We both felt more comfortable doing speed runs on starboard, even though port was the better (inbound) direction. I think that is because at our typical sailing spot, Kalmus Beach in Hyannis, we generally go out against the chop, and come in more parallel and over the back of the chop. So we are much more used to dealing with chop on the starboard track.

It is also interesting to compare Mike's track to Stroppo's tracks, since Stroppo was significantly faster than anyone else. Here is the polar diagram (Mike in blue, Stroppo in green):

Stroppo did not do any speed runs on starboard, so we can ignore the left half of the diagram. On the port side, Stroppo was going a few knots faster than Mike, with a bigger difference at deeper angles - up to about 127 degrees. Beyond that, Stroppo's speeds actually dropped, while Mike's speed still increased for another 20 degrees or so.
The reason for this can be found in the sail sizes: 4.7 for Mike, 6.0 for Stroppo. But Stroppo is about 60% heavier than Mike, and sail sizes scale (roughly) proportional with body weight. For comparable power, Stroppo should have been on a 7.5! That seems way too big for the wind, so let's do it the other way around: for comparable power to Stroppo's 6.0, Mike should have been on a 3.7!
Arguably, the linear sail size relation does not fully hold for race sails; however, a sail closer to 4 square meters would have been more similar in power to Stroppo's 6.0.
Watching the two of them on the water, it was quite apparent that Stroppo was on the (relative) smaller gear: Mike had a much easier time to get going, while Stroppo often had to wait for gusts. The (relative) bigger sail enable Mike to go deeper downwind; but the smaller sail gave Stroppo more control in the big gusts, which he skillfully converted to more speed. Looking at the fastest runs for both of them, the top-speed angles varied a bit, but in general, Mike's top speeds were reached at a roughly 15 degrees deeper angle.

Another observation from Stroppo's tracks was that he did not do "slingshots", where the angle suddenly changes. Instead, Stroppo's angle changed very gradually to deeper and deeper angles - here is an example (from his second-fastest 2-second run, 41.18 knots):
The near-constant acceleration over the entire run is quite impressive. His fastest run look very similar, but he apparently caught a good gust near the end of it, which gave him the extra boost to reach a 1-second top speed of 42.4 knots. Very impressive! Since the technique is quite different from the slingshot, it deserves its own name - how about "Stroppo Curves"?

I must admit that I find Mike's speeds, and the speeds in the high 30s to low 40s many others had, almost as impressive. Mike is more than a decade older than I am, and must have been quite overpowered in the gusts, but had way more control to convert the gusts to speed than I did. Perhaps the more appropriate board (43 cm speed vs. 59 cm slalom) helped, but there's definitely a skill difference, too. But whatever the causes were, it was very fascinating to be able to share a great day on a fantastic spot with so many good speedsurfers!

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