Monday, May 8, 2017

Fun With Eddie's Pictures

We had a couple of nice windsurf sessions at the "Kennedy Slicks" in Hyannis Port Harbor yesterday and today. Eddie took some pictures both days - yesterday from Hyannis Port, today from Kalmus, about a mile away. Here is a picture from yesterday:
I'm about 200 feet away from the pier in this picture. The wind had dropped a bit when the picture was taken, so I'm hanging really low. I had wanted to stop a few minutes earlier, but went out again when I saw that Eddie was just setting up to take pictures.

For comparison, here is the picture from today:
In this picture, either I have shrunken a lot since yesterday, or the wall is noticeably taller. Some shrinking happened - I was on a 7.0 today and a 7.8 yesterday, so the mast length was about 460 cm compared to 480. But mostly, the wall was taller.

Some of you may think I'm telling stories. Some may grow concerned about walls in the water suddenly growing. Some may astutely observe it's not the same section of wall in the two pictures. But those of you who have windsurfed at the Kennedy Slicks will remember that the spot is quite tide dependent. We don't have a lot of tide in the Nantucket Sound - the different between high and low tide today was only about 3 feet. According to the the tide tables, the tide was about 1 to 1.5 ft lower when today's picture was taken.

Since we've got the pictures, know the mast lengths, and have too much time on our hands, we can calculate the height of the wall above the water. Yesterday, it was about 140 cm; today, it was a bit above 200 cm. Most of the difference was from lower tide levels; the remainder could be from some small differences in height between the different sections, or from slight chances in wind direction that pushed more water towards shore yesterday compared to today (it was SSW-SW yesterday, compared to WSW-SW today).

Why does it matter, you ask? Well, thanks for asking! You just gave me an excuse to post today's GPS tracks:
I got my top speed of the day in the second run. All top 5 speeds were in the first 30 minutes; after that, I could not break 30 knots again, no matter how hard I tried. But the readings from the nearby iWindsurf meter show that the wind actually was a knot or two stronger in the second half of the session. So I should have been able to go faster after the first 30 minutes!

From the beginning to the end of the session, the tide dropped by about 0.8 ft, according to these tide graphs. That's not a lot, but it made quite a difference in the wind quality close to the wall - the wind became a lot gustier and weaker, at least in the really flat section within a 100 or 200 feet of the pier. Nina, who was freestyling and playing with waves, verified that this was a localized effect - when she sailed away further from the wall, the wind became steadier and stronger.

That a higher wall due to a lower tide will impede the wind more is not exactly rocket science, but what surprised me was that even a change of less than one foot made such a noticeable difference. When I stopped sailing, the predicted tide level was at 1.55 ft. At more than 2 feet, the wind gets a lot steadier; at 2.5 ft or higher, the disturbances from the wall are quite small, even when sailing within 150 ft of the wall. The higher tide levels come with a slight drawback: a lot of water can gush through the big holes that are in the first third of the wall close to shore, which creates "general unrest" in the water in the near-shore section. But then, I need to practice speedsurfing in chop, so I'll borrow a quote from Coach Ned: "It's all good".

1 comment:

  1. If you are finding a foot's difference in water level in noticeable in gustiness, that explains our torture. A man-made lake, ten feet low because of rain levels, surrounded by 100 ft pine trees. (SE US)
    OTOH, we are out of neoprene now, which is nice.