WIND, SPRAY, AND SPEED

"College students sure look young these days," I thought as I made my way through the common room of the housing co-op just off campus where Lee Helm lives. I was there to drop off her cellphone, which she had carelessly left in my chart table after a Wednesday night race. It was long after dark, and much easier for me to drop it off at her place than for her to bike to the marina.

The inside of the housing co-op was surprisingly nostalgic in its décor: psychedelic-themed wall paintings, peace signs everywhere, and posters promoting music acts dating back to my own college years. I thought I recognized an old James Taylor album playing at a low volume from one of the rooms.

When I asked one of the kids where I could find Lee, I was directed to a narrow flight of stairs. At the top, one poster caught my eye just across the hall from the door to Lee's room. It was a promotion for the 1990 Singlehanded Transpac, showing a small ultralight racer under spinnaker in a squall, in a very dynamic pose that evoked speed and danger, with spray flying from the bow. 

"Like my digs?" Lee asked as she invited me into her room. 

"Takes me right back to my own college years," I said. "And that's a great poster you found, the one right out in the hall."

The room was small, but she said she was lucky to have a single. There was sailing, windsurfing and paddling gear everywhere, and she had to move a stack of books from a chair to make a space for me to sit down. I was surprised to see that someone with such a structured intellectual mind would allow their living space to look like the inside of a dock box. 

"I love that painting too, Max. It was there when I moved in. There's nothing like DeWitt's artistry for technical accuracy combined with awesome use of colors. And like, it tells a story: There's only one person on deck 'cause it's the Singlehanded T-Pac.

The boat is flying along, but the driver can't let go of the helm to take down the big chute — the waves are too big for the autopilot. If he goes forward, the boat will crash. He's like, stuck there all night, exhausted but going really, really fast, till the wind speed drops a little at sunrise." 

"I've been there," I said, looking back out at the poster through the open doorway. "Only DeWitt can use those hot colors to evoke a wild ride in the moonlight. But if I were sailing like that, I could wake up the off-watch when it was their turn, so we got the chute down without a crash. How fast do you think the boat in that painting would be going?"

"Seventeen knots," Lee stated with authority. 

"Looks faster to me," I said. "How do you know it's 17?"

"The spray sheet gives it away," she explained. "You can tell how long the drops of spray are in the air by how high they go. It's just high school physics: Distance is speed times time, speed is time times acceleration. So to get the distance traveled by an object in constant acceleration — in this case, just gravity — integrate distance with respect to time and get the familiar formula, distance equals one half acceleration times time squared."

She took a blank IBM card from a large stack of cards in one corner of her very crowded desk, and wrote out the distance formula: S = 1/2 A T*2. 

"Let's use H instead of S for the height of the spray sheet. In the painting, I estimate the height of the spray sheet at three feet. If you drop something from three feet up, the formula is 3 = 1/2 32.2 T^2.

Solve for T, and we get…"

I handed her the forgotten phone, and Lee used her favorite calculator app to get 0.4317 seconds as the time for an object to fall from a height of three feet.

"Now double that, because the drop of water starts by being shot up from the bow. Then it stops, then it falls down. Multiply by two, and the drops in the spray sheet are, like, in the air for 0.8633 seconds."

"That doesn't tell us how fast the boat is going," I said.

"Totally. We also need the length of the spray sheet. Estimate 25 feet, assuming the boat is, like, an Olson 30 or something similar. If the drops are in the air for 0.8633 seconds, and they land 25 feet back along the side of the boat, and speed is just distance divided by time, the boat is moving at 25 feet during 0.8633 or 28.96 feet per second or about… divide feet per second by 1.6878 to get… 17 knots."

"That's a good trick," I admitted, no longer skeptical. "Does it always work?"

"Whenever you have a good picture of the bow spray sheet, and a known length for scale, like the length of the boat, you can estimate the spray sheet height and length. And like, do some arithmetic to combine the constants, and the formula becomes V = 1.189 L /sqrt(H), where V is speed in knots, L is the length of the spray sheet and H is height of the spray sheet."

Every square inch of the walls of Lee's small room was covered with photos, charts, or graphs. One photo was a naval ship, probably a destroyer of some sort, cruising along at speed with a very nice spray sheet at the forward waterline.

"Let's try it for that ship," I suggested. 

"Sure. The ship is a guided missile frigate, 567-ft LOA."

Lee found a tape measure under some clutter on her desk, took some measurements, and concluded that the spray sheet was 11.7 feet high and 67.5 feet long.

"Applying the formula," she said as she punched a couple of numbers into her phone, "we get 23.5 knots."

"Looks reasonable," I agreed. "Now what about that one?"

I pointed to a photo of an America's Cup foiling catamaran, producing a huge cloud of spray.

"Not as clean an image," Lee complained, "But let's see what we get. It's going to take a little more work because we don't have an orthographic side view." 

Lee used two measurements for scale: The 75-ft length of the hull was parallel to the length of the spray sheet. And the 87-foot rig, deck to masthead, was parallel to the height of the spray. 

"I get 6.06 feet for H and 74.58 feet for L. So we have… 1.189 times 74.58 divided by square root of 6.06… 36.0 knots."

"Believable," I agreed. "But is this actually useful?"

"Like, I wouldn't calibrate my knotmeter this way," she said. "Its main use is to analyze photos."

Not wanting to take up any more of Lee's study time, I made my way back down the narrow stairway, past the concert posters from the '60s, the swirling painted spirals on the walls, and a hint of the sweet smell of something that was not legal in my day. 

It was nice to know that nothing had really changed in the housing co-op.

— max ebb