Laplacian Sailing

Marathon, FL
No LL

Last night almost all of the US East Coast was hit by a winter storm. From Montgomery Alabama to Belfast Maine it resulted in up to a foot of snow. Down here it didn't result in snow, thank goodness, but we did get hit.

Big massive dark clouds passed over mid-afternoon, followed by a thin line of intense rain, and that followed by a night of uncomfortable winds. The winds kept me awake half the night.

Of course, over the years we spent many nights at anchor in winds much stronger than last nights. Most of those nights we slept sounder and felt more secure than last night. Why is that? The answer lies in what we engineers call power spectral density, and acceleration threshold. Non engineers are stuck with plain English.

blustering: blowing in violent and abrupt bursts;
gusty: blowing in puffs or short intermittent blasts;
breezy: abounding in or exposed to the wind or breezes

The adjectives that describe wind, all sound alike so they're not much help. I can describe it best by what we feel inside the boat.

If the wind blows just right, the mast and the wire stays that hold up the mast shudder. The shuddering can be felt throughout the boat. Also, if a gust of wind blows just right, the boad can abruptly change direction enough that we feel it in the seat of our pants. It is not the speed of the wind that causes this, but rather it's unsteadiness. In exreme cases, the boat heels over.

Three weeks ago, when anchored out, a cold front passed. Without warning came a single, abrupt and violent gust that I clocked at 65 knots (75 mph, or 33 m/s, or 120 kph) It heeled us over 45 degrees, but even before we finished saying WOW, it was past and the wind was calm again. We heeled so much because Tarwathie had no time to swing her nose in to the wind.

Almost always while at anchor, the boat is rocking from side to side, up and down, and the stern swings one way or the other. However, these actions are so gentle that you can't sense them at all through "the seat of your pants" If you can't see outside the windows, one could believe that there's no motion at all. During the day, it is the constantly moving beams of sunlight coming through the port holes that remind one of the boat's motion.

If you're not an engineer, stop reading right now.

A boat is a band pass filter for wind induced perturbations. The boat's interia filters out the high frequencies. The boat's ability to swing it's nose into the wind filters out the low frequencies. The transfer function is : s/(1+T1s)(1+T2s) where T1<1/(1+T3s) where T3<
Do the schools still teach Laplace Transforms in today's digital world?