Sailing Through the Trade Winds

The Trade Winds Falter

Tuesday, May 4, 2004 (Day 11 of the Easter Island Passage)

It was chilly last night so I had my hooded sweatshirt on, plus I used a small wool blanket that Marcie put in the cockpit so we can keep our legs and feet warm. It's not actually cold, but when you're just sitting there at night you can feel a chill. Over several days the wind has shifted, from southeast to east-southeast and finally east, so now the wind can blow directly into the cockpit; the dodger no longer blocks the wind.

We finally encountered our planned course line today, after being to the west of it since the beginning of the trip. We have been sailing as close to the wind as is practical, but the southeast winds prevented us from staying east of the course line as we initially intended. Instead, over time we wound up 50 miles west of it, and as the winds have gradually shifted further east we've been slowly working our way back to the course line (which is the great circle route).

Unfortunately, now that we are back on the great circle route and heading straight for Easter Island, we are now on a broad reach in light wind, which is not a great point of sail. The mainsail is blanketing the jib a lot, causing the jib to flutter noisily now and then. Plus the swell is now on our aft quarter, and it will take a little while to get used to the new motion of the boat.

Tonight's sunset.

The sky opposite from the sunset, where the moon will shortly rise into the Earth's gray-blue shadow band.

A mysterious orange moon rising up out of the ocean. I took this picture two and a half minutes after the previous picture.		One and a half minutes later.

Wednesday, May 5, 2004 (Day 12 of the Easter Island Passage)

This morning we discussed various ways to cope with the faltering wind. Our boat speed has decreased to between one and two knots and the jib is drawing poorly due to the light wind from aft. We discussed poling out the jib, or trying out the light-air drifter sail, or altering course. In the end, we decided to do an experiment and alter course to see if Vmg would improve due to a better point of sail. We managed to get another half knot of Vmg (which is a lot when it's only one to two knots to begin with), even though we were now sailing further off our desired track. Later on, we figured, we could turn downwind and pole out the jib, which would give us a better point of sail to make up the crosstrack error. By late morning, the wind picked up a little, but it was still nothing near the continuous 15 to 20 knots we had when we were solidly in the trade winds.

In case you're interested, I'll attempt an explanation of Vmg. As sailors, we can control our boat, but we surely can't control the wind, nor can we control where Easter Island is located. If we draw a straight line between the boat and the island, that's the shortest distance, but due to the wind direction, that might not be the most efficient point of sail. The boat sails most efficiently (that is, most quickly) when it's at a certain angle to the wind. With a given wind, it's entirely possible the boat will sail faster if we point it in a different direction other than directly at Easter Island. Now the question is: are we better off pointing away from the island and going faster, or pointing directly at the island and going slower? To answer this question, we need to do a little trigonometry to figure out what portion of our boat speed carries us towards the island when we're not pointing directly at it. Luckily, the GPS does the math for us and we only have to look at the display and read the Vmg figure (Velocity "made good" towards the destination).

As an example, imagine we are sailing away from the island; Vmg would be negative since we're moving farther away. Now imagine we are sailing in a very wide circle around the island, getting neither closer nor farther away; Vmg would be zero regardless of our boat speed. Now as we steer little by little towards the island, our Vmg increases little by little until when we are pointing directly at the island, Vmg is exactly equal to the boat's speed. So in our practical example above, we could read Vmg off the GPS before altering course, then make the experimental course change, then read Vmg again. In our case, Vmg improved a little, which is good. However, this doesn't take into account the fact that we are no longer pointing at the island, so we will clearly never get there. If we maintained the same course until we passed the island, we would miss it by an amount known as the crosstrack error (which the GPS also computes and displays). So although our Vmg improved, we began accumulating a crosstrack error. The hope is that sometime in the future the wind will change direction so we can efficiently sail on a new course that undoes the crosstrack error so we finally do reach the island.

Whew! Hope you followed that.