Monday, March 17, 2014

Passage to Nuka Hiva, Day 7.

Date: Mar 17, 2014
13° 45' N 120° 26' W
Previous 24 hour run: 118nm
Water Temp: 80
Wind: NE 17. Boat: 6.5 @ 232
Log: 11660
Distance to go: 1808nm (Distance from the start was 2620nm.)

I've been out for six days and four hours. The previous day's run (March 16th), over 24 hours, was 139nm.

The sailing has become more downwind, with the wind mainly in the NE quadrant, a little ENE which backed pretty quickly back to NE. I expect the winds to become more ENE soon.

I am currently on starboard tack, wing-and-wing with the genoa poled out. I'm running a little north of the line I have penciled in to a waypoint at 9N 126W. Its in this area, and as I approach it, that I will start to consider my options for crossing the ITCZ. I realize that some of you know a lot about the ITCZ, and some of you have no idea what those letters mean.

In the past, the ITCZ area has been called the doldrums. The new name for it is ITCZ. What's going on is that south of the ITCZ, which in this part of the world is generally 4-8 degrees north of the equator, you have SE trade winds blowing, north of the ITCZ you have NE trade winds blowing. If you notice - that means there are two bands of wind blowing together - the winds converge. In the region where winds converge at the surface, they can do nothing but go up. When you have wind going up, that's a low pressure zone - in this case a trough with the special name ITCZ. IT = inter tropical, between the tropics of Cancer and Capricorn. CZ = convergence zone, the winds converge. Low pressure zones are characterized by clouds, rain, and where the winds are converging, low wind. Air moving vertically up isn't wind, wind needs horizontal movement. Its also an area of lots of heat, and heat also causes air to rise. Rising hot air from the surface of the ocean is full of moisture, which as it rises condenses to form clouds and later rain clouds. After building for a while, the rain clouds have the rush of air that was moving up stall out, reverse, and then the water falls from the cloud and in the process creates movement of the air down. This air hits the surface and moves out in all directions, causing havoc with the local winds. Squall winds can be intense, 20-30 knots, appear quickly but normally don't last very long. You can see them approach though, at least if its not a dark night anyway.

When sailing in the winds above the ITCZ, they are mainly consistent, same south of the ITCZ. I think. Inside the ITCZ, and close to it, the winds are highly variable. There will be squalls down there, rain, clouds, high winds, low winds, winds from different directions. Its an area everybody has to go through to get to the South Pacific and generally you want to do it as fast as you can. In the old days, sailing ships could spend many days in the doldrums, waiting for some wind to arrive.

Of course I've never been there, so this is what I understand. Book learnin' you know.

Lots of people turn on their engines when they hit low winds in the ITCZ and just motor on through. The width of the zone varies from 50 to 300 miles wide. There are however, few sources of information available to help choose a place to cross the zone. Ideally you would cross at a narrow point which has some wind so you could sail the whole way. I would rather sail than motor, but am not a purist...

I've been downloading GRIB data weather files with wind and rain data, and this helps a little to choose where to cross. I can also download a text forecast. This morning's text forecast said a bunch of stuff, the gist of which is that the ITCZ currently is south of where it normally is (04N 121W to 01N 130W) with no significant convection. This is interesting, and may alter my choice of how soon I try to cross the zone. More on the ITCZ later...

Not much going on out here. Sailing along toward my destination. I had to alter course last night at 1:30am to avoid a close encounter with a freighter. I saw it on AIS over 20 miles away and noticed that the crossing the AIS display was showing varied between 2 miles ahead of the freighter, to 2 miles behind and everything inbetween including ramming straight into its side. I watched this develop for a while and the crossing stayed confused, so an hour out, I jibed. The jibe left me on a better course toward my destination as well as avoiding the encounter. I ended up crossing 3 miles behind it at 3am. I have had to alter course twice so far to avoid freighters. Surprising eh.

Well, carry on with your days. I'll do the same.

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1 comment:

  1. Good luck getting through the zone. Sounds like you might have a good window.