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Current position of the SSV Robert C. Seamans. Click on the vessel to view position history. Use the tools, top right, to change the map style or view data layers. Dates and times use GMT (Greenwich Mean Time).

SEA Currents: SSV Robert C. Seamans

May 26, 2020

Passage Planning

Rick Miller, Captain


Atmospheric circulation pattern. Image courtesy of NASA Earth Observatory

As the Robert C. Seamans sails from Hawai’i to California, we’re telling the story of the Pacific through ArcGIS Storymaps. We’re reposting some of the entries here, in our SEA Currents blog, but we encourage you to follow regular updates by going directly to Pacific Crossing 2020 StoryMaps.

The SSV Robert C. Seamans is underway from Honolulu, Hawaii bound for San Diego, California. A look at a map shows the shortest route is East Northeast (or about 065°T) and a distance of about 2,400 nm. So why is Seamans heading close to due north? It is all about the wind! Being a sailing ship, the Captain and crew of Seamans intend the sail the ship as much as possible to their destination. To do this, they’ll have to follow the relatively consistent wind pattern in this region.

What makes wind? Unequal heating of the earth’s surface is the answer. The sun’s radiation is more intense near the equator (the tropics) and much less progressing to the poles, while the land heats (and cools) quicker than water. This creates areas of high pressure and low pressure around the globe. Air moves from high pressure toward low pressure, and that is wind! The equatorial region is an area of low pressure. It is hot, and as hot air rises (think of hot air balloons!) it creates low pressure at the surface. A contrasting region near 30°North or South, has air aloft descending, creating an area of high pressure on the earth’s surface. There is another region of low pressure near 60°N or S and high pressure at the poles. The high pressure near 30°N is the feature Seamans is encountering on this passage. It is the North Pacific High, a persistent area of high pressure centered north and east of Hawaii (in the North Atlantic there is a similar feature known as the Bermuda High or Azores High).

Looking at the weather forecast map on the right, air from the center of the high moves straight away from the center in all directions toward lower pressure. Due to the earth’s spin on its axis, the Coriolis effect turns the wind to the right (in the northern hemisphere), creating a clockwise circulating wind around the high. The blue arrows show the wind direction around the North Pacific High. Sailing that shortest distance to San Diego, the ship would be going straight into the wind the entire passage (not efficient or fun). These are the NE Trade winds. The orange line shows the probably route of the ship as it transits around the high following the wind. They may reach as far as 40°N before heading east and then south. Transiting south and east offshore of the California coast, not only are the winds usually from the north northwest, but there is also the California current moving south. The ship will enjoy a great sailing wind with a boost from the current as it closes in on San Diego!

On a similar passage on the Seamans several summers with SEA Semester programs I’ve followed a similar route. It is a great passage! Starting in warm tropical trade winds the ship reaches into colder waters north and the prevailing westerlies. The remainder of the passage is in cold water and cooler air. The passage inevitably passes through a portion of the “North Pacific Garbage Patch” – but more on that in coming days.

- Captain Rick Miller

Categories: Robert C. Seamans, • Topics: pacificcrossing2020 • (0) Comments


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