That Big Boat Swing

If you have ever been at 15 feet, waiting for your dive boat to reach the end of its arc, maybe 100 feet or more away, and then waiting for it to swing back to you, you know the anxiety of wondering whether you will miss the line or the dive platform when it passes you again.

All vessels swing at anchor. They are at the mercy of wind and waves, and if the wind changes direction completely, they can even swing through more than 180°. For divers, this is significant. A small day boat will not swing over much distance, but a 46-foot Newton will swing farther, and the 120-foot Aggressor seems almost to disappear.

Liveaboards are getting bigger, something to keep in mind. If you are at the surface near the stern, it's relatively easy to approach and grab the ladder at the right time. But if you are diving from a boat that has a deco bar, you may have to wait at your first stop depth and watch for the bar to swing by.

The MV Odyssey, operating in Truk Lagoon, is a large vessel that is used directly as a dive platform. Divers will be aware of the swing as they wait for the wreck marker (often a small buoy) to come within sight before committing to the water.

In poorer visibility, it's best to locate the boat's anchor chain so that you will be at the point of the vessel's pivot before making your way to the stern but always stay away from the propellers. Alternatively, make your safety stop well away before making your way to the vessel on the surface.

Thankfully, most large liveaboards use Zodiacs or other support boats, so you won't encounter this phenomenon often.

-- Ben Davison