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River Science: Water Depth

Waiting for high tide...
A sailboat run aground at the mouth of a creek.

When traveling through uncharted waters, one of the most basic questions a ship's crew needs to be able to answer is also one of the most important: How deep is the water? In fact, as Juet's journal entries show, Captain Hudson and his crew repeatedly faced the danger of running aground while exploring the river.

Monitoring water depth is just as important for us on board the Replica Ship Half Moon. Of course, in 21st century the Hudson River has been throughly charted, and unlike Captain Hudson we have years of experience navigating this river, but a low tide and a thick silt deposit could still conspire to run our ship aground.

As you see here, even today boaters in unfamiliar waters can be caught out by unexpected tidal shifts.

Mouse over to sound the depths.
Bennett uses a lead line.

When in doubt, we take soundings (measure the water's depth) with the same tool Hudson's crew would have used: a lead line (or sounding line). Flags attached to the lead line mark depth in 6-foot increments (fathoms). The Half Moon has a draft of 8-1/2 feet (extending that far underwater), so it needs at least a fathom and a half of water to stay clear of the bottom.

In Hudson's day, crews would even use their lead lines to take samples of the river bottom. They could fill a small hollow on the bottom of the line's lead weight with tallow, then observed what stuck to this goo when they retrieved the line. Silt or sand made for a fine anchorage, but clean tallow warned of rocks lurking below, making anchorage dangerous if not impossible.

On this Voyage of Discovery, Greg and Ross took soundings during our data collection layover at Grave's End Bay to track changes in the height of tide for their student presentation.

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