Sun, Moon, and Tides: Salvaging the Keokuk’s Guns, Part 3

The first two posts in this set have related details of the salvage of guns from the USS Keokuk and discussed the sources we have for that story. Allow me to turn now to some “gritty” details and offer some of my own interpretation.

The key element in the recovery of the USS Keokuk’s guns is the light and tide data. Those factors limited the time allowed for the recovery crew to work.  So let me bring in data about the sunrise, sunset, moon rise, and moon set for April 20 to May 5 (from the US Naval Observatory site).  First the sun’s data for the time in question:

AprilMay63SunlightData

I’ve walked through the definitions of Beginning Morning Nautical Twilight (BMNT), Beginning Morning Civil Twilight (BMCT), End of Evening Civil Twilight (EECT), and End of Evening Nautical Twilight (EENT), but for a refresher, here’s a diagram:

The definition of each is based on the angle the sun is below the horizon.  So while we can say the sun set at around 7 PM during the period in question, the twilight ensured there was visibility until nearly 8 PM each night.  Likewise, “first light” was between 4:49 AM and 4:31 AM.  The bottom line here – “night work” was only possible between around 7:30 PM and 4:45 AM each day.

Next the moon’s rise, fall, and phase:

AprilMay63MoonData

The first important consideration with respect to the moon’s data is the percentage of illumination.  We all know of the fateful moonlight on May 1-3, 1863 and how that affected actions on certain Virginia battlefield.   At Charleston, those nights with full and almost full moon periods coincided with the retrieval operations.  In short, on the nights LaCoste’s crew needed the most natural illumination, they got it.

Second important point is the moon transit times.  Starting on the night of April 27-28 the moon transits were very favorable to those working at night on the waters – rising in the afternoon and setting after midnight.

But, there’s a third factor to consider with the moon – tides.  Yes, you probably recall the moon’s gravitational effects are the major influence on tides.  So while the moon was up, and shining brightly, the tides were also high.  I’ve taken the  tide data for the same dates from the National Oceanic and Atmospheric Administration’s (NOAA) tides page, placing it into this somewhat busy table.  Each day has it’s own tide data graph:

KeokukWorkWindows

Note, this data is “projected” tide levels, not actual.  Since NOAA did not have a sensor buoy out in 1863, the actual tide levels, which are affected by a number of other variables, are unknown.  The red diamonds indicate the projected level of the tide at hourly intervals.  For the period in question these ranged from just short of seven feet over sea level to just under sea level.  During the days of April when workers tore through the turret tops, the tides were not expected to exceed six feet.

The yellow highlights in those blocks indicate the “prime working hours” to the best of my calculations.  These would be during periods of night with the tides under two feet.  My estimates don’t match exactly to Johnson’s “two and a half hours’ labor on each night.”  During some of the best nights for work, from April 24-28, I think there were closer to four hours of prime working hours.

On the other hand, the first days of May offered scant few prime working hours for the crew.  For May 1-2 when the first gun was retrieved, the crews had barely an hour of darkness with low tide.  May 3 offered no prime working hours.  The best hours for work on May 4 and 5 were late evening.  The moon was full and up most of the night.

What’s more, the tide data runs against one element of the story related by Warren Ripley.  It was not a rising tide which pushed the first gun out of the turret, but rather a falling tide.  Call it a very fortuitous odd large wave.  But anyone who’s spent a day at the beach will attest such exist, even with falling tides.

There’s one more measure to consider here. Johnson indicates the Keokuk sank in 13 feet of water.  That matches to Navy charts made during the war.  Consider that Commander Rhind reported the Keokuk‘s draft, before going into action, was 9 feet.  If drawings of the Keokuk are accurate, the line of sight for the Dahlgrens was around 16 feet above the keel.  Even considering settling that would happen, the Keokuk‘s turrets should have remained exposed for more than just the lowest tides.  Given that calculation, would LaCoste and crew have worked on the turret tops or the sides?

Of course none of my data presented here takes into account the weather.  Even a slight storm would have deprived the Confederates of valuable working hours.  But Johnson does not mention any specific nights on which the weather prevented or minimized work.

In conclusion, I’ll offer these points for thought:

  • There were more “prime working hours” than generally believed (not counting for weather of course).
  • The turrets were more exposed than Johnson led us to believe.
  • LaCoste’s crew did not benefit from a rising tide during the recovery of the first gun.

None of those points would have significantly reduce the risk.  Nor do those offered conclusions diminish the daring nature of the operation.  And of course we can still say without doubt that this gun…

Charleston 4 May 10 060

… was recovered from the Keokuk in early May 1863 and used by Confederates for the defense of Charleston.

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