The balls sink in and slowly decelerate

Thursday, September 19th, 2019

The Castillo de San Marcos is Florida’s cannonball-eating Spanish fort:

The fort guarded the Spanish empire’s trade routes as well as the surrounding city of St. Augustine, and the English wanted to run this politically and economically important outpost for themselves. Led by Carolina’s governor James Moore, the English boats dropped their anchors and laid siege.

But even after nearly two months of being shelled with cannonballs and gunfire, the fort’s walls wouldn’t give. In fact, they appeared to be “swallowing” the British cannonballs, which then became embedded within the stone. Precisely how the walls did this remained a mystery for the next three centuries.

Cannonball hole and bullet holes in Castillo de San Marcos

Built from coquina — sedimentary rock formed from compressed shells of dead marine organisms — the walls suffered little damage from the British onslaught. As one Englishman described it, the rock “will not splinter but will give way to cannon ball as though you would stick a knife into cheese.”

[...]

Jannotti and the Sanika Subhash bought a few small coquina samples from the gift shop at Castillo de San Marcos, and shot small steel balls at them with speeds of 110 to 160 miles per hour. The idea was to mimic the collision conditions of a cannon firing, albeit in miniature. The researchers also used a high-speed camera that took 200,000 images per second to visualize how the coquina samples reacted to those impacts. They ran similar tests on other materials, namely sandstone and structural foam, in order to compare their properties with those of coquina.

[...]

On the contrary, coquina had a rare ability to absorb mechanical stress, which stemmed from its loosely connected inner structure. Although the little shell pieces that make up coquina are piled and pressed into each other for thousands of years, they aren’t cemented together, so they can shuffle around a bit.

So when a cannonball slammed into the coquina walls of Castillo de San Marcos, it crushed the shells it directly hit, but the surrounding particles simply reshuffled to make space for the ball. “Coquina is very porous and its shells are weakly bonded together,” Jannotti says. “It acts almost as natural foam — the balls sink in, and slowly decelerate.”

Comments

  1. Grasspunk says:

    My house is made of local limestone (dug out from a hole about 40 meters away which is now a pond) but I’d guess that it wouldn’t be as good a material for absorbing cannonball shells as coquina limestone since the particle size is smaller and wouldn’t deform the same way.

    Guess I had better find another way to protect us from the British fleet.

  2. Graham says:

    Sadly, the British fleet isn’t what it used to be. Sigh.

  3. Graham says:

    Also, and with reference to the title of this post:

    “…That’s what she said!”

    There. Now I feel better.

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