The best written description I have found is from Howard Chapelle's History of the American Sailing Navy (pp. 238-9) [in the book it is one long paragraph, so I have added some paragraph breaks for ease in reading - dp]
"In America . . . [by 1812] . . . the long gun on a pivot had now reached its height in professional popularity, which it was to maintain for sixty years of more, until it was replaced by the turret mount. . . . The mount most commonly used consisted of a metal ring, or 'circle' of from 9' to 12' in diameter, on deck and brought level athwartships by a wooden foundation.Chapelle then goes on to discuss the "great many variations" in the mount. So take the above only as a guide, to be replaced if you can get anything more specific to your ship.This circle was usually of iron, though copper and brass were also employed. The section of the circle was a shallow "U" shape, hollow side up, about 5 1/2" wide and 1" thick. The inside and outside rims of the top were raised 1/2" and were about 1/2" wide. Rollers traveled in the track thus formed.
These were on the bottom of two horizontal timbers, 8" to 12" square, called 'skids,' which were secured together by three or more blocks, or 'chocks,' and bolted. The skids were parallel and usually a couple of feet apart. On the top inside edge of each there was a rabbet running the full length of the skid. In this the bottom of the gun mount could slide. The skids were pivoted at the middle, or thereabouts, by a heavy pivot bolt, or pin, which passed through the center chock of the skids and thence through deck and a heavy timber plate in the deck and was often heavily bushed in the skid-chock, since the strains of recoil were largely concentrated on this structure.
The gun mount consisted, as a rule, of the standard broadside carriage without trucks, the bottom of the side brackets of which rested in the rabbets on the upper and inner side of the skids. Sometimes there were rollers on the underside of the brackets, or the trucks were retained and traveled in the grooves in the skids.
The gun was trained by prying the skids around by means of handspikes. Recoil was controlled by breechings - heavy rope secured to the breech of the gun and fastened either to ringbolts in the deck about the gun, or on neighboring bulwark stanchions. Small guns had breechings secured to the skids, but this put a greater strain on the pivot bolt than was desirable, so when the gun was brought to bear on a target the breechings were commonly hooked onto ringbolts in deck and rail. Neither gun nor mount was particularly suitable for firing on fast-moving targets."
As to why pivot mounts were so popular, I think that it had to do with weight. If the gun is not mounted on the ship's centerline, especially in smaller vessels like Niagara and Oneida, you have to have a second gun on the opposite side or the ship will list constantly. Instead, you can have one gun, of a larger caliber, for the same weight as two smaller ones. And, since most sea battles were fought one broadside at a time, you would have time to turn the cannon while the ship was tacking.
I suspect that the forward mount of the Oneida was not common, for similar
reasons. Although it had the advantage of allowing firing to the forward port
and starboard quarters, its position would have driven down the bows into every
wave. This could not have been good for its sailing qualities (this is a
guess, as I know next to nothing about sailing).
{Dan Pariser}
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