The U.S. Navy wanted a gunboat to cover PT boats, but selected the SC-497 submarine chaser as its starting point. The converted PGM-1 carried a three-inch dual-purpose gun, a 40 mm gun, multiple .50-caliber machine guns, and a mortar, yet retained a top speed of roughly 21 knots against PT boats capable of about 45 to 50 knots. The Fairmile D avoided that mismatch because its gunboat and torpedo variants shared similar operating characteristics. The American hull had ample weapons, but adapting a good submarine chaser without changing its power plant produced the wrong mobility for fast-attack support.
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PGM-1 subchaser conversion mismatch
The PGM-1 conversion added formidable guns to a submarine-chaser hull but left its 21-knot machinery unchanged, undermining its intended support of much faster PT boats.