“Sneak Ability”
Stern view of PT-117 at idle, August 4, 1942 Negative #1984.187.104782F
Over several years of development and sea trials of possible PT boat designs, the essential elements for a successful patrol torpedo gunboat evolved. As one Naval officer observed during evaluation of a manufacturer’s boat, “The three vital characteristics of a PT boat are: (a) SNEAK ABILITY; the ability to reach attack position undetected; (b) TORPEDO PUNCH; four fast torpedoes with large warheads; and (c) SPEED; giving the boat the ability to get away.”
Nowhere were these unique characteristics more important than for the PT boats serving in the South Pacific, especially against the so-called “Tokyo Express.” This was the main shipping supply line from northern Japanese bases to the now-occupied Philippines. The Japanese navy utilized enemy destroyers and large barges as part of their supply line to convey troops, food, and war material. The “Express” in the term indicated that shipment of supplies to southern Japanese forces occurred almost nightly. These supply ships steamed south slowly, close to shore and mostly at night. This was done to reduce noise and ship-wake visibility that could attract allied aircraft and PT boat patrols. For similar reasons, the American PT boat designers learned to counteract the Japanese by design features and tactics that would permit successful “sneak” attacks on the enemy shipping.
The problem of wake visibility was the most difficult challenge and engineers worked on this by testing various colored dyes or oils that could be let overboard in an attempt to color and disguise the PT boats’ wake. These attempts were mostly unsuccessful, so using only one of the powerful Packard 1500 horsepower engines on slow approach to an enemy target mitigated the problem. Also, the engine and shaft used was the center of the three, which had the deepest propeller, thus causing the least wake.
The engine noise issue was dealt with in a unique way by applying mufflers. These mufflers, pictured here in the stern view image of Elco’s PT 117, were attached at angles to the stern exhaust ports with a “damper plate” that was closed at idle or slow approach speed to direct the engine exhaust noise underneath the water and hence reduce engine noise. After striking the target, the damper plates, also referred to as baffles, were opened to allow direct exhaust to the night air reducing back pressure on the three fully engaged Packard engines. The “mosquito boats” would then retreat at an impressive 40-42 knots, usually much faster than any enemy ships in the attack zone.
A number of other PTs were damaged or destroyed by enemy ship or aircraft fire during their wartime service, but in large measure their uniquely designed quiet muffled approach and high-speed retreat capability limited damage to these boats.
(See related post “The Plywood Derby”, March 2017)
Guest post by Lee Greenwood, Rosenfeld Collection volunteer