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PT Boats of WWII
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PT Boats - General
Post a reply to: PT-3 Information
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[StartQuote] Here are my estimates and comments (opinion) on PT-3s hull design. Based on measurements, I came up with an at rest aspect ratio of about .25 and an on-step (plane) aspect ratio of about .34. The hull is relatively straight chine aft of midships (widest part and transom only differ by 2 ft) and the back portion of the hull only has a slight change in deadrise. The hull is also not concave in form, so together I would expect suction loads to be on the lesser side and would not expect to see much squatting of the boat on plane. Trim angle on plane (from photographs) estimated at about 2.5-3°. Because of the position of the fuel tanks (aft) and the weight of the engine room steel framing and engines, the center of gravity (CG) for PT-3 is pretty far aft. The center of buoyancy (CB) is guestimated at about 23-21 feet from the transom. As she starts to plane, CB would move aft and probably move very close to the boat's CG which I believe to be about 20-18 feet from the transom. She probably rides very well on glass calm based on other George Crouch designs. Deadrise is good at entry indicating potential for a smooth ride, however, in rougher sea states, her lack of a deep forefoot would probably result in some pounding forces, although the steep deadrise and slight convex shape of the bow would help. Having such a large hull sail area out of the water forward would probably make PT-3 very susceptible to beam wind forces when on plane. When operating at lower speeds, the CG being aft of the CB would probably make PT-3 susceptible to yawing motions in following seas. As for turning, she was probably good at slower speeds, but would suffer a bit at higher speeds due to not having the forefoot in contact with the water. I have no idea what PT-3s actual hump speed is. Probably somewhere about 20 kts (WAG). Even with the steel framing in the engine room, weight saving building techniques are obvious, so she was intended to be a planing hull design. This is a fun project to try and figure out this boat with lots of missing info. Hopefully the national archives will have some great PT-3 test info and not prove that I'm not totally full of crap
As my personal email signature states "A little bit of knowledge can be a very entertaining thing". [EndQuote]
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