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The harbour defence motor launch (HDML) was a British small motor vessel of the Second World War. The HDML was designed by W J Holt at the Admiralty in early 1939. During the war, 464 HDMLs were constructed, mainly by yacht builders, in the United Kingdom and a number of other allied countries. In view of their later expanded combat roles in some Commonwealth navies some HDMLs were re-designated as "seaward defence motor launches" (SDML) or "seaward defence boats" (SDB).〔(Sea Power Centre history page for SDB 1323 ) Australian Navy〕 ==Design and construction== HDMLs had a round bilge heavy displacement hull long with a beam of and a loaded draught of . Loaded displacement was 54 tons. The hull had a pronounced flare forward to throw the bow wave clear and provided considerable lift to prevent all but the heaviest seas from coming aboard. Although seaworthy, the boat had a considerable tendency to roll, especially when taking seas at anything other than directly ahead or astern. The cause was the round bilge midship section and a considerable reserve of stability, the effect of which was to impart a powerful righting moment if the boat was pushed over in a seaway. This, coupled with the round bilged hull and lack of bilge keels, would set up a rapid and violent rolling. One of the design criteria was that the boat had to be capable of turning inside the turning circle of a submerged submarine. To achieve this, HDMLs were fitted with two very large rudders and, to reduce resistance to turning, the keel ended before the stern. A side effect of this was that the boat lacked directional stability and was extremely difficult to hold on a straight course. The hull was of round bilge wooden construction, planked with two diagonally opposed skins with a layer of oiled calico between them – known as a “double-diagonal” technique. The hull was completed with frames or “timbers” riveted perpendicularly from the keel to the gunwale on the inside of the planking, forming a very strong hull. The hull was further strengthened by the addition of longitudinal stringers riveted inside the timbers together with further timbers, known as "web frames". They are fastened inside the stringers opposite every third main timber. HDMLs were fitted with a deeper section rubbing strake aft. Its purpose was to roll depth charges (kept in and delivered from racks on the side decks), clear of the hull and propellers. Most HDML hulls were planked in mahogany, but as the war progressed this became scarce, larch was used although this tended to lead to leaky hulls. The decks were also of double-diagonal construction and generally made of softwood. Boats operating in tropical waters (including the Mediterranean) were sheathed in copper below the waterline to prevent the attack of marine borers. In order to lessen the chances of the boat sinking in the event of damage to the hull, they were divided into six watertight compartments. Provided that the bulkheads were not damaged, the boat could remain afloat with any one compartment flooded. Some were constructed in the United States and nominally owned by the United States Navy, but delivered to the Royal Navy and other allies under Lend-Lease. Most were returned to the United States Navy at the end of the war before being sold to other countries, the majority to the Royal Netherlands Navy. 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Harbour Defence Motor Launch」の詳細全文を読む スポンサード リンク
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