View non-flash version
www.sname.org/sname/mt April 2013 April 2013 began to be used in the very late 1800s for strength with weight saving. For the Arrow , delivered in 1902, Charles Mosher used steel frames in the machinery space, else- where steel frames below the waterline and aluminum above, double-planked mahogany for hull skin, and wood deck except aluminum over machinery space. Wood continued to be used for hulls of performance vessels. American PT boats in World War II were mass produced using plywood for hull skin. Cold-molded production for hull skin (using two or three plies of thin planking or plywood adhesively bonded) continues to be used for con- struction of custom sport shing boats. After World War II, growth in the number of aluminum vessels accelerated to support offshore oil production. Molded berglass boat production also grew rapidly for rec- reational craft. e United States Navy, in the mid 1960s, procured 52-ft. landing crafts powered by dual 1,000 HP gas turbines, which proved to be more of a learning expe- rience than an advance in operational capability. Most recreational and military small craft were constructed from berglass, and larger performance vessels were aluminum. Around mid-1975, composite hull technology (various combinations of berglass, Kevlar, carbon and core mate- rials) began to be constructed. Currently, very lightweight hull structures are produced with carbon ber laminates infused with epoxy resins. Time-saving production techniques began to appear through advances in computer numerical control (CNC) manufacturing. Structural components fabricated from 2-D plate materials (aluminum, steel, plywood, etc.) could be nested e ciently with CAD capability. Reducing material scrap when cut by CNC machines, pieces are then tted together with reduced manual labor. Plastic and composite parts can have 3-D plugs and/or patterns machined with 3-, 5-, or 7-axis CNC equipment out of low-density foam from which production molds are made. Very large proto- type and limited production tooling can be milled directly with current equipment. e 2-D machining of plate materials and 3-D milling of plugs and patterns for composite construction reduces time and labor to delivery of a completed vessel by about 15 percent as compared to total manual fabrication. Current state It has been 150 years since powered monohulls slipped past hull speed. It took 40 years to go from hull speed to 40 knots and another 100 years to gain an additional 30 knots. And where are we now? Considering speed in calm, deep water as a measure of performance, Figure 5 shows the comparative position of several signi cant military vessels and modern motor yachts. Bare hull resistance-to- weight-ratio (R/W) curves scaled to a displacement of 500 mt are also presented in this gure for several round-bilge models from the National Physical Laboratory series and hard-chine models from DTMB Series 62. ese curves pro- vide guidance as to the speed ranges where each type hull form has preferred resistance characteristics. Speeds of military vessels CVN-77, LCS-1, CPIC, and the 80-ft. MK V patrol boat are indicated. e performance of some modern motor yachts is located at their reported maximum speed. Motor yachts Predator (72.8 m) and Silver (73.3 m) report maximum speeds of F nL = 0.54 and 0.57, respectively. Yachts Lazy Me and Pure One have higher Froude numbers than LCS-1 at F nL = 0.84 and 0.90, respec- tively. e Ermis (37.5 m) reports cruise speed of F nL = 1.00 and maximum speed of F nL= 1.62; the fastest motor yacht is Fortuna (41.5 m) at F nL = 1.89.The operational speed range of Destriero during its Atlantic crossing record event in 1992 is noted to be F nL = 0.89 at departure from New York and F nL = 1.27 on arrival in England. is wide range of Froude numbers shows that propulsion machinery composed of gas turbines powering waterjets can e ciently cope with variations in displace- ment due to changes in consumable fuel burn-o and/or cargo o -loading. An interesting point worth noting is that the R/W curves at heavy loading indicate a round-bilge hull would be the preferred choice while a hard-chine hull would have the least resistance at light load. When oper- ational requirements result in this overlapping hull form choice, best overall speed and range are attained when the hull form for light load is selected. It is important to r emember that, while achiev- ing low hull resistance in order to have a fast vessel with good fuel economy is a worthy goal, speed in a seaway without adequate dynamic stability and good ride quality will result in an ill-served owner. MTDonald L. Blount is president of Donald L. Blount and Associates, Inc. in Chesapeake, Virginia, providing engineering and consulting services for high-speed special-purpose co mmercial, military, and recreational craft. In August 1992, this 67.7 m vessel made a record-setting crossing of the Atlantic Ocean non-stop from New York to England in 58 hours, 34 minutes, averaging 53.1 knots.