Sec. 77.S 



PRELIMINARY DESIGN OF A MOTORBOAT 



825 



made in (|uLet water, either fresh or salt as 

 specified, with a clean, smooth, new bottom, and 

 with the propelling machinery delivering its 

 rated power or some specified fraction thereof, 

 corresponding to the 0.95 factor of Sec. 69.9. 



It is important for the naval architect as well 

 as for the marine engineer to realize that because 

 of the limit on mean effective pressure in the 

 cylinders, any reciprocating internal-combustion 

 engine can produce its rated power only at a 

 certain rate of rotation. At a lesser rotating speed, 

 the power is less. Furthermore, it may not be 

 possible for the engine to deliver its rated power 

 at a higher rotating speed than that for which it is 

 designed or adjusted. This makes it most impor- 

 tant that the small-craft propeller be one which, 

 at a desired boat speed, absorbs exactly the engine 

 output, less the transmission and shafting losses. 

 More power absorption means that the engine 

 and propeller both have to speed up to make the 

 powers match, if indeed this can be done. 



It is customary to overpower the modern 

 motorboat, just as a modern tug or fishing vessel 

 is overpowered, but not for exactly the same 

 reasons. Motorboat overpowering resembles more 

 nearly, in fact, the gross overpowering of the 

 modern (1955) passenger automobile. Engine life 

 is longer and wear and tear is less at reduced 

 powers. The engine power actually required for the 

 greater part of the time is delivered reliably 

 even though the engine may be somewhat out of 

 adjustment or the fuel may not be up to standard. 



For these reasons, and others not mentioned, 

 powering allowances are as necessary for a small 

 craft as for a large one. It is customary for these 

 to cover the power needed for: 



(a) Increase in weight due to thorough Avetting 

 of the boat structure, if of wood, corresponding 

 to a period of at least six months in the water and 

 in the weather, covering at least two seasons 



(b) Unavoidable increases in weight with time 

 in service, because of adding new equipment. 

 Demands for increased carrying capacity, over 

 and above design requirements, are not in this 

 category. Logically, they must be paid for 

 separately, either in increased power or reduced 

 speed. 



(c) Roughening of the bottom surface because of 

 rusting, pitting, oxidizing, flaking, peeling, uneven 

 calking, and fouling by marine organisms 



(d) Reduction of propelling-machinery efficiency 

 and output because of wear and tear, low grades 



of fuel, and excessive time between overhauls 

 (e) Damage to blades of propulsion devices from 

 numerous causes, probably of more frequent 

 occurrence on small craft than on large ones. 



77.8 Principal Requirements for a Preliminary 

 Design Study. The craft selected as the running 

 example in this chapter is a small motor tender 

 for the ABC ship designed in Chaps. 64 through 

 68. The first step in the preliminary hydrodynamic 

 de.sign of this motorboat is to outline the mission 

 of the craft; in other words, to state what it is 

 required to do. After consultation with the owners 

 and operators of the large vessel, this is set down 

 in items (1) and (2) of Table 77.a. It is followed 



TABLE 77.a— Motor Tender for ABC Ship; 

 Principal Hydrodynamic and Other Requirements 



MISSION — The craft described in these specifications is to: 



(1) Serve as a power launch for and to be carried on board 

 the ABC ship whose requirements are set forth in Tables 

 64.a through 64.g 



(2) Serve as an all-weather tender for the ferrying of 

 personnel, incidental packages, and miscellaneous portable 

 articles from ship to shore and vice versa, under special 

 conditions where the ship must anchor or lie-to. 



WEIGHT AND SIZE LIMITATIONS— To permit 

 hoisting it on board the ABC ship the boat is to: 



(3) Have a gross hoisting weight not exceeding 25,000 lb, 

 including a full supply of fuel, other consumables, parts, 

 tools, and two crew members, the latter assumed to weigh 

 350 lb. Hoisting is to be by a single hook on the ship. 



(4) Be capable of stowage, in a secure position for travel 

 at sea, on the weather deck either forward of or abaft the 

 passenger accommodations 



(5) Have an overall length not exceeding 40 ft. 



CARRYING CAPACITY AND ACCOMMODATIONS 

 — The tender is to: 



(6) Be capable of carrying one of the following items, or 

 any reasonable combination of all three, not exceeding 

 3,000 lb in weight: 



(a) Twelve passengers plus a maximum crew of four 



(b) Cargo in packages or luggage, in total weight not 

 to exceed 2,300 lb and in total volume not to exceed 125 

 ft', plus a maximum crew of four 



(c) Eight passengers and a maximum crew of four, plus 

 one litter patient 



(7) Provide protection from rain, wind, spray, and sun 

 for all passengers and cargo. 



SPEED AND ENDURANCE— The craft shall be able to: 



(8) Achieve a speed of: 



(a) 18 kt in smooth water with a half-load of fuel, a 

 crew of two, and two passengers and their personal baggage 



(b) 14 kt in smooth water with a full load of fuel and 

 cargo. 



(9) Run at full power for six hr without replenishing fuel. 



