Sec. 77.13 PRELIMINARY DESIGN OF A MOTORBOAT 831 



TABLE 77.C — Weight Distribution for Different Motorboat Types 



The data in this table supplement those set down graphically in Fig. 77. D. The Taylor quotients listed are based 

 upon the waterline length, where this is known. 



(c) Equipment and stores, 6 per cent or 1,080 lb 



(d) Electrical and electronics gear, 1.4 per cent 

 or 252 lb 



(e) Margin, 7 per cent or 1,260 lb. 



The estimated percentages or weights, or both, 

 are checked by the designer with those for planing 

 craft of somewhat similar type, size, and speed 

 range which he is able to gather from any sources 

 whatever. The rehabihty of these data must be 

 determined or assessed by the designer using 

 them. In some few cases it may be possible to 

 make direct weight estimates of certain items, 

 such as electronics gear, by deciding just what is 

 to be put into the boat. 



It appears from Fig. 77.D that the machinery 

 weight of 5,300 lb, with its percentage of 29.4, 

 may be on the high side. On the other hand, the 

 huU percentage of 28 may be low. Good engineer- 

 ing practice indicates that in this preliminary 

 stage the margin of 7 per cent is low; 10 per cent 

 would be much better. A second weight estimate 

 is definitely in order, before the design is too far 

 advanced. 



77.13 Second Weight Estimate. For a re- 

 vised estimate, somewhat more care is exercised 

 in arriving at reasonable figures. As is the case 



for the full-scale ABC ship designed in the earUer 

 chapters of this part, the weight figures given are 

 purely illustrative and are not intended to serve 

 as a rigid guide for any actual motor boat design. 

 The weights, powers, and even the types of 

 engine will change as the art of engineering 

 advances, but a good weight-estimating procedure 

 remains essentially the same. 



First, there are set down the weights specified 

 by the owner and operator, because these can be 

 rigidly controlled for the boat trials. These are: 



Useful load, 6 passengers and 2 crew at 



170 lb per person, 1,360 lb 



Cargo and baggage, 900 lb. 



The largest weight group under the designer's 

 control is that of the hull. For a fast planing boat 

 which is to travel at a T, of the order of 4.00, 

 and for a htill which has to be lifted by a crane or 

 derrick twice every time it is used, a percentage 

 of 33, based on the total boat weight, is probably 

 as small as should be considered. 



The next largest weight group is that of the 

 machinery. Two diesel engines, similar to the 

 GM HN-10 model, are capable of delivering a 

 total brake power of 330 to 450 horses, depending 

 upon the injectors used. The first estimate of the 



