2-24 BOAT HULL DESIGN 



For interior decks, non-tight cockpits, and cabin tops, the design load depends on the 

 intended use of the boat. The load for the interior deck of a privately owned vessel may be 

 taken as 40 pounds per square foot, while the passenger deck for a high speed ferry such as 

 a service craft for offshore drilling rig should be designed for approximately 60 pounds per 

 square foot. The above loadings are for the at-rest condition. The increase for accelera- 

 tions due to pounding in a seaway depends on the requirements of the service, and on the 

 arrangements. In high speed offshore ferry service, where it may be necessary to main- 

 tain high speed in rough seas, a "g" or gravity factor of two should be applied to the above 

 loads for flats in the forward portion of the vessel; that is the loads given above should 

 be doubled. 



An important design consideration for decks, particularly in walking areas, is deflec- 

 tion. The allowable deflection, as previously discussed in the section on shell design, is a 

 subjective matter. An allowable deflection of 1/ 200th of the span under the design loads 

 discussed above is recommended. 



Decks, particularly interior flats, are usually constructed of plywood or of sandwich 

 construction with a lightweight core and fiberglass faces. For maximum economy with 

 fiberglass construction, a single molding including deck, seats, and any other interior 

 accommodation is recommended where feasible. 



In small boats, with single skin construction, the use of contour, i.e. raised areas, 

 in lieu of stiffeners is very common. 



Gunwale: For open or partially decked boats, some form of gunwale must be provided 

 to strengthen the upper edge of the side. The most severe loading likely to occur on a 

 gunwale will vary according to the size of the boat. However all gunwales are subject to 

 impact at docks and from other boats. In a boat small enough to be man-handled, it is 

 possible that the boat will be rolled over on its gunwale for cleaning or storage. The 

 gunwale should therefore be able to withstand the weight of the boat concentrated at the 

 center of the longest span between gunwale supports. The factor of safety for this loading 

 condition may be quite small, say 2 on the ultimate wet strength, since it is not a normal 

 service load. 



For larger boats, the gunwale will be used, in many cases, as a step when boarding 

 or leaving the craft. The stability of the boat should, of course, be checked for this 

 condition and if found inadequate, the shape of the gunwale should discourage passengers 

 from attempting to step on it. In the event that the gunwale is intended for use as a step, 

 the necessary width, approximately 3 or 4 inches, should provide ample strength for boats 

 with conventional arrangements. For common gunwale types, see Chapter 3. 



Bulkheads: Transverse bulkheads in small pleasure boats are not usually intended to 

 withstand water pressure, as is the case in larger vessels. In some cases Coast Guard 

 Regulations (16) require bulkheads to resist water pressure. In the case of small com- 

 mercial vessels carrying more than six passengers the bulkhead must be able to withstand 

 a head of water to the top of the bulkhead. 



In addition to the possible requirement for withstanding a waterhead, bulkheads are 

 generally required to support the weight of equipment mounted on them, the loads imposed 

 by the deck and side framing and any local loads, as from a mast. A factor of safety of 4 on 

 the ultimate wet strength is recommended. 



