by Bruce N. Crandall
LOA 21', BEAM7' 11",DRAFT 10 1/2", DISPLACEMENT 1280 LBS.
Here’s a 21-footer designed for high speeds in rough water. Plywood construction makes her easy to build.
This is a semiround bottom, high-speed model designed for use on waterways which are too large and open to be safe with the average-sized outboard boat. While a developable-surface design such as this one is not as efficient at low planing speeds as some other types of bottom design, at somewhat higher planing speeds it becomes the most efficient, and in other words the fastest, of all equally soft-riding planing types. A boat of this size will attain planing speed between 19 and 21 mph, and at 29 mph and over this design is at its best efficiency. The hull is designed for planing speeds in rough or quite choppy water with motors up to over 100 hp. About 40 hp will be necessary to make the boat plane with a light load, and with 60 to 70 hp it will begin to reach its most efficient planing speeds. It will be noticed from the construction plan that there are a great many frames and bottom battens. The framework is extremely strong and forms such a good backing for the plywood that the boat will be able to stand the pounding of any amount of rough water as well as the roughest sort of trailer travel. Of course there is a lot of tedious work to laying out and cutting to shape so many frames but it is well worthwhile from the standpoint of strength as it results in the strongest and lightest type of construction. Equal strength could not be attained without the use of such closely spaced frames and battens unless plywood thicker than 3/8“ were used, and such plywood is virtually impossible for the amateur to handle in large sheets.
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