2-22 



BOAT HULL DESIGN 



For boats with high power, the transom design should be checked to insure that the 

 minimum dimensions given above are adequate. To aid the designer, Fig. 2-17 has been 

 prepared. This Figure provides the means of determining the required section modulus 

 of the transom. It has been prepared based on the following assumptions: 



The upper portion of the transom is 

 considered to be a simply supported beam 

 loaded with the thrust required to move 

 the boat. 



The thrust used in developing the 

 Figure was determined by taking the ef- 

 fective horsepower equal to 1/2 the motor 

 horsepower (1). 



To use Fig. 2-17, enter the graph at the 

 total horsepower to be used, read up to the 

 appropriate line for the speed and number of 

 motors, and read across to the value of the 

 parameter 



Z X ^ 



MOTOR HORSEPOWER 



(Z x fb>/& 



(2.6) 



Z = SECTION MODULUS IN INCHES 

 ffc = ALLOWABLE FLEXUAL STRESS IN PSI 

 B = BEAM AT TRANSOM IN INCHES 

 MPH = Ml LES PER HOUR 



Fig. 2-17. Transom section modulus 

 as a function of speed and power 



By using the known values of allowable flexural 

 stress, fb , and the beam of the transom B, 

 the required section modulus, Z , may be ob- 

 tained. A factor of safety of 6 is recommended 

 in determining the allowable stress. It should 

 be noted that a stronger transom is required 



to support one 40 horsepower motor than to support two 2 horsepower motors mounted side 



by side. 



The most common and practicable method of construction for transoms is a sandwich 

 arrangement with a plywood core and fiberglass facings. In computing the effective section 

 modulus of the transom., it is recommended that the following assumptions be used: 



The effective width of the transom should be taken as the maximum vertical 

 dimension of the motor clamping area as given in Table 2-1. 



Only the plies of plywood whose grain is horizontal, that is parallel to the span 

 being considered, should be taken as effective. These will normally be the face 

 plies, and alternating "center" plies within the plywood. This procedure is recom- 

 mended by Ref. 15. 



Where a seat or motor well is made an integral part of the transom, this should 

 be included in the section modulus. 



For an example of the use of Fig. 2-17, see Design Example 2-3 for the 19 foot runabout. 



Resonant vibration does not normally occur at the transom. The vibration felt there is a forced 

 vibration excited by the motor, but the fundamental frequency of the transom is so far from any nor - 

 mal operating range of the motor that resonance with this frequency is not a problem. 



The unsupported transom is used for boats with low power, but this not usually satis- 

 factory for boats in the high power range. Several methods of support are used, singly or 



