‘Studies of Interaction at Sea’

Extract

I note with interest Dr Barrass' comment in the January issue of the Forum (33, 139). He deals with three subjects: (i) the ultimate trim of tankers in shallow water; (ii) the question of increasing rpm in interaction situation; and (iii) the final turning diameter in shallow water.

Dr Barrass' curves (p. 140, Fig. 1) are useful approximations to squat and to ship characteristics but cannot be treated as definitive.

Fig. 2 is a particular interpretation of practice and not necessarily universally accepted, particularly in respect of the LCB position. In some recent VLCC work benefits have been obtained by moving the LCB aft in association with a buttock-flow stern. Furthermore, the dependence of block coefficient upon Froude number is not realistic, especially in the finer vessels; it is the prismatic coefficient that is dependent upon Froude number. It cannot be taken, therefore, that absolute curves for these factors exist as inferred from Fig. 2.

Turning to Fig. 1, the curves showing sinkage in confined channels are for the blockage ratio bracketing 0·1–0·3. This is a large range of blockage and as a result the curves can significantly underestimate and overestimate sinkage, depending upon where in this bracket a particular example falls. Similarly, the solid curves show sinkage in open water conditions over depth/draft ratios 1·1 – 1·2. This is quite a large range and a unique curve cannot represent these two values.