In the paper which he read on 19 April 1967, Captain Dickson described in some detail the investigations which have been carried out to determine the underkeel clearance which should be allowed in any given circumstance. These investigations have concentrated on the vertical movement of a tanker under different conditions, particularly in determining squat and the effect of wave motion. However, another factor in the underkeel clearance equation is the charted depth and this also is affected by ship movement, in this case the movement of the surveying vessel which obtained the soundings. In the following note the probable effect of this movement is discussed and suggestions made by which the effect might be reduced.

I would like to comment on the change in underkeel clearance due to the motion of a ship in a seaway (A. F. Dickson, this Journal, 20, 363).

Captain Dickson, in his conclusions, states that known techniques do not allow underkeel clearance to be calculated when ship motion is present. In fact there are a number of reliable ways of calculating the motions of a ship in waves (for example References (1) and (2), which treat the case of pitch and heave) which may help in this problem. These methods usually assume the ship is in deep water and is heading directly into the waves which are further assumed to be long-crested; but I believe it may also be possible to make reliable calculations for shallow-water effects and for waves which are, in reality, short-crested.

We would like to reply to some of the comments and all the criticisms made on our suggested amendments and additions to the existing (1960) Regulations for Preventing Collisions at Sea, which have appeared in this Journal(20, 103, 340).

to Captain G. Swallow, R.N.

Upon reflection we agree to some extent with the comment on our suggested re-wording of section 16 (b). However, for the very reasons he has pointed out, with reference to certain court cases, we feel that the mariner must be made absolutely certain about his position in law with regard to this section. This section, as it stands, can be very misleading in that it gives the mariner in charge of a power-driven vessel the impression that he need not always stop his engines if he hears a fog signal forward of his beam from another vessel, which has not been sighted. Yet in all the court rulings that we have seen, concerning the interpretation of this section, there is not one case where a mariner has been excused from failing to stop his engines on the grounds of the ‘apparent’ escape clauses, i.e. because of ascertainment of position and/or practicability.