Signalling and Safety at Sea. 23 



decisive on the question of collision or no collision. By 

 comparing what may be called the "danger bearing" and 

 the u danger rate of approach " with the actual bearing and 

 actual rate of approach he obtains complete assurance on 

 the serious issue before him. Thus, for example, with the 

 courses and speeds cited above the navigator finds that B 

 will bear XNE. from A, and A will bear SSW. from B, 

 and the rate of approach of the ships towards each other 

 will be 24 knots per hour: if collision is threatened and only 

 if collision is threatened. 



Accordingly, the navigator can tell whether collision is 

 threatened or not (a) by observing the actual bearing of the 

 other .ship or (b) by determining the rate of approach of the 

 ships. 



A very large percentage of cases presented to him may be 

 at once dismissed by determination of bearing. The bearing 

 of the other ship may be determined by radio-goniometer or 

 by submarine signalling. I do not think the accuracy of 

 such determinations will suffice for all cases of threatened 

 collision. This point must be further considered later. But 

 fairly accurate determination of bearing would suffice to 

 rule out many cases. Suppose, for instance, in the case of 

 A and B above, that the bearing of B from A is observed 

 to be more than a point divergent from NNE., i. e. from 

 the danger bearing ; it is then certain that collision will 

 not occur. The Officers on A and B might exchange a short 

 code signal expressing understanding on this point. And, 

 of course, both know that the existing courses and speeds 

 which insure safety must be carefully held and maintained. 



But if there is any close approximation of the observed 

 bearing to the pre-determined danger bearing, then the 

 determination of the rate of approach of the vessels would be 

 entered on at once. For one thing there is nothing in the 

 observation of bearing to tell the navigator the distance of 

 the other ship. Guessing this distance by the strength or 

 intensity of the signals may prove seriously inaccurate. And 

 with no knowledge of distance the sailor is placed in a very 

 anxious position, and one which may compel him to alter 

 course quite needlessly — as we shall see. 



The determination of the rate of approach is got by 

 successive determinations of the distance separating the 

 vessels. Now, knowing the danger rate, the sailor can tell 

 from the very first determination of distance ichen collision 

 would be due, supposing it to be threatened. And this tells 

 him whether he must act at once or whether be has plenty of 

 time. He knows, in fact, " there are so many minutes to go 



