The greatest joy of a pocket-book is that it is a personal possession immediately available to its owner without having to wait one’s turn for books in the chartroom or on library bookshelves. And I find the consensus of opinion is that the greatest usefulness will be obtained by turning away a bit from practicality (no one can teach how to splice a wire in one small page of print) and giving, instead, more guidance as to where information can be obtained and by acting as a personal aid to memory. I have therefore, with the publisher’s permission, plundered the rich storehouses of his many nautical manuals; I have retained quite a few gems from the knowledgeable Captain McKerrell and have inserted some items gleaned during my own sentence of 20 years hard labour at sea. For a technical author at this particular juncture the greatest difficulty of all is that many nations use the metric system but it is far from universal. Even as I have been compiling this book, the U.K. Government has announced that there will be no compulsory metrication only voluntary use of the metric system if desired. I have therefore retained some Imperial-measure calculations for at least another few years.