The writing-style is dated, but mostly easy-going and often compelling. Exclamation marks are used to denote moments of revelation and there's much "bless my soul, major, it's not often tragedy comes to Newhaven" type dialogue. Sterling stuff.
One notable difference between this Golden Age yarn and those historical detective fictions I've read by modern authors is that Crofts does little in the way of evoking atmospheres of the time through description. Journeys, even a trip on the boat-train to Dieppe, are usually over within a paragraph. Crofts wrote for a contemporary audience and not for historians yet-unborn, so backdrop details often seem taken for granted and merely sketched. But this hardly matters, for the `tache-twirling 1930s Boy's Own idyll of sleuthing with an engineering theme comes alive through the author's teasing voice and the exactitudes of his detective's reasoning. There are no female characters [notoriously hard to draw] and no attempt at a love-interest subplot, indicating Crofts knew his limits and knew his readership. This is a chap's book and, moreover, a book written for the kind of chap who had useful hobbies and who enjoyed tinkering in the garage.
The best bit, for me, comes in the final chapter. We're ten short pages from the close, on the edge of our fireside chairs, racing ahead yet wanting the fun to never end and Inspector French is staked out in the rain at 2AM, awaiting his quarry. What better time then, to spend a moment reflecting on those other cases he has investigated which also featured a sea-theme?
Crofts, you old sausage. But you needn't have worried. Even without Inspector French advertising your back-catalogue at this beautifully-judged point in the narrative, I'd still have been back for more.