The writing is very small, and the remaining letters - which are shown - aren't clear, but we can surmise that, like many sailors in naval dress, they have a ribbon (this is called a "tally") bearing a ship's name (intended to show the vessel the sailor serves on).
Looking at the frames on that page, I'd say the final letter is an "e", but even with the
edoutab string on its own, I think it is fair to say that the detectives have got hats with the name
Redoubtable on the band.
Although the Belgian Navy don't appear to ever have had such a vessel (they did have a ship named
De Moor though...), it is a name with a history in the French Navy from the eighteenth century on, since when it has been used for eleven craft. The most famous of these is quite possibly the ship against which
H.M.S. Victory was engaged when Admiral Lord Nelson was shot.
Update: I'd an idea that this had come up somewhere before, but I didn't get it when I first searched. Now I've come across the mention, which was made
here by chevet, in answer to a question set by yamilah. I knew I'd seen it!
Apparently the name
was readable in other places at one time.
Sure enough, if you look at the B&W newspaper strips - reprinted in the French volume
A la recherche du Trésor de Rackham le Rouge - it is much clearer throughout.
In fact, it would be fairer to say that the name has been removed from many frames of the colour - the tobacco-chewing scene on p.15 (p.43 in the B&W volume) must have had the name removed at some point, because it is there in B&W, but absolutely gone in the colour version, even although the frame and images are now bigger. This page is also accompanied by a short article, entitled punningly
Les redoutables détectives de la mer (
"The fearsome detectives of the sea").
You can see a similar hat if you search for images of a "bashi", to give it its proper name; it may make it easier to see that the line across the top, which is apparent in some frames, is in fact a chin-strap, which is folded up when not in use.
It is also notable that the caps worn in the tropics by the Detectives, are
winter gear (a summer/ tropical cap would be white, and more appropriate to the voyage they are on), adding another layer of nuance to the joke of their inappropriate garb.
