Were attacks on HX convoys evenly distributed?

Were attacks on HX convoys concentrated in particular years rather than spread evenly across the war?

Yes. Risk was higher in some years, so the 38-of-377 aggregate should not be read as a constant probability for every convoy.

The full-war HX totals combine very different phases of the Atlantic campaign. The probability of attack was higher in some years even though the route's aggregate record remained strong. Critical convoys also received deliberate protection, which contributed to their apparent good fortune. The 38 attacked sailings out of 377 therefore measure the system's overall result rather than a uniform one-in-ten risk on every date.