There were two variants of LCM or Landing Craft, Mechanised: the British-built LCM(1) and the US-made LCM(3). Both types played essential roles in the D-Day Landings, not least because they had a much more generous capacity than smaller craft like the LCA and LCVP, and were also less easy damaged than those boats, but they were smaller and more manoeuvrable than LCTs.
The LCM(1) was essentially a floating pontoon with sides added to it. Although the LCM(3) had a number of advantages including better stability, the LCM(1) was lighter in weight and therefore could be lowered by davits or boom with personnel or crew on board.
Above: An LCM(1) in the foreground, with two LCP(L)s behind. (Photo: Directorate of History and Heritage, Canadian Armed Forces)
Originally known as a ‘fifty-foot lighter’, the LCM(3) was a new design by Andrew Higgins of New Orleans. The pontoon origins of the LCM(1) meant that its cargo deck was above the waterline. Higgins lowered the cargo deck of the LCM(3) which increased the craft’s stability and therefore the load it could carry, up to a Sherman tank – however it was then too heavy to be lowered from davits.
Some Landing Craft, Mechanised that were used at Normandy were carried across the English Channel on board larger ships, but the majority crossed under their own power or under tow. The Americans used some LCM(3)s early on D-Day, such as those which landed obstacle clearance teams at both American beaches in the first thirty minutes after H-Hour. A few LCMs were used in a similar role on the Anglo-Canadian beaches, but the majority were part of the Build-Up Flotillas that helped unload larger ships after the assault.
Above: A jeep (plus its brave driver!) are loaded onto an LCM(3) of APA 13 USS Joseph T Dickman from an assault transport. This illustrates the greater size of an LCM compared to an LCA or LCVP for example. It also gives some idea of how much more time-consuming this sort of loading process was than simply loading troops. (Photo: US National Archives)
Sources for more information:
Adcock, Al, WWII US Landing Craft in Action. Warships No.17. (Carrollton, TX: Squadron/Signal, 2003).
Doyle, David, US Landing Craft of WWII, Vol.2. The LCT, LSM, LCS(L)(3) and LST. (Atglen, PA: Schiffer, 2020).