Harbour Defence Motor Launch
The Harbour Defence Motor Launch (HDML) was one of the most inappropriately named vehicles of WWII, as it rarely performed in its originally designed role. The idea for the HDML was based on the fear that, should war break out, enemy submarines would swarm into British harbors and wreak havoc among docked ships. As it turned out, HDMLs rarely were in harbor for anything but resupply and repair. The boats were about the size and displacement of American P.T. boats (p.W:DF90) but were considerably slower.
The boats were used for a wide variety of roles, including dispatch boats, inshore minesweepers and (equipped with radar) pathfinders for amphibious assaults.
The HDMLs were originally armed with a 3-pdr on the fore deck, a 20mm AA cannon on the aft deck, a pair of Lewis light machineguns and ten depth charges on two stern roll-off racks. Like the P.T. boats, armament might vary widely, either officially or from local improvisations. One of the most common changes was to replace the 3-pdr with another 20mm gun (reflected in the design below).
One of the more unusual additions to a small number of HDMLs was sails to extend their range to allow travel to the West Indies. Overall, some 560 boats were built (some in the US) and were used by British and Commonwealth forces around the world.
The HDML has a crew of ten. It mounts two 20mm Oerlikon cannons on open mounts fore and aft. A pair of .303 Lewis LMGs are also carried, but are generally kept stowed unless trouble is anticipated. Eight depth charges are carried. The engines burn 155 gallons of diesel fuel per hour of routine usage. The HDML can travel 2,300 miles at 11 mph.
Subassemblies: Medium Corvette chassis with Wooden option
+8; sealed Medium AFV superstructure [Body:T] +3; two full-rotation Small
Weapon open mounts.
Powertrain: 2¥119.5-kW marine diesel engines with
2¥119.5-kW water screws and 1,800-gallon standard tanks; 16,000-kWs
batteries
Occ: 3 CS Body, 3 CS Sup Cargo: 19 Sup,
1 OM
Armor
Body/Sup: 2/3W
OM1&2: 0/0
Weaponry
20mm Very Long Ground AC/20mm [OM1:F] (270).
20mm Very Long Ground AC/20mm [OM2:F] (270).
Depth charges [Body:R] (10).
Equipment
Body: Autopilot; 78-VSP bilge; 2¥bilge pumps; 6¥bunks;
30-VSP cargo hold; fire extinguisher; 240-man/days of provisions; 2¥very
large radio receivers and transmitters; 2-mile passive sonar. Sup:
Navigation instruments; searchlight; 2¥bunks. OM1&2: Universal
mounts.
Statistics
| Size: 72’¥16’¥?’ | Payload: 11.33 tons | Lwt: 54 tons |
| Volume: 524 | Maint.: 88 hours | Price: $59,200 |
HT: 11. HPs: 3,000 Body, 450 Sup, 75 OM1&2.
| wSpeed: 14 | gAccel: 0.7 | gDecel: 0.5(0.85) | gMR: 0.05 | gSR: 3 | Draft: 5.5' |
Design Notes
The Light Cutter chassis was slightly too large for this design, but
the design was still severely underweight and had to be increased by 20%
to match historical weight (note that a large amount of the “empty” space
was converted to bilge space to keep from increasing the design weight
by more than the suggested upper limit 20%). Design speed was 15 mph and
design draft was 3.8’; historical numbers were substituted.
The pair of .303 Lewis LMGs (Ground LMG) use pintle mounts that are light enough to ignore in the design system.
To extend range, a 240 gallon fuel tank could be carried on the deck.