Harbour Defence Motor Launch (HDML class)

Class overview.

Name: Harbour Defence Motor Launch (HDML)
Completed: 486
Active: none
General characteristics
Displacement: 54 tons (loaded)
Length: 72 ft (22 m)
Beam: 16 ft (4.9 m)
Draught: 5 ft (1.5 m) (loaded)
Propulsion: twin Gardner 8L3 engines, each 204 bhp (152 KW)
Speed: 12 knots
Range: 2,000 miles at 10 kts (1,650 gal)
Complement: two officers, two petty officers and eight ratings
Armament: typically twin 20mm Oerlikons, twin Vickers K machine guns and six depth charges.

HDML 1301 in Padstow with extra fuel tanks and stores for voyage to Malta.

We have found that a company in Durban,South Africa named,Spadbrow and another named Africa Marine in Mombassa,Kenya, built this class of boat in wood,it is probably the same class as Venture,now moored in Hout Bays harbour and privatly owned.

Design and construction.

HDMLs had a displacement hull of 72 feet (22 m) long with a beam of 16 feet (4.9 m) and a loaded draught of 5 feet (1.5 m). The loaded displacement was 54 tons. The hull had a pronounced flare forward to throw the bow wave clear, and providing considerable lift to prevent all but the heaviest seas from coming aboard. Although sea-kindly, there is a considerable tendency to roll, especially when taking seas at anything other than right angles. The cause, surprisingly, was their considerable reserve of stability, the effect of which was to impart a powerful righting moment if the ship was pushed over in a seaway. This, coupled with the round bilged hull and lack of bilge keels, would set up a rapid and violent rolling.

One of the design criteria, was that it had to be capable of turning within the turning circle of a submerged submarine. To achieve this, HDMLs were fitted with two very large rudders and, to reduce the resistance to turning, the keel ended 13 feet from the stern. A side effect of this was that the hull lacked directional stability, and was extremely difficult to hold on a straight course.

The hull was of round bilge wooden construction, planked with two diagonally opposed skins with a layer of oiled calico between them – known as “double-diagonal” construction. The hull is completed with frames or “timbers” riveted perpendicularly from the keel to the gunwale on the inside of the planking, forming a very strong hull. The hull is further strengthened by the addition of longitudinal stringers riveted inside the timbers together with further timbers, known as "web frames" fastened inside the stringers opposite every third main timber. HDMLs were fitted with a deeper section rubbing strake aft. Its purpose was to roll depth charges (kept and delivered from racks on the side decks) clear of the hull and propellers.

Most HDML hulls were planked in mahogany, but later in the war when this became scarce, larch was used although this tended to lead to leaky hulls. The decks were also of double-diagonal construction, and generally made of softwood. Ships operating in tropical waters (including the Mediterranean) were sheathed in copper below the waterline to prevent attack by marine borers.

In order to lessen the chances of ships sinking in the event of damage to the hull, they were divided into six watertight compartments. Provided that the bulkheads were not damaged, the ship could remain afloat with any one compartment flooded.