Friday, September 9, 2011

Vosper 70-ft type

The successful Vosper 70-ft type originated in the single 20.7-m (68-ft) boat built by Vosper as a private venture in 1935-6 and eventually commissioned by the navy as MTB 102. Built completely in wood, she was driven by three Isotta-Fraschini petrol engines with a maximum output of 2573 kW (3,450 bhp). These drove the boat at a maximum speed of nearly 44 kts in calm water, and were chosen as no British-built equivalent existed. Like all such engines they were extremely noisy, so a low-power Ford engine was also available to clutch into the wing shafts for a low-speed, but silent, approach, Centreline torpedo tubes were originally included, one forward and one aft. But the navy did not like them and developed the sided arrangement adopted thereafter, MTB 102 was notable also for being the first Royal Navy vessel fitted with the later universal 20-mm Oerlikon cannon.

Despite a further strong challenge by BPB with an alternative 21.34-m (70- ft) private-venture boat, the Admiralty chose the Vosper design as being the stronger in heavy seas. (Actually keeping these fast wooden hulls together in poor conditions was a problem that was never satisfactorily solved.) Four Vosper and two similar Thornycroft boats were ordered in 1938 and provided the basis for the war programme, with little design change except a nominal increase in length to improve seakeeping and cater for the growing load of topweight. Alternative engines were a problem; a few converted Merlins were coaxed from the Air Ministry but many boats had to take low-powered Hall-Scotts until the powerful American Packard became available. Oerlikons were virtually unobtainable, and the alternative 7,7-mm (0.303-in) machine-guns proved of little use against E-boats running as MGBs, Eventually 12.7-mm (0.5-in) machine-guns and 6-pdr guns were fitted which, with radar, made for an efficient night-fighting boat. Stronger but lighter hulls, together with uprated engines, meant that boats were carrying a 70-per cent increased equipment load by 1944 with little speed penalty. Orders for 193 boats were met between 1939 and 1945 with the exception of a few late boats which were cancelled.

Specification
Vosper 72.5-ft type
Displacement: 36 to 49 tons
Dimensions: length 22, l m (72,5 ft); beam 5.94 m (19.5 ft); draught 1.68 m (5.5 ft)
Propulsion: three petrol engines delivering 2983 kW (4,000 bhp) to three shafts
Speed: 40 kts
Endurance: 463 km (288 miles) at 40 kts
Armament: two 533-mm (21-in) torpedoes, and various combinations of 6-pdr gun, 20-mm cannon, and 12.7-mm (0.5-in) and 7,7-mm (0,303-in)machine-guns
Complement: 12 or 13
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Posted by Mitch Williamson at 9:31 PM
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