George
As mentioned, there are dimensioned drawings of RN torpedoes by John Lambert in various of his publications on Coastal Forces craft and weapons. For early and mid war design Vosper 70 foot boats, and those Vosper designs produced in the US for the RN 21 inch torpedoes were carried, usually the Mk VIII. This is the torpedo carried, for example, by MTB 77, the Vosper modelled by Italeri in 1/35 scale. The later 73 foot Vospers, designed in 1943, carried the smaller 18 inch aerial torpedo as discussed above, so it will depend on what kind of Vosper you are building. If you can’t get plans then Mikro Mir make a 1/35 scale Mk VIII torpedo as a kit in injection moulded plastic - you could use this to take off the dimensions. Various plastic aeroplane kits - Swordfish, Beaufighter etc, include 18 inch torpedoes. Hope this helps.
Steve