Operational 1/20th scale US Navy WW2 21" Mk18 Torpedo Tube
The modelling objective is an operational 1/20th scale WW2 US Navy Mk18 Torpedo tube. This was the torpedo tube used on PT109 and other similar vintage early war Elco PT boats prior to the fitting of the much lighter Roll Off Racks and the use of shorter, faster and lighter aircraft style 22" diameter mk13 torpedo's.
Top and side views.
Various views of the tube and support saddles.
The end view (bottom left) shows how the "training gear" operated, the handle (above that view) was used vertically through a right angled gearbox to operate a horizontal transverse screw thread which moved the front tube saddle in a curved slot in the front main deck support, there is also a locking support fitted to the deck under the very front of the tube and two aligning holes in a locking plate fixed to the bottom front of the tube. A locking pin was fitted in either of these hole positions when the tube was either in the stowed position or "trained" out to the firing position. Training was required to allow the torpedo's to clear the deck when fired and additionally to allow the rear torpedo to clear the rear of front torpedo tube.
The "impulse" unit at the rear top of the tube had a small replaceable explosive charge which, when fired, pushed the torpedo out of the tube and laiunched it on it's way. The impulse charge could be detonated electrically from the bridge or in case of electrical system failure it could be struck on a firing pin at the rear of the unit by the torpedoman with a mallet.
There is an excellent video by www.militaryvideo.com called - PT Boats of World War 2 - which shows a torpedoman operating the tube training gear with the handle. This video contains a few technical errors and some crappy music but for the serious enthusiast it is a must!
The 1/20th scale Model:
21" x 1/20th scale is 26.67 mm diameter which is the size that I mistakenly made my original Mk13 operational balsa wood and pvc pipe torpedo's, so much of the torpedo design is already done, however the torpedo for the tubes is a much longer version which allows for more weight and changes the balance point considerably.
I found some time ago that the 26.7mm OD (20mm nominal ID) pvc telecom tubing that I used fits perfectly in an aluminium Berocca tube (Berocca is an effervescent vitamin supplement tablet made by the multinational Roche, so they should be available world wide).
These Berocca tubes are a perfect scale fit and a perfect diameter, even the end 'fold over' is fairly right for the tube front so all that remains is to collect enough of them (12) to make up 4 complete tubes. They are easy to cut using a Dremel and a tiny cutoff wheel but protect your eyes, you only get one pair!
Top complete view and comparison with three Berocca tubes at 1/20th scale.
I also found that the ball out of a 'Mum' roll on deodorant is 1 inch or 25.4mm diameter and is easily cut in half and sanded to make torpedo 'ends'. If you heat a long length of pvc pipe gently and evenly around with a heat gun, paint stripper or similar device you can pull the ends apart and make the pipe slightly smaller in diameter where it was heated, then cut it where the diameter is 25.4mm, glue on the half ball and you have a very realistic tapered nose for your torpedo's.
Launching the torpedo with a tiny explosive charge is a possibility but will probably damage the torpedo's so compressed air is possibly a better alternative, although a puff of smoke just like the real thing does have a great appeal!
The tubes could be "trained" with a servo also but that may be more trouble than it's worth.
Much more to come.
(C) Copyright 2005, John Drain