A few days later the boat arrived, Photo 1. The basic hull and superstructure were originally quite well made but looked rather simplistic. It was 40 inches long, had four torpedoes, a couple of machine gun turrets but was pretty battered and the rest of the armament was missing or damaged. It had a single, rather odd looking electric motor but no sign of any radio control, Photo 2. Judging by the internal construction and remaining wiring it had probably been powered by a couple of ‘bell batteries’ and older readers may remember these rectangular dry cell batteries with brass screw terminals. The internal bulkheads were identified with printed numbers so I guessed it had originally been a kit rather than scratch built and I subsequently saw a similar model at the International Model Boat Show at Warwick with a label identifying it as a KeilKraft kit.
The Elco PT boat
While I did not plan to surpass Mr. Zulberti, I resolved to carry out an upgrade to the model I had been given by fitting radio control, three powered propellers and rudders, engineer some of the guns to rotate and fit working lights. After some further research I decided to base the model on one of the later production variants which could be fitted with a rotating radar scanner and the widest variety of weapons. The project therefore would have four major aspects, refurbishing the hull, a complete new superstructure, new propulsion system and the weapons.
Refurbishing the hull
While researching Elco PT boats, I discovered that John Haynes produced a detailed 1:24 scale model kit similar to the standard of boat that I wished to build. I wrote and asked if he would be prepared to sell me just the drawing for his kit and some of the weapons and fittings. I awaited a response with some trepidation as it seemed a bit cheeky and I have heard that some kit suppliers are very much of an all or nothing mind set. In fact, John was very helpful and after an exchange of emails and the associated funds, I quickly received the drawing and a very high quality set of white metal and resin parts to make the weapons.
I added spray strips to the hull sides using 3/32 inch square spruce. The towing eye and support plate, Photo 3, were made from a steel spring washer and a piece of brass sheet bonded to the hull with epoxy resin adhesive. The two sets of three engine exhausts fitted to the transom were a notable feature of Elco PT boats, Photo 4. Each had a mechanically operated two-way valve which at high power allowed the exhaust to vent directly aft for minimum power loss but with the penalty of high noise levels. At low power, the valves directed the flow down through silencers and the exhaust gases exited underwater. The additional back pressure reduced available engine power but also significantly reduced noise to allow stealthy operation. The exhaust valves were made from short lengths of 6mm brass tubing with brass wire bent to simulate the operating linkage. The silencer boxes were each made from two pieces of 6mm aluminium tubing glued together and filled with Milliput modelling putty to form the correct shape.
The new superstructure
The other parts of the superstructure including the engine air intake and Bofors gun ammunition rack were built up from sheet ply. The fabric cover over the ammunition rack, Photo 8 was made from a piece of an ancient handkerchief which had naturally reached a scale like flexibility and even before painting was pretty close to the required grey colour.
The propulsion system
The nearest scale propeller size was 30mm and based on my earlier experience with the TID tug I decided to use three MFA 457-RE540/1 motors. These would have a direct drive to the propellers rather than 6:1 gearing to a 70 mm propeller as in the tug. At this stage my inexperience was starting to show as I did not really know if this would be a good match of battery voltage, motor and prop size. However, I did know that it would fit the space available. I decided to conduct a few tests in the bath in order to try and work out the optimum layout and weight distribution and eventually decided on two 6v, 4ah, lead acid batteries connected in series to give 12v, with one battery at each end of the centre compartment. The original internal layout of the propulsion system was not photographed at the time but to illustrate this article a mock up was created by adapting the current standard, Photo 9.
Initially I was planning to drive all three motors from a single speed controller but I had in mind the possibility of using a mixer unit at some stage in the future so I decided to fit three electronic speed controllers. These were connected to the receiver by a pair of Y leads, daisy chained together. The battery eliminator circuit of one speed controller, the uppermost one in Photo 10, was used to supply the receiver with power. The positive wires from the remaining two controllers were disconnected, as per the manufacturer’s recommendation. The detail of this set up is difficult to photograph in the boat so for the purpose of this article it was simulated externally as in the picture.
There followed a whole series of trials on the bench to try and eliminate the ‘snatching’ fault. I suspected it was a motor control issue with perhaps some sort of interaction through the signal lines from the receiver to the speed controllers. One test was to try running the receiver from a separate battery and not using the speed controller BEC supply. Another was wrapping the signal lines in aluminium foil to screen them and using a W tail mixer to signal the three speed controllers. An alternative set of motors, in this case Graupner Speed 500E 12 V motors were tested but none of these changes made much difference.
The next logical step was to fit larger and more powerful motors, better matched to the battery voltage and propeller size. The chosen combination now was pair of Graupner Speed 600E 7.2v motors, each supplied from its own speed controller and 7.2v battery. This combination worked well, the speed was good, the ‘snatching’ eliminated and the final solution in sight.
The remaining changes made to the propulsion system were relatively simple. With just the two outer propellers driven and the W tail mixer in circuit, handling at low speed was excellent. In this configuration rudder and throttle lines from the receiver both go into the mixer. With pure throttle movements forward and reverse, both motors operate together. If rudder is applied with some throttle, the rudder servo works as normal but the motor on the outside of the turn increases in speed and that on the inside decreases. With the throttle neutral and just rudder applied, the boat would turn almost in its own length with one motor going forwards, the other backwards.
So, the final main battery configuration was a pair of 7.2v 4.3Ah NiMh batteries, one for each motor, Photo 13. However after about 20 minutes of sailing including quite a lot of high speed running, the battery voltage would drop to the point where the receiver failed to operate and the speed controllers began to behave erratically. A dedicated 4.8v receiver battery eliminated the problem and a battery voltage indicator (which can be just seen in Photo 12) was fitted to give a quick confidence check.
Development of the propulsion system was interesting and challenging but also frustrating and expensive. The cause of the motor snatching was never discovered but is now a thing of the past. Some overall performance measurements make an interesting comparison. In its original configuration with three motors and two 6v lead acid batteries the boat weighed 6.5kg and the power into each of the motors, with the propellers in the water was approximately 50 watts giving 150 watts in total. In the final configuration the boat weight had been reduced to 5.5 kg and the power into each of the two motors had been increased to approximately 135 watts giving 270 watts in total and transforming the performance.
Weapons - torpedoes
I re-used the bodies of the original torpedoes which came with the model from our family friend. However they were rubbed down and repainted and fitted with a pair of correctly shaped contra-rotating propellers from John Haynes. The roll-off racks were made from scratch with the operating linkage simulated by brass rod, Photo 14. I am not too fussy that a model is historically correct in every detail but I do like it to look as though it would actually work in practice. For a while I toyed with the idea of making up a working roll-off rack mechanism but eventually decided this could perhaps be an option in future. Several of the large PT boat models which have been built have had servo operated roll-off racks and torpedoes but it is not easy to do at 1:24 scale.
The 40 mm Bofors gun was again constructed from a John Haynes kit of parts and was mounted on a ply panel which covers the rear section of the deck, Photo 20. This panel provides access to the radio gear, rudder linkage, switches and connectors. It is not secured other than being a close fit in the hull and can be lifted by pulling up on the rear hatch which is raised off the deck just enough to get a couple of fingernails underneath it to grip. Again this gun is driven by a servo but in this case there was much more room and a sail winch servo was mounted below the gun, Photo 21. A servo drive disc was attached to the lower part of the gun mounting and this just pushes on to the servo splines, Photo 22. This arrangement works much better than the machine gun turrets with plenty of torque and smooth operation over approximately 270 degrees of travel.
Choice of colour and painting
Originally the model was to be sailed by my wife Elizabeth and she liked the original grey colour scheme better than the more common olive green found on most US Navy PT boats. At the end of the war, PT boat production was still in full swing in the USA and many of the final batches of boats were sold or lend leased to other nations. In 1951 ten Elco PT boats were supplied to the Norwegian Navy. It was therefore a joint decision that the model would be finished in Humbrol Sea Grey and carry a Norwegian flag. The first boat supplied was PT602 which was named Snøgg, which is Norwegian for ‘fast’. Elizabeth thought this would be a particularly good name, since given the combination of armament the boat carried, it was bound to be the kiss of death for anything it met in combat! I have tried to find details of the actual boat as supplied to the Norwegian Navy but so far without any luck.
On the pond
The finished boat looks good on the water, is very manoeuvrable and at high throttle settings achieves a scale planing speed, Photo 23. There is plenty of detail to add interest and when the boat has been displayed at club exhibitions and regattas it attracts the attention of the small boys who want to know what all the guns are for and whether any of them work.
I learned a lot about electric motor drives in the process of the boat’s development and fortunately most of the discarded items are now finding their way into other models. If I were to start all over again I would mechanise the machine gun turrets differently, but apart from that I am pleased with the end result, which is a pleasure to look at and sail and goes to show that you don’t need the latest most super detailed kit to gain pleasure from our hobby.
Gareth Jones... PT boat re-build... By Gunboat Driver1
by Gunboat Driver1
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