Operational 1/20th Scale Torpedo



The almost finished resin cast model


The objective is an operational 1/20th scale WW2 US Navy MkXIII Torpedo complete with dual counter-rotational propellers, This is almost complete as the photo's above and below show. I have a completed and relatively easy to make single prop model and it performs well except that the body of the "fish" rotates in the opposite direction to the propeller and that's just not good enough as it makes the torpedo wander, however for launching in a large lake the single prop version is easy to make and fairly cheap so you can afford to lose one or two. This is made from Balsa and Pipe and details are below.

I have been working for some time on a molded resin body torpedo and a printed circuit board, this has been experimental and I didn't really know how to do it all that well but I am giving it my best shot (and I've learned a lot about how not to do it) as lots of people want me to make them torpedo's and it takes forever to make them with pipe and balsa wood.

This is the beginning of the resin torpedo with the polished brass master of the rear section of the torpedo that I made, I tried making wax castings of this to use as the two halves of the outside mold and gave up on that idea after about 20 attempts and four different kinds of release agent, there seemed to be no predictability in the results, so I had a rubber mold made professionally by a jewellery mold maker. I then turned up an inner part on the modelling lathe and after a couple of false starts again with release agents and using different resins and the odd disaster or two I have FINALLY managed to make a successful hollow resin casting, Whoohoo! The rubber mold is smaller than the original brass by a few percent and this is apparently quite normal for that type of natural rubber mold.

The outside finish is actually pretty good, the lines that you can see are from brushing lashings of thick silicon release agent onto the inner brass "dolly" so that I didn't have to chisel it out of the resin yet again! There are a few small air bubbles and a few other slight problems but I am very pleased with the result of many months work.

So that's one made, I wonder if I can make two? Oh and I need to make the front part yet too!

The original thought was that this version would screw together with an "O"ring to keep it dry, didn't work!

And the new torpedo goes too! I have clocked the new resin torpedo at the equivalent to full size 72 Klm per hour (45mph) with two modified 130 Mabuchi motors and two 750mAH AAA NiMH batteries and it goes for more than 15 minutes on a full charge, I had to go and buy a new battery charger so that I could charge the NiMH batteries in a couple of hours as the old charger was taking day's. Sony make a fantastic small charger for two AA or AAA NiMH batteries for about AUD$50 (USD$30)


These are the new propellers that I had cast in solid silver (well they don't weigh much even with the casting sprues and it was cheaper than the casting fee to get some cast in brass) and I want to try molding resin props as well in the same mold so I just got a few made to try. As you can see the hubs are tapered to match the shape of the torpedo body so the front prop has a larger hub than the rear and different size holes -2mm and 1mm for the shaft within the shaft. I have made the blades the same size on both props to move the same amount of water to help prevent torpedo body rotation so the rear propeller is smaller in overall diameter by the difference in hub size, from all my photo's this appears to be true of the full size torpedo'ss as well and puzzled me for a while (quite logical when you think about it).


The Full size MkXIII Torpedo was 22.5" diameter (not 21" like the longer torpedo's) and 13'5"  long (not 13' 6") -  the existing 1/20scale model is 26mm dia and 206mm long. The new one will be 28.5mm diameter and 204.5mm long and so I will need to modify or make new roll off racks to fit the larger diameter. That's one problem with doing research, you mostly find out that you were wrong last time!

The first new front moulding sort of works, the outside is fine but the whole internal thread idea doesn't work, too thin and too hard to make - new idea required here!

  THE NEW PCB HAS ARRIVED Whoohoo (on Mayday - is that ominous!)

By the end of Mayday (1st May, 2003)

The completed circuit board with it's three "Ultra Low On Resistance"  MosFet transistors per motor. It has a couple of tiny issues but no major stuffups!

There is a reed switch, two s.m.t. (surface mount) resistors, three s.m.t. capacitors and seven s.m.t. transistors on this as well as the four battery clips and the two modified (hollow) motors. There's a good name - "hollow motor torpedo's"

A brand new complete "production" torpedo (it's a pity the photo doesn't do it justice but it will look better when it's painted) there are still a couple of issues to fix but it's damn close now. The front molding was made with a silicon rubber outside mold and a brass internal "dolly". I am going to try a silicon inner dolly when I can figure out how to make one as it won't stick to the resin, the brass one is very difficult to remove unless it has a really thick coating of silicon grease on it.

  Early Stuff

(It's fairly easy to see the differences in the shape)

fish.jpg (4079 bytes)                    fishswim.jpg (6595 bytes)

The original single prop balsa and pipe 1/20 scale "fish" in operation, don't worry, it doesn't go bang (well not yet anyway!) This is our MarkV.

4 blade Prop Development  

(Pre modelling lathe)


A bit rough yet, but it seems to perform extremely well, I have also made an opposite pitch version for the counter-rotational true scale model. The hub is made from an M4 brass dome nut and the blades from flat sheet. The blades were pre-bent and tightly fitted into saw slots in the hub before soldering into place, then twisting the blades and rough shaping. The dome of the hub will be cut off once I am happy with the performance and the centre thread filled with a brass screw and drilled out to the correct size (once I know what that is).

I am planning on casting these props in Resin or Zinc once they are finished, so let me know how many you want!

I am also experimenting with a two motor model with the shaft of one motor going through the hollow shaft of another, I have knocked out the steel motor spindles (2mm) and replaced them with hollow brass tube (2mm OD 1.5+mm ID), tricky but not difficult!

These photo's show the counter rotating props and in operation is looking fabulous! stay tuned for more developments here! This looks like THE plan!


Feb 2001 - This is the experimental 2 prop model with the finished props and a single N size battery (1.5Volt) This is looking really good! new circuit board coming soon with professional battery clips and hopefully we can fit in two batteries. This is the first torpedo housing with the smaller fins, I will be doing another body design because after much research this one's shape is not quite right and the horizontal and vertical fins were actually different.

Shhhhh!!! I also have some miniature switches that I have found for the detonator!


The Original PT-Boat.com - MarkVI - Single Prop

After much experimenting with more refined battery holders, types of batteries, props and so forth, I have settled on the following single prop "production" design. It seems it's very easy to move something slightly and have an unstable sinking torpedo or one that bobs about like a seal. I have made the tail fins larger than scale to mostly stop the body rotation with a single prop.

The Torpedo Factory 

  The Tail section pieces ready for assembly

The main body of the Torpedo is made from 25 mm telecom conduit tubing which just happens to tightly fit a Mabuchi 260 motor and the outside diameter is just over 26mm diameter - perfect! Both the front domed end and the tapered tail are made from Balsa which has been turned in a stand drill to fit within the tubing, to drill the prop shaft hole and to achieve the final shape - mostly with sandpaper.

The Balsa tail is turned down to fit inside the PVC tubing, glued in and once the glue is dry the final shape is applied. The tubing is also cut to length in the stand drill by using a mandrel that fits neatly inside the tubing covered in PTFE plumbing tape.

The inside tail section is cored out to allow for the motor end and the silicon rubber coupling from the 2mm motor shaft to the 2mm brass prop shaft. 


The outer prop shaft is either thin walled brass tubing or plastic tubing, both seem to work well, brass tubing is probably easier and better but slightly heavier.


The fins are slightly angled on the single prop version to reduce rotation and to straighten the path against the single prop's rotation, pretty difficult to make these go straight though!

The domed front is fitted in a similar way although only the rounded dome itself is balsa with a little step to fit into to tube. You need to make a very accurate dome or it will act to steer the torpedo in different directions as the body rotates with the single prop, makes the things very unpredictable.

I understand now why torpedo's HAD to have two propellers!

The electrics are mounted on home made printed circuit board made from 1/16" double sided PCB fibreglass laminate, available from most hobby shops. The copper sections are isolated by cutting through with a knife and pealing the copper away. The battery clips are made from some terminals from old electrical parts and are surface soldered on the top of the board. (not all that reliable, we have some new battery clips now) holes are drilled in the board for the reed switch wires and they are soldered top and bottom.


The batteries are 1.5V "N" size. The board has been made as small as possible to keep the weight down.

Slots have been sawed through the PCB for the motor terminals which slot in and are then soldered top and bottom. Use a sparing amount of Araldite to glue the motor to the board and to fill the breather holes at the front and back of the motor to stop water getting in, also glue the tabs that secure the metal motor body to the rear in the recesses to prevent the rear of the motor from being pulled off when seperating the torpedo halves. Some of the cheaper motors have fairly poor mechanical tolerance and I've found the better quality motors are not much more expensive and are certainly worth the extra few cents, sometimes the rear plastic motor end needs to be filed a little down to the same diameter as the metal jacket.

  The reed switch and magnet. 

In this type of reed switch the mounting foot can be broken off  to reduce the overall weight. The magnet is mounted on the Roll Off Rack and the reed is mounted inside the torpedo, the purpose of the reed switch is to allow the motor to start automatically once the torpedo rolls off the side of the PT-Boat, make sure the magnet is mounted around the right way or it won't work as these are magnet biased to stay ON when away from the main magnet! BEWARE the description, you need a reed that is OPEN when near the magnet!!!!! If you can find just the glass reed switch part you need the ones with one lead at one end (Com) and two leads at the other (NO/NC).


The single prop plans (see above for new 4 blade prop)


Printed Circuit Board Layout


The Counter-Rotating Prop Gearbox

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The fabricated parts and gears, they're a bit small!

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Experimental gearbox, worked well. It also gave me some idea's about using the same gear method to elevate the guns and particularly the operational rocket launchers.

torpedogearbox.jpg (24310 bytes)

A more refined model but not quite there yet.

torpedogearbox1.jpg (9051 bytes)


The new simpler 2003 gearbox

I had some new idea's on a simpler gearbox to make a simple twin prop version with a single motor in the new resin body.


This is built using two each of the "Scalextrix" type slot car (Crown (big) and Pinion (little)) gears, which are a bit bigger than the tiny AFX gears. This means that the inner shaft can be 2mm diameter which is the same as the diameter of the motor shaft and the same size as the shaft on the single prop torpedo. This will fit in the new resin body and with a bit of adjustment to the inside shape to hold the cross beam with the pinion gears this should be fairly quick and easy to make. The crown gears are modified by cutting off the "hat" section with a sharp scalpel and the holes need to be drilled out slightly to fit.


The Original Full Size Mk13 Aircraft Torpedo

The Mark 13 torpedo, compared with the others, is short and thick: its length is 13 1/2 feet, and its diameter, 22 1/2 inches. (The others all have the same diameter-21 inches-so they will fit the standard torpedo tubes.) The Mark 13 is designed for launching from aircraft and PT boats. Its range is 6,000 yards at an average speed of 33 1/2 knots, and it carries 600 pounds of high explosive.

Mark 14 and 23 types. The Mark 14 torpedo is fired from submarines. Its length is about 20 1/2 feet. It offers a choice of two speeds. At the high-speed setting it has a range of 4,500 yards, at an average speed of 46 knots. At the low-speed setting its range is 9,000 yards, and its average speed is 32 knots. It carries 600 pounds of high explosive.

The Mark 23 torpedo is exactly like the Mark 14, except that it has no speed-change mechanism. It operates only at high speed.

Mark 15 type. The Mark 15 torpedo is launched from the deck tubes of surface ships. It is 24 feet long, and carries an explosive charge of about 800 pounds.

It has three speeds:

26 1/2 knots (range 15,000 yards);

33 1/2 knots (range 10,000 yards);

and 45 knots (range 6,000 yards).

Mark 16 type. The Mark 16 is a “chemical” torpedo. It uses a strong solution of hydrogen peroxide, rather than compressed air, to support the combustion of its fuel. This feature gives the Mark 16 a relatively high speed and long range, and enables it to carry a relatively heavy charge of explosive.

Mark 18 type. The Mark 18 is the only nonhoming electric torpedo now in the Fleet. Its principal source of energy is a large lead-acid storage battery. It has a length of about 20 1/2 feet, and an effective range of 4,000 yards at an average speed of 29 knots.

H. Aircraft Torpedoes

12H1. Construction and use

Aircraft torpedoes came into common tactical use during World War II, as alternate weapons to aircraft bombs for use in attacks on surface ships. Torpedo attacks, however, ordinarily are not made against well-defended units, unless supporting attack is made simultaneously by other types of planes to divide the enemy antiaircraft fire. When properly employed, torpedo attack may force enemy ships to maneuver into an unfavorable position with respect to a main attack delivered by own ships, or accept the penalty of torpedo hits.

Aircraft torpedoes must be able to withstand heavy water impacts. They must also be capable of maintaining stable flight from plane to water, and a stable course through the water to the target. The Mark 13 torpedo, shown in
figure 12H1, is one of the older types still in service use. It is similar to the Mark 15 torpedo but differs in certain details of size and design, including the incorporation of special stabilization elements necessary for effective launching from aircraft.

An aircraft torpedo usually is suspended between two racks, which have suspension cables running between them and around the torpedo. A small stop bolt, projecting downward from the plane into a hole in the torpedo casing, serves to prevent fore-and-aft slipping of the torpedo in its cables. When one end of each cable is released, the torpedo falls away.

12H2. Mark l3 torpedo

The Mark 13 torpedo differs from the Mark 15 torpedo in the following ways:

1. The Mark 13 torpedo has better provision for air stabilization, being much shorter and slightly larger in diameter. It is 13 feet 5 inches long, and 22.42 inches in diameter.

2. The Mark 13 torpedo has greater capacity for withstanding water impact.

3. The Mark 13 torpedo contains a smaller explosive charge: 600 pounds of HBX.

4. The Mark 13 torpedo has a shorter designed range: 5,700 yards.

5. The Mark 13 torpedo has a single speed: 33.5 knots.

6. The Mark 13 torpedo has a water trip delay valve to prevent ignition until the torpedo enters the water.

7. The Mark 13 torpedo has a shroud ring around its tail vanes, which tends to minimize hooking and broaching upon water entry, and makes for greater stability during the water run.

8. The Mark 13 torpedo is rigged for launching with a box-shaped plywood stabilizer fitted over the fins and shroud ring. This stabilizer causes the torpedo to fall in a smooth curve, and to enter the water head first. The stabilizer breaks up on impact with the water. A parachute drogue stabilizer has been designed as a substitute for the box stabilizer.

9. The Mark 13 torpedo has a drag ring, in the form of a plywood tube open at each end, fitted over its head. The drag ring slows the torpedo’s rate of fall, tends to reduce wobbling, and acts as a shock absorber on water impact. The stabilizer and drag ring are shown in
figure 12H2. This drag ring is not used when the torpedo is rigged with a parachute stabilizer.

Near the after end of the torpedo is a starting lever. When the torpedo is installed on the plane, a toggle is hooked to this lever and is attached to the aircraft by a lanyard. When the torpedo is released, action of the lanyard and toggle trips the starting lever, but a water trip delay valve serves to prevent the combustion flask from lighting off until water entry. A gyro-locking mechanism is also provided. When the torpedo is installed on the plane, the gyro is locked with its axis parallel to the axis of the plane. The gyro begins to spin on release from the plane. The gyro will therefore keep the torpedo on the course determined by the direction of aircraft travel at the instant of release.

    E. Main Engine of a Mark 15 Torpedo

12E1. General

The main engine is located entirely within the afterbody, and is supported by A-frames secured to the after side of the turbine bulkhead. It consists of the turbine wheels, gear reduction train, and propeller shafts, along with the frames, spindles, shafts, and bearings that support these parts, and the oiling system that lubricates them. The main engine of a Mark 15 torpedo, viewed from the starboard side, is shown in
figure 12E1.

12E2. Functional description

The main engine converts turbine-wheel rotation into propeller rotation. In order to do this effectively, it must have several special features.

Because of the high velocity at which the combustion gases strike the turbines, the turbine wheels must turn at high speed in order to use the available energy efficiently. But, if the propellers are to operate efficiently, they must turn more slowly than the turbine wheels, and develop a higher torque. The main engine must therefore include a gear reduction train.

The torpedo is provided with two propellers, which rotate in opposite directions but at the same speed. This feature is necessary because the torque developed by a single propeller would tend to roll the torpedo in the opposite direction. As previously stated, the two turbine wheels also turn in opposite directions. But it is not possible to drive each propeller with a different turbine wheel, because the first turbine wheel develops a much higher torque than the second. The main engine must therefore combine the two unequal torques developed by the turbine wheels, and then divide this force equally between the two counter-rotating propellers.

Finally, the main engine must continuously lubricate its moving parts throughout the torpedo run.

12E3. Gear train

Figure 12E2 is a schematic diagram of the main engine. This illustration should be compared with figure 12E1, bearing in mind that the two views are from opposite sides. (The side gears are not shown in figure 12E2; they will be described later.)

Each of the two turbine wheels is mounted on a separate spindle. The first turbine spindle is short and hollow, and carries the first turbine pinion at its lower end. The second turbine spindle is longer, and passes through the opening in the first spindle. The second turbine pinion is mounted at the bottom of the second turbine spindle.

As they pass through the nozzles, the hot combustion gases expand and reach a high speed-about 4,000 feet per second. They strike the blades of the first (lower) turbine wheel, and spin it counterclockwise (as viewed from the top of the torpedo). The first turbine turns its spindle, and the first turbine pinion, counterclockwise. The pinion meshes with the upper main drive gear, and turns it clockwise. The drive gear turns the upper bevel pinion clockwise.

The combustion gases are deflected from the blades of the first turbine, and strike the blades of the second (upper) turbine. The second turbine spins clockwise (still looking down from the top). The second turbine turns the second turbine spindle, and the second turbine pinion. The second turbine pinion turns the lower main drive gear counterclockwise.

Each of the 2 bevel pinions meshes with both bevel gears. Working together, the 2 pinions turn the 2 bevel gears. The forward bevel gear turns counterclockwise (looking aft from the forward end of the engine). The after bevel gear turns the forward (outer) propeller shaft, which turns the forward propeller. The forward propeller shaft is hollow; the after propeller shaft turns inside it. The forward bevel gear turns the after (inner) propeller shaft, which turns the after propeller. Because the two propeller shafts are linked together through the bevel gears and bevel pinions, they turn in opposite directions at the same speed.

12E4. Turbines and turbine spindles

Figure 12E3 shows the turbine and spindle assembly; the spindle casing is at the left. In both turbines, the blades are of crescent-shaped cross section. On the end of each blade is a small projection to which the turbine band is riveted. The turbine band is made up of overlapping segments. The clearance at the butt ends of the segments gives them room to expand when they get hot. The blades of the second turbine curve in the opposite direction from those of the first turbine. And the blades of the second turbine are slightly larger than those of the first, so that the gases can keep expanding as they pass through the turbine.

The upper and middle bearings support the first turbine spindle. The second turbine spindle, which passes through the first, is supported by the lower bearing and the top bearing. (The top bearing does not show in
figure 12E3; it is located above the second turbine wheel.)

12E5. Crosshead assembly

The crosshead is shown in
figure 12E4. Its outer ends are supported in the two A-frames. Bronze bushings fit over the two crosshead shafts. These bushings serve as bearings for the main drive gears and bevel pinions. (Each drive gear and pinion combination is machined from a single forging.) In figure 12E4 the bushings are in place on the crosshead. Note the spiral oil grooves on the surface of the bushings. Notice also, to the right of the strut, a small pinion gear machined on the outside of the forward propeller shaft. This pinion supplies the power that drives the steering mechanism.

The after propeller shaft passes through the cross-head in a floating bronze bushing. The forward bevel gear is keyed to the after (inner) propeller shaft; the after bevel gear is keyed to the forward (outer) propeller shaft. The outer propeller shaft turns in a bearing in the engine frame strut. This bearing supports the shaft radially; it prevents any motion at right angles to the torpedo axis. Between the after bevel gear and the crosshead are a bearing washer and a thrust bearing.

12E6. Engine thrust

As the propellers turn, they develop a thrust, or pushing force. They transmit this thrust to their shafts. To drive the torpedo through the water, this thrust must be taken from the propeller shafts and applied to the shell of the torpedo. This is done in three ways:

The after bevel gear is driven by the two bevel pinions. Because of the slope of the gear teeth, the turning force of the bevel pinions tends to push the bevel gear aft. But the thrust of the forward propeller tends to push the bevel gear forward. The thrust of the propeller is stronger than that of the gears. A part of the thrust of the forward propeller therefore goes through the after bevel gear to the bevel pinions, and from there to the crosshead. The rest of the thrust from the forward propeller is applied to the crosshead directly, through a thrust bearing and washer.

The crosshead transmits the forward thrust through the A-frames and the turbine bulkhead to the shell of the torpedo. The after propeller shaft applies its thrust to a thrust bearing mounted on the after side of the turbine spindle casing. The spindle casing carries the thrust through the A-frames to the turbine bulkhead.

12E7. Engine balancing

Any rapidly rotating body develops a gyroscopic action, and resists any force that tends to turn its axis of rotation. The engine parts of a Mark 15 torpedo rotate fast enough to develop a considerable gyroscopic action. To keep this action from interfering with the steering mechanism of the torpedo, the main engine is balanced. The gyroscopic force of each of the principal rotating parts is balanced by the force of a similar part rotating in the opposite direction. For example, the gyro action of the first turbine wheel is balanced by that of the second. Other pairs of counter-rotating parts include the turbine pinions, the main drive gears, the bevel pinions, and the bevel gears.

12E10. Exhaust system

Figure 12E7 shows the exhaust system of a Mark 15 torpedo, looking down from the top. The two tubes carry exhaust gases from the space above the turbines to the tail section. Near the after end of the afterbody, each tube separates into two branches. (In figure 12E7, the lower branch of each tube is hidden.) The exhaust gases enter the tail section through four openings in the after bulkhead of the afterbody.

When the torpedo is under way, the main engine space is filled with a fog of oil. If this fog is allowed to mix with the hot exhaust gases it will burn. The torpedo will then leave a heavy wake of smoke. This is prevented in two ways:

1. Under the turbines, attached to the top engine frame, is a sheet steel pan called the turbine oil guard. This pan, together with thin horizontal and vertical bulkheads, keeps oil fog out of the exhaust system.

2. Above the upper turbine spindle bearing is a baffle, called the oil deflector ring. This ring keeps the oil in the spindle bearing from entering the turbine exhaust space.