All posts by BookCatat938

Torpedoes Part 3

Once I thought I had solved the problem of limiting the run of the torpedoes, i hoped to convert to rechargeable batteries.  I could not find rechargeable AAAA NiCad batteries, so decided to use 3.7 v LiPo batteries for the power.  I had also looked at 3 v Lithium batteries, but preferred that the batteries be rechargeable.

This photo shows the various batteries used or considered for use in the torpedoes.  The AAA s are on the right, the AAAA next and the rechargeable LiPoly 3.7 v on the left.  The LiPoly battery is a bit longer and that required some redesign of the torpedo.  Penalty, I guess, for the innate need to tinker, to fix things not broken.  The quarter is slightly larger diameter than the inside diameter of the PVC tubing used for the body of the torpedo.

I turned additional wooden parts for more torpedoes, eliminating the long insert and O-Ring grooves in the rear section, to shorten the drive shaft.  I also lengthened the nose section and shortened the center PVC section.  The plan was to hollow out the nose section a bit more and use it to hold both the switches, as the longer battery and the motor would take up most of the room in the PVC section.

On the left are two first run nose cones with slide switches installed, shown in open and closed position.  After some trials, I shortened the slides by half so they did not protrude into the PVC section.  On the right is a comparison of the first generation rear section, with longer insert and O-Ring grooves, against the second generation version, which is identical except for shorter insert.

To power the additional torpedoes, I ordered more of the small electric motors used in the first version.  These new ones, however, had slightly larger diameter shafts than the original ones I used requiring modification of the polystyrene tubing coupler connecting the motor to the prop shaft.  So it goes.  See above under “tinkering”.

As for the original, the props were cut from 0.016″ copper sheet.  I drilled a 1/8″ hole in the center of the discs to be shaped into props and once the props had been cut and filed into shape, I secured them to a shaft of 1/8″ solid brass rod by soldering a length of 5/31″ brass tube in front and in back of the prop.  The tubing is about 3/4 ” long forward of the prop, and the 6″ long brass rod has another piece of 5/32″ tubing soldered to the inside end with the tubing protruding about 1/4″ beyond the end of the rod.  This arrangement created two bearing surfaces (the 5/32″ tubing) when the shaft is installed in the 3/16″ brass tubing, 6″ long, used as the stuffing box.  This also permits light grease or oil to be placed in between the two bearing surfaces when the torpedo is assembled for lubrication and water seal.

With the shortened PVC section (to lower weight), the motor and longer battery were a tight fit, but by hollowing out the nose cone almost entirely and placing the switches in the nose, I managed to get everything to fit.  The battery comes with JST plug, so I used a JST female connector for that plug, and also a second pair of connectors to allow the switches to be disconnected and the nose cone removed when servicing/charging the battery.

 

Torpedoes Part 2

Unhappy with the mechanical switch I had designed for the torpedo motor control, I did a little research and decided to change to a magnetic switch to control the motor.

The mechanical switch required a hole in the torpedo wall for the pin to protrude through and I felt that the pin would drag on the wall of the tube during launch and also probably allow water to penetrate the torpedo once it was launched.  In these snaps, you can also see the O-Rings used for water seal between pipe and turned wooden components.

I found some magnetic reed switches that could be wire “always on”, so that I could use a magnet under the front part of the tube to keep the motor switched off until launched.

 

Also, I decided to join and seal the three parts of the torpedos with vinyl electrical tape.  I had originally designed them with O-ring seals, as in the photos above, but the tape seemed to work as well or better.  And I could use a bit of the same tape to seal the hole in the torpedo wall originally for the mechanical switch and convert the first 4 torpedoes to the magnetic switches.

Buoyancy testing was successful, with the torpedo floating slightly nose up just at the surface of the water.  Since I hoped to retrieve the torpedoes, I wanted them to float and also leave an obvious wake when running.  The drive train worked well and the propeller drove the torpedo through the water at a suitable speed, although the torpedo did rotate, but not severely.

The only issue remaining was how long the torpedo would run.  I had built the first torpedoes with the mechanical switches using two AAA batteries for power.  I used alkaline batteries rather than rechargeable since I was not sure about retrieval.  When I changed to the magnetic switches, I also changed to AAAA batteries.  The flotation and propulsion testing was even better with these batteries, but I still had no idea how long they would run.  And since I would be running the boat in the rather large Bayou St. John, a long torpedo run might be a problem for retrieval.

The solution was to design a tubular slide switch and install it in the nose of the torpedo, where a detonator might be.  When the switch is pulled out, it is on, but the torpedo does not run because the magnet under the front of the tube keeps the power off.  Once the torpedo is launched away from the magnet, the motor runs until the batteries run out or until the torpedo strikes and object head on and the impact pushes the slide switch in, turning off the power.  This would mean I would have to be fairly accurate in aiming the torpedoes and solid targets, such as the concrete steps along some of the bayou, or a canoe.

This photo shows the slide switch installed in the nose of the torpedo.

 

 

 

 

 

These photos show the torpedo in the tube.  On the left, the slide switch is pushed in, turning power off so the torpedo can be slid into the tube and the retaining block engaged with the launching pin.  On the right, the slide has been pulled out, “arming” the torpedo to run.  The magnet just visible on the underside of the tube in front of the forward pedestal keeps the power switched off until the torpedo is launched, when it should run until it bumps into something and pushes the slide in.

Addendum:

All this seemed an elegant solution and worked well in testing.  On the second water trial of the boat, in February, I did fire two torpedos, which launched perfectly, but I had forgotten to pull the switches out (on position) so the torpedoes did not run.  They went about 5 feet from the boat at the launch and then floated for a bit.  The water was just choppy enough that we soon lost sight of them and they probably sank pretty fast.  So it goes.  No definitive cure for stupidity, but I now have a “pre-flight” check list including arming the torpedoes.

Torpedo Launch Mechanism

I followed the original plan for the torpedoes and made 4 prototypes, but was unhappy with the rather in-elegant switch and feared it would be unreliable.  It did work as intended in the tubes, but was pretty tricky and touchy to adjust.  More later on torpedo re-design.

The first step was making the torpedo tubes.  I used thin wall 1 1/4″ plastic pipe for the tubes.  The torpedoes ended up a bit thinner than true scale.  The actual torpedoes were 21″ in diameter or about 1.3125 ” in scale, and by using the 3/4″ PVC pipe, the torpedoes on the model a bit under 1.25 “, but I figured that is close enough.  Because the torpedos are launched by springs from the tubes, I had to make the tubes a bit longer (1″) to accommodate the spring.  I found some springs just under 3/4″ diameter at the hardware store and epoxied plastic disks to the end of eight springs.  I use two springs per tube by gluing the second spring to the disc of the first.  The springs are held in the tubes by small wire pins placed across the tube near the back to catch the spring.  This arrangement permits removal of the springs if needed

The torpedo tubes were detailed with various materials, using paper strips for the reinforcing bands, plastic for other details, 1/16″ plywood for the pedestals, and 3/8” copper for the compressed air chambers on the top of each tube.

I made up some launching pins which are retracted by a servo.  One channel operated the port side tubes, and a second the starboard tubes.  Moving the channel control one way fires one tube, then the second direction fires the second tube.  The wire from the servo arm passes through a hole on the trigger and ends in a loop, so the trigger doesn’t move when “pushed” but retracts only when “pulled”.

The launching pins extend up through the deck, through the rear pedestal mount of each tube, and through the wall of the tube, protruding about 1/8 inch into the tube interior.  The pins engage small blocks on the torpedo.  When the pins are retracted by servo action, the torpedo is launched about 4 feet.

I used pretty beefy servos to make certain they were able to pull the pin when the torpedo was loaded in the tube against the spring pressure.

 

The loading process is pretty simple.  I use needle nose pliers to retract the pin on a tube, insert the torpedo with the retaining block down and slide it back against spring pressure until fully in the tube, then let the pin back up into the tube to engage the retaining block.

The launching system is simple and works well.

Drive Train

When figuring out the power system for the boat, I used several methods of scaling the power down from the original.  One method started with the horsepower of the original, another the displacement as a function of length, width, and depth.  Both methods resulted in an estimated power requirement of somewhere between 500 and 750 Watts.   Of course, the ultimate performance will be related also to the propeller size, pitch, etc and other factors affecting the RPM of the propeller shafts.  I thought that it would be smarter to run at a somewhat lower propeller shaft RPM and drive props that were larger than scale with a moderate pitch.

The boat is powered by three PropDrive 28 electric motors.  They are rated at about 1,100 rpm per volt and can handle up to 15 volts.  I had planned to use one large capacity 12 volt LiPo battery to power the boat motors and the air pump, but decided to start with lower voltage 7.4 volt LiPo batteries, one for each motor, figuring that the lower RPM might produce more realistic scale performance and that I could always increase the operating voltage if needed later on.  Also, having multiple batteries might avoid problems if one battery ran low and shut off.  The motors are small, smaller than a thimble of thread, just over an inch in diameter (28mm) and very powerful, rated at 189 W at 11.1 v (3S LiPo) and 290 W at 15 v (4S LiPo).  I estimated the power as about 110 – 120 W at 7.4 v (2S Lipo) for a total of 330 – 360 W for the boat, less than the estimated requirement.  But again, if the performance was lacking, I could up the batteries as necessary.

This shows the three motors installed in the hull and the stuffing boxes for the three props.  Aft of the center engine is a box built to hold ballast if needed.  In this picture, there is a one pound metal weight in the box, which can hold two such weights.   During initial buoyancy testing, I added two pounds of ballast in the box, but later, after addition of the torpedoes and other weight (crew figures), I could remove the ballast and the boat floated on the waterline.

I tried using a single ESC to control all three motors without success.  I found it might control two but not three motors.  In the end, I powered the boat with the three NTM PropDrive motors, each with individual battery and water cooled ESC.  For the ESC, I use the Turnigy AquaStar.  The center engine has an older version, and the two wing engines a newer version.

 

The air pump is powered by a separate 8.4 V NiCd battery.  And there are two 6 volt NiCd battery packs for accessories.

This picture shows the batteries installed in their boxes.  The three LiPo batteries for the motor are forward and run through the three toggle switches on the left.  The right hand toggle switch controls power to the air pump.  The slide switches control the accessory 6 v batteries on the left of the after battery box.

 

The props are plastic and rotate counter-clockwise, which is not to scale.  They are also larger than scale, which was done by design, hoping to compensate for lower shaft rotation speed with a bigger bite.  The props are inexpensive and easily replaced.  Since I will be running around in the local bayou, full of debris and hazards, replaceable props are a necessity.

The prop shafts are 3/16″ brass rod, running in stuffing boxes of 7/32″ brass tubing.  The prop struts were fabricated out of brass and copper and then epoxied to the hull.   The center prop shaft and engine mount are at a different angle than the outer shafts, which is consistent with the Higgins design.

 

Rudders are fabricated from copper sheet wrapped around 3/16″ brass rod and soldered to the rod and together.  Both the prop shafts and the rudder shafts are larger than strictly scale, again because of the operating environment.  The Higgins boats had two rudders unlike the ELCO boats which had three.  The rudders on the model are also oversize and extend deeper than scale to avoid problems with loss of rudder control due to prop wash/cavitation at speed, which had been a problem for me with earlier models using scale sized props and rudders.

Addendum: March 2017:

The boat had its first water trial in February and did pretty well.  The motors were coupled to the shafts using Dumas couplings and the nylon “dogbone” coupler sheared off its ears on two of the three motors.  There is a video of the boat running up to speed and you can see the loss of power as the couplings sheared.  Fortunately, the portside wing engine coupling held and the boat made it back to shore instead of drifting away into Lake Pontchartrain.

I reworked the couplers, replacing the nylon dogbones with soldered brass tubing.  This arrangement ended up working well on a second trial and produced pretty realistic performance, although the extreme torque and power of the motors caused problems keeping the drive coupling attached to the motor shaft.  The set screws loosened on two of the three motor shafts and once again, the boat made it back on a single engine.  I am still working on this issue, and will update on results.

Lights

The model is being equipped with LED lighting for things like anchor/mooring light, running lights, and searchlight.  The LED bulbs are installed in brass mounts fabricated from brass tubing and sheet stock.  Colored LED bulbs are used for running lights.  The lighting will be powered with a 5.5 – 6 volt power supply also used for Arduino, separate from the 12 volt power for the drive train.

Bow light:

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Stern light:

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Port running light:

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Starboard running light:

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Deck furniture and ventilators

The Higgins PT boats initially had 10 funnel ventilators as standard equipment plus powered fan ventilation in the engine room.  Ventilation was critical not only for the humans inhabiting the below deck space, but also for exhausting carbon monoxide and gasoline vapors that might accumulate in unventilated spaces.

In later boats, after the turrets were relocated aft, two of the tall funnel ventilators were removed and a smaller funnel vent added to the engine room hatch.

Funnel ventilators under construction.  The larger vents were built up using half inch plastic water pipe as a base and adding shaped card stock for the upper part of the vent.  These will probably be used for actual ventilation on the model, to provide cooling for the electric motors.  The smaller vents are made similarly but start with 3/8″ copper pipe as the base.  In this photo you can see some crew members being made from Sculpy clay, and several copper wire/aluminum foil armatures for more crew on the bench.

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Armament

Browning machine guns and turrets

The boat being modeled has the standard two turrets mounting twin Browning 50 caliber machine guns and a 20 mm Oerlikon auto cannon on the after deck.  For this model, all the guns are constructed with optical fiber in the barrels to simulate gun flashes when firing.  The fiber will be illuminated with LED lights controlled with Arduino modules/sketches.

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The gun barrels for the Brownings were made up from 1/16″ brass tubing for the barrels and polystyrene tubing for the cooling sleeves.  The holes in the cooling sleeve were drilled by hand using a jig made up from a 1/4″ hex nut on threaded rod as a jig/guide to spacing the holes around the tubing.  The brass tubing for the barrels will hold 1 mm optical fibre for the gun flashes.50cal_browning_guns

This snapshot shows the guns near completion with the gun cradle and turret mount in the background.  The ammunition cans and belt guides are also in place.  Details were made up from polystyrene sheet plastic, brass wire, and brass sheet stock.

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The guns ready to be mounted in cradles and the optical fibre installed.  The two “gunners” are ready to man their stations.

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20 mm Oerlikon auto cannon

This gun will be installed on the after deck, behind the engine access hatch. See part 2 for more information.

 

 

Torpedoes

The Higgins PT model I am building is of the earlier series and it is based on the Al Ross plans for the PT 265 class.  These boats were originally designed with torpedo tubes, but many had the tubes replaced with “roll off” torpedo rack/launchers before deployment or in the field.

PT 305, the boat being restored by the WW2 museum in New Orleans is of the 265 class, but in the field carried roll off torpedoes and gradually increased armament including the 20 mm, 37 mm, and 40 mm cannons typical of the late stages of the war.  I do not know whether the PT 305 ever had torpedo tubes installed at the time of construction, as PT configuration changed and evolved continually and rapidly during the war.

img_0306aThe 265 class was an intermediate stage in the evolution of the Higgins PT boats.  Because the boats were originally designed to carry torpedo tubes, the gun turrets were attached to the cabin at the aft corners and had short tails extending aft.  Once the tubes had been abandoned and the roll off racks were being installed on boats during construction, the gun turrets were moved aft about 30 inches to improve visibility from the cockpit, and the later boats had extensions from the cabin tho the turret and still maintained the short tails aft of the turrets.

The plan is for the model to have working torpedo tubes that launch working torpedoes.

img_0311aThe tubes are being fabricated using 1 1/4 “thin wall PVC pipe, the type used for under-sink drain pipes and the torpedoes will be launched with a compression spring.  The torpedoes will be pushed into the tube, compressing the spring and held in place with a pin extending up through the forward pylon holding the tube.  When the pin is retracted by servo action, the torpedo should launch.

The torpedoes are fabricated from lengths of 1” PVC water pipe with turned wooden head and tapered tail.  The propulsion system is a small electric motor, which just fits in the plastic tube, two AAA batteries in series and a switch that is held open by a pin extending through the wall of the plastic tube.  When the torpedo is in the tube, the pin is pushed upward and the switch is open.  As the torpedo leaves the tube, the pin drops down and the switch closes, starting the motor.

img_0308aIn developing the switch, I made up several prototype knife switches of brass using elastic bands to provide the closing mechanism.  These worked but were too tall to work well in the small space available, so I converted to a switch of spring steel, which has a lower profile and seems to have solved the problem of space.

In the photo, you can see the component parts of the torpedo, one of the initial prototype drive units, and the “new and improved” switch.

At present, I need to fabricate the four torpedoes, get them painted and sealed, do some buoyancy and water testing, then see if they will launch from the tubes as planned.

Deck Part 2

The second layer of deck planking was laid longitudinally.  First, the large access hatch was planked separately, and the planks were extended out from the frame of the hatch along its length to form an overlap along the side, and also extended from the rear of the hatch to the edge deck plank along the transom.

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Once the hatch was planked, it was replaced into the opening and the remainder of the deck planked.

 

Hatch coamings were built up using wood for the forward hatches, and laminated card stock for the engine room cabin hatch.  This hatch coaming also had wooden blocks placed so the hatch cover can be screwed down during operation.  This hatch is planned to be the main access for batteries, receiver, motors, etc at later stages of construction.

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Planking was carried around the cabin and turrets.  Planking was glued down and also secured with wooden pegs.  The pegs were made by splitting wooden tongue depressor sticks with a knife and then tapering them to a point.  These wooden pins were then dipped in glue and gently hammered into a hole in the deck plank and deck beam drilled with a #65 drill.

 

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