20 mm Oerlikon Auto Cannon
20 mm Oerlikon Auto Cannon
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.
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.
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.
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.
The guns ready to be mounted in cradles and the optical fibre installed. The two “gunners” are ready to man their stations.
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.
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.
The 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.
The 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.
In 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.
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.
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.
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.
Once the hull is free of the building board and on the cradle. the next steps included trimming the upper parts of the ribs and preparing to plank the deck.
The first step was installing deck beams on alternate frames, using the deck beams to also frame up the various hatches to be added or created later. I used the same plywood for the deck beams as for the ribs, supplemented by hard maple for some of the areas needing reinforcement, such as the position of the Samson Post. Then I added the perimeter board/rub strake to the deck, using hard maple, the same thickness as the deck planking to be added later.
The deck was also planked in two layers, as the hull, with the first layer laid on diagonally and the second layer longitudinally. The area of the main cabin was left un-planked and the cabin framing set up as shown as a guide the planking, which was stopped at the forward edge of the cabin.
Once the first layer of planking was in place, the main access hatch was laid out, running aft from the cabin. This was not a hatch as on the original, but designed to give easy and full access to the hull interior during the rest of the construction and after completion. Once this hatch was cut, trimmed and fitted, the cabin was sheathed in plywood, turrets made up from Creole/Cajun seasoning cans (of paper) and the second, longitudinal layer of planking could be laid.
Access to the interior through the large access hatch allowed some interior work to be done.
One early step was to install the tubing for the air supply for the “exhausts”. When the exhausts were made, I epoxy cemented short lengths of brass tubing into the opening of each, then let that tubing through the hull. Plastic air tubing for aquarium use was then used, with connectors and control valves, to make a manifold on each side for later connection to an air pump. I am planning to use a 12 volt air pump such as those used to aerate bait tanks on fishing boats as an air source.
The access hatch was sanded and fitted to the opening and supports installed along the side. The cabin itself fitted atop the first layer of deck planking. In this shot you can see the paper cans used to make up the gun turrets, to be added later.
The hull planks were milled from white pine. They were cut to a thickness of about 1/32″ and width of about 3/16 inch. The planking was in a double layer, as in the original boats, with the first layer glued onto the frames in a diagonal orientation, about 45 degrees from the frames in the mid portion of the hull, and as it ran for the remainder, especially in the bow. The first layer was slanted directed toward the bow above the chine, and opposite below the chine, to make a chevron pattern pointing aft. Then a layer of thin cotton fabric embedded in waterproof glue was laid over the first layer of planking, taking care to smooth it well and eliminate air bubbles. Once that layer was dry, the final layer of planking was installed, with the planks above the chine at a diagonal opposite the first layer, and the planks below the chine running nearly horizontal fore and aft.
The hull with the first layer of planking installed and the fabric layer applied over the planking. I used Titebond III waterproof glue to install the planking and to embed the fabric layer.
Second layer of planking done, chine/spray rails installed, and first coats of urethane marine varnish done. Note the direction of the planking as described above.
The hull cut free of the building board and the board converted to a cradle. The exhausts are carved from maple and painted gold/bronze. The exhausts are hollowed out and have plastic tubing for an air pump to create, I hope, somewhat realistic exhaust bubbling later on.
I used the Harold Hahn building method on this model. I first re-sized my hull plans/lines to the proper scale, then used the sections provided to layout the frames. When building the smaller models, I used the hull sections from the plan as bulkheads, inserting an additional bulkhead or two in the front because the curves in the bow needed additional support for the planking. In this larger model, I increased the number of frames by drawing two intermediate frames between each of the sections, to produce a set of frames about 30 in number. By re-drawing the plans to extend the frames up to a line above, which would represent the building board, I ended up with a set of frames that would allow building the hull upside down and would align easily. Each frame was glued to the building board and to the keel so that once they were set up, they were pretty rigid and would not distort when the hull was planked. I had done the smaller models similarly, but had cemented the bulkheads to the decking, but in this larger boat, I used a true building board. Once the boat was planked, I just had to cut the hull from the building board, and then trim it along the line of the bulwarks.
The frames and keel were cut from 1/8″ plywood. Once the frames were glued to the board and keel, the joints were reinforced with small blocks and later the keel was built up with two wooden strips. This created a small rabbet allowing the first layer of planking to slip beneath it for better strength.
This blog will follow the construction of an electric powered, RC controlled, scratch built model of a Higgins 78 foot PT boat. I have built several other similar boats in smaller scale, the largest a 1:20 model of a 77 foot ELCO. But having returned to New Orleans, living near City Park and Bayou St. John, where the Higgins PTs were built and initially launched, I wanted to build a boat to operate on the Bayou. The Bayou is a tidal estuary and the waters can be rough in windy weather. In addition, there are commonly floating hazards, from bottles and bags to driftwood and turtles. And the occasional gator. So I wanted a boat large enough and strong enough to operate safely. The smaller, 1:32 scale boats, would be fine on calm days in the inner part of the Bayou, but probably not able to function in rougher waters. I also wanted performance to at least appear to match the prototype with a boat that would plane and show a small rooster tail at speed. And a finished boat that would appear, at least from a modest distance to be realistically armed and have animated functions that would make it fun. These latter desired characteristics led to my exploration of computer chip controlled LED lighting and sound. The posts will be in reverse chronologic order and updated monthly.