Time for another update on the project! The following post chronicles the progress made during my Christmas vacation and the past week.
I started by gluing the two PVC endcaps that will make up the joint between the bow and stern WTC sections. Looking at the profile of the Type VIIb U-boat, you can see that the hull curves upward as we move towards the stern. Therefore, I decided to split the WTC into two sections:
The bow section contains of the following:
-Ballast tank servo-valve and switches
-Forward diving plane servo
-Torpedo control servo
-Main drive motors
The stern section will contain:
-Servos for stern control surfaces
Because of the curvature of the hull, the stern WTC will be slightly more elevated than the bow. The endcap joint that will connect the two sections together is shown below.
You can see that the stern endcap (left) is slightly offset because of the hull curvature. I later reinforced the joint with more epoxy.
Next, the holes for the motor shafts are drilled, along with larger holes you help secure the motor can:
The motors themselves are secured together with styrene sheets and four bolts:
Here are the motors test-fitted on the bow part of the endcap joint, before the two endcaps were glued together:
The black bolt will secure the styrene motor support to the endcap.
With the motors secured, I soldered the supply leads, along with the RF noise suppression capacitors:
Turning now to the stern section, I attached a piece of styrene to hold the Velcro that will support the ESC and drilled a hole that will allow the ESC and ballast pump wires to pass through:
The ESC is a Tamiya TEU-103BK designed for dual motors, from my monster truck.
Yup, that’s a lot of wires!
Next, I built the bow diving plane and torpedo control servo supports. These were fabricated using thick styrene sheets and brass square tubing. Hinges at the bottom will allow me to install and remove the servos after the supports will be glued to the bow WTC endcap:
To remove the servos, I simply have to undo the bolts and flip the rear support pieces down!
Finally, I began to tackle the ballast tank section. Originally, I had intended to use a sealed-ballast type setup similar to the ones used on the Sheerline subs. To do this, I had modified a windshield-washer pump and a medicine bottle to serve as a ballast tank. However, after some testing I discovered that at 7.2V, the pump only had enough strength to fill the ballast tank to about 70% full, this was due to the air being compressed inside the tank while water was being pumped in.
After doing some research on E-bay, I was able to obtain a nice gear pump and a blood pressure cuff bladder to convert this to a flexible ballast tank system:
The great thing about this pump is that it is able to pump water at a sufficient pressure with as little as 2.4V of input!
I then built the pump support frame that will be attached to the endcap joint. The black piece of plastic was obtained from motor cover of the windshield-washer pump, which happened to fit the gear pump motor perfectly:
The pump was then fixed to the endcap joint with two bolts:
The completed propulsion/ballast pump module!
In a flexible ballast system, a valve is required to prevent the flow of water from the rubber bladder to the outside when the pump is not running. The outward flow of water is mainly due to the elasticity of the rubber causing the bladder to deflate and also because of the increased interior WTC air pressure when the bladder is inflated. With an idea in mind, I proceeded to build my servo-actuated valve.
The concept is very simple, a servo with a stainless steel roller will pinch a flexible rubber tubing (connected to the bladder) in the neutral position, preventing the water from flowing out of the the bladder. To activate the pump, the servo will simply rotate one way and flip a DPST switch on. At the same time, because the steel roller is also part of the servo wheel, the pinch on the rubber tube will be released due to the wheel’s rotation. To reverse the pump flow, the servo simply has to rotate the other way and flip another DPST switch on.
The parts that will make up the servo roller mechanism, I obtained the steel rollers from a scavenged printer.
The roller all assembled, the bottom roller spins freely between the two servo wheels, whereas the top two rollers are glued to the plastic and will be used to flip the DPST switches on and off.
Installed on a Futaba S3004 ball-bearing servo.
The servo supports are fabricated and glued onto the component tray. There is a clearance of approximately 3mm below the bottom roller to accommodate the rubber tubing.
To hold the servo down, I built a upper styrene deck. This will be threaded through the upper steel WTC support rod. At the same time, it will also house the two AA batteries that will power the pump.
The servo support mounted on the component tray with the AA battery holder in place.
Now its time to build the DPDT switches! I was lucky enough to find these micro-switches at my local electronics surplus store. The little plastic tabs can be easily pressed down by the servo rollers.
A bit of superglue and styrene later, I have two DPST switches!
The switches are assembled onto the upper servo support, from the underside you can see how the servo will activate each switch by rotating one way or the other. In essence, the whole setup behaves like a DPDT switch.
Now its time to put everything together!
With the rubber tube in place, this is the neutral or pinched position.
The battery is a 7.2V NiMH 6800MAh, should provide plenty of power and runtime!
As you can see, I’ve modified the original bow component tray by shortening it down. Removing the steel support rods around the ballast tank gives the rubber bladder more capacity while making the system very modular! I can essentially use this unit in any 3″ WTC with a pump-based ballast system. The completion of the ballast tank control module is a crucial part and a major milestone of this project. The next step is to install the prop shaft and linkage seals along with building the stern component tray.
Thats all for now folks! Hope you enjoyed reading!