A small diameter hull can better resist crushing and has a smaller cross-section the reduces drag, but too small of a diameter and their is not enough displacement for the hull to be naturally buoyant. We selected 3 1/2", Sch 40, 6061 T6 aluminum pipe providing 3.4 lbs of positive buoyancy per foot and a theoretical crush depth of 5200 feet. The 4" outside diameter of 101.6mm can accommodate the thrusters, and the inside diameter of 3 1/2", 88mm will accommodate a VDSL2 transceiver, Ethernet hub, and cameras. Additional buoyancy or equipment can be added if needed by lengthening the hull. The hull is divided into seven compartments housing cameras, thrusters, lights, batteries and electronics. The three sections with thrusters and lights are free flooding so seven sections alternate between dry and wet. Couplers join the sections and incorporate bulk head connectors for wiring. Syntactic foam made from epoxy and glass spheres and/or a a lead keel internal to the hull would provide resistance to roll. There are two battery compartments, one forward and one aft, so pitch can be adjusted by moving batteries from one compartment to the other.
Line Boring Tool
The hull is devided into seven sections. Four of the sections are 1 atm, dry sections for batteries, cameras, and communication and control equipment. The other three house the thrusters, lights, and sonar equipment which is oversized and so housed in a separate compartment. The 3 1/2" aluminum pipe used for each section is sealed with o-rings on the couplers that connect each section.
The 3 1/2" schedule 40 aluminum pipe commonly is out of round by more than 20 thousands. In order to provide a good sealing surface for the o-rings the inside of the pipe needs to be machined round and for a close fit to the coupler. These sections are too long for the average metal lathe so we built a line boring tool to machine the ends of the pipes. Using larger 3/16" o-rings allows for more errors due to distortion and poor surface finish. These o-rings can easily withstand the high pressure provided there is less than 10 thousands of an inch between the two parts. The line boring tool, designed by Dave Allen, allows us to cut the inside diameter of the pipe so that it is out of round by less than 5 thousands. It uses a piece of angle iron to which the pipe is clamped. Two pillow blocks support a shaft with a cutter head treaded onto one. A hand drill is used to rotate the cutter.
3000 ft operating depth.
3 1/2" Sch 40 6061-T6 4" (101.6mm) OD, 3.548" (90.12mm)
ID, 0.452" (11.48mm) Wall, 3.4lb/ft
4" Sch 40 6061-T6 4.5" OD, 4.026" ID, .237" Wall,
Hull Coupler Designs
Steinn Hrutur came up with a number of possibilities for coupling sections of 3 1/2" Sch 40 pipe together.
Steinn also pointed out a design problem with regard to using an o-ring as a face seal at the end of a pipe. The problem is that the o-ring compresses leaving less than 1/2 of the wall of the pipe to support the loads applied from both ends, which is not sufficient for 1700 psi.
Our best design at this point is a simple coupler that slips into the pipe using a pair of o-rings, and which is held together with a pair of threaded rods that pass down the top and bottom of the ROV.
3 1/2" Sch 40 6061-T6 OD: 4" (101.6mm) OD,
ID: 3.548" (90.12mm) , Wakk: 0.452" (11.48mm),
Butyl or Silicone
When used as a static seal, the maximum recommended
Gland fill should be between between 60%
O-ring seal when the groove is cut into a flat surface and when the pressure is inward, the groove inside diameter is primary. This design technique minimizes movement of the O-ring in the groove due to pressure.
The ID of aluminum pipe is not all that accurate, so will the internal coupler can be machined round within a couple of thousands of an inch, it will fit inside a pipe that may vary. A typical Sch40 pipe the ID is 3.548 ("A" in the table above) plus or minus about .04". So if and interior coupler was turned to and 3.505" OD ("C" in the table above), it should clear the pipe by at least .003" and as much as .043". Will that work without machining the ID of the pipe?
Through Hull Connectors
SeaCon connectors IL-6-FS (Female Side) $41.20ea and the BH-6-MP (Male Side) $61.80, the locking collars are $7 each. seaconworldwide.com
Lowrance SonarHub Housing
Unfortunately the circuit board for the Lowrance SonarHub is one single board just under 7 x 7 inches, so the board must be adapted for the 3 1/2" ID pipe hull, or the hull must be adapted for the board. Options include cutting the board in half and soldering wire connections between the two sides. Making a 1 ATM housing to protect the boards components from the pressure sounds easy until we do the math. 7 x 7 inches = 49 square inches at 1700 psi = 83,300 pounds. Over 41 tons of force.
1) Spherical Hull
3) Syntactic Foam Potting
A more complicated approach that may increase the reliability would include multiple coats. The compression strength of a typical FR-4 fiberglass circuit board is 60,200 psi - flatwise. Resin encapsulating 5 micron, 0.0002 inch glass spheres is commonly used as flotation for deep sea vehicles. It has an good compression strength of 8000 psi, but low tensile strength. It also has a much lower shrinkage than typical epoxy. IC's and capacitors on the circuit board would first be coated with a thin layer of air entrained silicon to form a compressible buffer around these components. The board would then be think coating of "mg Chemicals" 832TC-450ML thermally conductive potting resin which uses aluminum oxide for thermal conduction (.682 W/mK) but which is not electrically conductive. The next layer would be epoxy infused with copper powder to produce a layer with an approximate heat conduction ability of 30 W/mK. This 6 to 10 mm layer will also bond 14 awg solid copper wire to the board in order to conduct heat away from the board. The end of the wires would pass off the edges of the board and be bent to form bands of copper down the sides of the board. Once the copper layer is cured the exposed copper wires would be embedded in salt dough which can be dissolved in water after the final layer of casting is cured. The final outer shell will be cast HYTAC-C syntactic foam poured into a mold of the desired fin shape from 1 to 3 inches thick. After curing the salt dough would be dissolved to form water channels over the exposed copper. This method would also not be reusable should the board fail and need to be replaced but a second board could be potted so replacement is available. This process also simplifies the cable connections as each conductor can by isolated and stripped to bare copper where it passes through the innermost thermally conductive potting layer to form a watertight seal to the conductor. The fin could be installed through a flooded section of the hull to limit it's cross-section. It could also form a mounting structure for the transducers or a dive plane should the ROV be used without the assistance of a forward towed array that controls depressing the unit when under tow.
4) Aluminum Casting
5) Aluminum Shell with External Frames and Syntactic Foam
6) Cut the Circuit Board and Soldier it Back Together
7) Cast Circuit Board Hull Housing
8) Oil Compensated Housing
A very simple solution is to build a light weight housing for the
board, and flood it with mineral oil. A flexible hose or
diaphragm is connected to the housing that allows the oil to be
compressed to match the external pressure.