Contents

Introduction

The aquatic domain is an exciting place for robotic exploration.  It is also a very challenging environment for robots.  Commercial seafaring robots represent the state-of-the-art, but cost many thousands of dollars.  They can also be quite large and require considerable support.  These are way out of reach for the average hobbyist who just wants to explore the local pond.  Fortunately, there is a very good book out there that can help.  It is "Build Your Own Underwater Robot" by Harry Bohm and Vickie Jensen.  It describes fairly inexpensive techniques for constructing a variety of submersible toys, remotely operated vehicles, and robotic platforms.  This article describes the construction of an Aquatic Robot Base (ARB) that uses many of the ideas from the book. 

Aquatic Robot Base

The Aquatic Robot Base (ARB) uses a two-motor differential steering system.  It steers just like a land vehicle, but instead of wheels or treads, it uses propellors.  The fundamental challenge in creating aquatic vehicle is water-proofing everything.  The ARB uses a waterproof sports equipment box to hold a battery and circuitry.  It uses two waterproof bilge pump motors for propulsion, with each motor driving a propellor.  The ARB could be mounted under a barge or hull to make a robotic or radio-controlled surface vessel.  Potentially, one could expand on the waterproofing and propulsion techniques to build a submersible robotic system. 

Propellors and Water Proof Housing Unit.

Motors

The ARB uses two 360 gallon-per-hour submersible bilge pump motors from Rule.  These are some of the smallest bilge pumps on the market, but they are great for propelling a small aquatic craft.  They are also designed to work underwater! Their operating voltage is typically 12V, but 6V works fine for the ARB.  These bilge pumps use an impellor to ingest water and pump it out a spout.  For this project, it is necessary to remove the spout assembly and fasten a propellor to the impellor.  This makes it possible to produce forward and reverse thrust by changing the motor direction.  An unmodified bilge pump will also provide thrust, but only in one direction because it can only push water out of its spout. 

First, snap off the blue strainer to modify the bilge pump to allow for forward and reverse directions.  Next, get a hacksaw to remove the spout assembly.  Start cutting on the white spout assembly about a quarter of an inch from the glued seam with the red motor casing.  This will leave a rim on the motor that will help mount it later.  A well-stocked hardware store may have a variety of small propellors for exhaust or cooling fans.  You may find one that fits nicely on the impellor.  A big glob of PC-11 brand marine-grade epoxy paste will fasten the propellor securely to the impellor.  Be sure to rough up the surfaces to be glued before applying the epoxy and be careful not to get any epoxy on the motor shaft. 

Motor Mounts

The motor mounts are made of aluminum bars (1 inch wide, 1/16 inch thick) and brass nuts and bolts to prevent corrosion.  The motors connect to two ends of a single aluminum bar.  They are held in place with two brackets, one for each motor.  The main bar has its ends bent up to form hooks that will grab the rims of each motor.  The brackets have hooks that also grab the rims of each motor.  It is possible to bend the hooks and brackets by hand with pliers.  (If the aluminum bar is any thicker, it will be nearly impossible to bend it by hand.) Before bending the ends into hooks, cut the corners off.  That will make the bending easier. 

Two brass bolts secure each bracket to the main bar.  Some foam tape on the main bar pushes the motors securely against the hooks and holds them in place.  Several layers of tape may be required to get enough tension to hold the motors in place without slipping.  The backing material should remain in place on the top piece of tape.  This prevents the tape from sticking to the motor and makes it easier to replace the motors if necessary.  A construction diagram with some rough dimensions appears below.  This mounting technique should scale easily to larger motors. 

Motor Mounting Diagram

Waterproof Box

The waterproof box on the ARB is a medium-sized Lexan utility box (4 x 6 x 7.5 inches) made by GSI Outoors.  A sporting goods store or online camping supply store may carry them.  This particular box is clear which makes it easy to inspect the inside.  There are a number of waterproof boxes on the market, some with depth ratings of 100 feet or more.  You may want to shop around to find the best size and style for your project. 

The motor mounting bar bolts directly to the box.  The bolts are secured from the inside with PC-11 brand marine-grade epoxy paste.  This ensures a strong watertight seal and firm attachment points for the motor mounting bar.  Several flat aluminum pieces reinforce the bolt holes to avoid overstressing and cracking the box.  Details of the construction are shown below.  It is only meant to show how things fit together, so the scale is not accurate. 

Exploded View of Waterproof Box

Putting everything together is not difficult, but it requires some patience.  Part A is a 0.75 inch 10-24 brass bolt.  Brass is used here to prevent corrosion.  Part D is a matching brass nut.  Parts B and C are aluminum panels to distribute the mounting force.  The first step is to drill holes for A in the back of box (hinge side).  Putting the bolts in back prevents the latches from getting in the way.  Be sure to leave at least an inch of space between the hinge and the main bar.  This space will have wiring holes.  It is wise to reinforce the drill points on the box with duct tape to help prevent cracking.  You may also want to work up to the final size bolt hole by drilling the first hole with a tiny drill bit and then gradually increasing the drill bit size.  Roughen up an area larger than part B around the bolt hole on the inside of the box with sandpaper.  The next step is to secure parts A, B, C, and D in place.  Fasten the bolts securely, but do not overtighten.  Then apply a generous glob of epoxy paste around the bolt hole on the inside of the box.  Make sure it completely covers parts A and B.  This will ensure a watertight seal.  Let everything dry for 24 hours.  After the epoxy has dried, you may want to test the box for leaks in a sink or bathtub.  Part E is a rubber gasket or spacer, slightly thicker than the bolt, with a middle hole that fits snugly around the bolt.  Part F represents the middle portion of the main motor mount bar.  It goes onto the bolts and rests on the rubber pieces.  Parts G are nylon wing nuts that hold the motor mount bar in place.  With this motor mounting design, it will be easy to remove the entire motor mount for maintenance.  When finished, the mount should resemble the drawing below. 

Assembled Box with Marine Epoxy Applied

Power

Now the motor wires need to go into the box.  Drill a row of four holes evenly spaced about 0.5 inches apart about a half inch above the main motor mount bar.  Then thread the motor wires through the holes.  Leave the wires long inside the box so you have plenty of slack to hook things up later.  Apply a lot of silicone rubber sealant on both the inside and outside of the box around each hole.  Be sure to force the sealant into the holes.  The silicone seal is flexible, but it can also be pulled or picked off in the future if you need to yank out the wires and change motors.  If you are not worried about replacing motors, you can use a stronger sealing material around the wiring holes. 

For power, a single 6V 4Ah lead-acid battery will work well.  At 6V, the continuous current draw of a single bilge pump motor running in air is approximately 650ma.  This current will go up when the motors are driving a propellor in the water, but the large battery will still provide a lot of run time.  (At 6V, the stall current is approximately 2.7A.  It is a good idea to add fuses to a final project.) This large battery also supplies a lot of ballast weight.  It takes a lot of weight to sink the ARB.  As pictured above, it will float even with the heavy battery inside.  A couple of adhesive backed hook-and-loop fasteners can secure the battery in place.  Fasten the fuzzy side of the strips to the largest side of the battery.  Then fasten the hook side of the strips to the front bottom of the box so the battery will sit in the most stable position.  The picture below shows the completed ARB without the battery. 

Completed ARB without the Battery

Going Further

The ARB described is this article is a flexible base for experiments with an aquatic vehicle.  It only lacks some type of control.  It also lacks streamlining.  The boxy ARB shown above will actually float fairly flat in the water once its battery is installed.  With the motors wired directly to the battery, it has a top cruising speed of about 1 mph.  A good follow-up project may be to make a streamlined surface-cruising hull for the ARB.  The motors are rated at 12V so you could add another battery and see how fast you can make it go.  (Placing a second 6V battery in the ARB shown above will make it sink.) Since the ARB is also watertight, it is a good start for a submarine.  As for control, add an RC receiver and motor controls to make an RC boat.  Try adding a microntroller and a compass or GPS to make a self-guided craft.  Experiment and have fun. 

Credits

The ARB was inspired by projects and contruction ideas in "Build Your Own Underwater Robot" by Harry Bohm and Vickie Jensen.  The book is published by Westcoast Words, Vancouver, B.C.  Canada (1997).