I finally got a free day to set up Faux Fish to take the tail photos. Unfortunately, it was raining when I got up this morning. At least it was melting the snow but I wouldn’t be able to take pictures outside. Being as my one car garage is presently occupied my choices became sort of limited.
We haven’t put up our Christmas decorations yet and my wife is out of town, soooo…
I have to admit my favorite part of this one-man operation was backing Faux Fish and its cart down the steps from my back yard to the front yard and then pulling it up the stairs to my front door. I really should charge my neighbors some sort of entertainment fee. Anyway I finally got it into the house.
The tail looks kind of scrawny without the skin segments doesn’t it?
Starting at the tail:
There is some distortion of the tail from the angle that the tail was photographed from, however the tail shape was copied from the tail of a Porbeagle shark.
The above four pictures show the complete tail mechanism on Faux Fish. The pink foam is insulation foam that is used for buoyancy. This piece and the flotation provided by the foam core in the tail skin sections allow the tail to be neutrally buoyant.
Here is the tail joint. Part of the challenge of building something with limited facilities is designing for construction within your fabrication capabilities. For example, I have found that control rod ends work very well as pivot points. They are relatively cheap, strong, accurate from base to the center of the hole, have built-in spherical bearings and don’t require welding or accurate machining to function, just drill 2 holes in your mounting plate. The ones shown here are made of nylon and cost about $9.00 each. I haven’t broken one yet. The metal ones in the same size range are about $7.00-20.00 depending on material. You can also get them with a male thread. I use these for pivots on the wings of Bogus Batoid. I will get into the cylinder functions a little later.
This tail segment is 18″ long from pivot to pivot.
This pivot joint is similar to the tail joint except that the cylinders are spaced further apart, not possible at the narrow end of the tail. The angle brackets are for attaching the tail skin segments.
This tail section is also 18″ long but much more substantial since it is the powered section. It is presently set up for being operated by push pull rods directly connected to the foot pedals. The rods connect to the section with clevises. (The rusty rods coming thru the large holes in the channel next to the hull. My threading dies won’t thread stainless steel rods that’s why I used regular carbon steel. That and about 1/4 the cost of ss.) The other drive options are water cylinders and pneumatic cylinders. The black rubber bumpers on this section can be adjusted using washers to limit travel of the tail and to reduce impact at the end of stroke. The whole tail is held on with one 1/2″ pin. This allows the whole tail to be easily removed for transporting the sub.
The first tail section beyond the hull is the steering section. It does not work when using the drive rods. In this application it just floats between the hull and moving tail sections. The drive rods have one main advantage and one main disadvantage. The advantage is a very simple drive drive, push the left foot pedal and the tail moves to the left, push the right pedal and the tail moves to the right. The disadvantage is that to steer you have to hold the left or right pedal down and stop pedalling for a moment. By holding the pedal down the tail is held in a full left or right position and acts as a rudder. When the sub turns the required amount you begin to pedal again. It works but you lose speed each time you need to turn. Fortunately we race in a straight line so this hasn’t been a problem. We even navigated the slalom course so it is effective.
When using the water or air cylinders for driving the tail the steering section allows the tail to be steered left or right while it continues oscillating. This is because the cylinders are trunnion mounted on the steering section and move with it. The steering section is pivoted by an operator controlled lever.
Well here is what the air cylinders do. They act as adjustable rate pneumatic springs. If you look at the photos you can see that when the tail is straight the cylinders are fully extended and the black balls are just touching the adjoining arm section. When the tail is flexed one spring at each joint is compressed while the other remains extended. The amount of spring force that each cylinder provides is varied by adjusting the pressure to the spring pair. The pressure to each set of springs can be controlled independently. The stiffness of the tail can be varied from easily bendable to completely rigid. The small black tubes from the cylinders go to an air chamber to allow the cylinders to retract without sucking water into the rod end of the cylinders. The cylinders provide no propulsion to the drive and could be replaced with regular springs. The main advantage is that the tail stiffness can be tuned to suit the operating conditions or varied while underway. I will be demonstrating this on an upcoming video.