PVC prototype with a tail

Here's the PVC prototype, now with a tail fin.  I've also refined the top tube (removing the upside down "V" from the earlier pass at this) and placed the axis for the outriggers at the center of the forward vertical tube.

Here, you can see the tail sticking up out of the water in the background.  The tail is made of balsa wood, shaped into a rough tear drop, with spar varnish.  The weight of the PVC places the center of gravity forward. Because the tail is on a fishing-rod, and because the fishing-rod has two sections with the tail attached to the end of the slenderest section, you may note that the tail is not quite horizontal.  I fixed this in the photos below (but note that the angle of the tail relative to the ground may be changed).  

Below is the craft with a payload (a light-weight boot).  It's a bit hard to tell from the picture, but the tail is now submerged.  The craft has good fore-aft and side-to-side balance.  I made the outriggers longer than necessary (toward the aft), because I wasn't sure if I'd need to move their floatation aft.  I didn't.

Below is a closer side-view.  

To test out the performance of the craft, I depressed and released the payload (being careful not to push forward or aft), producing a gentle oscillation of the payload.  The craft went forward!  Actually, it went much better than I expected.  I did it several times.  I thought there might be ambiguity regarding whether thrust was being generated, but there wasn't any doubt on any of the trials.  I could see the tail pushing water aft.  The oscillation rate was considerably slower than I expected, on the order of one full oscillation every 3 or 4 seconds. I had been thinking it would oscillate once per second or so.  It was clear that the water was damping the oscillation.

Next steps:  1) Increase the amount of floatation on the main thrust spar and shorten the outriggers.  I'd like to have less of the outriggers in the water. 2) Instead of the boot-payload, attach a small variable-speed electric motor and batteries to the top spar; the motor will oscillate the weight of the motor and batteries up and down, causing the top spar to oscillate.  Oscillation of the motor/batteries should produce sustained thrust. 3) Change the angle of the rear fin relative to the ground (discussed above).  Will this produce a steering effect?