I was always led to believe that on a coiled pipe engine, the ejected pulse always took the path of least resistance, and the water charge was sucked up the longer pipe to recharge the coil.

That seems to have been proven when making them. Short pipe at the bottom of the coil, long pipe to the top. Whether they operate on a different principle to the diaphram type, I have no idea. But the coiled piped ones always worked for me.

John

My boats, both with coil and with diaphragm, have pipes of equal length and I'm quite sure they exhaust through both pipes at the same time.

All the ones I have made have equal length tubes. I will have to experiment when the weather turns better, or maybe those in the southern hemisphere can play around?
Regards,
Gerald