Ron,DetroiTug wrote:Greg, Great post!
I think the biggest issue with a compound in a small boat where the load is varying wildly, is having a fixed cutoff on the LP. On large ships, I read and heard that every time a compound powered steam ship left port to make a long distance trip, the engineer would pull an indicator card on the LP and balance the cutoff to the load and speed for highest efficiency. I've heard reports of engines equipped with a pressure gauge on the receiver, that 0 psi was observed underway at certain speeds.
-Ron
Yes, the biggest difference between commercial vessels and private steam was the availability of professional full-time engineers, who (hopefully) knew valve gear well and could coax the most out of the steam for days, sometimes weeks, at a time where the opportunity to monitor an engine could be done in scientific manner. There are stories of engineers long in the tooth who would have their own little tricks for accruing the best performance out of the engine on their shift, some legitimate, some not so...!
I haven't yet managed to find any contemporary material on the design of receivers- the best Triples almost all used cast-in receivers, and I can't help feel that the volume of these is crucial- the balance between back-pressure and vacuum is a tricky one, and is definitely a good reason to use piston valves which can totally isolate the receivers- a pipe from slide valve HP chest onto a LP slide valve chest is simply a communication channel, not a precision vessel for holding steam pressure for a pre-determined amount of time. This would be greatly improved by having a piston valve on the HP and slide valve on the LP, an arrangement not uncommon on compounds of all sizes, and those I've observed modify their ST 6A's to this arrangement report a quite a difference in engine characteristics.
Greg