Quad versus twin compound

A special section just for steam engines and boilers, as without these you may as well fit a sail.
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Quad versus twin compound

Post by RGSP » Sat Aug 31, 2019 8:56 am

A couple of days ago I had an hour out in "Falcon", an 1890s steamboat now giving short tours from Stalham in North Norfolk. Falcon was given a new compound twin engine last winter, built by Steamboat Jack of this forum, and I'm pleased to say it's running nicely, and quietly. The previous engine was a lovely thing, a Simpson Strickland 2-crank quad compound, but it was so rattly that you couldn't hear what someone sitting next to you was saying. Furthermore the valve gear on those 2-crank quads is a nightmare to get precisely adjusted, and the old engine was reluctant to reverse. According to the boat crew, the new engine also needs a bit less coal than the quad did, though the boiler is unchanged. Falcon has a keel condenser, and as is very often the case it's too big, so the hot-well temperature only got to perhaps 30C after an hour's steady running, rather than the 70C I would be aiming at.

The boiler is an example of the very common Steam Boat Association design VFT102, but is in my experience unusual if not unique, in having an engine-driven fan. The old engine needed the fan to keep pressure up in the boiler, but on my trip recently, with the new engine, it was up at 170 psi almost the whole time, with the safety valve just hissing gently: I doubt whether the fan is now needed at all, but it does give a quick response to firing.

Of course I still don't know how the Simpson Strickland Quad would perform if coupled to 250 psi or more, and its valve gear rebuilt to give accurate timings. It would undoubtedly be better, and perhaps very good indeed. However, even our slightly staid steamboat world isn't immune from Boy Racers with their love of go-faster-stripes, and other gizmos, and I do wonder whether in 1900 the small quad was a form of expensive snobbery rather than something of significant benefit. In larger sizes no doubt the benefits would have been clearer, but going to a steeple 2-crank quad rather than a 3-crank triple must always have been a marginal advantage, and I suspect a 4-crank quad would have been much easier to maintain, perhaps evidenced by these being fairly common in larger ships.
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Re: Quad versus twin compound

Post by TriangleTom » Sun Sep 01, 2019 6:43 am

I've been looking at the Simpson Strickland engines for a while, and I was lucky enough to come across this quote on the Elliott Bay Website

"At 175 lbs. boiler presure the company claimed 10 indicated HP, and at 250 lbs., 14 HP. At these high pressures the engines suffered wear and were noisy, but the company's goal was high speed, and in 1878 speed won sales in a competative market for small power boats."

Given that in the 1870s the fastest machine you could reasonably expect to own was a steam launch, I'd say you are correct in your assumption that these were the "boy racers" of the day.
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