The Steamboating Forum

January this year, I bought a Stuart Turner Cygnet (5A) on the steamboatassocation.

My plan is to build a woodfired Blackstaff type watertube boiler, with a pressure of 4 baro (60PSI). The volume is 12 liter, so the boiler is category 1 of the PED. That have benefits for examination. The hull is a little one, 4 metres length, so I hope the engine has enough power, but it's not a speedboat.

The Stuart Cygnet is clearly used and some parts are worn, so it needs overhaul. This engine has a gearbox for reduction to a engine feedpump and a wet airpump. In my plan, the exhaust steam passes a feedwaterheater and goes to the exhaust in chimney. Possibly, there will be a keelcondenser but I've a question about the airpump.

The Cygnet engine has original an 'AC' membrane type fuel pump, as wet airpump. My question about this pump is; is it able to create a serious vacuüm (-0,8 baro)? Of course the condensor must have enough cooling capacity, but when this pump can't pump the water and air out of the condenser, it don't work.

Also, the steampassages of this engine are small. The 5A is designed to run at 100 PSI and atmosferic exhaust. When I want to work with a exhaust vacuüm, the steamflow in the passages becomes too high so the vacuüm behind the piston is much lower as the condenservacuüm. Has anyone experience of it?

Best regards,

Kees Hellinga, the Netherlands (excuses me for the bad English)

Your observations as to the possible difficulties with this arrangement are correct, however with such a small boat, the engine will probably perform adequately, even with the 60 PSI (4 Barg) steam pressure.

100 PSI (6.7 Barg) steam pressure is the biggest shortcoming in power output.

With keel condenser, the adequacy of the vacuum pump is almost entirely a function of the amount of air leakage into the system. I do not know the ultimate vacuum capacity of the "AC" pump, but every bit of vacuum helps, even with the ports in the engine undersized.

On my launch I had plans for condenser and feedwater heater, but once it was running satisfactorily without them, I keep delaying the added installations, and just cruise around with simple machine. The wet vacuum pump is used sometimes to transfer water between tanks sometimes, but that is all I use it for.

Good luck with your project. In many ways the old saying is true:

"The fun one can have on their boat is inversely proportional to its length"

The AC type diaphragm pump will pull a good enough vacuum: as Fred says, the actual figure depends on how much air leakage there is in the system, but approaching 0.8 Bar is possible.

Don't worry about port size: the 5A has a tiny swept volume , and the port area is big enough for it to run at remarkably high speed - much higher than you'll wan't for a boat.

Hello Kees.

I gather you want to avoid building a boiler that falls in the category that doesn't need inspection and I do understand that. But have a " high " pressure boiler would give you a lot more power and you could run the boat as a " puffer".
Living here in the Netherlands I know about the fearsome reputation of the old" stoomwezen" inspectorate and how notoriously difficult it to get a boiler passed .
Things have changed a tad since the " stoomwezen " ( board of trade boiler inspectorate for the non-Dutch speakers) days. After importing my launch " Araminta" from the UK and offering it up for inspection by the " Veritas" boiler inspector, I was surprised how easily it sailed through the inspection.
Total cost for the inspection and the boiler report was E 242.-- and the boiler now needs to be inspected once every 2 years.
So if I were you I'd give Veritas a ring and see what you need to do to get your boiler certified. A certified boiler also means it is easier to get insurance for your boat.

Hello Kees!
There are a lot of Dutch guys on this forum!
Nice meeting you on the steamlaunchevent in Medemblik!
We have a lot to talk about, hope to see you soon visiting my place.
Greetings,
Stoomsnor