The Heritage Steamboat Trust

Registered charity number 1129799.


All about the historic Thornycroft steam launch "Cygnet"

Cygnet is a 29ft riveted iron hull steam launch built in 1870 at Chiswick by the "then new" J.I.Thornycroft & Co as No 9.

It has not been modified or changed throughout its life so is a unique example of a mid Victorian steam launch.


Photos taken July 2010

Two photos taken after some of the muck was cleaned out of the bilges.

Note interesting water inlet connection for the injector; no skin shutoff valve though.

Photos below were taken in June 2010

Giffard Injector on lower left and single cylinder engine with a gunmetal base (under the dirt) with Stephenson reverse valve gear.

3.3/4 inch bore by 5 inch stroke, develops 9hp at 76 psi giving a boat speed of 7.8 knots.

Camera shot looking into the firebox tube plate, still sooty from 1970 fire!

Small pea whistle by the side of a twin Salters type safety valve. Note the adjusting screw to set the pressure!


The steam out for all services is taken via a flange outlet to a rather complex casting which incorporates the twin Salter type safety valves, the engine throttle valve and steam for the pressure gauge, the whistle, the injector, the blower and the bilge ejector.

The photo on the left shows the enclosed spring balance part of the safety valve which appears to be set at 40 psi, the red flange output is off to the blower, the larger flange with pipe out is to the engine.

There are three mountings rising up from the casting, one for the whistle and the other two are for the twin safety valves. These can be better seen on the photos above and below.

The check valve on the right part of the backhead is for the boiler hand pump feed.

Sorry it's not easy to see but we need to remove the boiler top to get a better photo.

The photo shown below is taken from above the boiler casing. The boiler backhead is at the bottom; note the take off flange mounting. The round object is the top of the pea whistle.


Coal bunker with firebars in the foreground

Looking down the Funnel, a large blast nozzle with?? is it the blower steam pipe? Very crude.

We now believe that the funnel and smokebox have been replaced at some time, no doubt rotted away.

Some experts taking stock??

So when was Cygnet Built?

This table is a copy of a list of boats built from the Thornycroft ledgers, these are still in existence. Note these show every boat built by Thornycroft.

The build date here is given as 1870 and matches the original builders plate, although there was another later plaque fitted giving an incorrect date of 1873 thought to have been fitted c1890's.

The 1870 build date fits in with the build date given for Miranda (no.10 - 1871). Miranda was the subject of river speed trials in 1871 and a  report of these trials was given to a lecture of the Institute of Naval Architects in 1871.