Steam Tug Engine and Boiler

For power source I decided to design my own steam engine.

For maximum flexibility a reversible, self-starting engine was required. An oscillating design was chosen for simplicity and because it offered the possibility of reversing by just swapping the steam and exhaust ports. To enable the engine to self start 2 double acting cylinders would be needed 90 out of phase.

The engine was designed with a split crank. This enabled solid bearings to be used for the main support bearings and the big-end bearings. The crank was assembled using locking pins and Loctite.


Engine design (click for larger image) layout plan.





This image shows the part completed engine.

At this stage the reversing valve was in place at the top, the 2 cylinders were finished and so was the crank.

Still to do were the con-rods and pistons, the exterior support frame and some paint.








A compressed air test confirmed that the engine worked as a whole and also that the reverse valve operated correctly. The engine ran quite smoothly even at speed.

The torque from the engine was good and hopefully enough to power the proposed boat design.

At 20psi the engine was measured to be running at 1000rpm.



Initially there were some leaks at the port faces. This was corrected with some polishing and lapping of the mating surfaces.

It was noticed that the reverse valve was also able to control the speed using its intermediate positions, but very fine control was needed to achieve this. It would need to be tested with the radio gear to see if this was a realistic method of regulation.



The Boiler

The boiler was made to a cheddar models design. It was an internally, fired 3.5" diameter, horizontal boiler.

These images show the boiler parts on the left and the completed machined parts above




This photo shows the steam plant part way through construction.

The engine was now mounted on the base and a condenser had been added. The boiler was not yet brazed but the stand was in place and a manual top-up pump had been purchased.






This photo shows the boiler plant before the plumbing was completed.

A steam test revealed that most of the plant worked as intended. However the outside engine frame, which held the cylinders against the port faces, was not quite central and after a short time the springs had worn a dimple in the cylinder body. This then caused the cylinders to pull away from the port faces during oscillating resulting in a waste of steam and an engine that wouldn't self-start in all positions of the crank.

So the frame was removed and replaced by 2 coil springs running down each side of the engine.


Another problem was highlighted when the plant was bolted into the hull. Despite best attempts to ensure that the output shaft of the steam engine and the propeller shaft were in line, the engine shaft had ended up about 0.5mm too high. This meant that use of the originally proposed universal coupling was not possible. Instead a coupling consisting of 2 discs, one with 2 driving pegs which located in the other which had 2 slots, was used. This joint was very tolerant of both radial and angular mis-alignment and worked well, although is was more noisy at speed.

Photo showing new coupling and cylinder retaining springs.