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Lady Jan steam tug

Does anyone know the design of this boiler?

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Ray Wood 230/07/2020 10:26:51
2847 forum posts
993 photos

Hi Mark,

Welcome to the world of Model Engineering many people are capable of making the parts and assembling them, but the testing and commissioning to make it work properly that's another story!!

Maybe extend the exhaust pipe to nearer the top of the chimney to stop any backpressure to the burner?

Regards Ray

Mark Knight 411/09/2020 22:15:56
21 forum posts
15 photos

Some progress to report with the steam tug. I spent a very enjoyable and informative morning over with Richard Simpson earlier this week. He inspected the boiler and performed a hydraulic pressure test, which it passed. Before a steam test is conducted, I have a few changes to the exhaust system to make. As reported above the exhaust was exiting into the chimney which caused a blockage and the flame to back-up. Richard stated this was not surprising and suggested the exhaust was taken right to the top of the chimney. Also I need an oil separator in the exhaust line. I’m now in the process of making one to fit in the available space. Once all this is done and plumbed in, I’ll be steaming it to see how it performs. If all ok, it’ll be back to Richard’s for a steam test and hopefully a certificate.

Mark Knight 425/12/2020 11:15:15
21 forum posts
15 photos


Finally got all the modifications finished and steamed the boiler. All went well, I adjusted the pressure relief valve to open at the working pressure of 40psi and it always keeps the pressure below 44psi (WP +10%) as required by the green book.

The exhaust now works fine as I’ve taken the pipe up the inside of the funnel to end above the top. This then has no affect on the flame travelling down the boiler flue.

I've added a centrifugal oil separator to the exhaust which works well along with the pipe and valve for emptying it.

I’ve still not attended to the leaking piston valves and also the reverser valve leaks. However the boiler still manages to hold working pressure with the engine running on the bench. Last job is to have a look at these and see if they can be improved. Anyone have any recommendations? Can piston ring seals be installed or do they just rely on very close clearances for good sealing?

Edited By Mark Knight 4 on 25/12/2020 11:16:33

Richard Simpson25/12/2020 23:02:39
1027 forum posts
256 photos

Mark, just so you are aware, we are now onto the Orange Book, which superseded the Green Book on the 1st May 2018.

The requirement that you refer to regarding the Working Pressure plus 10% for the safety valve has not changed though so nothing to worry about!

Glad to see she is progressing, she sounds like she is coming along nicely and your modifications have had the desired effect.


Edited By Richard Simpson on 25/12/2020 23:05:33

Mark Knight 426/12/2020 08:09:37
21 forum posts
15 photos

Hi Richard, thanks for this, I’ve now downloaded the orange book. I’ll be in touch once I’ve improved the valve leakage situation (and this Covid nonsense allows us to meet).

Richard Simpson26/12/2020 09:15:35
1027 forum posts
256 photos

Mark, can you post a picture of the engine and point to exactly the leaks are coming from?

Mark Knight 426/12/2020 13:25:09
21 forum posts
15 photos


Hi Richard, this snapshot from the YouTube video linked in my above post shows the engine piston valves bubbling on the top of the engine. There is some steam rising from these when the engine runs. However, I think the worst leak is coming from the reversing valve. It’s a piston type valve and when the piston is centralised (shutoff) there is still quite a plume of steam coming out of the exhaust. I think steam is leaking past the piston directly between the inlet and exhaust ports of the valve. If this was sealing correctly, it would stop any steam leaving the boiler. I’ve now re-reamed the bore of the valve and I’m in the process of making a new piston which will hopefully provide a better fit and seal.

Mark Knight 426/12/2020 13:39:48
21 forum posts
15 photos

I should also mention the process I’m going through with making this piston as there may be something more I can learn. I levelled my Myford lathe in accordance with the manual before I started and managed to get the turned diameter within 0.01mm down the length of the piston. I then turned the piston down such that it was a slight interference with the cylinder bore. The surface finish was very good from the turning but I then got it even better with some 800 grit wet and dry (the lathe was protected with a cover!). The piston was still a tight fit with the cylinder bore so finally I put some molybdenum disulphide on the piston and attempted to lap the interface with the cylinder. I put the lathe on a very slow speed and held the cylinder on the piston and gradually moved it in and out. It is loosening up slowly put still won’t fully go freely down the full length of the cylinder. I read about this method using molybdenum disulphide on a model steam forum but it had mixed opinions. Is there a better way of achieving a free but steam tight fit between piston and cylinder?

Edited By Mark Knight 4 on 26/12/2020 13:43:10

Richard Simpson26/12/2020 23:32:22
1027 forum posts
256 photos

Mark, without taking it apart fro a look I cannot say for sure but those slots you can see may be for valve adjustment, in which case we are looking at the top of the piston valve itself. If that is the case and you have leaks there your only options are to dismantle it and see if there are any internal sealing arrangements such as 'O' rings, which can be replaced. If not, and you are getting leaks past the valve then your only option is a repeat of the process you are going through with the main reversing valve, which is a real pain. You could try threading the outside of the boss that supports the valve and fitting a cap, which might just be an easier option.

Unfortunately piston valves are notorious for passing as there has to be a clearance for smooth running however make sure you run the engine with a good supply of steam oil in it as that helps to seal the piston. I don't think you can do any more than you are however the danger is that you are going to get the best possible fit you can when it is cold, which will change when it is up to temperature, and could lead to seizure if tolerances are too fine. Unfortunately the fit has to be a compromise between allowing free movement and resisting leaks. You will find you will have to compromise somewhere and that will inevitably mean putting up with a degree of leakage.

Mark Knight 428/12/2020 21:56:45
21 forum posts
15 photos


I’ve now finished the new piston for the reversing valve. The fit in the cylinder feels decent but there’s still too much leakage between the inlet and exhaust ports in the shutoff position for my liking. I don’t think the adjustable reamer was the ideal tool for producing an accurate round and straight cylinder as it was only cutting along the middle portion of the flutes. I think a fixed reamer of the correct diameter would be better. Anyway next step is to turn a couple of grooves and insert a pair of O rings. I’ve done some experimentation and it looks like about 7 thou of radial nip across the O ring will work without the ports slicing the ring with every pass. It’ll be interesting to see how long these rings last in this application.

Still not looked at the engine piston valves but going by the box of bits left by my Dad it appears these pistons have O rings at either end. I’ll hence take the pistons out and see if these rings have worn.

Edited By Mark Knight 4 on 28/12/2020 22:00:10

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