Small Oscillating engine with gear box.

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Tony Bird

Senior Member
Feb 20, 2011
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Cardiff, South Wales, UK

I have done a bit of R&D for a friend on a gear box for a model paddle boat. The steam engine that will power it is a single acting oscillator with an 8mm bore and 16mm stroke it is based on a Mamod size I think; so there isn't a lot of power to play with. A successful gear box with this size engine has been made using spur gears but a more compact one using a worm drive was desired. A couple of gear boxes using heavy brass gears have been made but they had too much friction. The hull to be driven is basically to the Mini Vap 50 rule it being just over 20" long and works quite well using the engine driving the paddles through a spur gear box.

The gear boxes that have too much friction.

So I am trying nylon gears.

The spur gear box using a 16:1 ratio drove a pair of 3.1/2"paddles easily. So a prototype engine and wheel and wheel gear box was made. The prototype gear box was designed so it could easily be made with different gear ratios. First an 18:1 ratio was tried; having a slightly lower ratio was used to overcome any higher friction that a worm and wheel gear box might have.

With the engine support fitted.

A cylinder was made using a K&S brass tube.

A balanced flywheel was made.

The steam ports were drilled in the port block using a jig.

As there is no means of lubrication in the model boat a long aluminium piston was used. I have found the combination of an aluminium piston working in a brass cylinder works well with saturated steam.

Checking the piston steam port clearance.

Checking the finished engine and gear box using steam.

With a boiler pressure of 20 psi it was found quite difficult to stop the engine by gripping the output shaft. So it was decide to build a lighter streamlined version using the same gear ratio of 18:1.

For the engine/gear box frame some 1" square tube with 1/8" thick walls was used.

The same cylinder, piston and flywheel sizes were used.

The finished engine.

The two engines/gearboxes built.

The gears used were supplied by S.H.G. Models; the push fit gears are more concentric in use than the gears with bushes and were used on the second engine. The cost of the worm and wheel was £3.50; I quite like using well made nylon gears they are quieter running than brass and seem to wear very well. I have some 20 year old model railway locomotives that regularly have a lot of use which whose nylon gears are still in good condition.

The second engines port block has been designed if necessary to be large enough to take a double acting cylinder. Using a thick wall square tube to make the engine mount means that bearings don’t have be used.

Video of prototype engine in steam.

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Streamline engine in steam.

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If there are any questions please ask; but from tomorrow I am away for some time to where the internet isn't that reliable so it might be a while before I can answer.

Regards Tony.

I have decided to make a hull to check out the engine and gearboxes made. I have had some plans for a Murray River paddle steamer for some years and they are being used as a bases for the test hull. I have had a meeting with the friend who is going to 3D print some paddle wheels for me. He has worked out how the paddles will be constructed; a very simple design with 12 blades.

I have started work on a prototype hull. The plan of the hull has been photocopied and has been increased in length to 54 cm to give more buoyancy and still still be less than the 50 Mini Vap rule.

The plain section on the drawing is the extra length.

A half section template was made from thin plywood it could have been from card but I didn't have any. The half section plan glued to the plywood.

The Half section was cut out.

And the paper removed.

The hull to be made is for testing and for positioning the boiler, engine in it so I am using timber that I have to hand. The hull shape will be to the extended drawing but the construction will be as open as possible to allow easy distribution of the various parts of engine boiler etc. The original instructions with the drawings specified balsa wood for construction. I don't have any balsa wood and for steam use a more robust hull might be an idea. So one of my usual sources of timer was used.

I don't make many boats from scratch so my constructional techniques tend to evolve so there are probably better ways of doing it. The coping saw was fitted with a new blade and cutting was started.

The two halves of the bottom of the hull were glued together.

Being a metalworker the result was filed rather than planed, I do have a plane somewhere.

To give the most access the deck was modified from the drawings.

The two sections of deck nailed together for filing.

The ends of the deck were glued and slotted for a piece of plywood foe extra strength.

The drawings called for formers to hold the bottom of the hull and deck apart but to give maximum access pillars were used.

That is as far as I have got with the hull.

Regards Tony.
Very nice indeed Tony.

I thought minivap was dead and buried years ago as I designed and made V-twin engines for them just after they started to be used. I gave about a dozen sets of parts to a friend many years ago, unfortunately he has now passed away and I don't know what happened to them.

I did modify (made it easier to make) a French designed long stroke engine which I sold for a few years, but I also made two paddler engines from the same design. On 5 psi, the two output shafts could raise a breize block on it's shaft at the max revs of 100 rpm. Very powerful indeed, and should easily power a 50" to 60" paddle tug. I still have one of each in my posession should ever I take up model boating again.

Both the vertical and horizontal versions.


Hi John,

A pair of very good looking engines you have made; it would be nice to see them powering a model boat.

A bit more has been done to the hull more pillars have been fitted.

And a inner skin has been fitted.

The hull so far has been sealed and hopefully the outer skin will be fitted tomorrow.

Regards Tony.
I made a total of 18 (in batches of 6) of the uprights and only two of the horizontal.

They are powering models all over the world (at least they were a few years ago), with not one complaint.
Stick with a simple design, make it to the best of your abilities and you can't go far wrong.
Where people go wrong with model boat installations, they make them far too complicated, with such things as gas attenuators, sight glass level sensing auto top up pumps etc, so they spend most of the time messing about with the steam plant trying to get it to run correctly rather than being on the water sailing.
I sailed my steamer almost every weekend, summer amd winter for ten years with no breakdowns, just a piston ring change every now and again to keep the power up and lots of steam oil.
It looks like you are aiming for the same thing.

Hi John,

I agree the simpler the better, less problems!

Today I skinned the hull with 1/16" (1.5mm) plywood.

Both sides being skinned at the same time.

The bows were done separately this is the second one being glued.

The hull as it looks at the moment.

Regards Tony.
I thought i knew most of the Murray paddle steamers but havent heard of the Jennifer Anne before. Great looking craft so far.
I would love to build a scale model of the Marion.
Hi Herbie,

Eris Kennedy the Australian who designed the Jenifer Anne in 2007 named her after his daughter and said that the model is representative of the type of paddle steam used on the Murray not a scale model. Apparently the models first sailing was on the Murrumbridge River a tributary of the Murray.

Regards Tony.

I caught up with some domestic engineering this morning and did a bit on the hull this afternoon.

The hull had been sealed and hung up to dry yesterday afternoon.

This morning a displacement test was done to see how much weight there is to play with for the rest of the construction.

About 2.4 kg nearly 5.5 lb to get the hull to its water line.

A start has been made on the sponsons which will hold the paddle wheel covers.

A dry run with a boiler and engine.

The engine and empty boiler come in at just under 1.5 lbs so there is a bit to play with.

Regards Tony.

Today I did a bit of metal work along with some wood work. I don't know what it does but the Murray River paddle boat has a very long stem post. Presumably made of wood; for a model this looked a bit vulnerable so one was made of aluminium and screwed in place.

The large rudder was cut out of plywood and a hinge made. The hinge is simply a couple of brass eyes screwed into he hull. The rudder has a brass tube glued to it and is held between the brass eyes with a brass rod. A slight kink in the brass rod stops it from falling out and allows it to be removed when necessary.

It was decided to make a drive system for the paddles so some bearing for the paddles shaft were made from some aluminium channel. The 18:1 drive is by Mamod type spring belt and some commercial 1:1 plastic pulleys.

The model now looks like this.

A short video of the engine, gear box and drive running on about 8 psi of air.

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Regards Tony.
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If I counted correctly, it looks like you have about the right rotation speed (depending on pressure to the engine of course) of between 60 and 100. I used to have my feathering ones running at around 90 RPM.
People have them going too fast and they lose any of the efficiency that non feathering floats have.

So it looks like it is going to be a good one.

Nice work at paddle wheel steam engine and the hull who is unable to be sunk. :)

The paddle boxes have been made.

A steam test was done and the boiler makes enough steam for the engine and probably any other engine that is likely to be fitted to the hull.

A video is at:

In Old South Wales the sun is shining and may it do so for the rest of the day as I will be playing with trains at the CMES.

Regards Tony.
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One of my treasured memories is of an unplanned journey on her from Bournemouth Pier to Poole Harbour one evening many years ago .
Hi Tony;

I really like the cooling "tower" in your first photos. Excellent use of questionable taste.

As to the tall stempost...maybe to push aside large tree branches?

Hi Charles,

I really like the cooling "tower" in your first photos. Excellent use of questionable taste.

You mean my desecration of a fine Indian hand engraved brass ogee specimen vase that was worth a fortune? I think it was £1.50 in a charity shop.

As to the tall stem post...maybe to push aside large tree branches?

Could be; some has suggested that it was a steering aid helpful on winding rivers? Maybe both?

Not a lot of progress on the hull some combing has been put around the deck and some undercoat applied.

I'll have to make a start on some paddles!

Regards Tony.
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Hi Tin,

What program did you use to generate the gears?

The plastic/nylon? gears are a commercial product available from the cost of the gears pulleys and belt was a little under£7.50.

When I repaired clocks for a living I cut gears in brass and steel but no nothing of the production of plastic gears.

Regards Tony.