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Conventional boilers

In their simplest form a boiler is a round tank of water with a fire underneath and in the case of my first engine (designed in 1967 by David Parker) it is little more than that. What we can do with these little boilers is to improve on the material normally used on model locomotives and to look at just how big a safety factor we really need. Model locomotives sometimes have a boiler shell that is twenty, thirty, even forty times stronger than it needs to be! ! The modern aircraft which we happily board and in which we fly away on our holiday works very safely on much lower factors of mechanical strength than forty to one! It is really quite safe to use a factor of four to one on a boiler of only 50 mm diameter which operates at perhaps 150 C. David's 'Comet' engine (see photograph and drawings) has a quite generous safety factor of 5:1 and the boiler is free of any long term danger due to corrosion. The boiler is made of 0.5mm nickel silver sheet, silver soldered with very simple water tubes of 6mm copper to increase its heating surface. It is spirit fired and I hydraulically tested mine to 3 times the working pressure with no problems, it worked consistently for over five years.

My experience with 'Comet' has led me to believe that it is perfectly feasible to build a model speed boat engine along similar lines as the airborne engine. Add reversing gear and a throttle valve and you could build a fast planing steam boat that is quiet, controllable and above all absolutely unique.. This is not silly thinking, the Comet's engine directly coupled to a 12''x8'' propeller turns at 3200 RPM. and develops, a static thrust of about 600 grams. The weight of this engine and boiler is little more than a 540 size electric motor and a standard 7cell 1500 ma Ni/Cad battery pack. The fastest steam powered model boats in the world are the tethered flash steam hydroplanes which reach speeds of up to 170 kph this brings me to a different kind of boiler that offers huge improvements in performance for model aircraft, the flash or Monotube Steam Generator.

Small monotube theory and practice

What is Flash Steam? There are two basic ways of generating high pressure steam almost everyone is familiar with the locomotive boiler and the land and sea born variants of the pressurised container, heated with either water or fire tubes and an external source of heat. In a flash boiler the heating surface is a single or it may be a series of tubes into which the cold feedwater is pumped against the developed pressure. The heat source is usually an oil, petrol, or gas flame directed against the tube. The tube is normally coiled and housed in a thin light weight case of stainless steel and the flame is directed through the centre of the coil. The development of high pressure steam is virtually instantaneous, hence the term flash steam. The little Groves 1936 engine that I have built goes from cold to full power in 8-10 seconds and the power is turned out all the time there is water and fuel. Throttling works within fairly narrow limits just by turning the heat supply down, the response is instant. The narrow band of power available can be satisfactorily extended using a more elaborate control system. The Groves engine is very simple and intended for free flight only where one power setting is all that is needed. The steam so generated can be superheated to the point of the engine's self distruction, where the lubricating oil is turned into carbon before it reaches the cylinder walls. I am fascinated by monotubes and in 2002 when the offer came I could not resist the temptation to buy all 15 feet of Skylark with its oil fired Monotube steam source. Like all experimenters I had to get it going ASAP in search of the first modification or improvement I could introduce. I am quite sure the previous owner would have been disappointed in me if I hadn't! There was one major departure from normal monotube practice; it was not a monotube at all it was a bi-tube, two parallel tubes wound alongside each other. (There is a theoretical advantage in doing this which I will not go into at this point.)

I have no complaint about Skylark, I launched her in the Chichester Ship Canal in April 2003 and everything operated much as the previous owner had said it should except, try as I might all I could get was 25psi and a slow walking pace. I was told that Skylark steamed well at 80-100psi and produced enough power to run at full hull speed which should be say 3.5mph, a fairly good walking pace. That is, it did when it had a monotube. I tried all I could to get that bi-tube to operate but it would not play ball. The previous owner gave me his bi-tube pressure equalising valve along with Skylark when I bought her and I experienced no sign of overheating of one tube with the other tube running much cooler. I can look down the funnel straight into the flame area and if anything is red hot I can see it at once. My impression is it did divide the flow exactly as intended. All I got was 25psi.

After a few weeks I gave up and made a new monotube from the self same two 20 foot (6.1 M) lengths of 3/16" (4 mm) Kunifer (copper alloy) brake pipe. They were joined with a screwed compression joint some way up the chimney out the way of the direct heat of the flame. Result, 60 psi whoopee progress!

(At this point I changed the propeller for a bigger one and as expected the pressure rose because the load was greater and now the boat operated exactly as its builder described. This matter of Balance between load and heat input will be covered under "Controls" when I write it!)

I do not think it was the bi-tube as a device that was the problem, my firm belief is that by switching from bi-tube to monotube I had DOUBLED the velocity of the feed water through the 3/16" (4 mm) Kunifer tube. This matter of water velocity has been discussed in the book "Experimental Flash Steam", it is so relevant to these notes that I have included the results in my table, I quote verbatim from the book:-.

Quote from Experimental Flash Steam, by Benson and Rayman my copy being published by Argus Books Ltd in 1973. The Experiments themselves were carried out by Mr Edgar T Westbury one of the most well known writers and designers of model engineering projects in UK. He is known the world over, the world of model engineering that is; (not a very big world!). Page 59.

“Tests carried out on three copper boilers each 11 Feet in length and 3/16" (4 mm), 1/4" (6.3 mm) and 5/16" (7.9 mm) diameters respectively and with a wall thickness of 0.03" (0.7 mm). Each coil was wound upon a circular tapered form, 2 1/4" (57 mm) inside diameter down to 1 1/2" (38 mm) diameter and spaced 1/8" ( 3.2 mm) apart. The casing left 1/8" (3.2 mm) gap at the largest coil. Water was fed from a water pump driven by an electric motor. Each boiler was fired by the same air-gas blowpipe 1 7/8" (48 mm) diameter, and various evaporation tests were conducted with a spring loaded outlet valve set to blow at 500psi.

In each test the 3/16" dia. boiler gave the best results and on a maximum evaporation test managed 27 cu. Inches per minute with the gas blowpipe flat out and the steam highly superheated. This represents about 1 lb per minute and seems a remarkable figure for only 11 feet of tube and the fact that this boiler had the lowest heating surface. In every test the 5/16" boiler gave the worst results. On repeating the experiments using thicker walled tube the relative results were confirmed but evaporation increased by about 12%! End of quote.

E.T. Westbury's test results on 3/16" tube is included in the table which follows. SO what is going on? 60lbs of highly superheated steam per hour from about 1/2 sq. ft heating surface! It must be said at once that this sort of hard driven performance is VERY wasteful of heat using only a few hundred degrees Centigrade from the flame. A Gas-Air torch typically burns at about 2200 degrees C at the flame cone. A thin piece of steel wire at the outlet to my tiny boilers glows red, about 800-900 C. We do not want red hot exhaust on a steam boat and I certainly do not get it on Skylark.

Why is velocity so obviously critical in the performance of monotubes? The previous owner of Skylark has a theory which I believe to be correct. As the water flows and heats at some variable point along the tube it begins to form tiny steam bubbles on the inside wall. These must tend to stick to the surface just as they do to the bottom of a saucepan when you boil water in it. The steam bubbles seem obstinately glued to the metal. In order for a monotube to operate efficiently the water must flow fast enough to scour bubbles away IMMEADIATLY they form. The conductivity of any vapour is thousands of times worse than water and maybe 100,000 times worse than copper alloys like Kunifer. If as I believe the dramatic increase in performance is due to the increased water velocity then I thought maybe I can juggle a few numbers and come up with a very interesting, if empirical figure that represents a likely MINIMUM velocity to aim for when designing very small monotubes like Skylark's and the really tiny tubes I have in Tiddler's Monotube.

Another result of this scouring is that scale and oxides do not form on the inside of the tube. Indeed if you cut open a well used section of a monotube you will see that the inside surface looks as if it had been lightly etched.

I could have directly compared the results of the bi-tube and monotube and left it at that but in addition to Westbury's 3/16" diameter tube experiment, a further source of data is available in an article published in the Model Engineer magazine in 1992. This was written by Bob Kirtley covering in great detail the construction of his world record breaking hydroplane Pisces II which raised the Class B Steam record from about 80 mph to 104 mph in one step. I have seen her go and it is a joy to behold, the noise is like no other, music to my ears and any other Monotubist's.

What I did was to compare directly the water velocities of the four separate cases and tabulate them as follows. The different values for Monotube area (Ma) for Pisces and Westbury are because of the differing tube wall thickness.

Boiler Details
Skylark Bi Tube
3/16” Dia.
Skylark Monotube
3/16” Dia.
Pisces II Monotube
3/16” Dia.
Westbury Test Monotube
3/16” Dia.
Pump Ram
Diameter (D)
0.953cm 0.953cm 0.653cm N/A
Stroke (S)
1.80cm 1.80cm 1.27 N/A
minimum (Sm)
300 300cm 2500 N/A
minimum (Vm)
384cc 384cc 1002cc 440cc
see text
area (Ma)
0.141cm2 0.075cm2 0.103cm2 0.081cm2
H2O Velocity (WV)
27.23 54.46 97.28 54.32

It was with smug satisfaction that I noticed the close tally between Skylark and the Westbury test data. To other Monotubists everywhere please give me the current particulars of your system so we can acquire a data bank for future use by others. All the tabled parameters plus pressures, temperatures, fuels, burners anything you can think of that may help others who will follow on after us.

I don't pretend that my observation and experience is a scientific study but it may prove more concrete than any other data that I have seen to date. It is further born out by the fact that if I slowly reduce the heat input setting on Skylark there comes a point at about 50 psi when the pressure drops very quickly from 50 to 25 psi; without a commensurate reduction in fuel flow. With the available data I would suggest that anyone contemplating a small monotube design for normal cruising speeds and pressures a WATER INPUT VELOCITY of about 50 metres per minute (0.8 Metres per second) should be a safe minimum to aim for at the systems normal 'cruising' power.

The importance of the above statement cannot be over emphasised as I believe it may be the key to solving much of the hit and miss approach that seems to dog this subject and is perhaps the root cause of the commonly heard mystique surrounding design and application of very small monotube steam plants. To design a plant we can now start from the same point as steam plant designers have always started; the steam consumption, this basic calculation is very clearly explained elsewhere in text books and I spare myself the need to repeat it here. This value can also be gained from users of the same and similar engine designs of course and it is perhaps a more reliable guide as well! In any case it is a good way of checking your own calculations. From the mass of steam reguired per hour the pump ram diameter and stroke can be decided and then the tube bore for a given water feed velocity becomes an additional but simple calculation. From my experience with Skylark and tiny monotubes for aircraft models I can suggest that it is an advantage if the water feed velocity drops well BELOW 0.8 metres per second for slow speed work, maybe as low as 0.4. I still get a reliable flow of steam at maybe 80 revolutions per minute but it is wet, as one would expect but very controllable.

This study, whilst interesting may be of very limited use much outside of the tube diameters in the table, I would hazard a guess that all would be well up to 3/8" (9 mm) bore tube. I have done some calculations on turbulant flow at the temperatures and pressures at which Skylarks monotube normally cruises and it seems to be that turbulent flow will occur at all the water flow rates we are likely to work at and it is unlikely to fall outside that rule of Thumb up to maybe 3/8" (9 mm) bore. More is needed in this arena to prove anything. It may just be that in full size practice perhaps the known point of turbulence is the lower end of any particular monotube's efficient and useful working range. Do we have a proffessional Monotubist out there ready to help us?

At this point it may be of interest to put a few figures together to illustrate what this all means to me. Skylarks boiler certainly produces about 50 lbs. of steam per hour at 100 psi and it is reasonably dry, the bore of the tube is 3mm (7square mm area) and there is about 10 metres of tube in the flame area. Now if we double the diameter of the bore from 3 to 6mm the area quadruples to 28 square mm and at about 1 metre per second water velocity you will get 200lbs of steam at 100 psi and I would expect you could do this with only 10 metres of tube if you throw enough heat at it. However more tube of greater diameter added as a feed water heater if you like to separate the sections mentally, will give a very good return in higher efficiency. This is exactly how Doble and others built their car monotubes; I am confident they knew exactly what they were doing and why! Looking at Dobles monotubes they had about 25- 30% of tube length at the smallest diameter and usually had a total of three diameters of tube in the furnace which must have helped reduce pumping frictional effort by perhaps 30-50%. When I build a bigger monotube I will do exactly what the old masters did.

Pisces has a water velocity twice that in Skylark and produces at least eight to ten times the power. I know this because Class B boats have similar rules be they Steam or IC Powered, the IC boats are usually fitted with 30cc racing two stroke glow motors which are known to be capable of 5 HP. A point I must make here Bob Kirtley's engine has a displacement of 13cc and yet has a performance comparable with 30cc IC engines!. A good steam Hydro is not that much slower than an IC boat and yet is generally 2-4 lbs heavier. I doubt if Skylark needs more than half a horse power to drive her as she goes at the moment.

Some Monotubes go and others Don't

The simple analysis given above is I believe the key to first time success in the construction of a small steam generator that performs in a very satisfactory manner and one that doesn't. I have seen on the net and in books many notions and designs, one of which involved the use of 300 feet of 1/4" copper tube to run a Stuart 5A, Westbury's experiment proved that tube length alone is not the answer to success in making steam but it must have a place in raising overall efficiency.
In some discussions about flash boilers the separation from a feedwater heat function and the steam raising and superheating area is viewed as a physical point of some mystic significance. If you look at Doble, White and Serpollet cars they certainly were efficient but never as far as I am aware did any of these great engineers write about the critical nature of water velocity at that transitional stage where the absorbtion of the latent heat takes place. Nor do they seem to consider that point of any significance in their designs. Their boilers had long lengths of tube of several diameters and ended up with a section where the velocity certainly was in excess of a metre per second. Without experiment into this we will never know the point where transition from liquid to gas takes place and how dynamic the change of that point along the tube will be as power (heat input) is changed by the operator; and having discussed it before with others and thought about it at length; I don't think it matters one jot! Of one thing I am certain, once the smallest bore tube has been reached that tube section must remain the same until it reaches the engine. I have seen set ups where the steam leaves the monotube and goes into manofolds and convoluted passages and is therefore allowed to expand cool and slow down as it gets to the engine----Why? Why do that? One answer I got was, "Well to me it is obviously being too hopefull expecting the steam to get to the engine through that tiny tube"! To my mind it has already gone through 30 or more feet of it, what effect is another two feet going to have? The small tube loses heat far slower than big tube and it is far more efficient to lag that diameter so my 4 mm steam tube goes straight to the steam chest. Get the steam to the engine ASAP and let as much expansion as possible take place there rather than en route, that is my view at the moment.
The crucial thing is in the design of the heat source, there is obviously an area/volume where the heat will be the most intense and that is where the small bore section has to be. If the water begins to vapourise in the feedwater heater so what, it will soon be accellerating along the hot bit and it won't stay liquid for long. When I fit the feedwater heater section it will be up what is now the funnel; it will be 6mm or 8mm tube and will be fed through a cone into the 4mm diameter tube until it gets to the steam chest.
This brings me back to the question of tube length. The longer the tube, the more heating area = greater efficiency. Yes, but there are limitations and in small sizes pumping effort is a very significant limitation. I want to try a feedwater heater section above the monotube in the smoke stack and because this area is unlikly to have boiling water in it, feedwater velocity is of much less significance. This then permits the use of bigger bore tube for the feedwater heating section of the system which will help with reducing pumping effort. Accepting that there is NO WAY of predicting where along the tube the water actually boils from measurement the exhaust temperature at the chimney top is only about 300 C. and boiling point at 100 psi is about 170 C. it is far less likely to boil water in the chimney than right in the fire. That is as far as the argument needs to go in my view.

The relative merits of flash or conventional Boiler

Conventional boiler merits.
1. The containment of a mass of heated water makes power control as simple as a carburettor on an IC engine.
2. The complications of pumps and pump control on a flash plant make the conventional boiler more reliable especially in the smallest sizes. 3. Pressure control in a heated pressurised tank, using a safety valve is essential for safety and very useful when throttling with a simple valve. 4. The pressure controlled tank of heated water is a store of instantly available energy in its latent heat. This latent heat is the source of the destructive power of a conventional boiler explosion.

1. Requires regular pressure testing.
2. Is heavier than any self respecting flash unit of the same power.
3. Limited in its capacity to safely contain very high pressures and temperatures 5. Generally a lower thermal efficiency than a flash plant.

The Latest Monotube I have Built

The pictures in this paragraph depict a monotube I built quite recently for use by a student at Southampton University and I make no apologies for the fact that it is a bigger version of H H Groves typical airborne Monotube design. The frame is of 1" x 1/16" 316 Stainless steel strip made to operate mounted in a vertical furnace of lagged stainless steel tube of about 5" bore. The most tedious job was striking all those slots in the spacers. I did this by mounting a 4mm thick angle grinder wheel in my off hand bench grinder and making a special tool rest which is grooved to support the thin material as it is cut. I wore out two wheels to get the job done which I thought was reasonable thin stainless is very hard on the wheel. I tried stacking all four up together and do them all in one go but it was far more hard work on the wheel and on me getting the cut made. The University TIG welded the frame together after I had tacked it with my Oxy/Acetylene.

The tube was 3/16 Dia. Copper/Nickel brake pipe which serves well enough as a monotube but I did melt the one I built for my little launch so the new monotube will be of Stainless Steel. I thought the Stainless would be a frightful price but it was not much more than the brake tube! Twelve Metres of 4mm stainless cost £40 and the same length of Copper/nickel was going to set me back £28---no contest!

A new Monotube for Skylark

The following pictures depict the new Skylark monotube as I built it and made the small adjustments necessary to make everything fit.
Frame Stretchers (3 of 4)
The frame stretchers and rings are of the same 1" (25mm) x 16swg. (1.6mm) 316 st/stl. that I mentioned in the previous paragraph. The struts or Stretchers have to be cut with 30+ slots in their 16" (406mm.) length. The whole assembly is inserted into a piece of 4" (100 mm) Mild Steel tube which is well insulated with Mineral Wool and layers of alluminium foil. The heated core is 4" (100 mm) Diameter and the outer case of the boiler is 15.5" (340 mm) diameter giving a total of 5.5" (140 mm) of surrounding insulation
Stretchers, Rings and 2 clamp jigs
The first two pictures shows the components of the frame, four spacer rings four vertical stretchers and two spacers which I made to hold the stretchers in position for welding. The third picture is the simple right angle section steel jig to which I clamp the rings and stretchers before welding. I have no TIG welder and I use Oxy/ Acetylene for all my stainless welding which of course uses much more heat in the process---which causes more distortion.
Alignment Welding Jig
Not surprisingly the biggest problem I found in making this particular frame WAS distortion; made much worse because I did not think about it enough! Last time, for Southampton University I just tacked it together using minimal tacks with Oxy/Acetylene, the University TIG welded it permanently which itself uses far less heat so there was very little distortion.
The complete new monotube
Next time I may use a piece of tube to line the rings up and I will weld from ONE END. This will allow the weld to distort the stretchers as much as it needs to and permit correction as required before welding the next ring into position. Gas welding stainless is not a big problem but it uses a glass flux which goes black and is a devil to remove so the appearance of the finished product is not wonderful. However a furnace rarely improves the appearance of anything so this is not too much of a worry! It ended up reasonably straight and was a good fit in the furnace tube.

The hot end and concentric coils

I changed the design at this stage as a little arithmetic told me I could get the whole of one 20 foot (6 metre) length of tube into an inner core only 16" (400 mm) long less a metre or so for lead out length. Looking at typical albeit lightly blown oil heaters it is very noticable that the flame tongue streaks straight through the middle of the furnace space so this time I tried to pack as much tube in that area as I can. This is why the outer coils look a bit ragged and badly spaced because there was space to spare on the outside. I am convinced that how the tubes are presented to the heat source and how near optimum the fuel/air ratio is kept is very important to overall efficiency in monotube steam sources.
How not to run Monotubes
This is born out by the fact that in the steam model hydroplane, changing from one big burner to three smaller ones yielded more steam and higher speeds. Skylark's outer boiler casing was made by the previous owner who deliberately made it look as much like a coal fired boiler as he could, just for the fun of doing it, I may change it one day but for the moment I quite like the joke of explaining how little coal it uses! I have now made two cores for this boiler and each was smaller than the last and smaller than the one which was in the boat when I bought it. This is how and why I created space for so much insulation. All the added insulation and concentration of heat proved too much for this the second core in Skylark! The copper/nickel alloy monotube became blocked with black copper oxide powder and caused this failure which melted the tube and damaged the pressure gauge. It made a pop no louder than opening a can of lager and made me one of the worlds totally converted monotubists. The new core whilst more compact carries an identical length of tube about 36 feet (10 metres) actually in the fire, I have done this deliberatly to gauge any increase or loss of boiler efficiency that may be caused by the changes of monotube form and insulation. This latest core has modified form being of the same overall diameter but 2" (50 mm) shorter and with more tube concentrated in the centre, nothing else has been done of any significance. I really want to know what is going on in these steam systems. If this change betters things, the next change will be a blue flame burner then a chimney feed water heater. All being modifications of the same boat, engine and propeller.

A Few Words about Materials

The core of the new boiler is 100% stainless steel however even this remarkably heat resistant steel is no match for prolonged use at red heat and beyond. If you look at the burnt out end of the last monotube the case is made of stainless steel chimney liner which I thought might last 40 hours, it began to fail well before that and I had to fit another inner liner to prolong its life. The problem is much worse than in a racing hydroplane where the boiler may only be used for a few hours in a years testing and racing. I want 100 hours continuous use as a starting point. The outer case (or liner) was of 0.005" (0.12 mm) thickness and it began to fail after maybe 10 or 15 hours of use, the new liner is of 0.062" (1.6 mm) mild steel which has an inner liner of 0.024" (0.06 mm) Stainless steel protecting the first 6" (152 mm) of the hot end of the assembly. You can see from the damage that not only the tube has burnt out, so has the frame which was made of 0.010" (0.25 mm) stainless steel sheet. Notably all this damage is concentrated in the first lower 5" (127 mm) of the assembly's total length of 18" (456 mm). In fact I am quite impressed by the longevity of mild steel in the furnace area clearly it lasts less time than stainless but not say 100 times less it has maybe a third the life of stainless in the same circumstances, with the lower 6" of the mild steel monotube case protected by the replacable inner liner I am fairly confident that I will get maybe 50-100 hours of use out of it.

Changing Materials and Metal Sections

The above notes on building this new monotube is not a recomendation for anyone to follow! I am already looking to new ways of making my next furnace and monotube built for 1000 hours of continuous use, provided the pump never fails to deliver feedwater I do believe the steel of the monotube itself will make it for 1000 hours but the steel liner and sheet frame no chance. Nimonic steels are a possibility but I do not want to bother with that, I think simpler and cheaper methods will work just as well. The tube itself is super cooled by the flow of water and steam through it. The outer case has no such benifit but it also has no structural function either, it supports nothing but itself, all it does is keep the hot gases close to the tube. I can see nothing but benifit by dispensing with a metal liner altogether and making a close fitting furnace space of furnace brick and genuine furnace cement with the monotube on a frame suspended concentricly within it. The frame has a support function which is of real utility especially if overheating does occur; in my experience it almost certainly will, one day. Sheet section steel is actually hopeless and although the new frame is over ten times thicker it will still only last perhaps 50 or 100 hours if current experience is anything to judge by. I am instead thinking in terms of using 0.187" (4.75 mm) or 5mm diameter stainless rod this will give a far greater thickness of material to waste away with far less exposed area per unit mass and an ever reducing area as it wears and burns away. I managed recently to aquire a few Stainless Steel welding rods of 2 mm diameter and provided one is reasonably deft and quick very sound welds can be made on this size (0.187") stainless rod with a standard ac welding set. The same basic form may remain with rings and stretchers but all made of stainless rod with the slots made of short lengths of straight rod. This construction may well allow use of spot welding to great advantage, a small jig of copper would help greatly to hold components at right angles. Food for thought.

At some future time I will also go into details like how to automatically control steam temperature (it is perhaps more important than the pressure). The likely limits to power up to the 3/8" (10 mm) bore to which the above table probably applies. Lubrication at the upper limits of temperature, the use of solid lubricants and posh oils. How to estimate a value for the real pressure inside the monotube, if indeed you want to know it

That is how I see small bore monotube design and is as far as I have taken it to date. More will follow as I have notes, pictures and drawings to make up perhaps a further 20 pages but the pay is not good and the workshop beckons at every idle moment but there will be additions to these pages as time permits.