small steam engine for a model boat.

  1. optima21 Forum Supporter

    halifax, England
    last year I decided that I wanted to build a small scale steam plant for a model boat, and the boat in question will be approximately 18" long and look something like this :-


    and is is designed to be just a static model for sitting on a shelf, so I think it would be nice to make one with a working steam engine and have it working by radio control, I've dabbled with small steam engines in this thread

    and adapting a model for radio control in this thread

    and after watching videos by leslie proper on youtube I decided a working steam model would be nice and he has made tiny piston valved steam engines

    and after a bit of light reading about model steam engines and boilers


    so that's the introduction to this project, and I'll only be dealing with the steam plant in this thread as the machining and metal working might be of interest to others.

    For the design of the steam plant Im starting off with a blank sheet of paper, and I keep changing my mind as to what I need to make, these are my main parts

    1) a boiler to convert water to steam
    2) a twin cylinder oscillating steam engine (with reversing regulator)
    3) an electric boiler feed pump and control electrics
    4) heating for the boiler
    5)lubricator for the steam engine
    6)oil/condensate separator and feed water heater

    and these are my current ideas (that can change at any time)

    1) Boiler

    originally I was planing on making a vertical firetube boiler from 35mm dia copper pipe and 3/16" brake pipes to make the fire tubes like this. (its called a firetube boiler because the fire goes though the smaller tubes in the boiler)


    but for various reasons decided not to do this and to make a horizonal watertube boiler like the boiler show below. its called a water tube boiler as the water to be heated is in the smaller tubes.


    my boiler will made from 28mm copper tube and will only be 45mm long so the water tubes will go across the boiler rather than along is length. the steam will leave the engine in a tube under the boiler in the fire so that it will heat the steam up, dry it out and give the steam more power that the wet (saturated) steam that leaves a boiler.

    2) Twin Cylinder Oscillating engine

    I decided to do an oscillating engine as they dont have many moving parts and are easy to reverse, just by changing which port the steam goes into the engine. In thinking of having a bore of about 6mm dia and a stroke of 8mm and was originally thinking or making a "V" engine like the one show below as I thought the crankshaft would have been easier to make.

    so it would have been similar to this but smaller (this is supplied with a regulator for reversing and lubricator)

    but a simpler engine (and possibly lighter) would be a twin vertical engine similar to the one below, it has a regulator but the steam is delivered though the body of the engine so the plumbing is much easier to do.

    3) Electric boiler feed pump.

    Im thinking of using a minature 3v 100 rpm motor like the one below


    to power a water pump with aprox 3mm bore by 3mm stroke using a scotch yoke to convert the rotary motion of the motor to the linear motion the pump will need


    I'll use an electric level sensor to operate the motor.

    4) Heating for the boiler.

    I'm thinking of making it butane fired, which will involve making a gas tank, control valve and burner as I think for long running it should give a cleaner burn than methylated spirit firing, but the spirit firing would be easier to do.

    5) If I was using wet steam I could get away without a lubricator for the engine but as the steam will be hot, lubrication of the cylinders will be required, the easiest option is to use a displacement lubricator like the one below. the lubricator of basically a sealed reservoir that is filled with oil and a small amount of steam is allowed into the reservoir. this will then condense into water. the water will sink to the bottom of the oil. the oil with then overflow into the steam pipe and will then go into the engine to lubricate it.


    6) oil/condensate separator and water feed heater.

    as the exhaust steam has oil in it, you have to separate the oil from the water in model boats so that you're not poluting ponds, the easiest way to do this is to have the exhaust steam from the engine into closed container like a cyclonic vacuum cleaner. the oil will stick to the wall of the container and then sink to the bottom and the steam can then exhaust from the container into the air as it will no be clean. if the feed water tube goes though this container it will pick up heat from the steam so should improve the thermal efficiency of the steam plant, as cold water being feed into the boiler will cause the steam pressure to drop.

    so thats about it, with a bit of luck it will work, and at this stage I have no idea what it will end up like. I'll try to post the work Ive done on the boiler tomorrow.
  2. fizzy Forum Supporter

    WOW wow wow!
    Nice project.
    hunter27, barking mat and optima21 like this.
  3. frank horton

    frank horton V twins are great but 4"s rule.........

    Soon 2 B Crete
    again WOW......
    optima21 likes this.
  4. addjunkie

    addjunkie Member

    Northumberland. Reet oot in the sticks
    The bit about the steam leaving through the fire to give it more power is called superheating, yes it gives it more ooomph but if you etop the steam flow to stop the engine, you could melt the tube. Super heated dteam is dangerous stuff.

    Normally in a water tube boiler steam is taken from the steam drum, or if getting complicated look up DType boilers. Basically your superheat section needs protecting from direct heat.

    But I look forward to following this, I like steam boats. But you should show the boat too.
    optima21 likes this.
  5. optima21 Forum Supporter

    halifax, England
    so once I decided that I was going to make a 28mm diameter boiler the next thing to check was how tightly I could bend 3/16" copper brake pipe to make the water tubes. so I ended up making a small tube bender and ended in with approximately 3/8" inside diameter without the tube kinking. here's the bender being used to bend the 3mm stainless steel steam pipe that will be under the boiler and go to the engine.


    and I needed 5 u bends in the brake pipe (plus a couple of spares).


    the flanged end plates for the boiler were made from 1.2mm thick copper from flattened 35mm copper tube, using a ball bearing on a piece of bar to form the flange. the end plate was annealed 3 times to do this and moving the metal about 15 degrees each time



    and then enlarging the holes in the end plates, but I ended up clamping them between some wood as I had images of me damaging the end of my fingers if I tried to hold them.


    then it was a case of turning the phosphor bronze fittings for the steam fittings, the finish with carbide inserts was awful and so I grabbed a brazed carbide too and it gave a decent finish. as the boiler is only 45mm long I realised that there wasnt enough space for the 4 steam fittings I needed if I wanted a chimney above the boiler and thought it would look wrong if there wasn't a chimney. I was just going to use a thread bush to fasten the boiler to the boiler case, but then decided to make the outer screw thread 1/4" x 40 tpi and threading the inside thread M4 x 0.5mm for the steam fittings



    and for facing off the bush to fit the hole in the end plate I mount them by the thread in the collet chuck with a drill bit in the other then of the collet to make sure it has even clamping. (this was the first one I did which was just a 3mm od thread before I decided I needed to have a steam fitting there

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  6. optima21 Forum Supporter

    halifax, England
    it took a bit of thinking about how to drill the boiler barrel accurately, and one issue was that it would need to be rotated 180 degrees to drill holes on the other side, so the easy answer was to mount it between a couple of pieces of square tube with a bolt between then, and then to align in in the milling machine, I clamped a measuring square in the vice and used a v block to get the boiler barrel at the the correct angle, its near enough and doesnt need clocking up this way.


    and then the holes for the water tubes were "drilled" in the bottom of the boiler barrel first. the were at an angle to the tube so used a 1/8" slit drill first and then opened up the hole with a 3/16 end mill. it was then turned over and the holes on the top were drilled (they were just a couple of 6mm holes)


    and the parts to make the boiler are like this


    and the steam dryer tube, water tubes and boiler barrel will be assembled something like this (upside down)


    just needs silver soldering together now, but I'll wait to see if I change my mind again, which is easier to do if I havn't soldered it together.

    yeah I've not called it superheating as Im hoping that the firebox area wont get too hot, just dry the steam, but still Im planning on using stainless steel tube for the steam pipe rather than copper tube. Im thinking of using a gas burner with 10 small jets that will play onto the water tubes to directly heat them, and hopefully that will mean that the steam pipe wont get too hot. Im not having a raging inferno like you'd have in a locomotive firebox

    and this is testing the conductivity of tap water, as I'll use this method to sense the water level in the boiler. thats just a led power by a single lipo cell, with two wires going into the water. one thing that surprised me was how the conductivity of the water changed, at 15C it was approx 115k ohms, but when boiling it dropped to about 13k ohms, but it means that the circuit to power the motor for the boiler feed pump doesnt need to be very sensitive

    Last edited: Feb 10, 2020
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  7. Migmac

    Migmac Forum Supporter

    Kintyre. Scotland
    You can have a WOW from me too
    addjunkie likes this.
  8. dyno-tron

    dyno-tron Member

    Near Chester, UK
    Great project, reminds me of my secondary school O level metal work project, making a mammod style traction engine. That was back when schools allowed the kids to do dangerous stuff :welder:

    Do you have a target date for completion or is it a "I'll do a bit when I have time" kind of thing?

    Looking forward to the next installment :thumbup:
  9. Bullet2012

    Bullet2012 Forum Supporter

    Good skills there sir.
    Parm and optima21 like this.
  10. optima21 Forum Supporter

    halifax, England
    I was hoping to have it completed by mid April, which would be possible if I was copying something from a published design, but Im spending alot more time thinking about what I, doing rather actually making things.

    my current plan is to complete the boiler, feeding the boiler with water, heating the boiler and then when that has been done, I'll get round to building an engine.

    so I was watching this video about gas burners for models

    and was going down this route, but wouldnt be using a ceramic matrix , I'd just be using a stainless steel plate screwed to a brass box to make the burner. the stainless steel plate would have holes in it and I could change the hole matrix later if required.

    so the burner began life as a chunk of brass and was flycut to a block 12mm x 44mm x 24mm, using my smallest flycutter, using a single point carbide cutter. and the reason for flycutting is that I like the surface finish they give.


    and threading a hole for connecting a pipe for connecting to the gas jet and air intake


    and then millled out as mcuh metal as possible to reduce the weight.


    and then drilling and tapping 16 M1.6mm holes to screw to top plate to.


    and then I woke up this morning at thought I was going the wrong way with this as it would be 13mm tall as a minimum and I thought it would be too tall so I had a rethink and to use some 3/8 brass tube with slots cut in it, as I made one years ago for my model steam lorry (another project yet to be completed), and for scale the boiler and flywheel are 2" diameter.


    and this is the burner for it, and run off my smallest Sievert blowtorch burner.


    so I thought I'd go down this route for this steam setup, so I cut the slots using my lathe with the tube in the toolpost as it was easier than setting it up on the milling machine. the clamp on the end for the bar is just so that I could cut the slots on the other side of the tube and get the angle correct (set up against an square on the lathe bed)


    and time to test.....lets just say its very temporary just to see how it would work. so I have my smallest burner on my sievert torch, the some 5/16" fuel pipe drilled out to 10mm to fit over the end of the sievert burner and the modified tube, and and at the other end a drill was stick in the tube to block the end of the tube.


    I found that I had difficulty lighting the gas and keeping it lit. I think it was due to the gas mixture going through the slits too fast, so a few extra holes and we end up with something like this.


    and just checking out the flame pattern on the boiler assembly (still not silver soldered together)


    and I managed to get the copper hot (went through the full range of tempering colours) without getting the stainless steam pipe glowing so its looking good so far. so I think I'll be continuing with the fueling and get that finished first.
    • DSC02463.JPG
  11. optima21 Forum Supporter

    halifax, England
    so the tube show above needed to be modified into a burner, the first thing I did was to silver solder a peice of brass bar into the end of the tube to cap the end of the tube and to provide a fixing for it.

    the other end of the tube will need to be screwed to the boiler casing, so that was made from a piece of flat brass bar and turned on a spigot to reduce weight and it will be silver soldered on to the burner tube


    a bush was also turned for a screw to fix the gas jet in the burner and another another piece of round bar was turned close the end of the tube. and the pieces ready to be silver soldered


    and after the silver soldering was done, the tube was drilled out to fit the gas jet


    and the air holes were drilled in the burner tube


    then it was time to make the gas jet. and I think I can call this a sucess...drilling a 0.25mm dia hole 2.5mm deep into 1/4" hex phosphor bronze


    and the completed burner and gas jet.


    next job will be the gas valve and gas tank.
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  12. optima21 Forum Supporter

    halifax, England
    things are going a bit slower than I'd hoped as its taking me forever to make a few nuts and bolts for this, and they've been made from phosphor bronze. the smallest bolts Ive made are 1.6mm thread, 2.5mm long with a 2.5mm a/f hex head, shown here on a pound coin for size. they were made to screw the end plates to the barrels of the boiler and gas tanks.


    most of the pipework I'll be using for the gas, water and steam is 2.4mm o/d so I needed to make some union nuts, like compression fittings for household plumbing. I could buy them but they would look too big and bulky for my liking. the thread on the inside of the nuts is M4 x 0.5mm pitch and they start off as 1/4" hex bar.

    the first thing to do is to drill a 2.5mm hole for the pipe to go though, then tap it 4mm deep, and then part it off slightly longer than required (5.5mm)


    then to face the end of the nut off to length (5mm) and get a decent finish


    and then to machine the hex smaller (I decided on having 5.5mm a/f nuts). I dont like having a screwed spigot as the nuts go slightly off centre, so instead use a round spigot that is the a sliding fit in the thread minor diameter.


    the corners of the nuts are then chamfered using the same spigot. the small nut just stops the union nut from moving.


    a couple of these union nuts were modified by having a small recess cut to seat a 4mm o/d o ring for sealing the gas valve in for the burner. just for size the boring bar has to have clearance in a 3.5mm hole.


    so thats how I make small nuts.

    I also decided that I would need some banjo bolts and they may give me more clearance in tight areas that having a bend in a pipe. Once again these were threaded M4 x 0.5mm and start off as 1/4" hex bar. they are threaded and then reduced in diameter below the head to allow more space for gases and fluids. the centre of the bolt is drilled 1.7m dia which is roughly the same bore as 2.4 mm tubing.


    then three 1.1mm holes are drilled radially,


    the bolt is then cut from the bar and the end faced. there is a small step 0.5mm deep just under the head so that it will centre the bolt in the spigot ( I'd done this before I realised the clamping nut for the collet was around the collet, so was lucky as I was just facing the end)


    the the bolt head is reduced to 5.5mm a/f


    and the corners of the head are chamfered


    I made twice as many as I think I'll need as I'll eventually need them one day, but its time consuming and fiddly to do, but they have to be accurate just for appearance only.
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  13. optima21 Forum Supporter

    halifax, England
    and so far I have these, and will have to make more later on. the two thin nuts at the top are 1/4" thread and the small bolts at the bottom are 1.6mm thread.

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  14. arther dailey Member

    loving this thread, darn good work:thumbup::thumbup:
    optima21 and gaz1 like this.
  15. optima21 Forum Supporter

    halifax, England
    time for another update, it seems that the amount of time spent making parts is inversely proportional to the size of them. I need to make a gas tank to fire the burner with and my best guess is that a 22mm/15mm end feed tee plumbing fitting will give a 30 to 45 minute run time. I needed make an end plate for the 15mm end to allow me to fit the lighter refil valve which was pinched from a £3 blowtorch off ebay and a threaded hole for the gas valve. this was turned to size in my lathe and then then a couple of holes were drilled in it on my milling machine


    then a bit of eccentric turning was done on one of the holes to reduce weight. my setup was to use a 4 jaw chuck mounted off centre on a faceplate, which might seem odd, but the bore of my 4 jaw wasnt large enough to do this mounted on the backplate, (unless I wasted metal by using a chucking piece), and I could still get the fine adjustment.


    it was then faced off in a 3 jaw chuck


    and it was then drilled (and threaded to fit the gas valve) this was necessary as I wouldn't want to accidently operate the refill valve


    I then needed to make some copper endplates for the 22mm ends, which were again "spun" over a former


    then I needed to drill the three equally spaced holes to bolt the 22mm end plates to the T piece, and I wanted the outside face of the endplates to be slightly proud of the the of the T piece. I decided the best way to do this was to make a spacer with a 1.5mm recess in it.


    a piece of allthread was mounted in my indexing fixture, and the spacer and end plate were fastened to that


    the T piece was then fastened to that. and the holes drilled at the end nearing the indexing fixture. It may not be to accurate to the nearest 0.01mm but as the parts are essentially fitted to one another its good enough, as I think trying to do this on a rotary table would taken even longer than doing it this way.


    and the next thing will be to try and silver solder the following parts into a boiler and gas tank

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  16. optima21 Forum Supporter

    halifax, England
    finally got the boiler and gas tank silver soldered together

    this is one of the end plates that has been bolted into the gas tank with a threaded bush in the end plate, all fluxed up ready to be silver soldered. To do this I decided to use hard silver solder for jewellery making which melts at 741 to 788c. its called hard wire because its hard to melt, not because its hard wire as its pretty easy to bend. I used high temperature solder so that I could later solder other sections with standard melting temperature silver solder (630 to 660c melting range). for this section I used Tenacity no.5 flux as it can be used upto 900c, as Easyflow flux can only be used up to 800c. the instructions for the tenacity flux say the best way to remove it is with caustic soda.......which doesn't really work, a sulphuric acid pickle works much better. Also with hindsight this was the wrong way to do things. I think I'd have been better off putting a few centre pop marks on the back of the threaded bush to give some clearance to let the solder be sucked in by caplillary action, and then riveting the bush to the end plate.


    then I needed to weld up a bracket to hold the stainless steel dryer tube in place. I "spot" welded some 0.9mm angle to some 2mm sheet steel with my tig welder.


    and a steam union was silver soldered to the dryer tube. as this was stainless steel I used the tenacity no. 5 flux again.


    and all assembled with silver solder rings at the water tube joints and all fluxed up ready to be heated up


    and the gas tank ready to have the top silver soldered in place


    and both the boiler and gas tank together


    and the underside of the boiler


    not the prettiest silver soldering I've ever done, but I was having trouble getting the high temperature silver solder to flow without overheating it, if I just used normal (630 to 660c melting) silver solder it would have looked better.

    both of them were then pressure tested, this should be done hydraulically buy filling them with water, but because of their size I just do it pneumatically with air and stick them in a bucket of water.


    on the subject of gas tanks, you need to be weary of the gas you are storing due to the pressures it will have to withstand, if butane is t approx 20C is pressure in 17psi, but at the same temperature propane is is 110 psi, and a butane/propane mix can be anywhere in between.
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  17. optima21 Forum Supporter

    halifax, England
    now its time to make a couple of valves, I need a valve to control the gas to the burner and a check (one way) valve for feeding water to the boiler. The bodies for the valves are each made of 2 pieces of phosphor bronze sliver soldered together with normal turning and threading (using taps and dies). the seat for the ball in the check valve was cut with the smaller 1/8" dia D bit that I made from an old centre drill last week as the centre of the hole needs to be raised.


    and the two bodies ready to be silver soldered together, using M10 nuts to stop them moving


    and drillling the holes though the side of the bodies


    and turning the spindle for the gas valve, this was done in stages as the main body was only 2mm diameter


    a 4 mm length was turned at a time


    this was then cut off and the other end was turned. the end is 1.0mm wide taper turned to a point.and held by the 2mm diameter in the collet chuck.


    turning the fitting to hold the end cap with the o ring in to seal the spindle


    for the check valve the best way to get best seal for the ball is to glue a hardened steel ball onto a piece of metal rod and gently tap that to get get the seat. this was done in the lathe to help with alignment and the rod used was a 2.4mm tig rod


    and turning the end cap for the check valve. the height of this needs to be adjusted to limit the lift of the ball when working

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  18. optima21 Forum Supporter

    halifax, England
    and the completed parts of both valves. Im not sure if the gas valve will work properly, so I may have to remake that and thats why I have only made a temporary lever to operate it. shown with a 5p piece for scale


    and the completed valves

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  19. optima21 Forum Supporter

    halifax, England
    well the gas valve worked well but its not sensitive enough sor the amount of gas I'll be using so I'll have to make another one. also Ive been informed that some regulations were introduced fairly recently that gas containers used in models need to be pressure tested to 400psi. I was just going to be using butane, but so many people are using butane/propane mixes these days and the temperature inside a model can get to 50C, so Im guessing its all in the interests of safety.

    I decided that I wanted an electric boiler feed pump and could use the resistance of water in the boiler to turn on the pump if the level gets too low. So I need a pair of electrodes to go into the boiler, and its not something you can buy in such a small size so its a case of making a couple of them. they would be made in 3 parts, the main body that screws into the boiler, the electrode and the insulator and would have to be water tight to 25psi and operate upto 120C, so here's what I came up with.

    the main body is phosphor bronze and threaded M4 x 0.5mm and drilled 2.5mm though the centre


    the other end was counterbored with a 4mm end mill for the insulator to stop against


    the electrode is a 1.6mm 316 tig rod, threaded on one end M1.6 so that it can screw into the insulator. and speaking of insulators, it seemed like PTFE was going to be a good option, but I needed part of it to be a tube, 1.6mm ID and 2.5mm OD, so its a bit flexible for machining on its own, so this is my way of doing it. I drilled and tapped some PTFE rod M1.6 to screw the electode into.


    the electrode was loctited into the ptfe insulator, not to retain it, but just so that when it went off and air gaps would be filled. the screwed end of the electrode was screwed into a brass spigot and the plain end went into a piece of phosphor bronze drilled 1.6mm to work as a bush in the tailstock (just for scale its 5mm squared paper that is in these pictures)


    and machining the insulator to shape. the electrode supporting it worked well. please don't tell me off about tool overhang, its not that much honest.


    and then the insulator was fitted into the body and the the lip of the body was then pressed over to seal it. when I said pressed it was just using light pressure on the tailstock. I had 3 dies to do this, as the lip was 4.7mm od, the first die used a centre drill which has an angle of 60 degrees, then a die that was just drilled which is near enough 120 degrees and then the last one was done using a 3/16" endmill to counterbore it.


    the working pressure of the boiler is likely to be 25psi, and both have been pressure tested to 50psi without any leaks. With hindsight it seems to be fairly easy to do, but it took a while to work out how I was going to do it, but got there in the end.
  20. optima21 Forum Supporter

    halifax, England
    one of little projects within this project is a water feed pump for the boiler, and because they don't make what I want, looks like I'll be making my own, so here's what my plans are.

    1) using a 3V N20 motor with 50rpm gearbox, this should provide 2cc's of water per minute, I've got 100 and 150 rpm motors on order. my way of thinking is that the 50 rpm motor would just about be able to keep up with demand, the 100 and 150 rpm motors should should provide plenty of water, but may not have enough torque to drive the pump.
    2) use a scotch yoke to convert the rotary motion of the motor/gearbox to the linear motion required to drive the piston in the pump.
    3) the pump with have a bore of 3mm and a pair of check valves built in to control the flow of water in and out of the pump.

    so for the first part was making the scotch yoke. as shown in this animation

    the slider in above animation I decided to make in two parts, one with the pin moving up and down and this would then be loctited into the ram to operate the pump. I could have done this by just chain drilling and filing, but decided to be a bit more accurate so co-ordinate drilled and milled some slots into some 2mm stick stainless steel



    then case of joining the dots up with a hacksaw and a bit of filing form the slider.

    it seems that in most of the animations of scotch yokes that the pins just run in the slider which isnt good for wear and so decided to use a die block in the slider like you'd find in the reversing gear in a steam train, so this was machined from phosphor bronze


    then a crank was made to fit to the motor, this was offset turned one way to drill and ream the hole for mounting to the motor and then offset turning the other way to machine the pin. I used a collet chuck in a 4 jaw chuck for this.


    flats were then milled on the crank to reduce weight and drill and take a hole for a grub screw to fix it to the motor


    and the it was milled to the correct thickness


    so there are the parts of the linkage with the motor, the washer was a spare one from my radio controlled truck.


    and assembled