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[dillinger 04]

Hello Everyone




I have a question which probably gets asked quite often on here but though I would ask anyway.
I have a set of plans for a simple oscillating steam engine designed for a small balsa wood boat .
no metal lathe required steam boiler fastened from a copper oil can spout cut to size to except a simple made pressure relief valve
light weight piston assembly made from a pair of fishing rod ferrules . My question how critical is it too equally match the steam engine to boiler in order to make it work? The plans do not indicate exact size but uses the word large my guess 4" base size too can does anyone know if it would be possible to scale down in size on the can and still have it work as I have a small laser cut wood boat kit in order to fit the boiler I would need to use a small can for boiler around 2" base size to fit the boat .  Any help would be appreciated

[carlfmiller]

There is a lot of expertise on this forum so you'll get answers for sure. I'll try to help by suggesting that first you calculate the displacement of your little engine. I assume the cylinders will be single acting (steam pressure on only one side of each piston) and then calculate in mm:  1/2 bore dia. squared, x pi x stroke length x 2. If you get a scientific way to "match" the engine and the boiler, most likely you will need to know the engine displacement and the boiler volume. I think you also need to specify the heat source you will be sing.


Now this is not a scientific answer, but here are two examples of 2" dia. boilers at work.  First, the 19"  lightweight Midwest launch with a 2" x 2" boiler fired with sterno, steaming a very small single cylinder single acting  oscillator. This works OK; Very small boiler, not a super hot fire, but only one cylinder.


https://www.youtube.com/watch?v=m3dF_Jnjin4

Another small steam plant is the MSM's 2.2 cc  "Avon" 2 cylinder double acting oscillator, here steaming from butane fired "Econo" boiler 2" dia by 3" high. This no doubt would be more than enough for your balsa boat but is just enough for this 40" 7lb boat.


https://www.youtube.com/watch?v=BejMNfIvXLQ


So your answer lies somewhere in between.  How the water is heated is a major variable here, and I have no idea how that is calculated.  (BTU's x heating area, probably) You can see from these two videos that your home made 2" dia boiler at 3" length  might be enough. If you can make it say 4" and have enough heat from your fire source, all the better I think.


Now the real experts will answer your Q. Good luck and please post pictures of the engine and the boat! (Don't forget to pressure test the boiler to at least 125psi, maybe more if you can)


-Carl

[dillinger 04]

Hello Carl


Thank you for responding to my post your input is appreciated I will try to ad pic's once I find a small copper or brass oil can
for the boiler made of pure c or b. I  am currently having trouble doing so one that will fit the little hull design while looking for one does anyone have any other ideas on what could be used for a very small boiler. Thanks   

[carlfmiller]

Dillinger maybe a small coincidence --but I just bought a very tiny 2 cylinder motor (on enginediy.com) and when I decided I need a boiler for it, I found the small Saito boiler OB-1  being sold on eBay by Galaxy RC in Poland for only $100.  I thought about making my own small boiler but the parts for it are almost the same cost!  Uses cheapest fuel ever, just denatured alcohol. So I have no idea if this is a "match" for your little emgine, but I think it will be ok, and perhaps it's something you'd like to consider as well. In my case, this OB-1 will a lot less likely to leak than a home made one. (maybe safer, as well)

-Carl

[Circlip]

125psi Carl? Are you running at 63psi? Initial test pressure on a newly constructed boiler is twice working pressure and subsequently one and a half.


  Regards  Ian

[carlfmiller]

I have no idea of the actually WP planned, but I am sure it's less than 63psi.  I suggested boiler pressure test of 125psi, because I think that's the max pressure most people with a compressor at home can generate.  As I mentioned the real experts can suggest answers to Dillinger's question. (For example, I would have thought 2x safety factor was normal, and a 60psi boiler pressure is pretty easy to reach) So you add info and thanks for that.


I just bought a Saito OB-1 small boiler and it will come without a pressure gage, so I'll enter into an unknown there! It think it will not have a control valve between engine and boiler, so this will be interesting.  I see video of it running a small engine just below 2 bar


-Carl

[AlexC]

I have no idea of the actually WP planned, but I am sure it's less than 63psi.  I suggested boiler pressure test of 125psi, because I think that's the max pressure most people with a compressor at home can generate. 


-Carl

Hi Guys,
Sorry Carl, but you should never pressure test a boiler using compressed air, you should fill it fully with cool water and attach a small hand water pump and a suitable calibrated pressure gauge.Slowly pump water in until the pressure reaches 2 x max working pressure... never higher as this will stress the boiler.
Water cannot be compressed and any leak will just cause a wet puddle... Air can compress and a leak can cause the seam to let the compressed air out explosively... which can cause injury.
Work safely.
AlexC retired boiler maker.

[carlfmiller]

Thanks Alex for the expert advice. It's news to me and maybe helpful for Dillinger as well. And now I understand 125psi is too high a pressure to use in any case. I guess 90psi might be a better top end test.


-Carl

[Circlip]

"Model stationary and marine steam engines" and "Model boilers and boilermaking" both by K.N.Harris. Purchase or download Carl. The "Bibles" for model steam enthusiasts.


  Regards Ian.

[JimG]

Thanks Alex for the expert advice. It's news to me and maybe helpful for Dillinger as well. And now I understand 125psi is too high a pressure to use in any case. I guess 90psi might be a better top end test.


-Carl

The OP is asking about a small oscilating engine so I would think that even this is way too high. These engines work at quite low pressure as any higher forces the cylinder off of the valve face. The  steam experts here will give a more accurate answer but i wouldn''t expect it to run at much more than 10/20psi. I know that the Mamod/Wilesco boilers have a safety valve that pops off at a low pressure. (When I worked as a school technician we used to get a safety check on our pressure vessels (autoclaves and a Mamod engine), the test for the Mamod was to steam it up , check for any visible leaks and see the safety valve pop, no actual pressure check.)
Jim