Posted on behalf of buello:
Just Starting Out
Joined: 17 Oct 2008
Posted: Mon Oct 20, 2008 5:01 am Post subject: Boiler Question
I have the following question(s)
In holland we are alowed to build a boiler that is about 10bar and contains 20liters. (pressure times volume must be smaller than 200. ie 10x20 or 8x25liter etc.etc) This is not very big I know but then can someone tell me....
1. Wat size (in hp) engine one can run on this type of boiler steadily. (Being type Roberts with water pipes).
2. I am wondering why one must condensate. Is it not more efficient to put the exhaust steam in to the top boiler and reheat.
3. I am not so good with welding. Is it possible to thread the pipes and use nuts to fasten them.
4. if threading may be used can what type of washers are needed.
5. Must one use special thickness copper pipes and special fittings.
Here we have copper water pipes 12mm of 15mm with of the shelf fittings. Types for soldering but also with "a grommit?" fastening by tightening them. Can this be used safely??
6. We have a lot of low bridges. Can you have the funnel hinged?
7. What will be the best pressure to have the boiler.
Thanks and please use the number in your replies.
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Lighting the Boiler
Joined: 01 Oct 2007
Location: Ambleside, Cumbria, UK
Posted: Fri Oct 24, 2008 9:52 am Post subject:
Dear Buello ,
Although your questions are simple enough some of the answers are not: so many variables can be involved that without going into considerable technical detail only very general answers are possible .
As I'm not an engineer I'll try to give some general answers and I'm sure some of our more technical members will correct me and fill in the gaps .
1. BOILER/ENGINE SIZE & POWER :
It is the boiler which is the heart of a steam plant . It provides the
power and the engine transfers this power to the prop-shaft . However
efficient the engine and whatever its size it can't put out more power
than it receives.
a. As a very rough guide a fire-tube boiler burning reasonable quality
coal will produce 5 lbs of steam per square foot of heating surface per
hour , a water-tube boiler about 10 lbs per hour . If you're burning oil
you will get slightly better figures due to the higher calorific value of
oil , if burning wood you will get considerably less .
b. By using small-bore tubes you can increase the heating surface
without increasing the volume of the boiler . So in theory the volume
doesn't matter . In practice a boiler with a large heating area
relative to its volume will be difficult to control and will not be a steady
I'm sure other members can come up with precise figures based on
their own boilers , my GUESS is that that you should be able to
produce 2 1/2 to 3 1/2 HP , quite enough for a 17 foot hull , just
enough for a 20-23 foot hull .
2.WHETHER TO CONDENSE OR NOT :
Very ,very roughly 1 cubic inch of water produces 1 cubic foot of steam.
So if you condense you only have to pump 1 cubic inch back into the
boiler : much, much simpler than pumping 1 cubic foot of steam.
a. I think you said in your introductory posting that you were thinking
of building a compound engine . To get any of the thoretical advantages
that a compound can give (very debatable advantages at this size) you
need a vacuum or partial vacuum on the exhaust side of the LP piston .
You will only get this by condensing the steam back into water.
b. To get the most efficiency out of the system you need to use as
much of the heat/energy in the steam as possible . This means that
the exhaust should not be very much above boiling temperature so
little is to be gained in pumping the exhaust straight back into the boiler
and all of the advantages of having a vacuum on the LP to be lost .
I'm sure your idea of pumping steam rather than condensate back into
the boiler might be possible and with a simple engine might even
result in a theoretical gain in efficiency ; but I don't think the theoretical
advantages would be worth more than the practical difficulties . If you
want to use a compound engine it would be counter-productive .
3.THREADED TUBES WITH NUTS :
I'm almost certain that this would not pass inspection in the UK . To
have the neccessary strength the tubes would have to be thicker than
otherwise needed . If fitted into round section mud drums and headers
the washers would need to be profiled/contoured to fit neatly rather than
be squashed to conform to the profile . I think this is a non-starter.
a. If you do not want to weld/braze/hard-solder the tubes expanding
them into the mud drums and header is a definite possibility . most
fire-tube and many water-tube boilers in the UK use this method . A 5%
expansion of the tube gives a very strong result and expanders are
reasonably cheap and very easy to use .
b. To use an expander you need to have access to the inside of the
tube where it enters the mud drum or header. This means that neither
of these should be too small but I have seen some very ingenious jigs/
devices enabling an expander to be used in mud drums of about 3
inches .(I'm not certain of this , but it was certainly less than 4 inches)
4. WASHERS :
See previous paragraph .
5. PIPE/TUBE THICKNESS :
a. In the UK it would not be permissible to use standard house pipes
in a boiler .
b. There is a discussion going on in this forum in this section under the
heading "Blackstaff Boiler Design (SBA Library)" which should be
c. I'm not sure of the facts , but you may need slightly thicker tubes
if you are going to expand them .
6. HINGED FUNNELS :
a. In the UK most boats are taken to rallies on trailers with their
funnels removed . It takes less than 5 minutes to put them into position
but there is no reason why you can't hinge the funnel rather than
removing it . The joint must be fairly precise otherwise you'll allow in
air which will reduce the draught through the boiler.
b. Don't forget that the funnel is only the outer casing around the
chimney/smoke-stack , this is a small detail that needs to be
considered when designing the geometry of the hinge system .
7. BOILER PRESSURE :
a. It would appear that the rules in The Netherlands limit you , as you
pointed out at the beginning of your posting. In general higher
pressures lead to higher efficiency but they also lead to heavier
b. As a rough rule it is not worth compounding below 120 lbs PSI and
above 150 lbs PSI it is worth going to triple expansion . A simple
engine will usually run quite happily down to 30-40 PSI though it will
probably be best at 80-120 PSI .
All this has been very general but I don't believe anything I have written is dangerous ! I hope some of our other forum members will have their say if they disagree .
If you want any reccomendations on useful books to read on these
subjects just say so.
I hope this has been of some help .
Yours Sincerely Edward .
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Just Starting Out
Joined: 17 Oct 2008
Posted: Sun Oct 26, 2008 4:40 am Post subject: Easy way out
Thanks for your reply. I have found a dutch guy that had solved the boiler problem.
This is the linkhttp://www.xs4all.nl/~meijersf/boiler.htm.
I have also solved all my problems. I have bought someone else's hobby.
He had already collected everything. I will have a launch hull (still needs finishing) a stuart swan twin and a boiler. Putting it all together will take a lot of effort so my questions from now on will change.
The engine I had in mind (that I wanted to build myself was) (in theory) a compound with the exhaust valve and intake valves from hp -> lp in the middle. I have not seen anybody do this before. I also thought that it is probably silly because it is easier to use copper piping to connect the exhaust to the lp intake. Also it would have been made from alluminuim keeping the weight down. If I do complete it I will post the design.
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Steam on Deck
Joined: 24 Nov 2007
Location: Cheshire, UK
Posted: Sun Oct 26, 2008 3:13 pm Post subject:
I have been following your posts with interest. It strikes me that you should join a steam launch club. The obvious local one for you would be http://www.stoomvaart.nl/site_eng.htm. However, I can recommend the SBA (As Hon. Sec. I admit bias!) as we do have a wide membership beyond the UK. We also have a full stock of back issues of the Funnel, the Association's magazine. I suspect that some of the ideas you mention may have been explored. Certainly there is a host of fascinating material that would excite somebody with your skills.
I am sure that you would cope with the technical English, as your posts demonstrate a good command of the language. That is more than can be said of myself. I have joined the ABV (France) and am finding their journal a real challenge (but great fun)!
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