Otter's boiler

As noted, the basic design of this boiler  follows the Lune Valley design, having multiple small coils emanating from a central drum.   In the original Lune Valley design, the tubes were steel, expanded into a forged flanged drum.  This  is rather impractical for home manufacture, so I did some adapting, using copper coils rather than steel, silver soldering the coils into steel nipples welded into the drum and welding the central pressure vessel together from standard pipe rather than using flanges. Note that the 4" and 7" caps and the 7" to 4" reducer are commercial forgings - reasonably inexpensive.  The weld-o-lets I used for all pipe connections are also small forgings.  The root passes on all the welds are 6010 DC and the cover passes are 7018.  After the coils were soldered in the entire assembly was pressure tested to 400 psi.  One of the welds securing a weld-o-let leaked a drop or two; it stopped almost immediately.

Some key design considerations if you're contemplating your own moderate pressure water tube boiler design for hobby steamboat use:

  • Maximize radiant heating surface - most of the heat should be transfered that way.
  • Allow 5 sq ft of surface area per engine horsepower.
  • Make sure any silver soldered joints exposed to radiant heat _cannot_ easily go dry.  A Yarrow style express boiler with copper tubes is a bad idea.
  • Make sure to provide 5 to 10 minutes steaming time between high and low water.  Using 3/4 gallon per engine horsepower will get you in the right ball park.
  • Allow room for the water level to rise when you blow the whistle, go racing, etc.  The Otter's boiler will easily see a 3" jump in water level with hard firing and full speed, so a steam dome is a good idea.
  • Allow enough firebox space so the flames don't touch the tubes.  This helps avoid severe sooting problems.

Here I am fitting the coils into the nipples for a test before soldering. In order to bend the coils, the copper tubes were filled with sand and wound on my 1942 10" Atlas by hand using a form made from maple. I turned the jackshaft pulley by hand with the lathe in the lowest gear, guiding the carriage with my other hand.  In general, winding the coils was not difficult, but was rather tedious.  After winding. the coils had to be annealed in order to splay them to fit the boiler (see previous picture).   This was done with a large weed burner.  After annealing, I used steel wool to clean the ends of the boilers.  

The support for the boiler was a temporary fixture on casters; this allowed the boiler to be readily spun about its vertical axis and made soldering, etc much easier.   If I were to do this again I'd make it higher - my back aches just looking at this picture.

The soldering procedure for each coil went as follows:  
  • Test fit coil in position.  Warm corrrect 2 steel nipples with  oxy-acy. torch.  
  • Brush  coil ends, inside of nipples with  medium strength hydrochloric acid (watch out for fumes as acid hits hot metal).  The temperature is right when the steel quickly turns a dull gray, free of rust or other discoloration.  
  • Brush inside of nipples with silver soldering flux.  
  • Place coil into nipples.  
  • Heat nipples with torch until flux turns glassy.  
  • Apply silver solder (Easy-flo) to nipple/coil joint.  When solder melts, walk solder around joint with torch flame.  
One coil joint leaked during hydro after completing all 108 joints; draining the water, reapplying flux, heat and solder fixed it.

Here's the casing for the boiler in a photo that  makes the frame pretty visible.  The rings top, center and bottom are made from 1" x 1/4" solid rolled on the roller I made to build the Otter's canopy (XXX insert link to future canopy page here).  The inner jacket is 22 gage 304 stainless; I chose this because I got it surplus at $2.50/lb.  If I had my druthers a more heat resistant grade would be better, but so far this appears to be working quite well.

This structure was difficult to get aligned correctly; the rings weren't exactly round and they didn't want to stay aligned vertically.  I think it might be easier to  just avoid the vertical members and just use the outer shell to hold it all together.

The view of the emoty casing from above.  The bottom is 12 gage or so steel; the supports seen as a X in the bottom are 1" x 1/8" on edge as is the inner ring.  On top of this is placed 1" Fiberfrax insulation, a disk of 16 gage stainless and a layer of lightweight insulating firebrick.  I originally lined the entire combustion chamber with this firebrick, but it proved too weak to handle the stress of trailering and thermal shock without being mortered into place.  The combustion chamber is now lined with 2" of Fiberfrax and an innermost surface of 321 stainless.  The later is warping slightly after several years of use.  

One can see here the fire door (the lower hole) and the burner entry (the upper tangenially cut hole).  If I redo this ever, I'm going to fit a larger fire door and some sort of grate or ash pan so I can raise initial steam on solid fuel.  Right now I use propane, but this is a pain to have in the boat.  Once steam is raised, of course, I use steam at 30 psi to spray diesel in a tangential swirling flame.  Since the flame wraps neatly around the firebox, we have little trouble with the fire going out due to a gust of wind or the like.

At this point the pressure vessel has been bolted in place and I've installed the 1" FiberFrax that forms the insulation for the outer jacket (18 ga 304 stainless).  A fellow steamboater arranged to have the outer jacket rolled for me.  Everything here was tacked in place while clamped into position, as it's very difficult to hold tolerances tight enough to do anything else.  

Wear a respirator when using FiberFrax - it's nasty stuff.

The plumbing has been added here.  From upper left clockwise the nipples are:
  • Auxillary steam feed for burner, Windermere kettle, etc.
  • Cold water feed.
  • Main steam output to engine.
  • Connection to top of water glass.
Note that all boiler nipples should (and are here) be schedule 80 pipe.  The white goo on the joints is Rector Seal 5 - great stuff.

Plumbing in place with economizer tubing (1/2" copper) added.  You can also clearly see the upper steam connection for the safety valve, stack draft and whistles.  Unlike the lower main and auxilary steam connections, this one doesn't have an internal pipe leading up into the steam dome.  This was a mistake.  I may yet replumb the sight gage to use this as the upper connection as the boiler appears to steam better with a high water level, and raising the level of the connection would allow the glass to show these higher
The Otter's boiler ready for hydro-testing.  The orange hose blew off from the pump during hydro, shooting 400 psi water (there was some air left somewhere) all over the place.

The Otter under test with her new boiler in 1996, just in time for the meet in Victoria BC.