The Eclipse return tubular marine boiler is a horizontal fire-tube boiler in which combustion gases travel forward through a furnace flue, reverse direction in a back combustion chamber, and return through a nest of small tubes to the smokebox. The tube nest is the critical component — it provides most of the heating surface and forces the gases through a long thermal path before they escape up the funnel. The design exists to pack high evaporation capacity into the short, low-headroom hull of a steam launch or small workboat. A typical 36 in × 60 in Eclipse evaporated 350–500 lb of water per hour at 100–120 psi.
How the Eclipse Return Tubular Marine Boiler Works
The Eclipse boiler is a return-flame fire-tube boiler built for marine service. Coal or oil burns on a grate inside a corrugated furnace flue running fore-and-aft through the lower half of the shell. Hot gases leave the back of the furnace, hit the rear combustion chamber — a water-jacketed box stayed against internal pressure — and reverse 180° to flow forward through a bank of 2 in to 3 in fire tubes packed into the upper half of the shell. They exit at the front tubeplate into the smokebox, then up the uptake to the funnel. The water sits around and above the furnace and tubes, with steam collecting in the dome at the top.
The geometry is the whole point. By folding the gas path back on itself, you get roughly twice the heating surface in the same shell length compared to a straight-through design. That matters on a 30 ft launch where you simply do not have the deck length for a horizontal locomotive-style boiler. The trade is structural — the rear combustion chamber is a flat-sided pressure vessel and needs through stays, palm stays, and girder stays to hold it together. If a stay corrodes through at the waterline or a stay nut works loose, the chamber crown bulges and you lose the boiler. That is the failure mode every marine surveyor looks for first.
Tolerances on the tubeplate matter. The tube holes must be reamed to expand the tubes cleanly — typically 0.005 in to 0.010 in clearance over the cold tube OD before rolling. Sloppy holes leak at pressure and cannot be re-rolled successfully. Tube spacing must leave at least 3/8 in of ligament between holes or the plate cracks between tubes under thermal cycling. Get any of this wrong and you will be pulling tubes in the first season.
Key Components
- Furnace flue (corrugated): The primary combustion chamber, typically 24 in to 36 in diameter, corrugated to resist collapse under external water pressure and to accommodate longitudinal expansion. Holds the grate and absorbs radiant heat directly into the surrounding water.
- Back combustion chamber: A stayed, water-jacketed box at the rear of the boiler where gases reverse direction. Built from 1/2 in to 5/8 in plate and held against internal pressure by through stays on roughly 4 in to 5 in pitch.
- Return fire tubes: Bank of 2 in to 3 in OD tubes — usually 60 to 120 of them — running from the back chamber to the front tubeplate. They provide 70% or more of the total heating surface and must be expanded into 0.005–0.010 in clearance holes.
- Front tubeplate and smokebox: The front tubeplate carries the tube ends and forms the rear wall of the smokebox. Smokebox door must seal tightly — air leaks here kill draught and dump unburnt fuel up the stack.
- Steam dome and stop valve: Raised dome on the top of the shell where dry steam collects above the waterline. The main stop valve mounts here to draw the driest possible steam to the engine.
- Stays (longitudinal, palm, girder): Tie the flat surfaces of the back chamber and end plates against internal pressure. Stay diameter is sized for working stress around 7,500 psi on solid stays — undersized stays are the most common cause of catastrophic failure in old return-tube boilers.
Real-World Applications of the Eclipse Return Tubular Marine Boiler
The Eclipse pattern and its close cousins — the Scotch marine, the Robertson, the small tug boiler — dominated steam launch and small workboat propulsion from roughly 1875 through 1920. Anywhere you needed 20 to 200 horsepower in a hull under 80 ft, a return tubular boiler was the default answer. The compactness and high evaporation rate per square foot of deck made it a natural fit for harbour craft, naphtha launches, steam yachts, and pinnaces.
- Steam launches: Original boiler fitted to many late-Victorian Thames launches built by Simpson Strickland and Forrestt of Wivenhoe — typically 30 ft to 45 ft hulls running 100–120 psi compound engines.
- Naval pinnaces: Royal Navy steam pinnaces and picket boats from the 1890s through WW1 used return-tube boilers in this size class, including the surviving Steam Pinnace 199 preserved at Portsmouth Historic Dockyard.
- Harbour tugs: Small dock tugs of the early 20th century, including vessels in service at Liverpool and Glasgow docks, ran return tubular boilers feeding twin-cylinder compound engines.
- Steam yachts: Private steam yachts up to 80 ft length — examples preserved at the Antique Boat Museum in Clayton, NY include vessels boilered with return-tube units.
- Working preserved vessels: SL Dolly on Lake Windermere (recovered 1962, boiler since renewed) and the steam launch Branksome at Coniston run return tubular boilers in active heritage service.
- Industrial donkey boilers: Shoreside auxiliary use — driving capstans, cargo winches, and bilge pumps at small commercial wharves where a marine-pattern boiler was already familiar to the engineer.
The Formula Behind the Eclipse Return Tubular Marine Boiler
The most useful number for sizing one of these boilers is the evaporation rate — how many pounds of water per hour you can convert to steam at working pressure. That sets the engine you can run and the bunker you need to carry. At the low end of the typical range, around 2.5 lb of water evaporated per square foot of heating surface per hour, you are running a clean-burning forced-draught oil firing with low excess air. At the nominal 3.5 lb/ft²/hr the boiler is working hard but sustainable on hand-fired Welsh steam coal. Push past 4.5 lb/ft²/hr with natural draught and you start priming — water carrying over into the steam line — and tube-end leakage begins as the tubeplate sees thermal gradients it was not designed for. The sweet spot for a long-lived boiler in heritage service sits at 3.0 to 3.5 lb/ft²/hr.
Variables
| Symbol | Meaning | Unit (SI) | Unit (Imperial) |
|---|---|---|---|
| Ws | Evaporation rate — pounds of water converted to steam per hour at working pressure | kg/hr | lb/hr |
| E | Specific evaporation rate per unit of heating surface, dependent on fuel, draught, and firing intensity | kg/(m²·hr) | lb/(ft²·hr) |
| AHS | Total heating surface — sum of furnace, combustion chamber, and tube external surface in contact with water | m² | ft² |
| Nt | Number of return fire tubes (used to compute A<sub>HS</sub> contribution from the tube nest) | — | — |
Worked Example: Eclipse Return Tubular Marine Boiler in a preserved steam picket boat
You are sizing the evaporation rate of a recommissioned 1902 Eclipse-pattern return tubular boiler being returned to service in a 42 ft preserved steam picket boat at a maritime heritage shipyard on Chesapeake Bay, where it will feed a twin-cylinder compound launch engine at 110 psi working pressure burning grade-2 fuel oil with a small forced-draught fan.
Given
- Shell ID = 48 in
- Shell length = 72 in
- Furnace flue OD = 24 in
- Furnace length = 60 in
- Number of fire tubes Nt = 84 —
- Tube OD = 2.0 in
- Tube length (between tubeplates) = 60 in
- Working pressure = 110 psi
Solution
Step 1 — compute the heating surface contribution from the furnace flue (external area in contact with water):
Step 2 — compute the heating surface from the tube nest. Each tube contributes its outer cylindrical surface, gas-side:
Add roughly 15 ft² for the back combustion chamber walls in contact with water, giving total AHS ≈ 266 ft². The tubes carry 83% of the heating surface — exactly why the return-tube layout exists.
Step 3 — compute the nominal evaporation rate at E = 3.5 lb/ft²/hr, the sustainable hand-fired or light forced-draught figure:
That is enough steam to run the compound engine flat-out at hull speed with margin for auxiliaries. At the low end of the operating range, a clean light fire at E = 2.5 lb/ft²/hr gives:
That is your cruising figure — light haze at the funnel, easy steaming, no priming, the kind of duty a heritage boiler will live a long life on. Push hard with the FD fan to E = 4.5 lb/ft²/hr and theory says:
In practice you will not see that. Above about 4.0 lb/ft²/hr on a 60 in tube length, the gas exit temperature climbs past 700°F, the tube-end ferrules start weeping, and you get wet steam carrying water into the engine. The boiler can do it for an hour to make a tide; do it all day and you are pulling tubes by the next survey.
Result
Nominal evaporation rate is 931 lb/hr at 110 psi — comfortably enough to feed a 25 IHP compound launch engine with reserve for the feed pump and whistle. The range from 665 lb/hr cruising to 1,197 lb/hr forced gives you a feel for the duty: cruise at the low figure for long boiler life, sit at nominal for a working day, only touch the high figure when you need to make a tide or fight a foul tide for a short spell. If your measured evaporation falls 20% below predicted, the usual culprits are: (1) air leakage at the smokebox door gasket killing draught and dumping heat up the stack, (2) sooted tubes — 1/16 in of soot on the gas side cuts heat transfer by roughly 30%, and (3) scaled tubeplate water-side, which insulates the tube ends and shows up as discoloured paint on the front tubeplate before tubes start to leak.
Eclipse Return Tubular Marine Boiler vs Alternatives
The return tubular boiler is one of three serious options for a small steam vessel. Compared against a water-tube boiler or a vertical fire-tube boiler, it sits in the middle on most axes — heavier than a water-tube, lighter than a Cornish, slower-steaming than a Yarrow but more forgiving of feedwater quality. The choice hinges on what you value most: rapid steaming, water reserve, simplicity of construction, or weight per horsepower.
| Property | Eclipse return tubular boiler | Yarrow water-tube boiler | Vertical fire-tube boiler |
|---|---|---|---|
| Time to raise steam from cold | 60–90 min | 20–30 min | 30–45 min |
| Heating surface per ft³ of boiler volume | ≈ 8 ft²/ft³ | ≈ 18 ft²/ft³ | ≈ 6 ft²/ft³ |
| Working pressure ceiling | Up to 200 psi | Up to 400 psi | Up to 150 psi |
| Tolerance for poor feedwater | Good — large water volume | Poor — tubes scale fast | Fair |
| Weight per IHP delivered | 80–120 lb/IHP | 30–50 lb/IHP | 100–140 lb/IHP |
| Survey/maintenance interval | Annual hydro, decadal full strip | Annual tube inspection | Annual hydro |
| Typical service life with good water | 40–60 years | 20–30 years | 30–40 years |
| Best application fit | Launches, picket boats, small tugs 20–80 ft | Fast naval craft, destroyers | Donkey boilers, very small launches |
Frequently Asked Questions About Eclipse Return Tubular Marine Boiler
Priming — water carrying over into the steam line — usually means you are evaporating faster than the steam can disengage from the water surface. In a return tubular boiler the steam release area is set by the waterline length × shell width, and once you push past about 4 lb/ft²/hr the bubbles do not have time to break before being drawn into the dome.
Two things to check. First, the working water level — many recommissioned boilers run a higher level than the original builder's plate specified because owners want a margin against low-water trips. An extra 2 in of water height cuts the steam release area dramatically. Second, dissolved solids — if total dissolved solids climb past 2,500 ppm the surface foams and primes at much lower firing rates. Blow down hard and refill with treated feedwater before condemning the boiler.
Pull a sample tube and measure wall thickness with an ultrasonic gauge. Original tubes were typically 0.105 in wall (12 BWG). If you measure above 0.085 in across the gas-touched length and the tube-end seats clean up with a re-roll, you can keep them — expect another 15 to 20 years.
Below 0.075 in wall, or any pitting deeper than 0.020 in on the waterside, re-tube the whole nest. Mixing new tubes with old in the same plate is poor practice — the new tubes are stiffer and carry disproportionate load through differential expansion, and the old tubes adjacent to them tend to leak first.
You are losing the back-end gas seal. In a properly tight return-tube boiler, gases enter the smokebox at 550–650°F. If the door reaches visible red heat you are seeing 900°F+ at the front tubeplate, which means either the back combustion chamber refractory has failed and gases are short-circuiting, or the ferrules around the rear tube ends have burned out and you are getting flame impingement straight onto the tubes.
Drop the smokebox door and inspect the front tubeplate for tube-end discolouration — bright blue or straw colour at individual tube ends maps directly to the failed ferrules at the other end. Fix the back end before you damage the front tubeplate, which is far harder to replace than a tube ferrule.
For coal firing on a return tubular boiler, the rule is roughly 12 to 15 lb of coal per square foot of grate per hour for sustained working. With Welsh steam coal at 14,500 BTU/lb and 70% boiler efficiency, that gives you about 10 lb of evaporation per square foot of grate per hour.
Oil firing changes the geometry. You delete the grate and fit a burner quarl, and the heat-release rate per cubic foot of furnace volume becomes the limit, not grate area. Size the burner for 100,000 to 150,000 BTU/hr per cubic foot of furnace volume. For the 60 in × 24 in furnace in the worked example that is roughly 16 ft³ of volume, supporting a 1.6 to 2.4 million BTU/hr burner — comfortably matched to the 931 lb/hr nominal evaporation.
Riveted seams seal partly through caulking and partly through differential expansion forcing the lap tight. If a seam is marginal, cold water at hydro pressure will not move it — the joint stays in its as-caulked geometry. Under steam the shell expands roughly 0.001 in/in over the temperature rise, and a lap that was half-caulked or has pitted rivet heads will open a film leak at temperature.
The fix is recaulking the seam with a pneumatic caulking tool while the boiler is cold and empty, working the plate edge over to re-seal. Do not be tempted to seal-weld a riveted seam unless the original construction certificate is being abandoned — welding a riveted lap concentrates stress at the weld toe and is a classic cause of cracking 5 to 10 years later.
Probably not. The original Eclipse was designed for saturated steam and the engine cylinder lubrication, packing, and valve materials were specified for it. Adding a superheater that delivers steam at 50–80°F of superheat will give you 5 to 8% better engine economy on paper, but you will need to replace the cylinder oil with a high-temperature mineral oil and re-pack the piston rod stuffing boxes with graphited asbestos-substitute that can take the heat.
If the goal is heritage authenticity, run saturated. If the goal is range under sail-assist or long passages, the superheater pays back — but budget for a complete engine top-end refresh at the same time. A superheater bolted onto a tired engine just exposes every weakness in the cylinders.
References & Further Reading
- Wikipedia contributors. Fire-tube boiler. Wikipedia
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