A Yarrow water tube boiler is a three-drum marine boiler with one upper steam drum and two lower water drums connected by straight, inclined water tubes that form a triangular cross-section over the furnace. Hot combustion gases rise between the tube banks, heating the water inside the tubes so it circulates upward by thermosiphon into the steam drum. The straight-tube layout raises steam quickly from cold and tolerates high firing rates, which is why the Royal Navy adopted it across destroyers from the 1890s through WWII to deliver up to 30,000 lb/hr per boiler at 250-400 psi.
The Yarrow Water Tube Boiler in Action
The Yarrow boiler works on natural circulation. You have one steam drum sitting up top and two water drums sitting low on either side of the firebox. Straight water tubes run between them, inclined inward so the whole arrangement forms a triangle with the fire underneath. Light the furnace and the water inside the tubes nearest the flame heats first, becomes less dense, and rises into the steam drum. Cooler water from the steam drum drops back down through the outer, cooler tubes into the water drums to replace it. That loop runs continuously as long as you keep firing.
Why straight tubes? Alfred Yarrow's design choice was deliberate — straight tubes can be cleaned mechanically with a tube brush from either end, they expand and contract predictably under thermal cycling, and they don't trap sediment in bends. Compare that to the Thornycroft boiler with its curved tubes, which raises steam slightly faster but is a nightmare to descale. Yarrow tubes are typically 1.5 to 2 inches outside diameter with a wall thickness around 0.1 inches, rolled and expanded into the drum tube plates. The expansion fit must be tight — a leak at a tube-to-drum joint under 300 psi sprays superheated water and will scald a stoker before the watch officer can shut the stop valve.
If circulation slows or stops you get tube starvation, where a tube runs dry on the fire side and overheats. Symptoms: bulged tubes, scale baking onto the inside surface, and in the worst case a tube rupture. The usual causes are scale buildup restricting flow (feed water that wasn't softened properly), firing too hard before the boiler is fully warm, or low water level masking the upper tube rows. Naval crews ran a strict water treatment regime — typically less than 1 ppm hardness — precisely to keep the tubes scale-free over a 5,000 hour service interval between major refits.
Key Components
- Steam drum (upper drum): Cylindrical pressure vessel running fore-and-aft along the top of the boiler, typically 36-48 inches diameter with 1-inch thick steel plate. It separates steam from water using internal baffles and a dry pipe, and feeds saturated steam to the main stop valve at 250-400 psi depending on the ship class.
- Water drums (lower drums): Two smaller cylindrical drums, typically 18-24 inches diameter, mounted port and starboard at the base of the tube banks. They collect the downcomer flow and distribute water evenly into the bottom of the generating tubes. Mud accumulates here and is blown down through a bottom valve once per watch on hard-fired vessels.
- Generating tubes: Straight steel tubes, 1.5-2 inches OD, expanded into the drum tube plates at both ends. Inclination angle is typically 30-40° from vertical. The bore must be held to ±0.005 inches at the rolled joint or you lose the seal under thermal cycling.
- Furnace and firebrick: Open furnace fired by oil burners (post-1910) or coal grates (earlier ships). Refractory brick lines the floor and side walls below the tube banks to protect the hull structure and reflect heat upward. Brick failure shows as glowing patches on the casing — a clear signal to bank fires.
- Forced draught casing: Sealed steel casing around the entire boiler maintained at 2-6 inches water gauge above atmospheric by closed stokeholds or fan-fed air ducts. This drives combustion air through the fire at high rates, allowing evaporation rates of 8-10 lb steam per lb fuel.
- Superheater (where fitted): Bank of U-bend tubes mounted in the gas path between the front and rear tube banks. Raises steam temperature from saturated (~400°F at 250 psi) to 600-700°F. Improves engine thermal efficiency by 10-15% but adds maintenance complexity.
Real-World Applications of the Yarrow Water Tube Boiler
The Yarrow boiler dominated warship propulsion for fifty years because it raised steam from cold faster than almost any rival, packed high evaporation rates into a small footprint, and survived the brutal duty cycle of naval service. You'll find it on destroyers, cruisers, gunboats, and a handful of preserved museum ships still in steaming condition today.
- Naval propulsion: Royal Navy A-class through V-class destroyers from 1913 onwards used three or four Yarrow boilers feeding Parsons or Brown-Curtis turbines at 250 psi.
- Heritage steamships: PS Waverley, the last seagoing paddle steamer in the world, was originally fitted with two Yarrow-type three-drum boilers built by Babcock to a Yarrow pattern.
- River gunboats: The Royal Navy's Insect-class river gunboats on the Yangtze in the 1920s ran twin Yarrow boilers feeding triple-expansion engines at 200 psi.
- Industrial power generation: Yarrow shore-based variants supplied process steam at jute mills around Dundee in the early 20th century, where Yarrow Shipbuilders sold land boilers as a sideline.
- Maritime preservation: HMS Cavalier at Chatham Historic Dockyard retains her original Admiralty three-drum Yarrow-pattern boilers as static exhibits, partially sectioned for visitor access.
- Steam yachts: Edwardian-era steam yachts above 100 ft length frequently used small Yarrow boilers because they raised steam in 30-40 minutes from cold, against 2+ hours for a Scotch boiler of equivalent output.
The Formula Behind the Yarrow Water Tube Boiler
When you're commissioning or recommissioning a Yarrow boiler you need to know how much steam it will actually deliver across the firing range you intend to use. At low firing rates the boiler loafs along at maybe 30-40% of rated output, useful for harbour idling but wasteful of fuel per pound of steam because radiation losses dominate. At nominal firing the boiler hits its design sweet spot — typically 8-9 lb of steam per lb of oil fired with combustion efficiency around 82%. Push to maximum continuous firing and evaporation rate rises again but per-pound efficiency drops back as stack temperature climbs and unburned hydrocarbons escape. The formula below predicts evaporation rate from fuel input, calorific value, and overall boiler efficiency.
Variables
| Symbol | Meaning | Unit (SI) | Unit (Imperial) |
|---|---|---|---|
| ṁsteam | Steam evaporation rate | kg/hr | lb/hr |
| ṁfuel | Fuel mass flow rate into burners | kg/hr | lb/hr |
| HHV | Higher heating value of the fuel | kJ/kg | BTU/lb |
| ηboiler | Overall boiler thermal efficiency (combustion + heat transfer) | dimensionless | dimensionless |
| hg | Specific enthalpy of saturated steam at operating pressure | kJ/kg | BTU/lb |
| hf | Specific enthalpy of feedwater entering the drum | kJ/kg | BTU/lb |
Worked Example: Yarrow Water Tube Boiler in a recommissioned 1936 destroyer Yarrow boiler
You are sizing the steam evaporation rate across three firing rates on a recommissioned 1936 Admiralty three-drum Yarrow boiler being returned to demonstration steaming aboard a preserved V&W class destroyer hulk at the Tyne shipyard heritage berth in Wallsend, where the boiler fires on marine residual fuel oil at 42,500 kJ/kg HHV and supplies saturated steam at 21 bar gauge to a single Parsons impulse-reaction turbine driving the starboard shaft. The trustees want evaporation rate verified at slow harbour idle (200 kg/hr fuel), nominal cruising (1,100 kg/hr fuel) and a brisk full-power demonstration burst (2,400 kg/hr fuel) before the open weekend. Boiler efficiency is taken as 0.68 at idle, 0.82 at cruise, and 0.78 at full power. Feedwater enters at 105°C (h<sub>f</sub> ≈ 440 kJ/kg) and saturated steam at 21 bar gauge has h<sub>g</sub> ≈ 2799 kJ/kg.
Given
- HHV = 42500 kJ/kg
- hg = 2799 kJ/kg
- hf = 440 kJ/kg
- ṁfuel,idle = 200 kg/hr
- ṁfuel,cruise = 1100 kg/hr
- ṁfuel,full = 2400 kg/hr
- η at idle / cruise / full = 0.68 / 0.82 / 0.78 —
Solution
Step 1 — calculate the latent + sensible enthalpy gain per kg of steam, the same at all three firing rates:
Step 2 — at nominal cruising fire of 1,100 kg/hr fuel with boiler efficiency 0.82:
That is the design sweet spot. The furnace runs cleanly, oil atomisation at the burners is stable, stack temperature sits around 320°C, and the turbine sees steady inlet pressure at the throttle. The watchkeeper can hold this rate indefinitely without overworking the firemen or stressing the tube joints.
Step 3 — at the low end of the typical operating range, harbour idle of 200 kg/hr fuel with efficiency dropped to 0.68 (radiation and casing losses dominate at low fire):
That is barely 15% of cruise output — enough to keep auxiliaries turning and the steam drum hot for a quick response if you need to move, but you're burning fuel inefficiently because the firebox is cool and combustion is incomplete. You'll see a faint dark haze at the funnel.
Step 4 — at the high end, full-power demonstration burst of 2,400 kg/hr fuel with efficiency falling to 0.78 because stack losses climb and some fuel passes unburned at maximum atomisation:
The forced-draught fans are roaring, the casing pressure climbs to 5 inches water gauge, and the turbine is being asked for full ahead. You can theoretically hold this for short bursts, but Admiralty practice limited continuous full-power firing to 4 hours before tube-end stress and refractory wear became measurable.
Result
At nominal cruising fire the boiler delivers 16,247 kg/hr (≈ 35,800 lb/hr) of saturated steam at 21 bar gauge — comfortably within the original 1936 design rating of about 38,000 lb/hr per boiler. Watch the steam drum gauge: pressure should hold steady within ±0.3 bar of the 21 bar setpoint without the safety valves lifting. Across the range, idle gives roughly 2,450 kg/hr, cruise 16,250 kg/hr, and full power 33,700 kg/hr, so the boiler covers a 14:1 turndown ratio with the cruise point clearly the efficiency sweet spot. If your measured evaporation rate falls more than 8% below the predicted cruise figure, the most likely causes are (1) scale buildup on the fire side of the generating tubes adding thermal resistance — feel for cold spots on the casing or check stack temperature creeping above 380°C, (2) burner tip erosion fattening the spray cone and dropping atomisation efficiency, or (3) air register leakage upstream of the casing dropping forced-draught pressure below the 4-inch w.g. minimum needed at full fire.
When to Use a Yarrow Water Tube Boiler and When Not To
The Yarrow boiler isn't the only three-drum design you'll encounter on a preservation project. The two main rivals are the Thornycroft (curved tubes) and the Babcock & Wilcox marine type (single steam drum, single water drum, sinuous header). Each picks a different compromise between steaming rate, maintenance access, and weight.
| Property | Yarrow water tube boiler | Thornycroft water tube boiler | Babcock & Wilcox marine boiler |
|---|---|---|---|
| Time to raise steam from cold | 30-40 min to 250 psi | 20-30 min to 250 psi (faster) | 60-90 min to 250 psi (slower) |
| Maximum evaporation rate per ton of boiler weight | ~10 lb steam / lb·hr per ton | ~11 lb steam / lb·hr per ton | ~7 lb steam / lb·hr per ton |
| Tube cleaning method and interval | Mechanical brush, straight pull-through, every 1500 hr | Curved tubes — chemical descale only, every 1000 hr | Mixed — partial mechanical, every 2000 hr |
| Tube replacement complexity | Low — straight tubes, expand and roll | High — each tube is a unique curve | Medium — sinuous headers add steps |
| Operating pressure ceiling | Up to 600 psi (late marks) | Up to 450 psi | Up to 850 psi (land variants higher) |
| Typical service life between major refit | 5,000 hr firing | 3,500 hr firing | 6,500 hr firing |
| Best application fit | Destroyers, fast warships, steam yachts | Torpedo boats, fast launches | Battleships, merchant ships, shore plant |
Frequently Asked Questions About Yarrow Water Tube Boiler
Almost always it's the feedwater temperature, not the boiler itself. The Admiralty figure of 30-40 minutes to 250 psi assumes feedwater entering at 80-100°C from a working feed heater. If you've recommissioned the boiler but the auxiliary steam line to the feed heater isn't live yet, you're feeding the drum at ambient (15-20°C) and adding roughly 350 kJ/kg of sensible heating that the burners now have to provide on top of the latent load. That extends raise time by 60-90%.
Diagnostic check: measure feedwater temperature at the economiser outlet. Below 70°C and you've found your culprit. The fix is either firing the auxiliary boiler first to warm the feed system, or accepting the longer raise time as a recommissioning quirk.
Pick Yarrow if the vessel needs frequent cold starts and short steaming sessions — heritage open weekends, parade days, demonstration runs. The straight-tube triangular layout has lower thermal mass than a Babcock and tolerates aggressive warming-through schedules without inducing tube-joint leaks.
Pick Babcock if you're running a continuous-duty role — a museum ship that steams daily for tourist trips, or a static demonstration plant. Babcock tolerates higher pressures, has a longer interval between refits, and is more forgiving of imperfect feedwater chemistry. Yarrow demands the water treatment discipline of a working warship.
You have localised circulation failure in those specific tubes. The water inside isn't flowing fast enough to carry away the heat, so the tube wall temperature climbs above the saturation temperature of the water and starts radiating visibly. Two common causes:
First, scale or sludge accumulation at the lower end of those tubes where they enter the water drum. Sediment settles into the bottom of the water drum and blocks the tube inlets nearest the mud accumulation. Blow down the water drums under steam and the glowing usually clears within 20 minutes.
Second, steam blanketing — if the tube has slightly larger bore than spec or sits at a shallower angle than the others, vapour bubbles can collect at the top instead of rising cleanly, starving the tube of liquid water. This is rarer and requires tube replacement.
That's a textbook signature of a tube leak inside the boiler casing. The leak is dumping water and steam into the gas path, where it absorbs heat without contributing to drum pressure, and the lost mass shows up as a falling drum level at the same time.
Confirm by checking the gauge glass — if level is dropping faster than the feed pump can replace, secure the fires immediately and isolate the boiler. Don't try to ride it out. A small leak at a rolled tube joint will enlarge under thermal stress within minutes once it starts. Naval practice was to bring the boiler offline within 5 minutes of confirmed tube leak indication.
Conversion is possible but rarely worth the engineering. The furnace volume and grate area sizing differ substantially. An oil-fired Yarrow has a smaller, taller furnace optimised for radiant transfer from a luminous flame; a coal-fired Yarrow needs a wide grate and a deeper combustion space for the fuel bed.
If you convert oil-to-coal without enlarging the furnace, expect evaporation rate to drop 30-40% because the coal can't burn fast enough in the limited grate area. Going coal-to-oil is easier — you fit burners through the firebox door and brick over the grate — but you'll over-heat the lower tube rows because oil flames sit closer to them than the original coal bed did.
Above 5 ppm total hardness and you'll see scale buildup of roughly 0.1 mm per 1000 firing hours on the inside of the generating tubes. That sounds trivial but scale has roughly 1/40th the thermal conductivity of steel, so a 0.1 mm layer adds the equivalent thermal resistance of 4 mm of additional tube wall. Stack temperature rises 15-20°C, evaporation rate falls 5-8%, and the fire-side tube metal temperature climbs because the heat can't get into the water fast enough.
Royal Navy practice held boiler water below 1 ppm hardness with a strict regime of distilled make-up plus phosphate dosing. For preservation work, aim for under 2 ppm and test weekly. Anything over 10 ppm and you're cooking the tubes.
References & Further Reading
- Wikipedia contributors. Yarrow boiler. Wikipedia
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