Thornycroft Boiler: Types, Merits, Demerits, and Applications

Thornycroft Boiler is a type of high-pressure water-tube boiler. It is named after its design Engineer, John Isaac Thornycroft. Its two designs are known as (1) speedy-type and (2) daring-type. The former was designed earlier. Both are used in marine work, such as in torpedo-boats and torpedo-boat destroyers. Both have higher evaporative capacity and handle fluctuating steam demand.

Before proceeding with the two types, it is mentioned that in the literature of water-tube boilers, such as Thornycroft and its contemporaries, there is no clear-cut method that defines the circulation route of water and steam in the boiler.

Water and steam find their path based on the thermal driving force, which is caused by the density difference between the two masses of fluids (water and steam). Moreover, it cannot be ascertained that the function of risers and downcomers is fixed.

At one moment, a water tube acts as a riser, and in the other as a downcomer and vice versa based on existing thermal conditions inside and outside the boiler.

The following section discusses the two designs of the Thornycroft boiler one by one.

The Speedy-type Thornycroft Boiler

Design and Construction

It is similar in construction to the Yarrow boiler. Just like it, the speedy-type Thorneycroft boiler consists of three drums (cylindrical vessels) placed in a manner that makes an equilateral triangle. In the setting, a steam drum is at the top, and two water drums are placed at the bottom at floor level. Such is an early design of the Thornycroft boiler, which became popular as a speedy type.

speedy type thornycroft boiler
Fig. 01 – Speedy-type Thornycroft Boiler

Inside the casing or shell, the upper steam drum is connected to the two lower water drums through a number of thin curved water tubes (called tube-bank).

The diameter of these thin water-tubes ranges from 1 to 1 1/4 inches, and they are fastened at their respective ends in the drums by rolling out.

These thin tubes are the generating tubes and are closely placed to each other in the lower ends where they fit in the water drum at each side compared with their upper ends fitted in the steam drum where spaces are left for the passing out of hot gases.

Furthermore, a small portion (normally two rows) of generating tubes are arranged at the outermost layer of the tube-bank in such a way as to form a water-tube wall.

These tubes are closely placed to each other and are nearer to the boiler casing. Their purpose is to enclose the hot gases and render their escape a bit struggling.

Only a small number of these thin tubes are fitted in the upper section of the steam-drum. A large number of these tubes is expanded in the water-section (which makes the lower section) of the steam drum.

The steam drum is also connected to the water drums through one large pipe at each side, as shown in figure 1.

The small generating tubes are for steam generation, whereas two large water pipes are for water circulation only from the lower section of the steam drum back to the water drum.

The generating tubes packed inside the steel-casing are lined with asbestos. The whole boiler is enclosed in the casing except for the large water-tube section.

The chimney is situated at the center of the boiler-case to exit the burned gases into the atmosphere.

Whereas the furnace is situated in the middle space between the two water drums. The hot gases rise and, by passing in the gaps between the generating tubes, discharge through the stack or chimney.


In its operation, the fuel-firing takes place in the furnace of the combustion chamber.

The hot burned gases produced rise above and pass over the thin water tubes and convert the water into steam.

This steam mixture ascends into the steam drum due to its lighter weight (or specific weight).

The steam is eventually collected above the water-level in the steam-drum. The deflectors in the steam drum deflect the water collected in the steam drum to the water-drums through outside circulating tubes. These tubes are called downcomers.

In this way, the conversion of water into steam sustains the circulation of water in a loop due to the difference in the density or specific weight of water and steam, which acts as a driving force of such a natural circulation.

The Daring-type Thornycroft Boiler

In the later designs, Thornycroft & Co. built a boiler compatible with combined boiler settings. It is known as the daring type and is heart-shaped. It was first used in torpedo-boat destroyers (TBDs).

Design and Construction

Its longitudinal and transverse views are shown in Figure 2.

Daring-type Thornycroft Boiler
Fig. 01 – Daring-type Thornycroft Boiler

In construction, it consists of a horizontal water drum and a large steam-drum lying above it. Both drums are connected by small diameter curved tubes on either side known as generating tubes.

They are closely placed at its two outermost rows, thus forming a water-wall as in the case of the speedy-type arrangement only to enclose the hot gases and increase their residence time inside the boiler. The spaces, however, are provided between the tubes at their lower ends.

Similarly, two innermost rows of generating tubes are brought together at their lower ends compared with an area nearer to their upper ends where spaces are left for the passing out of hot gases. A water wall is also created at the inner side of the tube-bank.

Also, there are 8 to 9 vertical tubes fitted between the two drums at the center line. They are about 4 inches in diameter.

Lastly, the outer portion of the combustion chamber is formed by tube-walls: two more water drums are connected to the steam-drum and make the combustion chamber’s outer wall. These two drums are connected at the rear side of the boiler through a cross-pipe.


Two fire-grates are provided: one on each side of the water drum. The hot gases rise above the grates and pass through the combustion chamber. Now, they enter the space between the inner and outer water-walls on either side.

Finally, the two streams of gases join in the heart-shaped central flue (pipe) of the boiler that leads to the funnel at the rear side of the boiler, from where gases take an exit into the atmosphere.

The generating tubes leave a water-steam mixture in the steam-drum (which is famously called a mingled mass of two fluids), where the baffle plates cause the separation between the two.

The steam is allowed to pass further into a steam pipe through narrow slits to increase its thermal quality further, while the water collected in the bottom portion of the steam drum returns to the water drum through vertical water tubes.


The merits of the Thornycroft boilers are mentioned below:

  1. Both speedy and daring types were used in the past in naval and maritime applications.
  2. The speedy-type boiler has a lower footprint and is a preferred choice where space is at a premium.
  3. The daring-type offers more power output and higher evaporative capacity compared with its speedy-type counterpart.
  4. The curved shape of the generating tubes is the hallmark of Thornycroft boilers: more residence time of the water-steam mass in the tubes means higher final steam temperature.
  5. The daring-type boiler is used in large vessels, whereas the speedy-type is fitted in small-scale vessels.


The demerits of the Thornycroft boilers are mentioned below:

  1. The maintenance of the thin curved tubes is challenging, almost impossible.
  2. The daring-type has more footprint and is not an appropriate choice where space is a premium.
  3. The speedy-type has limited steam generation capacity and, subsequently, lower power output.
  4. It is more susceptible to water hammer.
  5. Both types are used in small to medium size submarines.


  1. The earlier forms of Thornycroft boilers were designed for marine work, especially for small craft such as torpedo boats and torpedo-boat destroyers where it was necessary to generate steam quickly in a relatively smaller steam space.
  2. They were used in cruisers and warships.
  3. Their refined designs were subject to use in high-speed marine vessels.
  4. The daring-type was famously used in torpedo-boat destroyers, which were naval vessels used to protect large ships against torpedo attacks.