The Thornycroft Small Tube Boiler
And the Schulz-Thornycroft Boiler

Diagram of Thornycroft Daring boiler

Daring type    |    Launch type    |    Speedy type    |    Schulz type    |    How the Types Were Named

Small bent tubes are the characteristic of all the Thornycroft types. With these a few large tubes are generally associated. All Thornycroft boilers till recently had one feature in common, in that all the generating tubes delivered their contents into the upper barrel above the water-level. This arrangement, which affords large evaporating area, combined with the baffle plates or grids which the steam has to pass before reaching the lateral steam pipe, renders the steam very dry, and gives, it is claimed, great security from priming, even under rapid variations of power. Another advantage claimed is, that in the event of a tube splitting, the rush of steam keeps back the water. The latter types have some tubes entering the upper barrel below the water-line. The course of the gases are indicated by arrows -- there being a slight difference of direction between the two forms illustrated.    -- Jane's fighting Ships, 1906

The Thornycroft boilers are of three principal types, viz.:--"Speedy," "Daring," and "Launch."

The "Daring" type of Thornycroft boiler [named for the pioneer destroyer HMS Daring of 1893] consists of a large cylindrical steam-drum connected to a lower central water-drum by two groups of generating-tubes which form a pair of flues bounded by tube-walls, as in the case of the "Speedy" type of boiler. The fire-grates are on either side of the water-drum, and the fuel-gases enter the flues through spaces in the base of the outer tube-wall and then, having given up most of their heat, pass through similar spaces at the top of the inner tube-wall ino the central heart-shaped flue, and along to the smoke-box and funnel at the back of the boiler. In this heart-shaped flue are eight large downtakes, which are slightly bent for purposes of construction. The outer sides of the fire-boxes are formed by tube-walls leading from two small wing water-drums into the upper part of the steam-drum, the wing and central water-drums being connected by a cross connecting-pipe at the back of the boiler. The separate and feeding arrangements are similar to those described for the "Speedy" type, and as in the case of the latter, the tubes all deliver above the water-level.

To increase the heating-surface, two or three rows of tubes, with spaces all round, ar often fitted in the fire-box close to the outer wall, and in the largest boilers a flue is formed of the wing generating-tubes, similar to those of the two central groups ...
Total tube-surface of the boiler, in square feet:   4,020
Area of fire-grate, in square feet:   63.5
Ratio of the tube-surface to the fire-grate area:   63.3
Weight of the boiler and mountings with water, and with uptaeks to the level of the top of the boiler, in tons:   18.25
Weight per square foot of tube-surface in lbs:   10.17
IHP on trial with triple expansion engines:   2,000
IHP per ton of boiler:   110
Working-pressure of steam, in lbs per square inch:   220

The boiler is of mild-steel throughout. The casings are of thin steel, in two plates with a layer of asbestos millboard between them.

Thornycroft boiler under construction, Portsmouth Navy Yard
A Daring type Thornycroft boiler being constructed by Royal Portsmouth Dockyard, exactly like the plan above.


The "Launch" type of Thornycroft boiler ... is very similar to one half of a small "Daring" boiler, in which the tubes of the outer fire-box wall are rather large, and instead of being fastened into a wing water-drum, they are reduced in size and bent round horizontally, so as to form the fire-grate,and then fastened to the main water-drum. In this boiler, the fuel-gases may either be led under the steam-drum and past the downtakes as in the "Daring" type, or straight up as in the "Speedy" type. In small boilers it is usual to place the funnel on the top, and if the gases pass under the steam-drum they are usually taken to the side and over the top to the funnel.

The circulation of water in the fire-bar-tubes keeps them so cool that the galvanizing is not affected, and any clinker that may form does not adhere to the bars, as frequntly happens with small boilers.

The maximum IHP of this boiler, with a good non-condensing simple engine is 60.

It is claimed for the Thornycroft system of making the generating-tubes deliver into the steam-drum above the water level, that the circulation is double that when they deliver below it. The water steam are discharged from all the generating-tubes, and the return-water descends through all the downtakes. No reversal of current can take place in either set of tubes.

-- Walter S. Hutton, Steam-Boiler Construction: A Practical Handbook (London: Crosby, Lockwood, 1903).


The "Speedy" type.-- The Speedy type was first fitted (in units of large size) to HMS Speedy, and consists essentially of a central upper steam barrel and two smaller lower-wing barrels. A series of generating tubes is fitted between the upper barrel and each wing barrel. These tubes form practically the whole of the heating surface, and the inner row on each side is curved in such a manner as to form the top of the combustion chamber, and so protect the upper barrel. Two down-comers, of ample size, are fitted at one end of the boiler. The tubes are so arranged as to ensure the gases passing over the whole of their surface.

The modern Speedy type is being used in most navies for both coal and oil fuel, and consists of an upper steam barrel and two smaller cylindrical lower-wing barrels. A series of generating tubes, slightly curved at the ends to allow of free expansion, connects the upper and lower-wing barrels. The curvature also allows the tubes to be put into the drums radially, thus saving weight and also allowing a cylindrical-bottom barrel to be adopted. Down-take tubes are fitted at the end of the boiler. The modern Speedy type of boiler is well adapted for the use of oil fuel, as a large combustion space can be obtained, and owing to the curvature of the tubes distortion does not take place under the great heat and high rate of evaporation obtained.

In connection with all the above types a Thornycroft feed regulator is used. This is fitted in the upper barrel, and consists of a float, which rises and falls with the water level. This is suitably connected to a valve, throttling the admission of feed water if the level is high and admitting it freely if the water level is low. Another rod comes through the front of the boiler to allow of hand regulation.

-- Marine Engineering/Log, Volume XIV (Nov. 1909), 463-4 (Speedy article only).


The Schulz-Thornycroft Water-Tube Marine Boiler

Diagram of Schulz-Thornycroft boiler
Schulz-Thornycroft boiler used in the light cruiser SMS Königsberg.

The Schulz-Thornycroft type.-- This boiler is a modification of the "Daring" type of boiler. The arrangement of the tube-walls gives a somewhat different course to the fuel-gases. The Thornycroft-Schulz pattern makes the outer wings more important. The number of their tubes has been increased, such that they became the majority of the heating surface and also the main gas path for the exhaust gases. The wing drums became large enough to permit a man access inside, for cleaning and expanding new tubes into place. Two boilers of this type, each with flues differently arranged for the travel of the fuel-gases, are shown in Figs. 341B and 341C.

-- Hutton, op. cit., 253.

The Schulz-Thornycroft boiler was adopted after 1896 as the all but universal solution for the Imperial German Navy, and it remained predominant in that service through WWI. Small tube units were used to power torpedo boats and destroyers (in German nomenclature, destroyers were classed as First Class TBs). Large tube models powered cruisers and battleships. -- Ed.

SMS Deutschland on trials, 1906
Pride of the German fleet, the battleship Deutschland on trials in 1906.



The Ships the Boilers Were Named For

HMS Daring and her sister Decoy were one half of Britain's 1894 quartet of prototype destroyers; they were built by Thornycroft, while the other pair, Havock and Hornet, were built by Thornycroft's Glasgow-based rival, Yarrow. Completed in 1895, Daring attained 28.21 knots on trials, and carried a crew of around 50 men. In addition to her three torpedo tubes, the vessel carried one 3" 12-pounder quick-firing gun forward plus two 3-pounders. It is noted in company literature from the period that "these boats were stronger built compared to the Yarrow Boats." Having made her mark, Daring was broken up in 1912.   Enlarge

Also built by Thornycroft, commissioned 1894, HMS Speedy was an Alarm class torpedo gunboat designed by Sir William White, Director of Naval Construction for the Royal Navy. She is seen here at right, leaving Portsmouth in a watercolor by Charles Dixon. Speedy was powered by the boilers that bore her name, and which were quite a successful product for Thornycroft, undergoing trials in 1893 against sister-ships equipped with locomotive boilers; the results were written up glowingly by White. In power and fuel economy, the Thornycroft product outdistanced the boilers installed in Speedy's sister ships.   Complete info

Launch was not a specific vessel, but a type of boat. The launch pictured is the Phoebe, which operates on the Adirondack lakes of Upstate New York. Courtesy Hank Wevers.