In the early 1930s, competition in
development of long-range flying boats for intercontinental passenger
service was becoming increasingly intense. Great Britain had nothing to
match the new American Sikorsky flying boats that were making headlines
all over the world, and the authorities in Britain felt something should
be done. In 1934, the British postmaster general declared that all
first-class Royal Mail sent overseas was to travel by air, effectively
establishing a subsidy for the development of intercontinental air
transportation. In response, British Imperial Airways announced a
competition for an order for 28 flying boats, each weighing 16.4 tonnes
(18 tons) and having a range of 1,130 kilometres (700 miles) with a
capacity of 24 passengers.
The contract went almost directly to Short Brothers of Rochester in
England. Short had long experience in building flying boats for the
military and for Imperial Airways. However, none of these flying boats
were in the class of size and sophistication requested by Imperial
Airways. The business opportunity was too great to pass up despite the
risk, and so Oswald Short, head of the company, began a crash program to
design a flying boat far beyond anything the company had ever built.
The head of the design team was Arthur (later Sir Arthur) Gouge. The
design he produced, the Short "S.23", was a clean and elegant aircraft,
with a wingspan of 35 meters (114 feet), a length of 27 meters (88 feet),
an empty weight of 10.9 tonnes (24,000 pounds), and a loaded weight of
18.4 tonnes (40,500 pounds). The S.23 was powered by four Bristol Pegasus
air-cooled radial engines, each providing 686 kW (920 HP). Cruise speed
was 265 KPH (165 MPH), and maximum speed was 320 KPH (200 MPH). The S.23
featured a new hull design and a new flap scheme to reduce landing speed
and run. The big flying boat had two decks: an upper deck for the flight
crew and mail, and a lower deck with luxury passenger accommodations.
The first S.23, named "CANOPUS", performed its first flight on 4 July
1936. The S.23s were the first of a series of Shorts flying boats for
commercial service, collectively known as the "Empire" boats. A total of
41 S.23s were built, all with names beginning with the letter "C", and so
they were also referred to as the "C-class" boats.
While the S.23 was a great step forward for Short Brothers, it was still
not quite the equal of the big Sikorsky and Boeing Clippers that were
opening up worldwide commercial routes. The S.23 was relatively overweight
and restricted in range and payload. It still performed reliable service
in connecting Great Britain with the distant regions of the British
Empire: South Africa, India, Singapore, Australia.
* The limited range of the C-class boats meant that they could not operate
on the high-profile transatlantic route, which was an embarrassment. In
1937, the second and third C-class boats, the CALEDONIA and CAMBRIA, were
stripped down and given additional fuel tanks to make the transatlantic
run, though their payload was minimal.
The British were so desperate to stay in the race for transatlantic
commercial flight that they then came up with an extraordinary scheme, in
which a beefed-up variant of the S.23 carried a smaller four-engine
floatplane, the "S.20". A single example was built, with the carrier
aircraft named MAIA and the piggyback S.20 named MERCURY, with flight
tests in 1937 leading to a mid-air launch of MERCURY in 1938.
The MAIA-MERCURY scheme amounted to little more than a stopgap and a
publicity stunt while Short Brothers worked on a better solution. In 1938,
they delivered the first of an improved C-class boat, the "S.30", with
Pegasus 22 engines providing 753 kW (1,010 HP) each.
Eight S.30s were built, with four configured for mid-flight refuelling
from Handley-Page Harrow cargo aircraft. Limited transatlantic operations
were conducted in coordination with Harrow tankers operating out of
Ireland and Newfoundland, until World War II intervened and put a stop to
the flights. Another C-class variant, the "S.33", never got into
However, three of the bigger and better "S.26" G-class boats were built,
with the first, the GOLDEN HIND, delivered in September 1939. The S.26
boats were powered by four Bristol Hercules engines, each providing 1,030
kW (1,380 HP). The G-class boats had a loaded weight of 34 tonnes (75,000
pounds), a range of more than 4,800 kilometres (3,000 miles), and were
intended for transatlantic mail shipment.
* During World War II, the Empire boats were pressed into military
service. Four S.30s were used for ocean patrol; they were fitted with
later-mark Pegasus engines, and armed with twin Boulton-Paul turrets --
each with four 7.7 millimetre (0.303 calibre) machine guns -- plus racks
for external stores. The three S.26 G-class boats had similar combat fit,
but featured three Boulton-Paul quad turrets. Only one of these seven, an
S.26, survived military service. It returned to commercial operation until
scrapped in 1954.
The Empire boats would be little more than a footnote in aviation history
except for the fact that this family of aircraft included a military type,
the "S.25" or "Sunderland", which would become one of the most famous
flying boats ever built.
S.25 / Sunderland Mk 1
the first S.23 was under development, the British military was taking
actions that would result in a purely military version of the big Shorts
flying boats. A 1933 British Air Ministry requirement designated "R.2/33"
called for a next-generation flying boat for ocean reconnaissance. The new
flying boat was to have four engines, but could be either a monoplane or
The R.2/33 specification occurred roughly in parallel with the Imperial
Airways requirement, and while Shorts worked on the S.23, the company also
worked on a response to the Air Ministry's need at a lower priority. The
military flying boat variant was designated S.25, and the design was
submitted to the Air Ministry in 1934. Sanders-Roe also designed a flying
boat designated the "A.33" for the R.2/33 competition.
The military ordered prototypes of the S.25 and S.33 for evaluation. The
first S.25, now named the "Sunderland Mark I", flew from the River Medway
on 16 October 1937.
The prototype had been designed with the expectation that a 37 millimetre
cannon would be mounted in the nose, but this weapon was deleted. The
armament change meant a shift in centre of gravity that led to a modified
wing and some other changes. The prototype was also fitted with Bristol
Pegasus X engines, each providing 709 kW (950 HP), since the planned
Pegasus XXII engines with 753 kW (1,010 HP) each were not available at the
The prototype went back to the shop for modifications after its initial
flights. It flew again with a new wing and Pegasus XXII engines on 7 March
1938. Official enthusiasm for the type was so great that in March 1936,
even before the first flight of the Sunderland prototype, the Air Ministry
had ordered 21 production examples of the new flying boat.
Delivery of the SaRo A.33 was delayed and did not fly until October 1938.
The aircraft was written off after it suffered a structural failure during
high-speed taxi trials, and no other prototypes were ever built.
* The Sunderland Mark I had much in common with the S.23, but had a
different and deeper hull. The installation of nose and tail turrets gave
the Sunderland a considerably different appearance from the Empire flying
The FN.11 nose turret mounted a single 7.7 millimetre machine gun, and
could be winched back from the nose to allow entrance to and exit from the
aircraft through a forward hatch when the flying boat was docked. The new
FN.13 tail turret mounted four 7.7 millimetre guns. A single hand-held 7.7
millimetre gun was mounted on either side of the fuselage, above and
behind the wing, firing through an oval port with a fairing and sliding
wing, as mentioned, had been modified after the first flight of the S.25
prototype, being swept back 4.25 degrees to compensate for the heavy tail
turret, and as a result the Sunderland's engines and wing floats were
canted slightly out from the aircraft's centreline. Although the wing
loading was much higher than that of any previous RAF flying boat, the new
flap system kept the takeoff run to reasonable length.
Sunderland in Combat
The thick wings carried the four Pegasus XXII engines and accommodated six
drum fuel tanks with a total capacity of 9,200 litres (2,430 US gallons).
Four more fuel tanks would later be added behind the rear wing spar to
give a total fuel capacity of 11,602 litres (3,037 US gallons). Offensive
armament load was 900 kilograms (2,000 pounds) of bombs, mines, or
(eventually) depth charges. Ordnance was carried inside the fuselage and
winched out under the wings through doors on each side of the fuselage.
The aircraft was of metal construction, except for most of the control
surfaces, which had metal frames and were covered by fabric. As with the
S.23, the Sunderland's fuselage contained two decks. Of course, the
Sunderland was not a luxury liner like the Empire boats, but it had a
number of niceties useful for keeping its crew of seven comfortable during
long and exhausting ocean patrols and operations from remote locations,
such as six bunks and a galley with a stove. The number of crew would
increase in later marks to eleven or more.
Although the Sunderland was not an amphibian, beaching gear allowed it to
be pulled up on land. Two-wheeled struts could be attached to either side
of the fuselage, while a small two-wheel trolley with a tow bar could be
fitted under the rear of the hull.
The RAF received its first Sunderland Mark I in June 1938, when the second
production aircraft was flown to Singapore. By the outbreak of war in
Europe in September 1939, the RAF Coastal Command was operating 40
Although British antisubmarine efforts were disorganized and ineffectual
at first, Sunderlands quickly proved useful in the rescue of crews of
torpedoed ships. On 21 September 1939, two Sunderlands rescued the entire
34 man crew of the torpedoed merchantman KENSINGTON COURT from the North
Sea. As British antisubmarine measures improved, the Sunderland began to
show its claws as well. A Royal Australian Air Force (RAAF) Sunderland
performed the type's first unassisted kill of a U-boat on 17 July 1940.
As the British honed their combat skills, the Sunderland Mark I received
various improvements to make it more effective. The nose turret was
upgraded to two millimetre guns instead of one. New propellers, and
pneumatic rubber wing de-icing boots, were fitted as well.
Although the 7.7 millimetre guns lacked range and hitting power and the
British would in time understand the need for more formidable weapons, the
Sunderland had a fair number of them, and it was a well-built machine that
was hard to destroy. On 3 April 1940, a Sunderland operating off Norway
was attacked by six German Junkers Ju-88 fighters, and managed to shoot
one down, damage another enough to send it off to a forced landing, and
drive off the rest. The Germans were supposed to have nicknamed the
Sunderland the "Fliegende Stachelsweine (Flying Porcupine)".
Sunderlands also proved themselves in the Mediterranean theatre. They
performed valiantly in performing evacuations during the German seizure of
Crete, and one performed a reconnaissance mission to observe the Italian
fleet at anchor Taranto before the famous Royal Navy Fleet Air Arm's
torpedo attack on 11 November 1940.
Beginning in October 1941 Sunderlands were fitted with "ASV (Anti-Surface
Vessel)" Mark II radar. This was a primitive low-frequency radar system
operating at a wavelength of 1.5 meters, featuring a row of four prominent
Yagi "stickleback" aerials on top of the rear fuselage, two rows of four
smaller aerials on either side of the fuselage beneath the stickleback
antennas, and a single receiving aerial mounted under each wing outboard
of the float and angled outward.
A total of 75 Sunderland Mark Is were built, produced at Shorts factories
at Rochester in England and Belfast in Northern Ireland, as well as 15 of
the 75 built by Blackburn at Dumbarton.
Sunderland Mark II / III / IIIA
In August 1941, production moved to the "Sunderland Mark II", which
featured Pegasus XVIII engines with two-speed superchargers and provided
794 kW (1,065 HP) each. The tail turret was changed to an FN.4A turret
that retained the four 7.7 millimetre guns of its predecessor, but
provided twice the ammunition capacity, with a total of 1,000 rounds per
Late production Mark IIs
also had an FN.7 dorsal turret, mounted offset to the right just behind
the wings, and fitted with twin 7.7 millimetre machine guns, replacing the
hand-held guns mounted in the fuselage ports.
Only 43 Mark IIs were
built, with 5 of the 43 manufactured by Blackburn. Production quickly went
on in December 1941 to the Sunderland Mark III. This variant featured a
revised hull configuration, tested on a Mark I the previous June, that
provided improved seaworthiness, which had suffered as the weight of the
Sunderland increased with new marks and field changes. In earlier
Sunderlands, the hull "step" that that allowed a flying boat to "unstick"
from the surface of the sea was abrupt, but in the "Sunderland Mark III"
it was a smooth curve.
The Mark III would turn
out to be the definitive Sunderland variant, with a total of 461 built.
Most were built by Short Brothers at Rochester, Belfast, and a new plant
at Lake Windemere, but 170 of the total were built by Blackburn. The
Sunderland Mark III would prove to be one of the RAF Coastal Command's
major weapons against the U-boats, along with the Consolidated PBY
Sunderland Mark III:
feet 9 inches
feet 3 inches
feet 2 inches
MPH / 180 KT
MI / 2,610 NMI
New weapons made the
flying boats more deadly in combat. The ineffectual anti-submarine bombs,
which in some cases were known to bounce up and hit their launch aircraft,
were replaced by early 1943 by much more effective Torpex depth charges
that would sink to a shallow depth and then explode. This eliminated the
problem of bounce-back, and the shock wave propagating through the water
had greater effect.
Although the bright Leigh searchlight was rarely fitted to Sunderlands,
ASV Mark 2 radar allowed the flying boats to effectively target U-boats
operating on the surface, until the German submarines began to carry a
radar warning system known as "Metox", also known as the Cross of Biscay
due to the appearance of its receiving antenna. Kills fell off drastically
until ASV Mark III radar was introduced in early 1943. ASV Mark III
operated in the centimetric band and used antennas mounted in blisters
under the wings outboard of the floats, instead of the cluttered
stickleback aerials. Sunderland Mark IIIs fitted with ASV Mark III were
designated "Sunderland Mark IIIAs".
Centimetric radar was invisible to Metox and completely baffled the
Germans at first. Admiral Doenitz, commander of the German U-boat force,
suspected at first that the British were being informed of submarine
movements by spies, and there is a story that a Britisher prisoner, a a
smooth liar, confused the Germans by saying the aircraft were homing in on
the Cross of Biscay.
In any case, the Germans responded by fitting U-boats with one or two 37
millimetre and twin quad 20 millimetre flak guns to shoot it out with the
attackers. While Sunderlands could suppress flak to an extent by hosing
down the U-boat with their nose-turret guns, the U-boats had the edge by
far in range and hitting power, although many Sunderlands were field
fitted with four fixed 7.7 millimetre machine guns to improve their
ability to hit back. One Sunderland that was mortally wounded by U-boat
flak is said to have deliberately crashed into the submarine.
Along with the forward-firing guns, Sunderlands were often field-fitted
with hand-held 7.7 millimetre and later 12.7 millimetre (0.50 calibre)
machine guns on either side of the fuselage.
The Sunderland was under the command of Australian Flight Lieutenant Colin
Walker. The crew was on an antisubmarine patrol and also searching for
remains of an airliner that had left Gibraltar the day before, to be shot
down over the Bay of Biscay with the loss of all crew and passengers,
including British film star Leslie Howard, known for his starring role in
THE SCARLET PIMPERNEL and supporting work in GONE WITH THE WIND.
In the late afternoon, one of the crew spotted the eight Ju-88s. Bombs and
depth charges were dumped while Walker redlined the engines. Two Ju-88s
made passes at the flying boat, one from each side, scoring hits while the
Sunderland went through wild "corkscrew" evasive manoeuvres. The fighters
managed to knock out one engine.
On the third pass of the fighters, the top-turret gunner managed to shoot
one down. Another Ju-88 disabled the tail turret, but the next fighter
that made a pass was bracketed by the top and nose turrets and shot down
as well. Still another fighter attacked, smashing the Sunderland's radio
gear, wounding most of the crew in varying degrees and mortally wounding
one of the side gunners. A Ju-88 tried to attack from the rear, but the
tail turret gunner had managed to regain some control over the turret and
shot down the German fighter.
The surviving fighters pressed home their attacks, despite the losses. The
nose gunner chewed up one of the fighters and set one of its engines on
fire. Two more of the attackers were thoroughly shot up, and the other two
finally decided they'd had enough and departed. The Sunderland was a
wreck. The crew threw everything they could overboard and nursed the
aircraft back to the Cornish coast, where Walker managed to land and beach
it. The crew waded ashore, carrying their dead comrade, while the surf
broke up the Sunderland.
Walker received the Distinguished Service Order, and several of the other
crew received medals as well. Walker went on to a ground job, while the
rest of the crew were given a new Sunderland. That Sunderland and its crew
disappeared without a trace over the Bay of Biscay two months later, after
reporting by radio that they were under attack by six Ju-88s.
What remains puzzling about this incident is that some sources claim
German records show no losses of Ju-88s over the Bay of Biscay that day.
It is of course easy to believe that the crew of the Sunderland might have
have exaggerated the number of enemy fighters shot down, but it is hard to
believe that the whole thing was a complete fabrication. The exact details
of what happened that day are almost certainly now lost to history.
* Eventually, Sunderlands would claim the destruction of a total of 28
U-boats, and assist in the sinking of seven more. Along with the Atlantic
missions, Sunderlands were also used to patrol the Indian Ocean, and were
used in Burma to re-supply British "Chindit" commando camps behind
Sunderland MARK V
Although a "Sunderland Mark IV" was developed, it proved to be different
enough from the Sunderland line to be given a different name, and did not
see combat in any case. It is discussed in the following section.
The next actual production version was the "Sunderland Mark V", which
evolved out of crew concerns over the lack of power of the Pegasus
engines. The weight creep that afflicted the Sunderland resulted in
running the Pegasus engines on combat power as a normal procedure, and the
overburdened engines had to be replaced on a regular basis.
Australian Sunderland crews suggested that the Pegasus engines be replaced
by Pratt & Whitney R-1830-9OB Twin Wasp two-row radial engines. The
14-cylinder Twin Wasps provided 895 kW (1,200 HP) each and were in use on
RAF Catalinas and Dakotas, making logistics and maintenance
straightforward. Two Mark IIIs were taken off the production lines in
early 1944 and fitted with the American engines.
Trials were conducted in early 1944, and the conversion proved all that
was expected. The new engines provided greater performance with no real
penalty in range. In particular, a Twin Wasp Sunderland could stay
airborne if two engines were knocked out on the same wing, while a
standard Mark III would steadily lose altitude.
Production was converted to the Twin Wasp Sunderland, which was designated
the Sunderland Mark V, and the first Mark V reached operational units in
February 1945. Defensive armament fits were similar to those of the Mark
III, but the Mark V was equipped with new centimetric ASV Mark VIC radar,
which had been fitted to some of the last production Mark IIIs as well.
* 155 Sunderland Mark Vs were built, and another 33 Mark IIIs were
converted to Mark V specification. With the end of the war, large
contracts for the Sunderland were cancelled, and the last of these great
flying boats was delivered in June 1946, with total production of 749
aircraft. At the time, a number of new Sunderlands built at Belfast were
simply taken out to sea and scuttled, since there was nothing else to do
with them. However, despite this indignity, there was plenty of life left
in the Sunderland.
passenger conversions / Seaford / postwar use
Sunderland evolved from a family of commercial flying boats, and would
find itself used in that capacity as well. In late 1942, British Overseas
Airways Corporation (BOAC) obtained six Sunderland Mark IIIs and stripped
them down for service as mail carriers, with primitive accommodations for
seven passengers. They were used for mail service to Nigeria and India.
BOAC obtained more Mark IIIs and gradually came up with better
accommodations for 24 passengers, including sleeping berths for 16. These
conversions were given the name "Hythe", and BOAC would have 29 Hythes by
the end of the war.
Another civilian conversion of the Sunderland was the post-war "Sandringham".
The "Sandringham Mark I" used Pegasus engines while the "Sandringham Mark
II" used Twin Wasp engines. Apparently most or all of the Sandringhams
were modified from existing Sunderlands, but details of the Sandringham
* The Sunderland Mark IV, mentioned in the previous section, was an
outgrowth of a 1942 Air Ministry specification, "R.8/42", for a generally
improved Sunderland with more powerful Hercules engines, better defensive
armament, and other enhancements. The new Sunderland was intended for
service in the Pacific.
Relative to the Mark III, the Mark IV had a stronger wing, bigger
tailplanes, and a longer fuselage with some changes in form. The armament
was greatly improved, consisting of two fixed forward-firing 12.7
millimetre guns in the nose, a Brockhouse nose turret with twin 12.7
millimetre machine guns, twin 20 millimetre Hispano cannon mounted in a
B-17 dorsal turret, twin 12.7 millimetre guns in a Martin tail turret, and
a 12.7 millimetre machine gun in a hand-held position on each side of the
The changes were so substantial that the new aircraft was redesignated the
"S.45 Seaford". Two prototypes and thirty production examples were
ordered, and the first prototype flew in April 1945, well after the
introduction of the Sunderland IV and too late to see combat.
The prototypes were powered by Hercules XVII radial engines with 1,253 kW
(1,680 HP), but production aircraft used Hercules XIXs with 1,283 kW
(1,720 HP). Only eight production Seafords were completed and never got
beyond operational trials with the RAF.
The second production Seaford was loaned to BOAC in 1946 for evaluation as
a civil airliner. BOAC liked the aircraft, and so 12 Seafords then being
laid down were completed as "Solent Mark 2". Most of the RAF Seafords were
rebuilt as "Solent Mark 3s".
* The Sunderland served on after World War II. During the Berlin Airlift
in 1948, Sunderlands shipped food into the British Sector of the besieged
city, landing on Lake Havel. They also engaged in maritime patrols over
the Yellow Sea in the Korean War, and somewhat oddly served in a
counterinsurgency role during the British war against Malayan guerrillas.
Sunderlands helped supply a British Greenland expedition from 1951 through
1954. In 1954, the RAF began to phase out its last Sunderland squadrons,
with the type fading out of service through the rest of the decade.
However, 19 Sunderlands had been reconditioned in Belfast in 1951 for the
French naval air arm, the Aeronavale, and 16 more were reconditioned in
England for the Royal New Zealand Air Force (RNZAF). The Aeronavale
Sunderlands operated until 1960, and the RNZAF Sunderlands served until
the mid-1960s, when they were replaced by the Lockheed P-3C Orion maritime
patrol aircraft. A number of Sunderlands were operated by the South
African Air Force until 1958, when they were replaced by Avro Shackletons.
Several Sunderlands survive on static display, and at least one
Sandringham, owned by well-known American warbird collector Kermit Weeks,
is still flying. Efforts are under way to get more Sunderlands back into