Seaplane Lighter H21

In action
Seaplane Lighter H21

Before ships and techniques were developed to allow aircraft to take off and land at sea, the Royal Naval Air Service used seaplane lighters to support seaborne aircraft operations. These lighters were towed behind warships at speeds of up to 30 knots, and were used to operate both flying boats (including the Curtiss H12) and fighter aircraft (such as the Sopwith Camel). For flying boats the aircraft was loaded onto a hinged trolley ashore and embarked from a slipway to rails on the vessel and then lashed down.

SEAPLANE LIGHTER H21 was designed by Thornycroft, in collaboration a serving Naval Officer, with for a specific purpose in mind: to extend the range of the successful Felixstowe flying boats by loading them onto a lighter which could then be towed at high speed to a launch point closer to the German coast.

SEAPLANE LIGHTER H21 was built by the Royal Engineers in 1918 at Richborough, a port constructed by them near Sandwich, Kent, to re-supply the armies fighting in France during the First World War. Her structure, particularly the combination of a fast planing hull shape with a semi-submersible docking bay, is unique, with some of the equipment used for flooding and blowing her ballast tanks still in place. SEAPLANE LIGHTER H 21 is the only substantially complete seaplane lighter in the country.

The design of SEAPLANE LIGHTER H21 included an impressive high speed bow. Fifty seaplane lighters were ordered, but building ceased at SEAPLANE LIGHTER H32 due to the ending of the War. The first 25 had galvanised steel hulls, but the remainder had steel hulls in order to reduce costs.

At sea, seaplane lighters could be flooded in order to lower them in the water to enable the embarkation and disembarkation of an aircraft. The water would then be pumped out using onboard compressed air bottles to restore buoyancy. SEAPLANE LIGHTERS were supplied with sufficient compressed air to perform two complete operations.

Several of the seaplane lighters built were used to carry and launch a single high performance fighter aircraft as a defence against the high flying Zeppelins used by the German Navy for reconnaissance over the North Sea in the First World War. One successful attack was made by a Lt Culley and resulted in the destruction of a Zeppelin in 1918.

For fighter aircraft the lighters were modified further in 1918 by being fitted with a temporary elevated inclined wooden deck from which the aircraft took off. There was no way of landing on the seaplane lighters, so the aircraft either had to land ashore or ditch alongside the vessel.

As built in 1918 SEAPLANE LIGHTER H21’s shape was ideal for her unique role. A single much larger towable Flying Boat Dock was built for the RAF in 1922 to enable maintenance work to be carried out in some of the remote bases used by flying boats in the inter war years. There was no direct aviation development of the towed lighters, except that similar emergency measures were introduced in the Second World War to counter the threat posed by the Focke Wulf Condors operating over the Atlantic in 1941.

SEAPLANE LIGHTER H21 was subsequently operated by Thornycroft as a cargo barge on the River Thames from Platts-Eyott Island, Sunbury from 1932 to 1966, when she had the registration number T3.

Where is she now?

SEAPLANE LIGHTER No.H21 is at the Fleet Air Arm Museum in Yeovilton, Somerset, where she is in dry berth. She is undergoing restoration work in stages, with careful examination taking place of every area of the craft from its paintwork remnants to all of its hardware and fittings. Plans are in hand to build a new demonstration hall to display her, together with a Sopwith Camel, on an elevated deck.


1997 The Times Historic boat that gave pilots a sinking feeling.

1998 Survivors Register - World Ship Society British Armed Forces Small Craft Historical Society.