The Scooter

–1916’s coastal motor boat “AT FORTY MILES AN HOUR” was the Auckland Star’s headline on 17 May 1924, above a photograph of an amazing motor launch at speed on the Waitemata – along with a startled seagull.

The caption told much of the story. Admiral Field’s dispatch boat, or C.M.B. No. 123, as she was known to the officers and men of the Special Service Squadron, was racing at top speed off the Prince’s wharf.

A single step hydroplane, she was built for Admiralty service by Messrs. John I. Thornycroft and Co. Ltd, of Southampton. At 40-feet long with a beam of eight feet six inches, she was driven by a 250hp, 12-cylinder engine.

She was capable of a sustained speed of 35 knots, equal to 40 land miles. The speedy little craft could accommodate 14 passengers in a comfortable, covered cockpit. She consumed 15-18 gallons of petrol an hour when going at full speed.

This particular vessel was built in 1917 and did constant work on the Belgium coast for two years during the war, when she was fitted with a torpedo tube, four Lewis guns and two depth charges. At top speed she could make the run to the Thames in exactly one hour, or to Kawau in 45 minutes.

The Royal Navy’s Special Service Squadron was in Auckland on its Empire Tour, a circumnavigation to demonstrate Britain’s naval might and to thank the Empire countries for their contribution to the Great War.

The Squadron consisted of the battle cruisers Hood and Repulse and the light cruisers Delhi, Danae, Dragon, Dauntless and Dunedin – plus the Australian cruiser Adelaide. Dunedin arrived to take the place of Chatham as flagship of the New Zealand Division, Royal Navy. CMB 123 was carried on the davits of one of the ships, a bon-bon for the Colonials.

But what were the origins of this dramatic craft doing 40mph on the Waitemata?

In the early summer of 1915 three young Naval Officers from

the Harwich Destroyer Force conceived the idea of a small, extremely fast motor boat that could be carried in the davits of a light cruiser and was capable of firing a single, self-propelled Whitehead torpedo.

Such a boat could be dropped as near as possible to the enemy coast to carry on at high speed through or over German minefields to attack shipping and shore targets. It could also sow mines in the ports of occupied Belgium where the enemy had constructed havens for its large submarine fleet then harrying British shipping in the Atlantic.

The Navy gave the officers authority to approach the contractors and engineers J.I. Thornycroft & Co Ltd with their plan. The initial concept was for a planing hydroplane that would discharge its torpedo over the stern without having to turn for the attack.

Thornycroft was not only a major contractor for steel warships to the Royal Navy but had also been at the forefront of experimental racing craft. So John Thornycroft’s design for the eight-cylinder 110hp 26-footers Miranda IV and Zigarella of 1910 provided a good a prototype. In an interview that year he said:

“As to the Miranda type, a few notes may be interesting. The main idea is to combine the boat and hydroplane, formed in a practical way, and the Miranda III did this in a way which encourages me to believe in the evolution of this class of vessel.

“She has an overall length of 22 feet and a beam of 6 feet 10 inches, which gives plenty of engine space. She tapers away greatly at the stern. Built of single skin mahogany, she has an almost flat bottom, which begins to run up astern about 3 feet aft of the centre.

“At this point there is a small ‘step’, little more than a notch; in fact, it seems, however, to break the adhesive water from the after hull sufficiently well. Under the bows is a vertical web carrying a spade-shaped plate which just touches the surface at skimming speed, the rudder being just abaft of this.

“From one-third to half the boat’s length forward is clear of the water at speed, as of course is the tail, aft of the main plane. This plane at speed gives a supporting surface of some 6 feet fore and aft by say 6 feet transversely, 36 square feet in all, while the fore ‘foot’ has about 10 square feet.

“With an 18-inch propeller the boat, loaded with four passengers, the total weight being about 1¼ tons, has planed successfully, and I think would do even with a considerably larger weight. The foot has a useful action in steadying the boat in choppy water and preventing the tendency of the hydroplane towards an erratic ‘wriggling’ motion.

“I hope to see the type ‘skim’ down to as low as 16 knots before long; at present 20 knots is about the figure. All this field is so new that there is a lot to be learned, and the speeds shown by the hydroplanes recently defy all the usual calculations. I think some compromise between hydroplane and boat will be the surviving form and there is room for indefinite improvement in this direction.”

By 1915, too, light, high-powered aero engines were being built in quantity. The new design, highly secret and codenamed ‘Vedette-skimmer’, was a wooden, unarmoured motor boat that could work up to a speed of 35 knots in a matter of minutes and discharge its torpedo over the stern while steering directly for its objective.

The first boat produced came up to the expectations of the specification. The Admiralty ordered 12 boats to be built as soon as possible and established a base on the South Eastern Railway Company’s pier at Queenborough, Kent. The first three boats were ready for service in April 1916. To maintain utmost secrecy during the training period the crews took the boats out only at night.

Each boat had a crew of two RNVR officers and a mechanic (ERA) who spent several weeks at Thornycroft’s Basingstoke works to see the engines and boats built and the trials run. Because of his exceptional knowledge of the Belgian, Dutch and German coast, Irishman Lt. Erskine Childers, RNVR, joined the flotilla as Navigator. His novel The Riddle of the Sands was the sensation of 1903 (and is still a wonderful read).

The new craft received the official designation ‘Coastal Motor Boats’ which gave nothing away, but got the term ‘Scooters’ from their crews. And scoot they did. In 1916 planing hulls were entirely novel outside the ultra-expensive speedboat racing waters of the Solent, Monaco and North America.

This first batch of CMBS were 40ft LOA with a beam of eight feet and a draught of only two feet. The hull was a double skin of mahogany, the frames and knees were of American elm, and stringers were of oregon.

The hull was designed to be resilient to withstand high-speed slamming in a choppy sea. At planing speeds the hydroplane step lifted the centre part of the hull to reduce resistance. A spiral propeller was fitted for jumping booms. The torpedo sight, which consisted of a metal triangle somewhat resembling a sextant in a horizontal plane, was fitted at the forepart of the cockpit.

A special design of lifting gear was fitted to the forward engine bearer and to the torpedo trough aft, for slinging under davits. However, all the action to come along the Belgian coast was from their own bases at Dover and Dunkirk. A fuel tank with a capacity of 100 gallons of aviation spirit was installed in the cockpit amidships and beneath the Commanding Officer’s seat.

A single 18-inch diameter Whitehead torpedo was carried in a cradle in a trough aft on the centreline, and was launched by a bell-head ram which was impelled by a cordite charge exploded in a steel bottle. 1,500 grains of No. 37 cordite provided the power, giving a pressure in the explosion bottle of four tons, but reduced to 1,000 lbs pressure on reaching the ram.

The torpedo firing lever was conveniently to the right hand of the Commanding Officer. The torpedo weighed three quarters of a ton and when being launched its initial speed was 13 knots. The whole rig looked scarily Heath Robinson but it worked, although the CMB had to sheer away as soon as the torpedo was launched!

In place of a torpedo, depth charges or mines could be launched from the after trough. There were two twin Lewis guns on portable swivel mountings fore and aft in the cockpit.

But the heart of the boat and its bigger descendants was its aircraft engine, and a variety was used, all V12s of 250hp-350hp, marinised and branded by Thornycroft but built by Sunbeam, Green and Napier.

In the next article I’ll deal with the CMBS’ triumphs and defeats in the Channel, the Baltic and the Caspian Sea and their legacy as the grandfathers of all planing torpedo boats.

This article was sparked by the image of CMB123 appearing on the Waitemata Woodys website (https://waitematawoodys. com/2015/09/18/mystery-launch-18-09-2015/) and my subsequent visit to Imperial War Museum, Duxford. Many of these images were supplied by and are copyright to the Imperial War Museum. B