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Coastal Motor Boat

During World War I, the British Royal Navy needed a small, inconspicuous combat vessel with the ability to sneak up on enemy German ships anchored in their bases. Three junior officers of the Harwich destroyer proposed a Coastal Motor Boat (CMB) be designed light enough to skim the water's surface undetected without activating the minefields below while carrying small torpedoes to attack the ships, and then make a stealth retreat. Their superior officers approved the idea and asked for plans and specs for a CMB in the summer of 1915.

Coastal Motor Boat Design

Speed and weight were major considerations in the CMB boat design. The vessels had to be lightweight so they could travel at a minimum speed of 30 knots when fully loaded with fuel, soldiers and weapons. Standard maritime internal combustion engines were hard to come by, so the designers used modified aircraft engines made by Sunbeam and Napier. To be effective, the boats had to be armed with torpedoes, depth charges and light machine guns. Thornycraft, a company with a good reputation for building small, fast watercraft, was contracted to build the original CMBs.

Early CMBs

The first CMBs were 40-foot models that quickly proved too small to do the job since they could only hold one 18-inch torpedo. The single torpedo was not launchable from a conventional torpedo tube because the boat could not withstand the tube weight. Instead, it was fired through a trough that faced backwards, which made the torpedo hit the water tail first and then be activated and realigned to aim for its target through manipulating a trip wire. This required the pilot of the CMB to shoot the torpedo and then make a sharp U-turn and hasty departure to avoid being hit by its own explosive.

CMB Evolution

After building twelve of the original CMBs, Thornycroft designed and built an improved model in 1917. The upgraded CMB was 55 feet long and sturdy enough to carry two torpedoes instead of one. It also offered the option of mixing weaponry to include one torpedo coupled with four depth charges.

Most Famous CMB Conflict

Although CMBs, which eventually became known as CMB4s, were instrumental in the 1917 Zebrugge conflict, the most famous battle that involved the small boat was in the Baltic in 1919. Lieutenant Augustus Agar spotted the enemy cruiser Oleg near the Tolboukin Lighthouse at Petrograd. He and his two officers navigated their CMB4 into nearby waters and fired. The Oleg sank in twelve minutes.

Agar earned the Victoria Cross for his actions and his officers were also decorated for their victory. The win was considered an essential contribution to the freedom of the three Baltic states Estonia, Latvia and Lithuania.

CMB4 Retirement

After the sinking of the Oleg, the CMB4 was retired and put on display at the Imperial War Museum in Britain. For a short period, Thornycroft borrowed the boat for a final exhibition of their boats before they shut down operations, at which time the CMB4 returned to rest at the Imperial War Museum.

There have been no other Coastal Motor Boats built since the ones involved in these historic conflicts although they remain popular among model makers. There is also a group called the Coastal Motorboat Heritage Trust that is dedicated to preserving the history of the boat and stages reenactments of CMB conflicts.