We've used horsepower since the dawn of time. While the horse fell from favor a couple hundred years ago we still use the term when talking about the power output of an engine or motor.
An average work horse achieves a maximum of just under 15 hp at a sprint, while a human at peak production achieves around five horsepower. Interestingly, a horse exerting 1 horsepower can lift 330 pounds of coal 100 feet per minute, 33 pounds of coal 1000 feet in one minute, or 1000 pounds 33 feet in one minute.
I did some volunteer work for the Fort Wayne Railroad Society many years ago getting the 765 ready for the rails. She's a huge 2-8-4 Berkshire Locomotive built by Lima Locomotive Works in 1944 for the Nickel Plate Road. Her two massive pistons weigh in at 920 pounds and produce 2754 horsepower each! If you've ever been around one of these engines and heard the sharp report from the stack when she's pulling a heavy load, it's a sound you'll never forget.
From the early days of power boating people wanted to go fast. In 1910 two brothers near Detroit, Michigan formed the Smith Ryan Boat Company. They focused on building fast, economical runabouts for the masses and sleek racing boats for the wealthy. Garfield Wood, a boat racer, bought the company in 1916 and it was ran by the brothers until 1922. The brothers, in 1924, started another boat building company in Algonac, Michigan named Chris-Craft. Garfield Wood then changed the name from Smith Ryan to Garwood and focused on racing.
Garfield set a speed record of 74.8 MPH on the Detroit River in 1920 using his new boat, Miss America. She was a wood racing boat powered by two Liberty V-12's producing 400 horsepower each. In 1931 he broke the 100mph mark at 102.25. His quest for speed cumulated with Miss America X. With four Packard 1800 horsepower V12's he hit 124.9 mph!
With all the horsepower available I can understand why maritime safety became a concern. Anyone who has been on a plane and hit someone's wake or a wave at and odd angle knows how quickly the boat will pitch and roll. There are several YouTube videos of people being thrown from their boats when this happens. For many years boat manufactures have installed Engine Cut-Off Switches(ECOS) as a safety measure, should something happen to the operator of the vessel. While use of the switch has been only recommended, It is now a law.
On April 1, 2021 a new federal law goes into effect that requires the operator of a boat with an installed Engine Cut-Off Switch (ECOS) to use the ECOS link. The link is usually a coiled bungee cord lanyard clipped onto the operator's person, Personal Floatation Device (PFD) or clothing and the other end attached to the cut-off switch, but there are plenty of variations on the market, including electronic wireless devices. The law applies on all "Navigable Waters of the US".
As of April 1st, 2021 the use of an Engine Cut-Off Switch will be mandatory on recreational vessels:
Don't become a statistic, Wear your Lifejacket and your Engine Cut-Off Switch lanyard.