mid to late fifties. Initially rated at 16 horsepower, this engine was soon beefed up to 20. There was also an "H" version, which produced more power but carried no rating. "H" version Mercurys were mostly used for racing. In 1949, the company also introduced its first large outboard, the approximately 40 cubic inch, 4 cylinder in line, "Thunderbolt". This engine was rated at 25 horsepower, but actually put out around 28. There was soon an "H" version designed for high rpm use. This version put out nearly 40 horsepower, while being rated at "25+". The Thunderbolt engine benefited from upgrades and updates, eventually becoming the Mark 40, Mark 50, and Mark 55 engines of the mid and late fifties. All of these were rated quite accurately at 40 horsepower. The Mark 55H (high speed) developed more, but carried no rating. Due to a rather large gap between the Mark 25 (20 hp) and the Mark 55 (40 hp), Mercury brought out the 4 in line, approximately 30 cubic inch "Mark 30" (30 horsepower) engine for 1956, and produced versions of its "TurboFour" engine for several years. It, too, was available as an "H" version, producing much extra horsepower for racing and sporting use. Why these Mark engines (and other Mark engines) carried number names which were not horsepower ratings, not displacement numbers, and not consistent from one motor to another, is a real mystery. Why was the 20 hp Mercury a Mark 25, and the 40 hp Mercury a Mark 55, while the 30 hp job was the Mark 30? No one really knows. Of course, there were also smaller Mercs during both the K era and the Mark era, such as the "Super 5" and later, such engines as the "Mark 6" and the "Mark 10". 1950s In 1957 Kiekhaefer started testing at a Florida lake he called "Lake X" to keep the location secret. Later that year the Kiekhaefer Mercury company designed a new inline 6-cylinder, 60 cubic inch, engine named "Mark 75". Like its 2- and 4-cylinder brethren, the Mark 75 featured internal reed valves. The reed valve blocks served as intermediate main bearings. Because the valves occupied space already necessary for the main bearings instead of mounting to an extension of the crankcase, crankcase volume was minimized, resulting in a higher crankcase compression ratio, and more power in proportion to displacement than the competition. Mercury's