Lower Unit Design

by Chuck Petersen

In 1948, Carl Kiekhafer instructed his chief engineer, Reginald Rice, chief draftsman, Bill Spaeth, and Dick Williams to produce a racing gear case interchangeable with the standard or “fishing” model. The propeller shaft and gears were inserted from the rear end of the propeller shaft housing, and the gear case contained a water pump in both versions. During the next 14 years, over 17,000 Quicksilver gear cases were produced. During that time, the lower fin or steering was lengthened so that the unit would be lifted higher out of the water to take advantage of propeller improvements while still maintaining steering capabilities.

What most current enthusiasts fail to note; however, is that the “fishing” units of that era were not the stout full-shift models seen in the modern era. Standard Mercury gear case designs in the late 1940’s were very streamlined non-shift models and when properly mated with brass, two blade propellers, could perform quite well. Michigan Wheel Company had developed their two blade AJ (Aqua Jet) propeller while Mercury offered cupped stainless steel under the name “Kaminc” which stood for Kiekhafer Aeromarine Motors, Inc. Perhaps the finest examples of two blade brass props were made by Johnson Oakland of California. If you find one of these beauties attached to an old Super 10 or MK40 Merc, don’t lose it!!!

While the Quicksilver model is certainly the answer for pure racing hydro or runabout hulls, the early non-shift models with trick propellers is really the best compromise for the popular “gentleman’s” sport hulls seen at many of out shows. The best test platform I have run across recently is my son’s G.W. Invader, 10’ model produced in the late 1960’s. This fiberglass classic is too heavy and the transom height is too tall for the “Hydro Short” Quicksilver unit. The first gear case configuration I tried out following hull restoration was a MK55 full-shift model with a 44CID MK58 race prepped powerhead. This worked okay, but seemed heavy for the boat. In addition, transom height was limited due to the water pump inlet and propeller choice. The solution was transferring the powerhead to a late 1940’s, early 1950’s KG-9 version standard tower housing with a non-shift gear case.

While losing the convenience of neutral-reverse shifting, the gain in rpm and handling was marked. In addition, weight reduction achieved with the simple all-aluminum housing is significant. While mixing the late 1940’s and late 1950’s components may not be popular with purists, the performance achieved is too sweet to ignore.
Stock racers have always mixed available parts for both performance and durability motives. The stock class “D” Mercmotors I raced in the late 1980’s had components mixed and matched from 1954-1964 model years I am limiting my current restoration activities to high performance race model Mercury mills including an original MK20H and trick 44CID modified powerhead for my B&H Hemp Hydro. I keep my “need for speed” fulfilled by snowmobiling with my family in Northern Wisconsin over the winter season.

Best wishes and a Happy Holiday Season to all at BSLOL from all of us in the Petersen house. God Bless!