Posted by MichaelHanson on September 10, 2002 at 19:37:07:
In Reply to: Short shaft outboard motors posted by Jon Strom on August 21, 2002 at 22:15:54:
From 1947 to the early fifties, Kiekhafer Mercury made the Mercury Super 10 outboard. It had just under 20 cubic inches and came in two models; 1) the "Hurricane" with a standard 15 inch shaft (clamp to cavitation plate), a gear ratio of about 2 to 1, and a small standard three-blade aluminum prop (for medium loads) and 2) the "Cruiser" with a standard 15 inch shaft ( clamp to cavitation plate), a gear ratio of about 3 to 1, and a huge standard two-blade prop (for heavy loads). Mercury sold the "Quicksilver" racing lower unit separately. It had a 12 inch shaft (clamp to cavitation plate) which was designed for transoms 15 inches tall (so that the cavitation plate ran three inches above the water surface after planing off). This put "Michigan Wheel's" famous matching two-blade brass prop about 1 inch out of the water after planning off (most racers placed "rev sticks" under the clamp and the top of the transom to raise the engine even higher). The "Quicksilver" lower unit had a 1 to 1 gear ratio (which allowed the "bullet" of the lower unit to be much smaller in diameter than the standard bullets of either model), a cone-shaped bullet nose, a narrow and more streamlined shaft housing and skeg, and a low water pickup on the bottom of the bullet. Today, these outboards are rare, and parts for them are rarer. They are usually in the hands of aging racers or collectors. Mercury still makes similar racing lower units with similar specifications for its new stock outboards. Until recently, OMC did as well. In my opinion, unless you race in the APBA, all of these may be overkill. Props for them can cost hundreds of dollars. Cavitation and high r.p.m. during planning quickly wear out the powerhead, cooling components and delicate small gears in the gearcase ("bullet"). The term "short-shaft" is usually used to distuish one standard outboard from another which has a long shaft (such as one intended for use as an auxilliary motor on a large cabin cruiser a sail boat with a high motor mount). Many times ordinary outbord motors are available in two standard shaft lengths, short and long. I generally think of a "shortshaft" outboard as an ordinary stock (non-racing) outboard with the shorter of the two standard shaft lengths usually available for that standard model outboard. Of all the "deficiencies" of an outboard, shaft length is the easiest to correct. Just, make your boat transom taller by the same amount that you regard you shaft length as being too long. "Jacking up" your outboard (such that the cavitation plate runs above the water line) is a trial and error process with lots of variables from cavitation, "rev sticks", prop, trim (tilt) and cooling concerns (not to mention losing the outboard by dropping it into the water). Unless you are a racer in the APBA with a professionally built race boat, I recommend running your cavitation plate near the waterline and buy a decent light-load prop. Remember, K.I.S.S is not just a rock band.