I often hear asked how one can tell the difference between a "long shaft" and a "short shaft" outboard. Another frequent question is how one determines if an existing boat is set-up for a long shaft or short shaft engine. And finally, should a home-built boat be built to accept a long shaft or short shaft engine?.

click images to enlarge

First, a little history; through some form of evolution, it evolved in the early days of outboard motors that they would , generally, be designed to be mounted on a boat transom measuring 15 inches in height. At a later point in time, when it become apparent that increased freeboard might be desirable on larger boats, a second "standard" of a 20-inch-high transom was established. The 15 inch engines were referred to as "standard" or "short" shaft engines, while the 20 inch engines were referred to as "long" shaft engines.

Through most of the 50's and 60's, nearly all outboards (there were exceptions) where available in either version, although as the engines become larger and larger, exceeding 100 hp in the early 60's, and exceeding 150 hp in the mid 70's, most of the bigger engines were only available in the 20 inch "long" shaft version.

In the late 80's, with the increased popularity of deep-V offshore boats, outboards designed for 25 inch transoms began to be available, although these are almost always engines in excess of 100 hp. Only the smallest of engines, say under 40 hp, are still available in the 15 inch "short" shaft version.

Because of confusion caused by the introduction of the 25 inch engines, which are sometimes referred to as "long" shaft engines, leaving the 20 inchers to be called "short' shafts, I suggest we dispense with all the "short" and "long" nomenclature, and instead refer to the particular engines by the "inch" measurements, and since I only discuss "old"engines, that will limit us to 15 inch and 20 inch engines.

First of all, how to identifying a 15 inch engine from a 20 inch engine; This is fairly easy with OMC (Johnson, Evinrude, and Gale) engines, as the 20 inch versions from the 50's and 60's, of no more than 40 hp, always had an adapter piece added to the "leg" of the engine. Shown here are photos of two 1957 Johnson 18 hp outboard motors, identical except that one is a 15 inch engine, and the other is a 20 inch engine. It is fairly easy to pick out the adapter section added just above the lower unit of the 20 inch engine.

If you carry a tape measure or yard stick, you can always measure the distance from the underside of the outboard's mounting clamps (the point which actually sits on top of the transom) to the horizontal cavitation plate located just above the propeller. this dimension will be about 16 or 17 inches on a "15 inch" motor, and will be about 21 or 22
inches on a "20 inch motor."

The parts needed to convert various Evinrude models from 15-inch versions to 20-inch versions click HERE for full size view

The sharp-minded reader (and what Duckworks reader would not be sharp-minded?) will immediately ask, "can a 15 inch engine be converted to a 20 inch version, and also the other-way-around?" The short answer is "yes," the engines can be converted back and forth, but there are several parts and pieces necessary to make the conversion. The vertical driveshaft comes in two lengths, as do the water tubes that conduct cooling water from the water pump in the lower unit to the powerhead, and one would need the appropriate parts. Also, the vertical "shift shaft" comes in two lengths as well; however, there was a five-inch-long coupler available to allow a 15 inch engine to be converted to a 20 inch version without disassembling the lower unit to install the 20 inch shift shaft. If your engine has this extended coupler, replacing it with a standard coupler will work, but you still need the other pieces. So it is probably best to try to buy the version of the engine that you need, rather than to count on a conversion.

Now, how does one identify which engine a particular boat is set-up to use? Again the yardstick or ruler will come in handy; one merely measures the height of the transom at the motor mounting point. If that measurement is 15 to 16 inches, then the transom is for a 15 inch outboard. A measurement of 20 to 21 inches indicates that a 20 inch engine is called for. Just one complication; the "height' of the transom is the vertical measurement from the engine mounting point down to the plane of the boat's bottom. Since the transom will have some "rake" to it, the yard stick will only touch the top of the transom, not the bottom. the measurement is NOT made by holding the yardstick directly on the transom. Note that few transoms will measure at exactly 15 or 20 inches, and that most motors have about an extra inch or so of length. It generally will not cause any harm to have the lower unit a little deeper in the water than what would be ideal, but an engine mounted too shallow may have problems with cooling water circulation and also with propeller ventilation (air getting into the prop, causing it to lose it's "bite" on the water and causing the engine to "rev-up" while the boat slows down). Also, as a home-boat-builder you should be aware that how you build the boat can affect how "deep" the motor needs to be mounted. For example, If the boat has a large "keel" or "skid" on its bottom, this protrusion needs to be tapered or streamlined so as not to affect the water flowing into the propeller; otherwise, the motor needs to be mounted deeper so that the propeller is below the level of the offending item.

OMC recommended transom and splashwell dimensions click HERE for full size view

As to whether you should build your boat with a 15 inch or a 20 inch transom, well that depends upon the design of the boat and also how you intend to obtain your outboard motor. A design that specifies a 15 inch transom can usually be modified to a 20 inch transom with few ill effects. but one must carefully consider freeboard concerns when modifying a design specifying a 20 inch transom, to a 15 inch transom. In either case, a dialogue with the craft's designer is probably in order for those builders who are unsure as to the implications of such a design change.

Considerations of the boat's design aside, it is wise to keep in mind that, unless one is buying a new outboard, one must build one's boats to accept those outboard motors that are going to be prevalent in the market; in other words, unless you already have your outboard, do not build your boat to specs. that require an engine that will be difficult to obtain in the used motor market in your locality.

For example, if one intends to power one's boat with a used OMC-built outboard of under 20 hp, and older than 1970, then the 15 inch transom is the correct choice, as very few of these engines will be found in 20 inch versions; I would guess that about 98% of such engines that I have seen are 15 inchers.

Parts diagram for 18 hp engine click HERE for full size view

If, however one intends to use a 40 hp OMC outboard dating from, say 1962 until 1982, then you will probably find used engines split about 50:50 between 15 inch and 20 inch versions. And larger engines, say 60 hp and above, 1962 and newer, will almost always be 20 inch versions, say probably about 95% or so.

These percentages are my guesses, based on looking at an awful lot of outboards, and not based on any hard research. Also, local characteristics may affect those percentages; geographic areas containing large numbers of sailboats, for example, will offer more 20 inch versions of smaller outboards than geographic areas with few sailboats.

A perusal of local boat dealers' used motor displays will give one an idea of just what is available in one's particular area. A little research will provide some guidance to those who wish to build their vessel before they have secured their "prime mover."

Well, that's been a rather long-winded discussion on short- and long shaft outboards.

Happy Motor'n

Max