In an earlier RDP article, “Taking the Mystery Out of the X in X Dimension” it was discussed how that specific measurement was derived at before the rigger cuts a transom hole for the installation of a stern drive on a new boat. In the overwhelming majority of cases, boat manufacturers have pre-determined where that critical cut should be made to deliver ‘acceptable’ all-round performance for the ‘average’ boat buyer – minimizing propeller ventilation or struggle getting on plane, overall good trim response, an absence of propeller ventilation at speed or during turning, good prop efficiency numbers throughout the entire throttle range, etc...

To avoid guesswork, most boat builders develop transom drilling templates for specific models. This simplifies the operation and minimizes the chance for errors to occur.

Boat buyers are not generally expected to be experts in determining the ‘optimum’ X-dimension for their particular boat. That’s something you depend on the boat builder to know. Besides, it isn’t always a simple task for the average boater to locate the critical points on the transom to accurately measure the X-dimension once the boat is assembled.

Assuming that’s true, what is important? The key is the optimizing the vertical location of the propeller shaft in relation to the bottom (keel) of a hull. In very general terms, what you ultimately want is the least amount of gearcase dragging through the water as possible without sacrificing propeller thrust/efficiency. The higher a propeller is to the surface (or in some cases above the surface) of the water, the greater the potential for a loss of thrust/efficiency. (we’ll talk more about special surfacing drives, propellers and outboards in a future article). Likewise, inefficient performance will occur if the propeller shaft is positioned too far below the bottom of the hull.

(Picture of a stern drive "Surfacing Setup").

Enough of this abstract talk for now, let’s look at some real world applications. Let’s start with your average single stern drive family boat (18’ to 24’ in length with a Bravo 1 drive). It’s probably not set-up for maximum speed by the boat manufacturer but rather for good all-round performance. If you want to check this for yourself, lower the drive on the trailer at the point where the gearcase propshaft is as horizontal to the bottom (keel) of the hull as possible. You should be able to get this pretty close just by eye-balling, but if you want to be really precise, take a long straight edge (6’ or more) and hold it up (horizontally) against the keel of the hull with about 4’ of it touching the bottom. That should leave about 2’ or more sticking out behind the transom. Then adjust the drive unit with the trim switch and line up the gearcase propshaft with the straight edge. When it’s perfectly in-line you’re now at ‘zero’ trim angle. Measure up from the propshaft and determine the distance between the centerline of the propshaft and the bottom (keel of the hull). This measurement tells you how deep below the hull bottom your propshaft is.


On average, for single stern drive boats, propshaft centerlines may vary between 8 or 9 inches to only 3 to 5 inches below the bottom at the transom. A nine inch propshaft depth (13-1/2 Inch X-dimension for a standard Mercury Bravo 1) would be considered a conservative/deep/low set-up where three inches below (a 19-1/4 inch X-dimension) would tend to be more aggressive and favorable to a performance set-up. Remember, the less gearcase you have dragging through the water without losing propeller efficiency, the better it is in terms of improving overall performance.

(Typical low "Propeller Height" / "X Dimension")


(Typical "High Propeller Height" / "X Dimension")

In the majority of cases where a boater is looking to gain a little extra speed or overall performance, moving the propshaft closer (vertically up) toward the surface of the water may potentially result in a gain in speed. Generally, most boat manufacturers are conservative when it comes to selecting an X-dimension. They are often willing to sacrifice a couple of miles an hour at wide open throttle to insure easier planning and better overall handling at moderate cruise speeds. If, however, more top end speed is a priority, locating the optimum propeller shaft height is an important part of the entire performance equation.


So, how much ‘more’ speed will a boat gain by raising the propshaft in relation to the bottom of the hull? Unfortunately, there’s no set or predictable formula to calculate this. Depending on a myriad of factors (hull design, gross weight, horsepower, propeller selection, etc., etc.) the resulting improvement could range from miniscule to significant. The only way to know for sure is to try it.

For stern drive applications, there are really only two options available to adjust propeller shaft height (up or down). One way (the hard and expensive way) is to remount the stern drive, up or down, on the transom. This is not a simple task involving plugging and cutting a new transom hole and in some instances making a corresponding adjustment to the placement of the engine. Depending on the physical limitations of the engine compartment, this might even require a reconfiguration of the motor hatch. This option is simply a lot of work and usually costly as well. This could also be referred to as "Moving the X Dimension" which in turn is also moving "Propellor Height".

Assuming the propshaft height adjustment needs to go vertically higher, there are a limited few other options, but they too tend to be expensive. This could involve buying a complete shorter stern drive leg. For example, Mercury Racing offers a two-inch shorter gearcase option for its Bravo XR Sport leg. IMCO Marine, a popular aftermarket maker of performance stern drive products, also offers “shorty” drives for several of its units like the SCX4 (four inches shorter) and the SCX (two inches shorter). However, before you invest in one of these shortened drives, fully research what additional accessories may be needed to complete the adaption process. (By changing the length of the outdrive you are now affecting "Propeller Height" without affecting the "X Dimension")

(Picture courtesy of CP Marine / IMCO Marine)

If you want to go in the opposite direction (lower the propshaft in relationship to the bottom of the hull), drive spacers are the logical and most economical way to go. Not all stern drive models, however, will accommodate the use of spacers so again do your homework before purchasing product that you can’t use. The most common drive model accepting spacers is the popular Mercury Bravo 1. IMCO Marine, California also offers spacers for many of its IMCO brand drive units in half-inch increments up to three-inches. When ordering, you need to be specific about which IMCO drive model the spacer will be used with since the external profiles of their drives do differ. These spacers come in a kit format (with all necessary accessories including properly sized bolts) and are priced between $500 to $800 per drive. The Nevada based division of IMCO concentrates on spacer kits for stock Bravo drives only. The range of lengthening (deepening) spacers also cover a half-inch to three-inch span.


When it comes to purely performance stern drive applications where top speed is the ultimate goal, propshaft location close to (within 1 to 3 inches of the bottom of the boat), and sometimes even above the bottom (1 to 3 inches above) can potentially deliver some significant increases at top speed. If it helps to convert this into X-dimension height, it can range from a 19-1/2” X-dimension up to as high as 25” and possibly even more for certain extreme race boat set-ups.

(Typical X Dimension Height, with a IMCO lower unit - Note the amount of space between the top of the bell housing and the rub rail! An Easy and quick way to check your X Dimension against your friends with the same model boats!)

Before we conclude this portion of the discussion, we need to mention two additional factors. What about transom extension boxes and hulls with notched/recessed transoms? If you’re considering an extension/stand-off box (like Mercury’s ITS system) which locates the drive unit 7 to 9 inches farther behind the transom, remember, take into account that for every three or four inches of set-back, plan to locate the propshaft approximately one-inch higher in relation to the bottom of the boat. These are just approximations because every boat is different, but it will get you into the proper ballpark to start with.

(Mercury ITS system installed on a Lavey Sebring)

In a similar way, notched/recessed transoms require the same strategy for set up. If your hull has a six-inch notch, you need to take that into account. Again, a six inch back by six inch deep notch is very likely to require a corresponding 1-1/2 to two inch higher X-dimension than a similar hull with a conventional straight transom.

(Notched Transom with a pair of SSM#6 Surfacing Drives)

Another point to keep in mind if you’re raising the propshaft in relation to the bottom of the hull, maintaining proper water pressure and water volume are critical, especially if your engine is raw water cooled. Investing in water pressure and water flow volume gauges is invaluable. If water pick-ups are not getting a constant flow and pressure of water to the engine and accessories, all kinds of unpleasant overheating situations may occur.

Coming soon, we’ll discuss outboard set-ups, transom jack plates and some tips on propeller selection.