Although many performance boaters assume that pure horsepower solves all problems, it doesn't. In most cases, what makes a great performing boat is a combination of proper set-up, balance, propeller selection, gear ratio, hull design and of course, horsepower. And within these critical factors that determine performance, there's one particular component that seems to remain a mystery to most stern drive boaters; the X-dimension.


So what is it? First, it is a term of measurement exclusive to stern drive powered boats. So, get a picture of a boat transom in your mind. Then, imagine where the engine is located inside the boat. Next, think about where the crankshaft of the engine is, and where the drive shaft comes through the transom of the hull connecting it to the stern drive. And finally, lower your eyes to the lowest point of the hull (the center keel is the lowest point on single engine vee-bottoms). The X-dimension is the measured distance between where the centerline of the engine's crankshaft/drive shaft passes through the transom and the bottom (keel) of the hull.


Since the Mercury Bravo I is the most common stern drive in use on performance boats, let's use it for the sake of the discussion. According to Mercury, the length, or span of distance, between where the Bravo I connects to the crankshaft of the engine at the transom and the centerline of the propeller shaft is 21-3/4". So in order to determine where you want the propeller shaft in relationship to the bottom of the hull, you have to calculate your X-dimension accordingly. And don't forget, the key to achieving maximum performance is the ability to correctly locate the propeller shaft at precisely the most efficient height in relationship to the bottom of the hull. If it's too high, or too low, performance potential will be minimized.


For example, if you want your propeller shaft to be even (on a parallel) with the bottom of the boat, the X-dimension would therefore be 21-3/4" (which happens to be a relatively high position for most applications). If you wanted the propeller shaft to be 6-inches below the bottom, then the X-dimension would be 15-3/4" (a rather low setting for a performance boat).

There is no one optimum X-dimension that fits all boats. Depending on hull design, weight, balance, propeller selection and available horsepower, X-dimensions will vary between boat brands and even within different models from the same manufacturer. There's no ideal magic number. If you want a very general guideline to put you in the ballpark for a performance boat (single engine), the X-dimension range is normally between 16-inches up to 18-1/2-inches. That puts the propeller shaft on a stock Bravo I somewhere between 5-3/4-inches to 4-inches below the bottom of the hull. This X-dimension range delivers good all-around recreational performance in most instances; easy to get on plane without propeller ventilation, good handling and turning characteristics, good response to trim, slightly above average top end speed and good low speed maneuverability and steering control. For race boats, or boats intended to maximize top speed with only minimal concern for other performance dexterity, X-dimensions of 20 inches up to 23-inches may be used, thus putting the propshaft several inches above the bottom of the boat in a near surfacing-propeller condition.


What Happens If My Boat Has the Wrong X-Dimension? How Do I Know?

X-dimension does affect overall performance, especially when coming on plane and at higher speeds. If you're experiencing an abnormal amount of propeller ventilation (the prop spins, engine rpms climb quickly and there is a sensation of slippage/minimal thrust) during acceleration and you've tried a number of possible solutions (different styles/sizes of props and various trim settings), chances are an improper (too high) X-dimension might be the cause. Conversely, if your boat seems to be noticeably slower on the top end than similar boats with similar horsepower, that's another common sign that the X-dimension (too low) needs to be checked. In general, if your X-dimension is not within the efficient limits of your hull, a loss in performance will result whether it is set too high, or too low.

How Do I Measure for the X-dimension?

Whether you're laying out a fresh transom that has not been cut for the stern drive, or you're working with an already mounted drive on an older hull, it is always recommended that you level the hull on the trailer, or the cradle, supporting it when taking this measurement. Again, we're assuming that this is a single engine vee-bottom application, so we begin by locating the lowest point (the keel) at the center of the hull, and measuring up on a perpendicular from that bottom point. This does not mean simply laying a tape measure on to the transom and making a mark. Why? Because virtually all transoms are angled from 8 to 16 degrees depending on the hull design. If you measure along the transom, and don't create a perpendicular, you'll be off on your measurement by as much as 3/4 to 1-1/2-inches.


In order to create a perpendicular from the keel up, it often helps to affix a straightedge to the keel that extends out behind the boat to use as a measuring guide and point of reference. Once the perpendicular has been established, it's relatively easy to then measure up with a tape and mark the point where you want the centerline of the engine crankshaft to intersect with the transom. In most cases, determining the X-dimension is done at the factory by professional riggers. However, for do-it-yourselfers, templates for hole drilling are available from most stern drive manufacturers.

What Should I Do If I Want to Change the X-Dimension of My Boat?

You really have two choices. First, the hard way. You can un-rig your boat, requiring the drive(s) to be removed as well as pulling the engine(s) from the motor compartment. Then, you face the daunting task of plugging, rebuilding and cutting a new hole in the transom as well as adjusting the engine mounts so the motor(s) will align properly with the new X-dimension. It's a big job, and probably best left to the experts. The alternative is to invest in a new aftermarket "shortened" lower gearcase such as offered by IMCO (Extreme Advantage lower units, retailing for approximately $4,600 to $5,000). IMCO provides a selection of three "shorty" gearcases (1", 2" and 3") that bolt directly to a Bravo I drive. In effect, these aftermarket drives, along with the 2-inch shorter SportMaster lower unit by MerCruiser for their Bravo XR drive, raise the X-dimension without needing to alter the transom or the position of the engine(s). If you have a performance situation that needs the propeller shaft to be deeper below the bottom of the hull, the solution is easier and less costly. Aftermarket shim kits that will extend (deepen) the stock Bravo I gearcase from 1/2- inch up to 3-inches in 1/2-inch increments and can be installed relatively inexpensively.


What Kind of Performance Gains Can I Expect By Optimizing the X-dimension?

Unfortunately, no one can answer that question definitively since every application is different. Generally speaking, if the X-dimension is grossly too low, gains of as much as 5 to 7 miles an hour may result at full throttle by raising the propshaft two, three or more inches in relation to the bottom. If the boat only requires minimal adjustment, however, the improvement may be as little as only a mile an hour or even less. What you ultimately want to achieve is the least amount of gearcase in contact with the water (eliminating as much parasitic drag as possible) while still maintaining maximum thrust and efficiency from the propeller.

What Else Do I Need to Know?

First, some additional general comments. Cats and/or air entrapment hulls normally respond better to slightly higher X-dimensions in order to perform properly compared to like sized vee-bottoms. And likewise, step/ventilated vee-bottom hull designs usually perform better with higher X-dimensions than conventional non-step vee hulls.


When you change the X-dimension (up or down), you should also expect to consider a change in propeller (pitch/diameter/number of blades/degree of rake/shape/contour, etc.). As your propeller shaft rises closer to the surface of the water (higher X-dimensions), generally, propeller pitch and/or diameter needs to be increased. Therefore, more conventional three-blade designs will no longer be as efficient as four or even five blade configurations. Additional blades not only make it easier for your boat to come on to a plane, but mid-range efficiency (higher speed at lower rpms) will improve plus potentially more top end performance in some cases.


One factor you must be aware of when running higher X-dimensions is potentially inadequate water pressure/volume for cooling. This is especially critical for older stern drive packages that are not equipped with a closed cooling system, and depend on a constant supply of raw water which is inducted into the system by intake holes located along the lower unit external casing. When these stock intakes get too close to the surface of the water at higher speeds, less water and more air is captured by the inlets thus starving the water pump and potentially causing an engine overheating condition to occur. To avoid that, it is sometimes necessary to use an external transom-mounted thru-hull pick-up which is totally separate from the lower unit so adequate water pressure and volume is maintained.


Occasionally in high performance applications, set-back extension boxes are used to move the entire stern drive and propeller farther aft of the transom in order to gain more trim leverage and successfully use extremely high X-dimensions. For example, the Mercury Racing ITS system which is available with the XR Bravo Drive moves the drive and propeller seven inches farther behind the hull. IMCO's aftermarket extension box adds an additional 13-inches to the set-back of the drive and prop. In cases such as these, a general rule of thumb is to increase the X-dimension by 1 to 1-1/2-inches for each four to six inches of set-back compared to a standard transom mounting.


Over the past decade, boat manufacturers have found that incorporating the use of higher X-dimensions thanks to better hull designs and significant advancements in propeller technology have dramatically improved the performance and efficiency of their products. Ten years ago, what was considered a standard or industry average X-dimension height, is now regarded as low or not acceptable in today's competitive marketplace. With so much importance placed upon operating efficiencies, greater fuel economy and increased performance, optimizing the X-dimension is just one of many ways that boat builders are coping with the challenges of boating today and tomorrow.

Words : Bob Brown
Images : Bob Brown / RDP Forums