Outboard General FAQs

Where can I purchase Mercury branded patches, decals, clothing or other merchandise?

Mercury offers a complete line of branded merchandise, and it's all available in the Mercury DockStore at www.mercurydockstore.com.

Please explain EPA restrictions on engine modifications, high altitude carburetor jets, or gearcase changes.

EPA regulations prohibit changes to engines that could affect emissions limits. Per the Service manual, "The dealer and/or consumer is not to modify the engine in any manner that would alter the horsepower or allow emission levels to exceed their predetermined factory specifications." The only changes permitted are carburetor jet and gearcase/gear ratio changes for high altitude operation as detailed in applicable Mercury Marine Service manuals.

You can order all available literature online. You could also order all literature through any authorized Mercury dealer. Please have your outboard or sterndrive serial number before placing the order.

I need to replace my existing engine. What are my options?

Due to the large number of different engine/boat configurations and the changes made to engine packages over the years, we suggest you contact your dealer, engine power center and/or boat manufacturer for specific guidance in your case. Information on repower options is available in the Repower section of this Web site.

How do I obtain horsepower curves, torque curves, or fuel consumption data?

Horsepower and torque curves are not published because they are considered confidential company information. Fuel consumption data is not published because it is affected by factors beyond our control, such as boat hull type, temperature, humidity, altitude, gear ratios, propeller, etc. In some cases, fuel consumption or other performance data is available from your boat manufacturer for a specific boat/engine combination.

Some engine test data is available in the Engine Test section of this Web site.

Can you provide contact information on antique or old outboard motors?

The Antique Outboard Motor Club can provide information for collectors, restorers and people who want to determine the value of old outboards. The club has chapters throughout the U.S. and Canada. Information is available on-line at www.aomci.org.

How can I determine when my outboard was built?

Every Mercury outboard has a serial number label located on the transom bracket area of the engine. The latest serial number labels display a 2-digit number in a box at the lower right portion of the label. These digits coincide with the last two digits of the year in which the outboard was manufactured. For example, if an outboard was manufactured in 2010, the number "10" would appear in the box at the lower right portion of the label.

Can I install aftermarket accessories like stabilizer "fins" to help my boat get on plane faster?

While these add-on devices may reduce the time it takes to get the boat on plane, they may also cause or contribute to abnormal or even unsafe handling of the boat and failure of the anti-ventilation plate. Using a product often referred to as a stabilizer fin is a modification of the gearcase that may change the manner in which the boat operates on the water. With the unit trimmed fully down (in or under), a reduction of the time necessary to get a boat on plane may result. However, some V-bottom boats using a stabilizer fin may exhibit a tendency to roll over far to one side as soon as planing occurs. The direction and degree of the roll will be dictated by prop rotation, weight distribution, and degree of trim under. The boat roll can cause passengers to be ejected. Trimming up or out can reduce or eliminate the roll. Because it is not possible to know how a given boat will respond to the use of such a product without thorough testing, Mercury cannot recommend that such products be used or not used. Also, use of these products may cause or contribute to the failure of the anti-ventilation plate on the gearcase. Failures caused, or contributed to, by modification are not covered by the limited warranty.

Why does my boat perform differently on a hot day versus a cool evening?

It is a known fact that weather conditions exert a profound effect on power output of internal combustion engines. Therefore, established horsepower ratings refer to the power that the engine will produce at its rated rpm under a specific combination of weather conditions.

Corporations internationally have settled on adoption of I.S.O. (International Standards Organization) engine test standards, as set forth in I.S.O. 3046 standardizing the computation of horsepower from data obtained on the dynamometer, correcting all values to the power that the engine will produce at sea level, at 30% relative humidity at 70 deg.F (20 deg.C) temperature and a barometric pressure of 29.61 inches of mercury.

Summer Conditions of high temperature, low barometric pressure and high humidity all combine to reduce the engine power. This, in turn, is reflected in decreased boat speeds--as much as 2 or 3 miles-per-hour (3 or 5 Km per-hour) in some cases. Nothing will regain this speed for the boater, but the coming of cool, dry weather.

In pointing out the practical consequences of weather effects, an engine--running on a hot, humid summer day--may encounter a loss of as much as 14% of the horsepower it would produce on a dry, brisk spring or fall day. The horsepower, that any internal combustion engine produces, depends upon the density of the air that it consumes and, in turn, this density is dependent upon the temperature of the air, its barometric pressure and water vapor (or humidity) content.

Accompanying this weather-inspired loss of power is a second but more subtle loss. At rigging time in early spring, the engine was equipped with a propeller that allowed the engine to turn within its recommended rpm range at full throttle. With the coming of the summer weather and the consequent drop in available horsepower, this propeller will, in effect, become too large. Consequently, the engine operates at less than its recommended rpm.

Due to the horsepower/rpm characteristics of an engine, this will result in further loss of horsepower at the propeller with another decrease in boat speed. This secondary loss, however, can be regained by switching to a smaller pitch propeller that allows the engine to again run at recommended rpm.

For boaters to realize optimum engine performance under changing weather conditions, it is essential that the engine have the proper propeller to allow it to operate at or near the top end of the recommended maximum rpm range at wide-open-throttle with a normal boat load.

Not only does this allow the engine to develop full power, but equally important is the fact that the engine also will be operating in an rpm range that discourages damaging detonation. This, of course, enhances overall reliability and durability of the engine.

What other things besides atmospheric conditions will affect my boat performance?

WEIGHT DISTRIBUTION
Proper positioning of the weight inside the boat (persons and gear) has a significant effect on the boat's performance, for example:

  • Shifting weight to the rear (stern)
    • Generally increases top speed.
    • If in excess, can cause the boat to porpoise.
    • Can make the bow bounce excessively in choppy water.
    • Will increase the danger of the following - wave splashing into the boat when coming off plane.
  • Shifting weight to the front (bow)
    • Improves ease of planing off.
    • Generally improves rough water ride.
    • If excessive, can make the boat veer left and right (bow steer).

BOTTOM
For maximum speed, a boat bottom should be nearly a flat plane where it contacts the water and particularly straight and smooth in fore-and-aft direction.

  • Hook: Exists when bottom is concave in fore-and-aft direction when viewed from the side. When boat is planing, 'hook' causes more lift on bottom near transom and allows bow to drop, thus greatly increasing wetted surface and reducing boat speed. 'Hook' frequently is caused by supporting boat too far ahead of transom while hauling on a trailer or during storage.
  • Rocker: The reverse of hook and much less common. 'Rocker' exists if bottom is convex in fore-and-aft direction when viewed from the side, and boat has strong tendency to porpoise.
  • Surface Roughness: Moss, barnacles, etc., on boat or corrosion of outboard's gear housing increase skin friction and cause speed loss. Clean surfaces when necessary.

WATER ABSORPTION
It is imperative that all through hull fasteners be coated with a quality marine sealer at time of installation. Water intrusion into the transom core and/or inner hull will result in additional boat weight (reduced boat performance), hull decay and eventual structural failure.

CAVITATION
Cavitation is caused by water vapor bubbles forming either from a sharp edge or angle on the gear case or from an irregularity in the propeller blade itself. These vapor bubbles flow back and collapse when striking the surface of the propeller blade resulting in the erosion of the propeller blade surface. If allowed to continue, eventual blade failure (breakage) will occur.

I want to upgrade to a more powerful engine. How much more performance can I expect?

This depends on many factors such as hull type, weight distribution, maximum horsepower rating of the boat, etc., and cannot be accurately determined without more information. We suggest you contact your dealer, engine power center, or boat manufacturer for specific guidance in your case. Information on repower options is available in the Repower section of this Web site.

What precautions should I take when trailering my outboard powered boat with the engine attached?

Trailer your boat with the outboard tilted down (vertical operating position). Shift the outboard to forward gear. This prevents the propeller from spinning freely. If additional ground clearance is required, the outboard should be tilted up using an accessory outboard support device. Refer to your local dealer for recommendations. Additional clearance may be required for railroad crossings, driveways and trailer bouncing.

IMPORTANT: Do not rely on the power trim/tilt system or tilt support lever to maintain proper ground clearance for trailering. The outboard tilt support lever is not intended to support the outboard for trailering.

What precautions should I take when operating my engine in freezing temperatures?

When using your outboard or having your outboard moored in freezing or near freezing temperature, keep the outboard tilted down at all times so the gear case is submerged. This prevents trapped water in gear case from freezing and causing possible damage to the water pump and/or other components.

If there is a chance of ice forming on the water, the outboard should be removed and drained completely of water. If ice should form at the water level inside the outboard drive shaft housing, it will block water flow to the engine causing possible damage.

What precautions should I take when operating my engine in salt water or polluted water?

We recommend that you flush the internal water passages of your outboard with freshwater after each use in salt or polluted water. This will prevent a buildup of deposits from clogging the water passages.

Refer to "Flushing The Cooling System" procedure in the Maintenance Section or your Operations and Maintenance Manual supplied with your engine.

If you keep your boat moored in the water, always tilt the outboard so the gear case is completely out of the water (except in freezing temperature) when not in use.

Wash down the outboard exterior and flush out the exhaust outlet of the propeller and gear case with freshwater after each use. Each month, spray Mercury Precision Lubricants or Quicksilver Marine Lubricants Corrosion Guard on external metal surfaces (DO NOT spray on corrosion control anodes as this will reduce the effectiveness of the anodes).