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Basic Trailer Maintenance

Knowing your boat is maintained in top shape gives you confidence every time you leave the dock. Whether you do it yourself or choose a Mercury Authorized Dealer for your service and maintenance needs, every boat owner should be familiar with the basic principles of marine maintenance. 

Can You Tow It?

Towing capacity is determined by the tow vehicle manufacturer, and the vehicle owner’s manual is the best source of information on towing capacity. You’ll also usually find this information on a sticker inside the vehicle door jamb, or on the manufacturer’s website. The Gross Combined Vehicle Weight (GCVW) is the most critical rating to determine. The GCVW is the maximum combined weight of the fully loaded tow vehicle with passengers, fuel and gear plus the weight of your boat (including its fuel and gear) and the trailer. The best way to accurately check GCVW is on a public scale at a truck stop or municipal facility. Weigh your rig with the fuel and gear you take to the water. Also pay attention to the maximum weight ratings of the trailer and the trailer hitch.

Tire Pressure

Tire failures caused by low air pressure are the most common cause of trailer trouble. Underinflation can cause a tire to overheat, delaminate and fail – a dangerous circumstance at highway speeds. Check trailer tire air pressure – including the spare tire – before every trip and set it to the pressure listed on the sidewall of the tire or on the capacity sticker on the trailer, which may be 60 psi or more. It’s especially important to check the tires after the trailer has been sitting unused during the offseason. Visually inspecting the tire treads and sidewalls for wear or damage before every trip is also a good practice.

Wheel Bearings

Failed wheel bearings are another common cause of trailer trouble. A boat trailer axle hub should be equipped with a wheel bearing protector (the Bearing Buddy® brand is a common term for all bearing protectors), which uses a spring-loaded piston to hold grease in the wheel hub and has a grease fitting in its center. This prevents water from entering the hub and corroding the bearing assembly. Read the directions that come with your trailer for details on adding or replacing bearing grease. Even with bearing protectors it’s a good idea to inspect and repack the wheel bearings with fresh grease every three to five seasons. Mercury Extreme Grease is an excellent general-purpose choice for use in trailer bearings that will be frequently exposed to water.

Trailer Brakes

All but the lightest boat/trailer combinations will be equipped with brakes, which require occasional maintenance to ensure safe performance. Check your trailer owner’s manual for specific brake maintenance instructions.

Keep Brakes Clean

If you launch your boat in saltwater it’s important to rinse the trailer brakes and wheels with freshwater as soon as possible after you retrieve the boat. This helps prevent corrosion of brake components and wheel lug nuts. Many marinas have a freshwater hose available for this purpose, and some trailers may be equipped with an onboard hose connection and system to rinse the brakes.

Brake Fluid

For trailers with hydraulic brakes, check the brake fluid reservoir in the master cylinder (typically mounted on the trailer tongue) a couple of times a season, following instructions in the trailer owner’s manual. Fluid level will go down as the pads wear. If the level is suddenly very low you could have a leak in the brake system, which should be repaired immediately.

Brake Pads or Shoes

Trailer brakes may be disc-type (with brake pads) or drum-type (with brake shoes), both of which wear with use, just like the brakes on your tow vehicle. Have the pads or shoes inspected every few seasons by a trailer shop or a general auto repair service.

Trailer Hitch & Receiver

Applying grease or spray lubricant to the mounting ball, draw bar, hitch retaining pin and receiver mount makes hooking up your trailer easier and reduces wear and rust formation on the components. Also inspect the safety chains that connect the trailer tongue to the receiver hitch on your tow vehicle for weak or broken links.

Trailer Lights

Your vehicle’s marker lights are probably obstructed by the boat, so the trailer lights are essential for safety. Check all of your trailer lights every time you hook up the trailer and double-check them during gas stops or whenever convenient to catch a failure on the road. If your lights stop working, here are some things to check:


A missing or loose ground connection is a common cause of trailer light failure. The white wire on your trailer harness is the ground, and it may be secured with a screw to the trailer frame. Remove the screw and use sandpaper to clean the contact point to bare metal.


Inspect the wiring and protect it with electrical tape or wire conduit where it touches or passes through the frame and has an opportunity to chafe through the insulation. This can short the current to ground and cause lights to fail.


Most new trailers have LED lights that are durable and infrequently need replacement, but the incandescent bulbs in older trailer lights can break or burn out and need to be replaced. Apply a dab of dielectric grease to bulb sockets to prevent corrosion.

Next Topics

Maintenance: What's In It For You?

Boating is an activity that fuels pride of ownership, and for many owners a strong do-it-yourself attitude. Knowing you’ve covered the basics of maintenance gives you confidence that your boat is in tip-top shape every time you leave the dock.

Fuel Basics

Using the correct fuel in your Mercury Marine® engine and properly maintaining your fuel system can help ensure many seasons of reliable service. For more guidance on fuel requirements for your engine, see the fuel section of your Mercury® operation and maintenance manual.

Preventing Corrosion

Your engine lives and works in the water, where it’s constantly exposed to elements that can cause corrosion.

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