Tow Vehicle Maintenance for Boat Owners (Transmission, Differential, Brakes, Hitch)

If you tow a boat regularly, follow the severe-service column in your owner's manual, not the normal one. Ask your shop to log oil, transmission fluid, differential and brake service by date and mileage. Keep receipts in the glovebox; some warranties ask for proof of service on vehicles used for towing.

Heat is what wears out an automatic transmission under load. BoatUS notes that factory tow packages often add transmission and engine cooling, a heavier frame mount and other upgrades that an aftermarket hitch alone doesn't include. To help:

• Use Tow/Haul mode if your vehicle has it. BoatUS explains it keeps the transmission from shifting into overdrive and hunting between gears. On most vehicles it turns off with the engine, so turn it back on after every fuel stop.
• On long grades, downshift instead of riding the brakes, as BoatUS's towing tips suggest; the engine helps hold your speed.
• If your vehicle shows transmission temperature, watch it on hills and at the ramp.
• Check the fluid level and condition the way your manual says (some transmissions have no dipstick and need a shop). Dark fluid, a burnt smell, slipping or harsh shifts mean it's time for service.
• Ask your dealer whether an auxiliary transmission cooler makes sense if you tow often and your vehicle doesn't have the factory tow package.

BoatUS's article on ramp traction explains that water can get into a differential through the vent tube or condensation, and that backing the rear axle into the water doesn't help. Water and heat break down the gear oil and the friction modifier that limited-slip clutches need. BoatUS recommends that owners who back their rear tires into the water drain the rear differential at least at the end of every boating season and look at the oil:

• Milky or yellow-brown: water got in.
• Burnt smell: the limited-slip clutches are worn out.
• Metal flakes on the plug: too much wear inside.

If there's water but nothing else is wrong, refill with the gear oil your manual specifies plus the friction modifier it calls for. Burnt oil or metal means a shop visit. Our 2WD ramp traction guide has more on keeping a limited-slip working.

Back in only as far as you need to float or load the boat. If your rear tires are going under water every time, the trailer may need a tongue extension, or the bunks and winch post may need adjusting so the boat loads at a shallower depth. Keep the exhaust tailpipe and rear axle vent out of the water when you can. Set the parking brake before anyone gets out.

Extra weight means extra brake wear and heat. Have pads, rotors, brake fluid and lines checked at the intervals in your severe-service schedule, and sooner if the pedal feels soft or the brakes pulse or squeal. If your trailer has electric brakes, set the brake controller before each trip (Mercury's checklist includes this). If it has surge brakes, the trailer's own system does the work; see our surge brake guide.

Mercury's pre-tow checklist puts tire pressure on the tow vehicle (and especially the trailer) near the top. Many vehicles list a higher pressure for heavy loads on the door-jamb placard or in the manual; use it when towing. BoatUS warns that low pressure overheats the sidewall and leads to failure. Look for cuts, bulges and uneven wear, and make sure the spare is inflated.

BoatUS's checklist says the hitch should be bolted securely to the frame and shouldn't be rusty or otherwise compromised. Each time you hook up:

• The ball size must match the coupler, and the ball, ball mount and receiver must all be rated for the load.
• The hitch pin and clip (or locking pin) are in place and not worn.
• The trailer sits level. BoatUS's hitch setup article recommends choosing a ball mount with the right rise or drop so the trailer frame is parallel to the ground.
• Check the receiver's mounting bolts for rust and tightness at least once a season.

Corroded plugs cause most "the trailer lights quit" moments. Brush out the vehicle's socket and the trailer plug, coat the contacts lightly with dielectric grease, and close the socket cover when you're not towing. BoatUS's connector article notes there's no single plug that fits every rig; you may need an adapter, such as 7-way to 5-flat. Don't use the hitch ball as the ground (BoatUS says so plainly); the trailer needs its own ground wire. Test every light before each trip, as Mercury's checklist suggests.

Cross the safety chains under the tongue and hook them to the vehicle's chain loops, with enough slack to turn but not to drag. If the trailer has brakes, BoatUS says to attach the breakaway cable to the hitch, never to the safety chains. Tongue weight affects how the vehicle steers and brakes. BoatUS's hitch article says most vehicle makers want 10–15% of the loaded trailer weight on the hitch, and that tongue weight counts against the truck's payload. Check it with a tongue scale whenever you change boats or gear.

Salt sitting on the frame, brake lines, exhaust and suspension is what rusts tow vehicles from below. BoatUS suggests stopping at a car wash on the way home from the ramp to use the handheld sprayer, or washing at home within a few hours of leaving. Rinse:

• The receiver, hitch pin, ball mount and trailer plug
• Wheel wells, brakes and the area around the rear axle
• Frame rails and the spare tire carrier

A sprinkler under the vehicle (BoatUS suggests one under the trailer) reaches spots a hose misses. Do it in fresh water too after a muddy ramp.

At the end of the season, or at the shop:

• Change the rear differential oil if it went under water (step 3).
• Have the transmission fluid checked or changed on the severe-service interval.
• Inspect the brakes and brake lines.
• Look under the vehicle for rust on the frame, hitch, brake lines and fuel lines.
• Pull the ball mount, clean the receiver and coat the pin with grease before storage.
• Have the battery load-tested (not just a voltage check) and look at the drive belt, hoses, air filter and wipers. A Bassmaster pro angler's tow-vehicle list includes all of these, since a dead battery or broken belt strands you just as surely as a trailer problem.

Before a long haul, Sport Fishing magazine adds: weigh the loaded rig at a certified scale instead of guessing, check tires (including the spare), brakes, belts and hoses, and make sure your vehicle's jack can actually lift the loaded trailer and your lug wrench fits the trailer's lug nuts. On the road, it suggests pulling over every couple of hours to check the coupler, safety chains, light plug and the boat's straps.