Towing a Boat? Don’t Damage Your Car


It happens every weekend. A person towing a boat backs his vehicle down a boat ramp to launch the vessel. It’s fun to submerge the rear tires a little too far into the water. Besides, it gets the boat into deeper water, which is good for the boat. But you also might be wondering, “will towing damage my car?” It can, but if you’re careful, you don’t have to worry about damage from towing a boat.

Your Towing Mistake Has Consequences

But a few weeks after you were towing the boat, a roaring noise develops from under the vehicle. At the local repair shop our driver gets bad news. Water had entered the rear axle and diluted the gear lube, which caused very expensive damage to the bearings and the even more expensive gears. What happened to ruin the fun?

Rear axles generate a lot of heat as the vehicle is driven. Heat expands the air inside the axle, naturally, so there is a small vent on top to relieve the pressure. Water entered through the vent, aided by the rapid cooling action of water surrounding a hot axle assembly.

A Small, Often-Overlooked Maintenance Item

The vents are pretty small, so another problem is that they often become clogged with dust and dirt, which sticks to escaping axle lube vapors. Not good, as a clogged vent will allow pressure to build up inside the axle, which will then force axle lube out, passed the axle seals. If you’ve ever had to replace axle seals that always seemed to leak for no apparent reason, a clogged axle vent might have been the cause.

Even with a clogged vent, our boat enthusiast’s car wouldn’t have been safe from damage. Lake or river water can get sucked into the axle through the axle and pinion seals. Those seals are not very high off the ground, at about the level of wheel center. Certainly not high enough to withstand deep water from that towing incident.

Heat Is Your Transmission’s Enemy

A few weeks after paying for an expensive axle overhaul, the vehicle’s automatic transmission goes bad. Back at the repair shop our driver hears more discouraging news. Excess heat from towing a boat had broken down the thin transmission fluid, severely decreasing its ability to lubricate the transmission’s expensive internal moving parts. Without proper lubrication, transmission parts failed.

So where did all the excess heat come from? Our driver wasn’t doing anything unusual, except for the water mishap. Simple. Automatic transmissions don’t have manual clutches to disengage the engine from the drivetrain at idle when stopped in traffic. Instead, they have a device that allows hydraulic slippage so the engine can continue to run while the car is stopped in gear. Called the torque convertor, its slipping action generates tons of heat, just like slipping a manual clutch can overheat a flywheel and clutch assembly to the point that you can smell it burning.

The torque convertor’s heat goes right into the transmission fluid, which is as thin as light cooking oil. Also when the vehicle is moving, the weight of the trailer and its load will often demand more torque convertor slippage. That slippage allows the engine to rev up a little higher to better pull the added weight of trailer and boat. More heat.

Defending Against Heat Is Easy And Inexpensive

After the transmission overhaul, the shop advises the owner to buy and install an auxiliary transmission fluid cooler. It looks like a little radiator, and will typically measure about 6 inches by 9 inches. It should be installed in front of the vehicle’s radiator, in the flow of fresh air coming through the grill. That will lower the temperature of the transmission fluid quite a bit. Enough to prevent an overhaul, in many cases.

Installation takes only a couple of hours. It makes for a pleasant Saturday afternoon project, using ordinary hand tools. It’s very inexpensive, too. Buy the transmission cooler online or in an auto parts store.

Unfortunately, that’s not the end of the sad story on damage from towing. As the TV commercials say, “But wait, there’s more!”


Brakes Don’t Do Well In Deep Water

A few weeks later the vehicle’s brakes start acting funny. The rear brakes seem to be making a burning smell, and the brakes don’t seem to be working right.

Back at the repair shop, where our driver is now one of their favorite customers, he gets more discouraging news. Water had entered the brake system, drawn in around the one-way lip seals in the rear brake calipers. Water instantly dissolves brake fluid, and causes erratic brake action.

The lip seals are made to contain the tremendous operating pressure within a brake system. The harder you brake, the more hydraulic pressure pushes the lip seals out against the caliper bores. But that design feature also means that water can easily, too easily, get drawn passed them into the system.

The same thing can happen to brakes when a vehicle is driven through deep water. Well, not that deep, just deep enough to bathe the brakes. Cars are not designed to be driven through deep water.

Our driver could have easily prevented those three expensive repairs. So don’t back a vehicle into deep water when launching a boat, install an inexpensive automatic transmission fluid cooler, and don’t drive through deep water under any conditions. Happy boating!