How to Inspect, Inflate and Replace Boat Trailer Tires

Find the size (for example, a code starting with ST), the load range letter (B, C, D, E and so on: higher letters carry more weight and usually need more pressure), the maximum load and the maximum inflation. Then find the placard on the trailer. Write the placard's cold pressure and tire size in BoatMend so you have it at the ramp.

All the tires on the trailer, spare included, should be the same type, size and construction. Never mix radial and bias-ply tires on the same trailer.

Check before you drive, or after the trailer has sat for a few hours. Pressure climbs as the tire warms up, so a reading taken after a drive will look too high. Remove the cap, press the gauge on firmly, and compare with the placard (or the tire maker's guidance if you have non-original tires).

Discover Boating notes that tires naturally lose a pound or two of pressure a month, so a trailer that sat all winter is almost always low. Trailer tires usually need much more pressure than car tires. Mercury and Discover Boating both note it can be 60 psi or more, so a cheap car gauge may not even read high enough. Don't judge pressure by eye: a stiff trailer tire can be badly low and still look fine.

Fill to the placard pressure. Never go over the maximum pressure molded on the sidewall, and never over the pressure rating of the wheel. Recheck with the gauge after filling. Put the caps back on.

Low pressure is the number one cause of trailer tire failures: the tire flexes too much, heats up and can come apart. Overinflation is not the fix either. It makes the tire easier to damage on a pothole or curb.

With the tire inflated, wet the valve stem with soapy water or press it sideways with a finger and listen. Bubbles or a hiss mean a leaking stem or core. Old rubber stems crack too. A slow leak you keep "topping off" is often the stem.

On one side of each tire is a code that starts with DOT. The last four digits are the build date: the first two are the week, the last two are the year. 3517 means the 35th week of 2017. Very old tires may show only three digits, which means they were built before 2000 and are long overdue.

Check all of them, including the spare. The code may be on the inside face of the tire only, so look on both sides.

Tires age even when they sit in the garage, and the tread will usually look fine long after the rubber has gone weak. Discover Boating's checklist says tires more than six years old should be replaced, citing the National Transportation Safety Board. Many tire makers and trailer shops suggest replacing trailer tires sooner, often somewhere between three and six years, especially if the trailer lives outdoors in the sun. There is no single legal limit. Use the age, the storage conditions and the condition you find in the next steps together, and when in doubt, replace. Check your tire maker's own guidance.

Look closely at both sidewalls and the bottom of the tread grooves in good light. Replace the tire if you see:

• Fine "spiderweb" cracks in the sidewall or between the tread blocks (dry rot or ozone cracking)
• Bulges, blisters or a wavy sidewall
• Cuts or gouges deep enough to show cord or fabric
• Scuffed or chewed sidewalls from curbs or the fender

BoatUS calls spiderweb cracking evidence of dry rot and imminent failure. Don't wait to see if it holds.

Put a penny upside down in a tread groove. If you can see the top of Lincoln's head, the tread is worn out. Then look at how it wore:

• Center worn more than edges: usually overinflation
• Both edges worn: usually underinflation
• One edge, or one tire much worse than the others: a bent axle, bad alignment or a bearing problem. Have the axle checked before you put new tires on, or the new tires will wear the same way.

Your loaded rig (boat, motor, fuel, gear and the trailer itself) has to be within what the tires can carry. The total is the load rating on each sidewall times the number of tires, but tandem and triple axles carry less than that simple math suggests, so use the trailer placard and the tire maker's load tables, not a rule of thumb. Weigh the whole loaded rig on a public scale (truck stops and some landfills have them) at least once. Many owners find their boat is heavier than the brochure said. If you're over or close to the limit, talk to a tire shop about the next load range up, and make sure the wheels are rated for it too.

The spare must be the same size and type as the road tires, on a wheel that fits your hubs, inflated to the same pressure and not older than the tires it would replace. Check that its carrier is tight. Pack a jack that fits under your trailer's frame, a lug wrench that fits your nuts, and wheel chocks.

With the trailer on the ground and the wheels chocked, check each lug nut with a torque wrench set to the value in your trailer or axle manual (it depends on the wheel, the nut style and the stud size; don't use a car number). Tighten in a star pattern. Dexter explains that trailer wheels need to be re-torqued several times after they are mounted until the torque stabilizes; follow your trailer maker's re-torque schedule after any wheel or tire change. Never over-tighten: that can damage the wheel.

Switching wheels? Dexter warns that aluminum wheels are thicker where they bolt on and may leave too little stud for the nut to grip, so check stud length and the torque with the wheel maker. Any tire you mount must be approved for that rim size, and neither its load capacity nor its inflation pressure may exceed the wheel's own rating. A mismatched tire and wheel can separate violently.

Between trips, keep the tires out of the sun with tire covers or a shaded spot. BoatUS warns that parking on grass for a long time lets moisture get into the rubber. Park on concrete, or on boards or plywood. For long layups, check the pressure every few weeks and roll the trailer a little now and then so the same spot doesn't carry the load all winter.