How to Choose, Install and Adjust Trailer Guide-Ons (Posts or Bunks)

If you mostly launch on calm, well-built ramps and just want to see the trailer, posts are enough. If you fight wind, current or a tidal ramp, or you load single-handed, consider bunk-board guide-ons. BoatUS's writer switched to bunk guides after watching a friend use them to load in a strong side current. On a long or heavy boat, some owners pair tall posts (for visibility) with side bunks (for strength).

Posts should stick up well above the water when the trailer is at loading depth, and reach high enough to be seen from the helm. Common kits come in more than one height; pick one that ends a little above the gunwale or rub rail, not far above it where it can catch on a bow rail or swim ladder. Bunk boards should sit at the widest, flattest part of the hull side, below the rub rail and above the chine. Check that nothing will hit the trailer lights, fenders, steps or winch post.

Guide-ons work best at the back of the trailer, where the boat first meets the trailer. BoatUS's how-to rests each upright against the frame and moves it close to the side of the boat at every possible spot, then picks the four mounting points with no interference. Mounting next to a crossmember usually makes the post stiffer. Check that a tightened U-bolt won't pinch a light wire, brake line or bunk bracket.

Fit the U-bolts around the frame rail and through the guide-on brackets. BoatUS's install needed only two wrenches and a screwdriver for that kit. Snug the nuts but don't fully tighten yet; you'll be sliding things in and out to fit the boat. Make sure the brackets sit square on the frame so the uprights stand straight.

Slide the uprights in or out until there's a small gap between each guide and the hull, the same on both sides. You want the boat guided, not squeezed. Too tight and the hull rubs every time the trailer flexes; too loose and the boat can still land crooked. Many owners aim for about an inch or two per side on posts. With bunk guides, BoatUS's writer held the board loosely against the side of the boat to set the angle of the swivel brackets before tightening.

Tighten the U-bolt nuts evenly so the bracket doesn't twist, then tighten the upright clamps. Sight down the trailer from behind: both guides should be the same height and the same distance from the centerline. Push hard on the top of each one. A little flex is normal on PVC posts; a bracket that slides or rotates is not. Fit the end caps or plugs.

With the boat strapped down, look at each guide-on where it's closest to the hull. Bounce the back of the trailer by hand. If a post touches the gelcoat or the rub rail, move it out slightly. BoatUS notes that properly installed bunk guides, together with good transom straps, also help keep the boat on the trailer while you tow, so a light touch on bunk-board carpet is fine; a bare post or bracket touching the hull is not.

Guide-ons that stick out past the trailer fenders can push your rig past your state's maximum width. Measure the widest point of the rig (boat, fenders and guide-ons). If it's over your state's limit, slide the uprights in or see our oversize trailer permits guide.

Some post kits have LED lights in the top of each post. They help you find the trailer at night and in murky water. Keep these points in mind:

• They are extras. They don't replace the required tail, stop, turn and side marker lights.
• Tap into the trailer's running-light (tail) circuit with sealed, heat-shrink connectors, and run the wire along the frame with clips, away from moving parts.
• Use LEDs rated for submersion. Most lighted posts are made to go under water at the ramp.
• Test with the trailer plugged in. BoatUS's connector guide explains how the tow vehicle's wiring powers all the trailer lights, and some vehicles need an adapter for LED trailer lights.

On a calm day, practice loading. Back in until the bunks are wet and the posts are well out of the water, then idle the boat in between them. The guides should catch a drifting bow and steer it toward the centerline without stopping it dead. Watch from the dock with a helper the first few times. After you pull out on level ground, check the boat sits centered and readjust if needed.

At the start of the season and after any hard bump:

• Check U-bolt nuts and clamp bolts for tightness and rust.
• Replace cracked PVC sleeves, split roller covers and worn or torn bunk carpet before bare wood or staples touch the hull.
• Spin rollers by hand; replace any that are seized or flat.
• Check lighted posts work and their wiring isn't chafed.
• Rinse with fresh water after saltwater use, and add a guide-on check to your monthly trailer look-over (Mercury's checklist already includes bunks and rollers).