How to Add or Upgrade Boat Cleats (Sizing, Placement and Backing Plates)

Common upgrades: midship cleats for spring lines (many small boats have none), bigger bow and stern cleats to take thicker lines for storms, an extra cleat for a dinghy or fender lines, or replacing horn cleats with pop-ups where people walk. Look at how you actually tie up at your slip or dock and where the lines want to lead.

The rule of thumb cleat makers use, as the BoatUS Foundation reports it, is roughly 1 inch of cleat length for every 1/16 inch of line diameter:

• 3/8-inch line: about a 6-inch cleat
• 1/2-inch line: about an 8-inch cleat
• 5/8-inch line: about a 10-inch cleat

The Foundation's advice is to buy the biggest cleat your budget and the deck space allow. Practical Sailor points out how quickly this grows for storm lines: it says storm-mooring a 35-foot boat calls for 7/8-inch line and 14-inch cleats, while few boats that size have cleats over 10 inches. Size for the heaviest line you will use, such as your hurricane lines, not your everyday ones (see the dock lines and hurricane prep guides). A larger cleat with smooth, rounded horns also reduces chafe.

The BoatUS Foundation recommends a high-strength metal such as stainless steel, bronze or aluminum and similar alloys. In its tests, four-hole cleats with small feet tended to break at the feet, so look for feet that are large relative to the cleat and have plenty of metal around the bolt holes. Nylon and plastic cleats are for flags, fenders and light lines, not for mooring or docking. In salt water, aluminum or zinc cleats with stainless bolts can corrode where the metals touch; use the fasteners the cleat maker specifies and isolate them as directed.

Good cleat placement follows the lines:

• Bow and stern cleats take the bow and stern lines, which Discover Boating runs forward and aft to the dock.
• Spring lines run from a midship cleat (or the farthest-aft cleat if there is no midship one) to the dock, and they are what keep the boat from surging forward and back. Discover Boating calls the spring line the essential line when docking and also uses a line from the midship cleat straight across to the dock to stop the boat swinging out.
• Chocks and fairleads should be close to the cleat with no change in direction between them. The BoatUS Foundation notes that sharp bends weaken line, and Practical Sailor says pressure against sharp edges can cut a line's strength by 20 to 50 percent.

Also keep the cleat clear of walkways where it will catch toes, hatch lids and the swing of the anchor or windlass. Lay a line from the cleat through the chock to where the dock would be and look at the angle before you drill.

Go below and look at the underside of the deck where the cleat will go: is it solid fiberglass or a cored sandwich, is there room for a backing plate, and what runs nearby? The BoatUS Foundation advises avoiding soft-cored areas such as balsa for cleats; if you can't, the core has to be removed and the area reinforced, a job it says is generally best for a professional. Ideally the builder solidified the deck where cleats go, but BoatUS's surveyor warns that you'll often have to deal with the core yourself (step 8).

BoatUS shows why plates matter: two 5/16-inch bolts with washers give only about 2 square inches to resist pulling through, while a simple 3-by-6-inch plate gives 18 square inches, almost nine times more. The BoatUS Foundation suggests an under-deck plate about twice the cleat's length and half its length wide. Material options from BoatUS:

• Aluminum: at least 1/4 inch thick (3/8 inch if needed), easy to cut; taper the edges.
• Stainless steel: stiffer and stronger, harder to cut; 1/4 inch is usually plenty.
• G10 fiberglass board: corrosion-proof and stiff, usually 1/4 inch.
• Marine plywood: void-free with waterproof glue, tapered edges, sealed with several coats of epoxy, with the bolt holes drilled oversize, filled with epoxy and redrilled so water can't reach the wood. The plate must sit flat against the underside; fill any unevenness with thickened epoxy first.

Cover the area with masking tape to protect the gelcoat. Mark the holes using the cleat's template or the cleat itself. BoatUS's surveyor runs the drill in reverse until it's through the gelcoat, to avoid chipping, then switches to forward. Drill straight down. Look at the shavings as you go: they tell you whether the deck is solid or cored, and with what.

• Plywood core: BoatUS says there's no need to remove it. Just wet the exposed edges in each hole with a few coats of epoxy so water can't migrate into the wood.
• Balsa or other soft core: remove the core around each bolt and replace it with epoxy. BoatUS's method works from below: cut through the bottom skin and core with a 1-inch (or larger) hole saw centered on each hole, stopping before the top skin and picking out the last bit of core by hand so you don't damage the deck surface. Tape the bottom of the hole, flood it with epoxy to soak the exposed core, remove the excess after a few minutes, then fill completely with thickened epoxy, stirring out air bubbles with a bent paper clip. Once cured, redrill the bolt holes through the plugs. The plugs keep water out and stop the nuts from crushing the core.

Bevel the top of each hole with a countersink bit; BoatUS says this helps prevent gelcoat cracks and gives the sealant a better seat. Dry-fit the cleat, bolts, backing plate, washers and nuts. Fix any fit problems now, because once the sealant is on, as BoatUS's surveyor puts it, everything gets very messy.

Apply a marine bedding sealant to the cleat's base and around the holes. BoatUS suggests a polysulfide or hybrid polyether sealant and advises against permanent adhesives such as 3M 5200, which make removing and rebedding the cleat later very difficult (see the marine sealants guide). Push the bolts through, fit the backing plate, washers and nylon-insert locknuts from below.

With a helper holding the bolt heads, tighten the nuts until sealant just starts to squeeze out around the base, then stop. Leave it overnight so the sealant sets into a gasket. The next day, tighten to the cleat maker's spec. Don't overtighten: BoatUS warns that it can crush the laminate and deform the hardware. Trim cured squeeze-out with a sharp plastic scraper or knife.

These usually come with a precut backing plate that doubles as a template. BoatUS's method: hold the plate in place, drill one hole and put a bolt through it, drill the rest, fit bolts in all of them, tighten two opposite ones to lock the template, then cut the opening with a rotary tool or jigsaw. Seal any exposed core exactly as in step 8, since the larger opening exposes more of it. Remember the mechanism sits below the deck: check it won't hit anything underneath, and that water entering through it will drain where it does no harm.

Boating Magazine adds a few points from its own retrofit: many retractable cleats come with a drain cup underneath, and on a closed gunwale or deck you run a small hose from the cup's barb to a proper drain fitting (open gunwales that already drain to a self-bailing cockpit may not need it). If you're replacing a regular cleat, choose a retractable one at least the same size; its bigger footprint may hide the old holes, otherwise fill them. Check that the studs or bolts are long enough for your deck plus backing plate before you cut. Some pull-up models need only two large holes instead of a central cutout, if you'd rather not jigsaw the deck.

If the new cleat doesn't cover the old holes, fill them with thickened epoxy and finish with gelcoat or paint, or mount the new cleat on a base pad that covers them, as BoatUS describes. Once the sealant has cured, put a line on the cleat and load it firmly in the directions it will be pulled, while someone below watches the backing plate and the deck around it for any movement. Check the bolts and look for water stains below at the start of each season and after any storm.