How to Check a Boat for Transom and Stringer Rot

Clean the transom so you can see it. Look for:

• Cracks in the gelcoat around the motor mounting bolts, the corners of the transom and the drain plug. Spider-web or star cracks radiating from the motor bolts are a classic sign of a transom that flexes.
• Brown or rusty stains weeping from screw holes, the drain plug or the motor bolts. That is often water carrying rot out of the core.
• Fresh caulk smeared over old holes, or many old unused holes.
• A transom that looks bowed or pushed in where the motor mounts.

On an outboard boat with the motor tilted down and the boat on its trailer, grab the lower unit and push and pull it firmly, leaning some body weight into it, while a helper watches the top edge of the transom and the motor brackets. On a sterndrive, a helper can push and pull on the drive.

A sound transom does not move. If the top of the transom flexes, the gap between the motor bracket and the transom opens and closes, or you hear creaking, treat the transom as structurally compromised.

Tap the transom with a plastic mallet or sounding hammer every inch or two, in a grid, from the inside and outside where you can reach. Solid laminate over good wood gives a sharp, crisp click. A dull thud means a void, separated layers (delamination) or wet, rotten core. Mark the dull areas with painter's tape and photograph them.

Compare with an area you know is solid, like the hull bottom: you are listening for a change in sound, not a perfect note.

Look closely at the motor mounting bolts and the drain plug tube from inside the boat. Wet wood, black or crumbly wood around the bolt holes, or water seeping out when you remove a fitting are bad signs. Do not drill test holes in someone else's boat. On your own boat, a surveyor may take a small core sample to confirm what the meter and tap test suggest.

Walk slowly over the whole deck, especially near the transom, around the fuel tank access and along the centerline where the stringers run underneath. Feel for spots that give, bounce or feel spongy under your feet. Tap test the deck as well. Soft deck spots often mean the wood underneath is wet, and the stringers below may be too.

Open every hatch, inspection plate and the bilge. Look at the stringers where you can see them:

• Cracks in the fiberglass along the stringers or where they meet the hull
• Fiberglass that sounds hollow or has separated from the hull
• Black, mushy or crumbly wood showing through
• Standing water that does not drain, or sawdust-like debris in the bilge
• Cracks in the gelcoat or resin on top of a stringer, especially at the engine mounts. It may be only surface resin, but it can also be wet wood that froze and split the laminate, and either way it is a path for water: have it checked before you buy
• Soaked, heavy foam around the stringers. Waterlogged flotation foam and rotten stringers usually go together (see our waterlogged foam guide)

Press on exposed stringer edges with a screwdriver handle (not the tip). Firm is good; soft or crumbly is not.

A moisture meter held against the fiberglass gives a reading that rises when there is water in the laminate or the core below it. It is a useful clue, but not proof: readings change with the meter type, the laminate thickness, paint, temperature, and whether the boat was just washed or rained on. Take readings on a dry day, after the boat has been out of the water, and compare suspect areas to known-good areas of the same boat.

High readings plus a dull tap sound plus flex or stains is a strong combination. That is the time to bring in a professional.

If you have found no warning signs so far, take note on your next run: the hull sides should not visibly ripple or shift when you hit waves, and you should not hear new creaks from under the deck. BoatUS notes that hull sides that ripple and shift in waves are a sign of rotten stringers.

Map the problem areas on a simple sketch of the boat: transom, deck and each stringer. Add photos. This helps a surveyor or repair shop give you a realistic quote, and it is the record you need if you are deciding whether to buy, sell or repair.

• No signs of rot: keep it that way. Re-seal any screw, drain or transducer hole that looks dry or cracked with a marine sealant, and keep the bilge dry. Our deck hardware rebedding and through-bolting guides show how to seal fasteners so the core stays dry; on its own transom rebuild, the WEST System team coated the inside of every hole drilled through the wood with epoxy for the same reason.
• A small, local soft spot away from the structure (for example, in a seat or a small deck area): a fiberglass shop may be able to repair it locally.
• Transom flex, cracks around the motor mounts, soft stringers or a soft deck over a large area: this is structural. Get a marine surveyor or a reputable fiberglass shop to inspect it before you use the boat again, and get a written quote. Compare the repair cost with the value of the boat; with many older boats, BoatUS notes the repair costs more than the finished boat is worth.

Do these checks before you make an offer, and make any deal subject to a survey. Discover Boating lists a water-logged transom among the significant problems a professional survey is meant to catch. Unless you want a big restoration project and have the skills and budget for it, walk away from a boat with structural rot.