The line where hull and deck meet is the longest joint on the boat. When it leaks, water usually runs down behind the linings and the source appears to be somewhere else entirely.
Joint types
- Flanged and bolted
- Hull and deck flanges overlap and are bolted and bonded, together with the toerail.
- Bonded
- Joined with structural adhesive and laminate; few or no bolts.
- Shoebox
- The deck sits inside the hull's edge; the joint is reinforced with laminate from inside.
Signs of leaks and damage
- Water staining on linings along the topsides, damp in lockers.
- Rust marks and cracked sealant around toerail bolts.
- Impact marks at the bow; the joint opening.
- A crack or ripple along the joint line outside.
Checks and repair
The joint is inspected from inside with borescope and torch, from outside by eye and sounding; wet areas get moisture readings. Repair is usually removing and rebedding the toerail; with structural damage, the joint is reinforced with laminate from inside.
Types of joint
On production boats, hull and deck are moulded separately and then joined. The joint is made on a flange at the hull's top edge, with the deck sitting on or inside it. It is held by adhesive, bolts or both; on some boats it is reinforced by laminating from inside. The joint usually sits under a rubbing strake or moulding and cannot be seen from outside.
Signs of a leak
- Water marks in lockers, especially at the hull edge.
- Rust weeping or gaps under the rubbing strake.
- Rain or seawater coming inside in a seaway.
- Joint bolts rusted on the inside.
Why it matters
Water entering at the joint damages the interior and causes mould, and also reaches the core at the deck edge and the fastenings. As the joint weakens, movement between hull and deck under load can begin, which increases the leak and fatigues the structure.
Repair
For a local leak, the rubbing strake is removed, the joint cleaned and resealed. For a widespread, structural problem, extra laminating from inside or renewing the fastening may be needed. The method follows the joint type.
On survey
Where the joint is accessible from inside, it is seen, moisture is read and fastenings are checked; outside, the rubbing strake line is examined for marks and gaps.
Prevention
Most joint leaks start at a damaged rubbing strake after a hard berthing. Checking the strake after any knock, and resealing early, keeps a small problem from spreading along the joint.
On hard-used boats
On charter boats and boats that berth often, the joint takes more load from quay knocks. Dents and cracks in the rubbing strake suggest the joint may be affected too. On these boats, checking the joint from inside is an important part of a pre-purchase survey.
Maintenance
The seal under the rubbing strake is checked at intervals. If the strake's screws have loosened or the strake has shifted, water enters the joint there. A gap found early is closed with a small sealing job.
Mould on the backs of lockers, lifting linings and water on locker floors after rain are often the first signs of a joint leak. When they appear, the joint is checked outside along the strake and inside the lockers together, to find the exact spot.
Repairing before finding the source often just moves the leak elsewhere, so the source is always found first.