What core material should I use for a transom or stringer?

What core material should I use for a transom or stringer?

Choosing the right core material is critical for structural marine components like transoms and stringers. These areas endure high stress, water exposure, and dynamic loads — so durability, strength, and water resistance are non-negotiable.

Coosa Bluewater

The most popular and recommended core material for transoms and stringers is Coosa Bluewater. It’s available in 20–32 lb/ft³ densities, reinforced with fiberglass, and is fully resistant to rot, water absorption, and delamination. Its superior compressive and screw-holding strength makes it ideal for:

• Transoms
• Engineered stringers
• Deck reinforcements

Divinycell H

Divinycell H is a closed-cell PVC foam core that offers a high strength-to-weight ratio with excellent fatigue resistance. It’s suitable for stringers and transoms in performance builds or where weight savings are important. Best used with epoxy or vinylester resin systems.

AirCell T Foam

AirCell T is a vinylester-compatible foam core with excellent water resistance and thermal properties. While it’s lighter than Coosa, it may be used in stringer systems for smaller vessels or where structural loads are moderate.

For a side-by-side breakdown, check out our Core Material Comparison Chart »