Better than new: Epoxy fillets Part I


Epoxy fillets can make a bond between components even stronger than the components themselves, ensuring that loads can be transferred between structures and panels effectively, with no risk of stress fractures or other structural issues. In this blog, David Johnson explains what they are and how they work. In Part II, we overview the five steps that are the hallmark of a perfect fillet.

All wooden boat builders need to create strong, durable bonds between the inside of the hull and structural components such as bulkheads and stiffeners. Critically, these bonds must be capable of transferring large cyclic loads between panels and components, leaving no weak spots anywhere.

Epoxy fillets really can be stronger than the components themselves.

For modern boat builders, the best way to create strong bonds between structural components is to reinforce them with epoxy ‘fillets’. These are formed with thickened so-called ‘radiused’ epoxy on the inside corner of wood junctions and are often used where plywood parts join, such as planking junctions, or where bulkheads and stiffeners mate to the hull.

In the past the ‘stitch and tape’ method of boatbuilding brought many designs, both power and sail, to DIY kit or plan build to worldwide use. This method used copper twisted wire to hold panels together and subsequently glass fibre tape and polyester resin as a means of making structural joints. However, the fact that polyester resin doesn’t bond well to wood negatively impacted quality and durability, increasing the risk of structural weaknesses and increasing the risk that expensive, in-use maintenance and repairs were required.

In sharp contrast to polyester, an epoxy resin/hardener mix is characterised by its high tensile strength, durability and its excellent adhesion to wood. Epoxy fillets really can be stronger than the components themselves.

Filleting with epoxy also plays a key role in the ‘stitch and glue’ construction method for plywood boats, which is based on the principles of metal welding. First, panels are held together at key points using wires or cable ties. Epoxy fillets are then added along the entire length of the bonds to create strong, durable hulls that can also be sheathed with fibre reinforcement and epoxy if required.

Click here to see the five steps for making the perfect fillet.

 David Johnson is Technical Operations Manager at West System International. He can be contacted here.


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