MagicalArmchair
Well-known member
After my mild steel truss rotted away and a painful extraction process of the old truss from solid resin, I am in the process of redesigning a new wood/glass-fibre/epoxy composite truss to support the foot.
A Ballad owner who had the same problem completed the repair as below. The whole supporting truss structure is 1 inch marine ply.
A cap of oak goes across the top of the raised section of the aft bulkhead to the small transverse bulkhead forward with the grain running perpendicular to the boats centreline into which the foot is secured. It has been sailing for two years since this repair.
As the truss will be transferring the full 3 tonnes of rig load to the keel, I called Robbins timber to get the figures on the compressive strength of their 1 inch Super Elite Plus ply. They don't have that figure and it sounded as if they had never been asked to quote it, which did baffle me. The compressive strength of white oak is well documented and is circa 7,440 psi (pounds per square inch, so about 3.3 metric tonnes), the cap to the truss is about 8 x 5 x 1 inches, meaning an oak truss beneath that could carry the weight of the rig many times over.
My initial response had been to follow what the other owner had done and use Plywood throughout with an oak cap on top, with the addition of GRP layup over the whole thing, however, now without the maths to back the whole thing up, I am now uncomfortable doing this and am swinging back to using plywood for the bulkheads and white oak for the cap and truss .
Questions for all you marine jedis out there:
A Ballad owner who had the same problem completed the repair as below. The whole supporting truss structure is 1 inch marine ply.
A cap of oak goes across the top of the raised section of the aft bulkhead to the small transverse bulkhead forward with the grain running perpendicular to the boats centreline into which the foot is secured. It has been sailing for two years since this repair.
As the truss will be transferring the full 3 tonnes of rig load to the keel, I called Robbins timber to get the figures on the compressive strength of their 1 inch Super Elite Plus ply. They don't have that figure and it sounded as if they had never been asked to quote it, which did baffle me. The compressive strength of white oak is well documented and is circa 7,440 psi (pounds per square inch, so about 3.3 metric tonnes), the cap to the truss is about 8 x 5 x 1 inches, meaning an oak truss beneath that could carry the weight of the rig many times over.
My initial response had been to follow what the other owner had done and use Plywood throughout with an oak cap on top, with the addition of GRP layup over the whole thing, however, now without the maths to back the whole thing up, I am now uncomfortable doing this and am swinging back to using plywood for the bulkheads and white oak for the cap and truss .
Questions for all you marine jedis out there:
- Does anyone know of any studies on the compressive strength of Plywood? Or a manufacturer who publishes those figures?
- Am I being a bit precious? Robbins do a cracking product which is used throughout the boat building world. Should I trust it in the way the other owner has done even without the documented proof that it can take the compressive loads? Whatever the compressive strength is of the wood core, we ARE making a composite truss after all that will have its strength greatly augmented by GRP layup and West System epoxy (inferior polyester resin and woven rovings come in at circa 20,000 psi, and epoxy is far times stronger than polyester).
