Robbins Plywoods compressive strength

MagicalArmchair

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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.

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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).
 

MagicalArmchair

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I really liked the idea of engineering plastics for the truss, and Tufnol has a compressive strength of some £29k psi (13 tonnes on one inch square!! That's comparable to steel.). Can't rot either, no galvanic corrosion - what's to go wrong? Thanks MoodySabre, interesting figures

Edit - one problem with materials that don't give (such as stainless or some forms of phenolic) is that is it is not sympathetic to the surrounding structure, being that GRP with either polyester or epoxy can flex 4%, leading to a greater possibility of it breaking away from the hull. Over thinking it? Certainly a disease I suffer...
 
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nemodreams

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Robbins super elite plus is layed up with all sapele veneers if I remenber rightly. Maybe just check out the compressive strength of sapele ?
Would not be for off the mark.
I find it amazing that Robbins are not able to quote a figure for such a top end product ?
 

MagicalArmchair

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I thought along those tracks too. Sapele has a MPa of 60.4 Mpa (3.9 tonnes per square inch, so along the 8 inch truss that's 31 tonnes, so plenty of margin with a 3 tonne rig) . White oak is 50.8 MPa (3.2 tonnes per square inch).

I have read that the compressive strength of plywood is lower than its solid wood counterpart, but by what margin? Going by the above, the ply manufacturing process would need to have stripped out 90% of the Sapeles strength compared to the solid wood for it to be a problem, and I very much doubt that would be the case. In all conscience though I can't use a material in a critical structural location where the manufacturer cannot tell me at what point it will fail. I suppose the truss will be layered with epoxy and layup anyway (which will provide the real strength), so I should probably stop worrying about it and go for the most cost effective material for the job.

All that said, I can buy Whale Tufnol, where they will happily reel off the material specifications, for nearly the exact delivered cost of the Marine Ply option. The Tufnol is 3.5 times stronger than the Ply (the whole truss could take 104 tonnes before failure!). I'm a bit concerned about machining it though, I suppose carbide jigsaw blades would be the way to go when cutting and a slow feed. They tell me if the water does get through the epoxy it will only swell slightly and it will not delaminate or lose strength.

I've emailed Robbins to question once more the fact they don't have the compressive strength figures of their product, I'll post back what they respond with.
 

MagicalArmchair

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No thread drift at all LittleSister, its a very valid question. The original steel truss I can only imagine was designed to direct the rig stress's directly to the keel, however, when they encapsulated the steel truss in resin, this effect was lost and the forces would have been distributed to the hull and keel alike (which actually presents no problems at all). If Albin Marine would have just filled that void with more thickened resin there never would have been this problem, only the addition of this mild steel truss has caused the failure of the whole step - home goal for the Ballad 40 years on.

I may go for the Tufnol option then.
 

Concerto

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Personally Mark I would go with the Tufnol option. The only question I would check is how good is the surface bond with epoxy. I doubt if it will be a problem, but worth checking.

The new truss will not be operating in isolation as there will be additional support from the epoxy glass lay up. If the top is covered with the same, or thicker, lay up then the compressive loading will have been massively distributed between the different materials to the keel. So which ever material you use will be over engineered for your use, besides I doubt if you will ever load the rig to close to maximum compression loading - even if racing.
 
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