supplementary beams: which material?

[Kris]

I need some 3,40m or so supplenentary beams to construct a frame for a bathing ladder and are wondering what people here usually preferr in regards to materials.

I have seen all sorts of materials being used, from aluminium bars to laminated plywood strips.

On a similar construction that I have seen they used hardwood, but in that length and thinkness it is a bit hard to come by here and also expensive. Let alone transport issues at that length.

Fiberglassing regular wood beams seems to be generally discouraged, although I am not 100% sure why. I guess different thermal expansion and or movement characteristics of natural wood and risk of unchecked hidden wood rot propagating through it?

So I am currently thinking the classic Wharram style of laminating strips of marine plywood and fiberglassing them would be best?

Can they be just laminated flat without any pre-tension to create a slight arch like I have seen in the Wharram designs, or is it too weak over such a length without that?

Also what would be the minimum number of layers and recommended thickness of the Plywood for holding the weight of a human?

For the ease of construction I am currently thinking to use 4 layers of 16mm plywood (cut to 100mm strips) overlapping 50 percent of their length, so 3600mm length in total, resulting in a 100mm x 64mm beam.

Am I also correct in assuming that if these beams are oriented with the plywood vertically it should be somewhat stronger for a person standing on it?

[Robert Hughes]

In a long solid beam, plywood has half the strength of solid timber.  This is because 50% of the plies are running across or through the beam, not along it, and contribute almost no longitudinal stiffness.  Box or I-beams are a different (and more complicated) matter, for instance because the web of an I-beam carries different stresses to the cap strips.

When I wanted to add a forward walkway to my Tiki, I found the simplest and most cost-effective solution was to buy an aluminium ladder...

[Kris]

This is also what I gathered from further web-searches.

What about epoxy-glueing some regular wood plank together? Like for example 15mm ones in 4-5 layers? I assume the epoxy would penetrate quite deeply into the wood that way and thus hidden rot would be somewhat mitigated?

[Kris]

So I checked again in the original building plans, and apparently the original Pahi 42 plans also laminate planks (18mm, up to 10 layers) of softwood for the main beams. Somehow the Pahi 42 I have didn't follow the original plans in that regard though and it looks more like a mix of hardwood and plywood under the paint, but not 100% sure.