Ply Construction

[Lovako]

I’m in need of info concerning the building of a center box between beams 2 and 3 on a Pahi 42ft. It is not about building of the box, but that of material used.

My old one needs to be rebuilt because of rot. I used 9mm ply but was wondering if I can build it lighter with the same strength by using 6mm ply covered in chop strand or woven fibre. The fibre on the outside will also protect against water splashes? Any help will be much appreciated.

[Ian R]

Your solution would be regarded as a "bodge" in my eyes, though I am not over familliar with that boat.

You know that, so you asked for someone to tell you  (honestly?) ..........6mm is dinghy material.

[Jose Alberto Garibaldi]

Lovako - I don´t know much about the Pahi 42 boxes, but some suggestions on the method. You could try what you have in mind, but I think it might end up being difficult to have a good result without having some sort of mould to place the chop strand on beforehand. But again, you might already had advanced some solutions to that. I had used the sandwich method, and that might be an alternative path to what you had been thinking. You could make panels with skins of thin, softwood plywood, maybe around 3 mm, and place a central core of extruded polyesterin insulation, made 4 to 6 mm, and border the sides with cedar similar in thickness to the core, and glue both the core and the sides to the skin. You would need to press with some well distributed weights to glue the centre insulation parts, and clamps for the cedar borders. You could try combinations - ie. a bottom and side panels, or side panels and  bottom solid ply, or...etc. Internal divisions are useful, and add strength. You would not to bad if you also fillets in all cases. If you cover the resulting assembly with some fibreglass outside for waterproofing, that would be relatively light,  and quite strong. Be careful with the gluing if you don't want to add weight. Best wishes in the building, 

JAG

[Ian R]

Well exactly- you can't do a proper job with fibreglass by just layering onto thin ply. We have all seen some awful stuff done like that, it's very heavy, lumpy, and dirty and not strong at all. Not a good rep. for Wharram boats.

Sorry to be brusque, I have been told that I'm too hard, but then I remember well plenty of advice of that nature given to me over the last 50 years and it sticks-  probably better than chopped strand...

I would guess that the easiest/cheapest, fairly light way would be 9mm  or thicker  fir ply properly sheathed then filled with a filler coat and sanded off. That may even be what the design says...Fir- ply is a bit wobbly, but battened would be alright.

In the UK cedar is very expensive just now.  A bientot -

[Lovako]

I never asked concerning the build of the box, BUT concerning the different streangths between 9mm ply, 6mm ply sheathed in one layer of fiberglass or 6mm ply sheathed in one layer glassfibre both sides.

By the way if you have ever done some glassfibre jobs it wont be "lumpy" or "dirty"

How do you think a Wharram is built? Ply sheathed in glassfibre, which indecently  is not lumpy, dirty or weak.

You also said: "Sorry to be brusque, I have been told that I'm too hard . . " You never got the hint ???

[Ian R]

Unlike most of the people on here I have built a Wharram as it happens!

Lorra people wanting to buy on "Wharram Builders and Friends" ;  not so many building.

I shared a professional workshop with a professional fibreglass laminator so I have seen and sniffed it all.

Please don't drag down Wharrams by slapping chopped strand on ply.

Suck eggs etc etc.

[Ian R]

Bonsoir

[Ian R]

Ian R said:

Unlike most of the people on here I have built a Wharram as it happens!

Lorra people wanting to buy on "Wharram Builders and Friends" ;  not so many building.

I shared a professional workshop with a professional fibreglass laminator so I have seen and sniffed it all.

Please don't drag down Wharrams by slapping chopped strand on ply.

Suck eggs etc etc.

[Ian R]

If you were meaning fibreglass cloth in epoxy, that's a different thing.  Since you mentioned chopped strand that would indicate you don't know the difference between epoxy/fibreglass laminating and  fibreglass laminating.

In which case get someone else to fix your boat. No apologies for being brusque....you might kill someone.

[Lovako]

Smart arse, i happen to have built my 42ft not a 31 dingy.

I used cloth.

Please get off my discussion

[Robert]

I am also rebuilding the boxes on my Pahi 42.

Would suggest avoiding chopped strand because of poor strength to weight ratio. Skins of woven cloth on both sides is a good idea—especially for water and abrasion resistance.

In fiberglass sandwich construction, increasing core thickness gives better stiffness at lower weight compared to increasing skin thickness.

Ribs/tophat reinforcements can increase strength with little extra weight. I have to put some thought into the best strategy.

Please share any insights you come up with.

[Lovako]

Hi Robert, Im all ears when people answer me in a decent way.

I built a large box hanging between beam 2 and 3 on a lip with x3 25mm stainless safety pins on each beam. My box lasted 10 years but must be rebuilt as the frame was built from soft wood. I now intend to use Sui, a hard wood that can stand the elements very well  “weatherproof” found here in Madagascar. I will build in the same way except my floor frame will be 70mm and not the 100mm I used previously.

My thinking is, instead of using 9mm ply I wondered if I can go for a thinner ply glass cloth covered to save weight? Not sure it will work as cloth/resin is a heavy in it’s self.

Maybe I need to do an experiment, weigh and see. :-( only once I get back to MADland, im stuck in SA till borders open.

Your input is much appreciated.

[Dean Wilkerson]

I agree that CSM is a poor choice, but woven fiber makes sense.     However there is another option if stiffness is the goal, and that is foam sandwich.    You could also use thin ply on one face in lieu of glass if you want that wood look, provided you use the proper procedure to get a good bond.   A piece of 1" divinicell for example with two glass faces would yield a very strong stiff floor that would still be quite light weight.

Here are a few files that may be useful... I don't know how to interpret the top line (thickness) in this first chart, but flexural strength and weight are pretty obvious......... where do the 7 and 37 figures come from and what do they mean?