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]

Hello again Lovato:

Yes you may have built a 42ft boat, but let's face it, one boat is not the same as another.

My boat is almost 30 yrs old and looks new: how about your boat?

I have got to laugh, you even try to ban me for commenting on your ideas (sic)!

 

Well I wish you a happy Christmas wherever you are and good sailing.

[Iain & Cherry Mae Cairns]

Ian R, instead of pulling people down with any idea's they may have about their boat, sail your perfect boat out into the middle of the bay,, throw the anchor over the bow ,, however don't let go.

Lovato is asking for collective thought, and you berate him.

Dude !!!!

[Dean Wilkerson]

Chopped Strand Mat adds weight, but very little strength.  Fiberglass epoxy adds an immense amount of strength, so much so that foam sandwich can be a direct replacement for plywood in many instances.  There is some disagreement as to what can be used for direct replacement.  Infusion makes a big difference in weight, avoiding excess resin while insuring a properly saturated efficient structure, epoxy resin beyond what is actually needed is just useless weight.  It will also ensure a good bond to the core.    You can find information on boatdesign.net, and other places.  My  inclination would be to use a fairly thick core.... core thickness greatly increases stiffness.   I won't give equivalency figures, because I feel some are radically conservative, and others are on the "optimistic" side.  Of course a foam sandwich structure is also basically immune to rot.   Marine PVC foams are often drilled and or sliced to provide adhesion and allow flex when being laid up.    I wouldn't build something like this with foam without doing my "due diligence", which would include lots of reading, and some test pieces.  

     Plywood is not an "efficient" structural material in terms of strength to weight ratio, usually requiring lots of additional structure because it lacks sufficient stiffness to span large areas without support... unless you resort to excessively thick plywood.   Ideally the highest tensile strength should be on at the surface, declining toward the center.   The I beam is the model of structural efficiency, and foam core somewhat mimics this model.   I used to have some truck bed material that was an extruded aluminum material that essentially amounted to many small Ibeams joined together forming a solid deck.  Incredibly stiff and rugged.  To me that is a near perfect model of what you are trying to achieve with foam core.  In a perfect world we would have a foam that varies in density from high density surface to low density at the center like an egg just beyond the "soft boil" stage....... and it would be shot through with fiber... something we will of course never see........... how could you manufacture it?     I like foam sandwich for the fact that you can build a boat without stringers (to catch moisture and dirt and grow mold, and foam adds insulation... reducing or eliminating condensation, and properly done foam sandwich runs about 66% of the weight of plywood on stringers for the same strength, and of course zero rot.   With your project, the only benefits are reduced weight and elimination of rot...... both of which are significant.     If interested in this topic, I recommend using Google extensively and using your own judgment.   Rob Denny designer of the Harry Proa is one of the gurus of foam sandwich and you can find a lot of his writings on the web.   IMHO he leans more toward the "optimistic", than the "conservative" side, but he practices what he preaches, and has a lot of experience with infused foam sandwich.

[Roger]

I know from my own projects that 6mm (1/4") plywood glassed on both sides with 6oz cloth is extremely stiff and strong. I don't have any data, but would expect it to be at least as strong as 9mm ply, but is also better protected from the elements and wear and tear. WEST have data on relative stiffness and strength of glass and ply combinations if you look around on the interwebs.

[Lovako]

Hi Roger, thanks for your reply, imensly appreciated.

I understand all you say and am thankful for constructive information.

I just need to remind all that here where I am i only have ply and fibreglass to work with - no foam sandwich is available. It is the "sticks" Not even epoxy glue is available. We make use of thickened resin with baby powder.

We are 300 years behind time. I am realy so appreciative of all input.

Thanks and take care

Dries - Madagascar