Structural Interior Furniture Options

What a great site!. Poking around and I've found a lot of useful information. Maybe my question's been answered and I just haven't found it yet. Regardless, I'm going to start fresh.

Am redoing the interior of my 36 year-old (new to me) 32’ ultra-light displacement boat. This boat is used for racing and being an ultra-light, is sensitive to weight.

The interior uses 9mm plywood and has significant areas of rot and checking. The ply used appears to be marine plywood in that the pieces I’ve removed are coming in around 35lbs/ft^3. The ply is unsheathed.

The furniture serves a structural purpose as it forms an interior grid running almost the length of the boat. Presumably, this helps counteract forces that want to bow the ends upward which would detract from racing performance.

So my question is what to replace the 9mm ply with. Off the bat, seems I could use 9mm okume at around 28lbs/ft^3 saving around 20%. I could then spend some of that gain on sheathing both sides with say 12oz biax (or uni?). If my calcs are right, this puts this hand layup (including the resin) slightly heavier than the untreated 9mm marine ply. My presumption being that okume has comparable characteristics to the marine ply.

Means that using okume, it seems I can get a reasonable weight savings going uncovered, or sheath with 12oz fabric and be roughly weight-neutral but gain stiffness in each of the components.

Alternatively… In most sections, I’m not constrained by thickness. This opens up the possibility of using a foam (corecell A500 or other) core material. My understanding is that around ½”, there’s no performance gain to using foam versus ply. But once the thickness of the core increases, the benefits of a structural lamination improves rapidly. So is there a combination of foam at certain thicknesses combined with x number of skins of y weight that improve performance versus untreated 9mm marine plywood? Performance being a combination of panel stiffness and weight.

Ideally, I’ll end up with all the fields in the table below populated.



Any and all insight and/or advice into this would be greatly appreciated.

 

You want light, there’s carbon fiber and epoxy, but if the old shelves are just failing after 36 years, I’d be inclined to replace same.

 

I don't know how much money you're willing to spend but there are composite products out there that are feather light (compared to equivalent plywood) and quite strong. The problem is cost. Have you heard of Coosa Composites? This stuff is light to extremely light depending on what panel you choose. Coosa is easy to work with, I always wear a mask though. I can't afford to use them for interior joinery but you may not mind the cost if light weight is that important to you.

Lightweight Fiberglass Composite Boards | Coosa Composites http://www.coosacomposites.com

You might also want to stop into Boulter Plywood Corp. over in Medford and ask them for some advice. They know their way around panels over there.

 

The longitudinal cabinet front faces and horizontal seats give fore-and-aft structure to the hull. Yes, anything non-structural can easily drop down to 6mm ply. But this is a very simple layout. Most every piece of the interior contributes to the boat's structure.

Not terribly interested in carbon due to cost.

 

Thanks... Yes, I looked into Coosa. Unless I'm missing something, their Bluewater 26 doesn't offer much of a weight advantage versus 9mm okume. It's expensive and would consider but the advantages seem nominal.

Boulter is a great recommendation. Was there last month picking their brains and getting some 6mm and some hard woods for another project.

 

Saw a suggestion elsewhere that I can longer find that the same weight composite panel will always be stiffer than the corresponding plywood. Presumably, the analog is also true. That for the same stiffness, the composite panel will always be lighter so long as there's enough thickness to capture the benefits of the composite structure. Suppose I could make up some test pieces but am hoping to minimize the number of false starts.

A 4' x 8' sheet of 0.75" Core-cell A500 weighs 10lbs. Add 2 layers of 12oz fabric, one each side (biax or uni), adds 11lbs for a total layup of 21lbs or 40% less than the marine ply and 20% less than bare okume. With apologies to all the engineers who will likely blanch at the question, is the resulting panel less stiff or "similarly", "modestly" or "significantly" stiffer than the 9mm marine ply?

Thanks again.

 

I have some build experience. Panel is stiffer than 9mm ply a bit as described.

A 12mm core panel with that layup is pretty solid.

If you can describe the structures dimensions better; I'd give an opinion.

But I believe you are confusing angles here. A piece of furniture is probably perpendicular to the side of the hull? So the strength is on the perpendicular?

All you really need is a nice stiff seat?

3/4" core
12 oz on top
2x 12 oz in tension on bottom
Taped to hull well

 

Knockdown

The full highth/width of the furniture is not structural. Only a few inches near the hull would be structural. The original builder used a single thickness of ply to simplify construction. You might look into decreasing the thickness of the non-critical portions

 

It's clear I've not been entirely clear. The boat has five bulkheads none of which are changing. Bulkhead #1 is one-third height and runs athwartships the width of the boat. It starts roughly two feet forward of the mast. A solid full-width horizontal platform runs from the bow to the leading edge of bulkhead #1.

Bulkhead #2 is the full height chainplate bulkhead.

Between bulkhead #1 and #2 is a vertical piece of ply that constitutes the vertical face of a built-in cabinet/seat. The horizontal sitting surface is fixed in place with a cut-out and lid to provide access.

Bulkheads 3 and 4 are two-thirds height bulkheads. Between them are vertical panels. Between #2 and #3, the panel rises to the height of the #3 bulkhead. The vertical panel between #3 and 4 is only 14" high to make this a seat though there's a seatback outboard of the panels 1, 2 and 3.

Bulkhead #5 is just forward of the rudder post.

Between #4 and 5 is a structural tube.

The continuous red dashed line is roughly 14" off the hull and 2' outboard of the centerline. I believe that having structure along this line adds significant longitudinal stiffness to the boat. The horizontal panels, tabbed to the vertical panels and vertical surface of the hull add additional structure. I also believe this structure is more effective than two long (one each side) longitudinal stringers in that by being off the floor there's distance between the hull and deck. Or am I off the mark? So if any of this is correct, I'm looking for panels that maintain stiffness along the red line but at less weight than the existing 9mm ply. And by going thicker with the panels all I'm sacrificing is a bit of interior volume.

 

Each panel is roughly 18" fore-and-aft; Panels 1 and 3 are 14" high and #2 is more like 24". Goal though is to make them like one monolithic piece of material separated only by the bulkheads.

 

Were the ply panels bonded to the hull with glass tapes?

Bottom and sides?

 

Yes, along the bottom edge. They were glued to bulkheads with 0.5"x0.5" lengths of wood to add strength to the joint. When I redo I'll likely use a fillet to give a more elegant interior joint.

 

Make the verticals with 12mm corecell 12 oz each side; tape in with a couple staggered 4" 1708 tapes each side.

Make the horizontals, if seats, as I siggested, 18mm foam wth one 12 oz top and two 12oz bottom. Same tape scheme.

For any horizontals not seats; a single layer of 12 oz is fine each side and same tape scheme.

This all assumes a reasonable bulkhead spacing I don't believe you stated.

calculate the total weights using 100% resin for the tapes and for the layup and compare to the ply...

if you are heavy, let us know; you could drop the tapes down to 1208 as well I'd say....

I am jist a builder. No credential.

I'd say you really only need 4" or so of core as hull stiffeners. After that, it is all furniture..so the tapes are doing all the work...

 

Thanks much! Roughly 18" fore-and-aft between the bulkheads

 

You don't say what the hull is made
Of either. But at 18"; it seems unlikely you need much hull stiffener.