Why not *very* thick core in hull & deck?

POLYURETHANE foam can be formulated to be soft and spongy or as hard as wood . its up to the chemist that makes the stuff .
I worked with a chemist for a time and we did spray insulation inside of steel boats plus spa pools heat insulation after they had been plumbed !! We also did packaging for a computer and electronics company polyurethane spray packaging for very delicate and breakable items wrapped in plastic bags and the air sucked out and then suspended over a blow of foam into a strong carton till the foam hardened enough to hold the item and then filled it completely and close the lid tightly !!
its amazing stuff what can be done with it !! :idea:

 

I've no idea what the OP meant by "typical" when it comes to core thickness in a sandwich construction hull. I recall that the divynicell core in the hull of a 350-ton mine hunter hull that we worked on was 40mm-50mm everywhere below the waterline...and of course that was designed for (and actually tested to verify) a very high shock resistance.

 

An ideal name for a boat built with 6" foam........"Thicker Than a Brick"

 

This is already being done in ships decks. I am not sure if they are using polystyrene or reactive 2-pack. The later is rather heavy and expensive. It must be progressively sprayed as overheating can occur. A very good system on flat profiles. Compressional strength of polystyrene will surprise you. It can be easily cut to shape by hot-wiring around patterns. But – it dissolves in petrol and (maybe) diesel. And, how do you pressurise the foam and weld plate over it? Trade secret sorry.

We are building flat barge decks from 6 mm alloy, using this system. They can take a 15 ton point-loading without oil-canning. Don't knock the OP

M C

Floatainer Systems

 

The OP asked a simple question and got some good answers. One way to look at the issue is to consider that we are talking about a refrigerator instead of a boat. More insulation thickness is better but the disadvantages in space and expense is obvious. Just so in a boat. When the designer gets the level of any property needed for the application, he stops adding more unless there is no downside.

I have an icebox with 8 1/2" foam on the bottom. Overkill? Probably. But the only downside was minor cost and minor labor, so the box is super insulated compared to others and works quite well. Doing the same thing on the boat hull would devastate the interior space.

 

Done properly this system can eliminate beams or stiffeners. Anyway, what sort of boat are we talking about?
MC

 

Try this link:
http://web.archive.org/web/20100806082633/http://www.bourneboats.com.au/

 

15-ton point loading on a sandwich with a 6mm alloy skin ? I want some of that core !!!

 

limitations of foam core sickness

This option is possible but requires sufficient shear stiffness between outer skin layers. The major problem is that inner foam core wouldn't have the same stiffness compared to laminate skins. To illustrate this behavior simply imagine ice-cream or butter layer between two thin plywood sheets. In order to make really thick cores one would need to provide shear connections between skins. This maybe labor intensive and is not worth in commercial production. It is entirely possible and the hull would be stiff, unsinkable. The problem to watch out for are:

1. Intense stress concentrations in critical areas (stanchions, mast, engine mounts; and deck, hull, keel connection areas)
2. Osmosis water infiltration
3. Puncture volnurability

and maybe some more.

It is definitely do-able on a small scale construction for a remote builder, but probably cost prohibitive for larger hulls and commercial production.

 

Many good points so far, thank you all.

Kvsgkvng, the math seems to tell me the stress reduces with thickness, in fact as a factor of 1/2d where d is the thickness of the core. So a core 4x as thick has only 1/8 the shear force. Therefore, this should allow the core material to get lighter faster than the core volume increases: use a 1 lb foam instead of an 8 lb foam, but 4x as thick, so the weight of the core is half as much, and the cost will be about half as much (since you mostly pay for the chemicals making up the foam, not the air).

When cores are very thin, they have the same load as a ply in the skin. Its only when cores are many times as thick as a ply that something like foam or balsa or paper honeycomb can be used.

So the math, and practice, is the opposite of what you suggest. Very thick cores converts loads like stanchions, deck loads, or sea forces into compression on the core instead of shear loads between the faces.

At least, that is what the math looks like to me. Not quite what I expected, but since it maps to practice, perhaps I understand this aspect of structure.

Warning: I am a computer scientist, not a structural engineer. So I could be, and I think I must be, missing something important here.

Why do I care:

The goal is to be able to lay out the interior without any concern whatsoever for structure. To be able to just have an open, cavernous space. To be able to change the utilization and interior layout without any concern over any structure.

Imagine a boat with no frames, no bulkheads, no floors: all the structure is simply in a very rigid skin (hull and deck), made so because the core is "very" thick.

As mentioned above, a Boston Whaler is an example. A surfboard is an example of 100% filling the space between hull and deck to no apparent disadvantage. But both are small scale.

Let's scale it up, say, 60 feet long, 10 feet wide, 7 feet from the inside hull skin vertically to the inside deck skin.

Lets say this is a 6-8 knot (slow speed) light displacement (so weight matters) powerboat (low powered, no rig loads, no significant point loads beyond davits, stanchions), but fully ocean going.

No flat surfaces, no developable surfaces, no straight lines. Deck has camber.

Presume core cost is irrelevant. Presume getting core to conform to shape is easy. Presume volume taken by core up to, say, 1.5 feet thick is not a problem.

I am trying to find where increasing core thickness to increase distance between frames to be more than 60 feet (so no frames are needed) becomes a structural issue.

A friend simply doubled the core thickness on his boat during construction to all good, no negative effect. The boat has been heavily used for over three decades. A big positive was it made the boat unsinkable, like a Boston Whaler. That boat is Sarrissa:

http://www.powerandmotoryacht.com/sightlines/still-cruising-after-all-these-years

 

The skins have to be certain thickness due the point loads etc. The core is optimal thickness when the structure is strong enough, after that added core thickness is wasted space and money..

 

A mistake in your math. The shear stress in the core due to bending is proportional to 1/2d as you noted. But the factor of 2 is a constant. So if the thinner core has a thickness of dorig it will have a shear stress proportional to 1/(2*dorig). The thickness of a core 4x thicker will be 4*dorig, and the shear stress will be proportional to 1/(2*(4*dorig)). The ratio of this shear stress is 1/(2*(4*dorig)) / 1/(2*dorig) which is 1/4, not 1/8 of the thinner core.

Assuming the shear strength of the core varies as it's density and assuming the shear stress due to bending determines the required core properties, then the required core weight per unit area will remain constant as the core thickness increases.

But as others have noted other factors affect minimum face thickness and required core properties.

 

OK, I think I get it:

While skins could be made thinner with a thicker core, the minimum skin thickness quickly becomes dominated by other factors, such as surface toughness, point loads, etc. If one computes the minimum skin based on these other factors, then there is only so much core required, and that much core is... not much.

Sound about right?

 

I posed the question awhile ago in a "club sandwich" approach but a few 'thick foam' cases are mentioned.

http://www.boatdesign.net/forums/boat-design/single-skin-vs-sandwich-fiberglass-43941.html

This thread is interesting also as the concept makes a much stronger foam core laminate and I think paves the way for thicker foam cores than are normally used.

http://www.boatdesign.net/forums/fiberglass-composite-boat-building/anyone-tried-45617.html

Here's another one...
http://www.boatdesign.net/forums/fiberglass-composite-boat-building/finally-43951.html

I've posted before about polyiso foam.

http://en.wikipedia.org/wiki/Polyisocyanurate

http://www.elliottfoam.com/features.html

It's what Carolina Skiff used in their early boats in the form of 3-6" thick "beams". The outer hull would be laminated and then the athwartships foam beams would be put in with a laminate on the vertical face of each and then the inner sole/floor/deck laminated. The outer hull and inner sole were tied together by the vertical laminate, resulting in a very strong boat.

A slight variation of the method was used in commercial below deck fish holds, where an insulated structural bulkhead would be built using 8-12" square or rectangular foam beams. A foam beam was set on the sole and glass laminated over the top, down the sides and unto the sole to tie the sole and bulkhead together. The 2nd beam was set on top of the 1st and glass laminated over the top and down the sides unto the 1st beam to tie them together, the 3rd done the same way to tie it into the 2nd beam, etc, etc.

Depending on how you use it and what you need it for, or how you can adapt your plans to fit the material, the foam can be used in marine situations, but it can be bought at wholesale, commercial roofing insulation prices. Since there is a lot more roofing going on in the world than composite boatbuilding and there is a lot more competition for sales of construction foams than marine foams, the prices are multiples less. Wholesalers are usually quite ready to sell to individuals, even without tax numbers and such. Since roofing foams are now being recycled to avoid filling landfills, you can get used foams of various sizes, shapes and conditions for pennies on the dollar of even the wholesale roofing prices. Even at the wholesale supplies they can have unused overbuys, returns etc for dirt cheap.

Aside from being impervious to all the resins, all the polyiso I've used has fiberglass filaments embedded in it, sort of like in concrete but not nearly as much.

 

I can't see sense in making any boat's Living Space less insulated than a typical house in VERY temperate California, which will be at least 1/2 sheetrock, 3.5" fiberglass, tarpaper and 1/2" plywood and 1/2" exterior plywood.

Since boats don't have attics and can't plug into 'the grid' for AC, I'd think any "live aboard" size boat should have at least 2X normal house insulation rating....most likely with space saving but little more expensive foam.

Shouldn't add much to cost of boat, but should make boat so much more livable all year around.

Plus the whole buoyancy and strength thing.

How about an extra layer? Outer fiberglass, metal or wood hull and layer of foam that works good for that, and then thin layer of glass and some kind of 'interior' foam that repairs easy to embed systems and interior panels.