Inner and outer core density variation

I was pondering this for a while and finally tried to find any information on this. I could not find much which means this is either a stupid idea or rather clever.

In my understanding the stresses in the core of a foam core construction are highest close to the skins and lowest in the center.

Could a lighter or stronger boat be built by having a very light foam in the center, sandwiched between higher density foam if compared to a single density foam core?

I do realize construction could be a lot harder and there are more possibilities of defects but while answering please assume it can be made without defects.

 

My non-engineering seat of the pants answer is that any composite is only as strong as it’s weakest component.

 

That's a no-brainer that means nothing in practical terms because the weakest component of a compound can be located at a point where the stress is lower than anywhere else in the compound.
Answering @Qmaran, the fact that the density is higher does not imply that the material is more resistant to the stresses experienced by the core. Tension/compression stresses in a well designed core are never an issue, the core tends to work with very high compliance factors, so putting in two cores with different densities is not likely to help. On the other hand, the interlaminar tensions between both nuclei can represent a problem. My opinion is that it is better that you forget that solution.

 

Following your reasoning there would be no explanation for the wildly different foam densities that exist ranging from 30 to 300 kg/m2. I can see that a very light but thick core would be very stiff and strong but would fare terribly with a hammer blow. To withstand a hammer blow you would need a denser core or way thicker skins. My reasoning is that by having a heavier "outer core" you get the hammer blow resistance and the lighter inner core gives you more thickness for high stiffness and strength without the weight penalty of a thick and uniformly dense core. But I am not an engineer so I might be missing something.

 

...the essence of cored composites

 

@Qmaran, in general hull laminates are not designed to withstand hammer blows. Even icebreaker hulls are designed more with ice abrasion in mind than hammer blows.
The best thing to withstand hammer blows is to place a steel core with a surface tempering and forget about the weak outer layers of foam or fiber.
My reasoning may or may not be correct, but it corresponds to what must be taken into account when calculating the structure of a ship subjected to the regulatory design pressures.
In structures everything is possible if it is done well, so I am not going to tell you not to do what you are thinking. If, for whatever reason, you need to place two cores with different physical and mechanical properties, take care that the interlaminar shear stress does not make one core slide over the other. As simple as that.

 

This is where it already starts to go wrong.
The core, is designed to carry the shear load...which is ostensibly uniform across the section...not as you describe.
You're referring to bending stress not shear stress.

Indeed you are...

 

In elementary engineering classes, stresses are decomposed into a few simple types and you learn models to handle those neat little loads. But the real world is messy and complicated.

Puncture resistance isn't impact resistance isn't peel resistance isn't rolling load tolerance isn't cyclic load tolerance isn't slamming tolerance isn't fastener holding ability isn't trailering and lifting tolerance. None of these decompose neatly into shear, tension or compression. Furthermore, the foams and the GRP are all non-isotropic, so figuring out load paths is tricky.

Boats that are small enough for a backyard build will normally have adequate global strength once the hull is thick and strong enough to survive the bumps, bangs and other indignities they all suffer over time.

For crush and puncture resistance, which are major concerns for small craft on a weight budget, the GRP can be thinner if the core has a greater elastic modulus and compressive strength. Having said that, I usually consider any foam lighter than 200 density unsuitable for hulls anyway, so if that is the light stuff, what did you have in mind for the heavy stuff?

One way this is done in practice is to use a low density additives in resin to wet out the foam. The idea is to get a hard (not brittle) smooth sandable surface before laying down glass. There are also coremat products that take a lot of the guesswork out of it.

So using what's out there - divinycell 200 density, a light sandable bog for fairing, coremat 610 density (various thicknesses to suit), then GRP at 1500 density or so.

(If what you were thinking was 40 density foam, then 2mm 80 density foam, then GRP - the answer is no - a solid laminate is better, cheaper, faster and stronger for the same weight.)

 

I enjoy the comment, but trying to understand how you expect all cores to be 200.

Even the cost of Gurit M200 is about 3x M80.

The cost is also a weight cost. M200 is obviously more than double the weight.

On balance, you make a lot of great points, but I am already nervous about my boat getting heavy from using too large fillets and 1708 tapes versus 1700, etc.

 

>a solid laminate is better< By solid laminate do you mean no foam or a single density foam core?

To give some more information on the intended use, it is for a light weight 16m proa with unstayed mast. Looking at other similar boats being built (like Harry Proa) I never see cores approaching 200 density. Usually more like 2cm 80 or 90 density foam or in that ballpark. I guess you are thinking of a large lead keel monohull when suggesting 200 and higher density?

 

How about posting a dimensioned sketch showing the typical rudder installation?Lets not overlook the reality that the force needed to steer the boat has to be transmitted to the skin of the rudder(s) and the core will have a large part to play .Applying several varieties of core material just invites poor bonding between them and can be very time consuming-which eats up the apparent cost saving.As we now know that the proposed boat is a 16 metre proa,is the rudder going to pivot through 180 degrees when it is at the leading end of the boat or will it be locked into the straight ahead position and have the flow reversed over the foil section?

 

I didn't know he was building a 16 meter proa. That definitely needs some thought to global strength, and the needed puncture resistance, scuff resistance, crush resistance, etc might all come when the global requirements are met. Then the core choice has more to do with ease of construction and having a core that can take and hold the desired shape before the laminating is done. Extra thickness can help there, so a a lower density gets used. Doesn't make the finished boat any better, just easier to build.