single skin vs. sandwich fiberglass

Has anyone ever done multiple cores, such as skin-core-skin-core-skin-core-skin? Would that offer any advantage? It seems like it could be a real tough hull, where each 'layer' would be exponentially harder to break or breach, as far as getting a hole in it that would let in water.

I read somewhere of a guy who took the foam core thing in a different direction than the thin, high priced "marine" cores. He took some kind of cheap block of foam, shaped the outside and glassed it, and then carved out the inside and glassed it, leaving the foam thick, like 12" or something. The strength came mostly from the foam itself, the glass just kind of sealed it up and gave it a tougher wearing surface. Has anyone ever heard more of that experiment/method?

 

I belief the RNLI severn class is build that way, somewhat like 5 layers of 1 inch corecell with glassfibre in between layers.

 

That's very interesting! I found this about their construction...

They must be tough, being all-weather ocean capable rescue boats...





I wonder if they use this sort of diagonal strip method...?

 

Yes - multiple cores are called - "the club sandwich" method. its described very well in Ken Hankinsons book "fiberglass boatbuilding for amateurs"
It is simply using thinner core- layers of glass, core-glass etc It add stiffness not necessarily puncture resistance. And there is a method-and i tried to find it today but failed to produce the vids on youtube- of a guy in australia, that builds an armiture of rebar -and a builds the boat inside out--then sprays expandable foam about 12 inches thick - fairs it off and then glasses it up.
I have seen only two examples of this type of boat being built...however in this case- the rigidity comes from the rebar framework NOT the foam.
try googleing it--i cant remember the guys name or the method but he uses a armiture then spray foams the entire hull--it does make a beautifully fair round bilge hull..but it is expensive due to the amount of foam needed,,,
I also have no idea how durable this method is...
hope that helps

 

Not so simple ! Cored hulls are quite vulnerable to damage from collision. Particulalry as the boat gets larger and heavier. The outer skin is too thin to offer much in the way of shear strength and the foam offers none, so the outer laminate fails and very shortly afterwards in a separate event the inner laminate fails. Once the force is high enough to shear the outer laminate it will also shear the inner one if the force is still applied and the object is long enough.

There are several structural reasons why cored hulls fare poorly in collision but a lot of it amounts to the fact that the skins fail in isolation rather than in unison and the foam does nothing structurally except to hold the skins in place.

Tearing can be a big problem for example, and even an anchor chain or a wire hawser can cut significantly through the hull of a foam cored boat, something almost impossible for the solid laminate equivalent.


I don't think cored hulls should be used below the waterline on displacement craft except where weight is the critical design criteria.

I posted on another thread recently:
http://www.boatdesign.net/forums/materials/weight-aluminium-vs-grp-yacht-7405-2.html#post572903

 

That might have been La Cage, formerly of www.bourneboats.com.au. He built steel mesh hulls, poured in low density insulation foam, and glassed around it.

Since his website is down, and he hasn't been around since 02-02-2008, the methods success cant have been terrific.

 

The foam doesn't count, it has no practical structural contribution no matter what type of foam you use.

 

Nobody has mentioned stiffness in this discussion - which is the biggest reason to use foam core instead of solid glass.

The stiffness of a panel is proportional to the thickness cubed.
The strength of a panel is proportional to the thickness squared.

So making a sandwich panel of significantly greater thickness, increases the the strength to weight ratio ALOT, and similarly the stiffness increase is phenomenal.

This is important when determining the frame and stringer/stiffener spacing for the boats structural requirements. You can have greater frame and stringer spacings if the panel it supports is much stiffer.

In general, most boats have a stiffness problem, not a strength problem. What i mean is, if the panel is stiff enough, it will almost certainly be strong enough and by a considerable margin. Boats needs to be stiff, so that over time the flexing is not excessive and therefore the boat destroys itself via fatigue cracking, joint flexing and delaminating issues just to name a few.

So it makes sense to use a cored construction where you have a stiffness problem as you can create a structure that is much stronger and stiffer, compared to the same weight structure in solid glass. Alternatively, you can design the same strength and stiffness but with much less weight, which means a more fuel efficient boat.

The problem with using relatively thin skins on a compressible core such as foam, is that you have a poor impact resistance... - The thin skin is easily fractured by the impulse of a point load, and the core beneath it offers little compression resistance so the whole structure in that area fails easily. This is why many boats choose to use solid glass below the waterline, but sandwich structure for the rest of it in order to save weight.

 

If you calculate the moment of inertia (decisive for stiffness), you'll see the inner glass layers contribute almost nothing. Add to that the fact that a multiple core construction is time consuming and almost impossible to repair, the idea has very little value.

 

The inner glass layers are an integral part of the sandwich panel structure, whether in tension or compression (dependant on the bending direction), how can you say "the inner glass layers contribute almost nothing"....? i cannot find another way of describing this comment other than absurd?

panel bending stiffness, D = (Ef*tf* h^2*b) / 2

where;
Ef = modulus of elasticity of the facing skins (fiberglass type, fibre direction and resin matrix dependant)
tf = thickness of the facing skins
h = thickness of the panel
b = breadth of the panel

And rough panel deflection equation not including the smaller shear component; sigma = (kb*P*l^3) / 2
where;
kb - bending deflection coefficient - dependent on type of loading ie. cantilever, simply supported, uniformly distributed, one end supported etc etc etc...
P = applied load
l = panel span

 

Agreed, but with club sandwich approaches, CDK is correct in that the inner 'glass portions of the laminate contribute very little, owing to their relationship with the center of gyration. The laminate is only useful with some distance from each other, well bonded to a suitable core. The added strength and stiffness of this approach is measurable, but negligible, progressively less so the farther away from the center you get, hence the point of why have these extra laminates there in the first place. Simply put you can save considerably by not employing this technique and just increase the skin separation with a single, thicker core.

 

ah ok, i didnt realize he was talking about the "club sandwich inner layers"... indeed putting glass between layers of core material contributes virtually nothing to stiffness and adds unnecessary weight and cost... a colloquial WOFTM as we say here waste of ........ time and money

 

Very few of my contributions in this forum have been rated as absurd....

In all calculations concerning bending or warping of bodies, the distance from the axis is the key factor. In formulas it turns up as square or 3rd power.

Calculating the true resistance to bending of a single core laminate is impossible without integrals/differentials because the foam core will be distorted (compressed) under load. For that reason the core must be interrupted at regular intervals. Some builders use plastic I-profiles or wood to maintain equal skin distance, others (like Glastron) simply add a few layers of glass between core lengths and accept the fact that it ruins the smoothness of the outer skin after a few years.

 

bourne boat??

This problem is easily solved - if your building a heavier cruiser--by inceasing laminate thicknesses...a tugboat for instance would use 1" thick core and 3-4 layers per side of triaxial--that would be stiff and almost bulletproof..steel of course is the best for impact resistance but- the core acts as insulation--thereby cutting costs in that area.

btw--i did come across the bourne method...i cannot see this hull being much stronger than the thickness of the laminate--and the foam would absorb a lot of water getting trapped.
theres not much info on it--this was all i could find..

http://bourneboats.blogspot.ca/
www.youtube.com/watch?feature=player_embedded&v=f7_hCYMNb8w

http://www.spray-insulation.co.uk/bourne_method.htm

 

And he was told that in no uncertain terms. he was just a crazy old farmer with an interesting hobby.

He managed to sell his first creation - and the new owner sailed it from southern australia to Sydney ..... where it ended up in the 'Boatpoint' boats for sale website with the heading 'Urgent Sales - all offers considered'