Cored Hulls

[holmek]

O.K. Let's start this debate all over again. The purpose of coring is to create a distance between the outer and inner skins of fibreglass to create structural stiffness. so. - - It matters little whether the core is balsa/foam or other hi-tech material, the core is not structural, the issue is the GAP. NOW, should that core material get saturated, what matter? Sure, there is a (slight) increase in boat mass, but even if the core should get totally removed at this stage, the GAP(even be it an AIR gap) is the strength. This gap will be maintained in both the saturated and removed scenarios since there are numerous points, thru hulls/deck joins/winch pads/keel floors etc to preserve the integrity of the distance between the outer and inner hull skins. What comments forumites?

[skipmac]

If the core is removed the "numerous" attachment points you mention will be completely insufficient to maintain the attachment and rigidity between the inner and outer skins.

For a cored structure to work the inner and outer skins must maintain their structural linkage throughout. Plus with the core removed, fiberglass is quite flexible enough for the skins to bend together and touch, even over a pretty short span.

FYI I do own a boat with a cored hull. 1984 Pearson, in the water since new except for bottom paint and the core is pristine. Even got to see it since I had the bottom shaved to remove 25 years of layered on bottom paint and superficial blistering in the gel coat. Under the gel coat the fiberglass is quite transparent and you can clearly see the balsa core and individual blocks.

[daddle]

It does matter, structurally, whether it is foam, balsa, Hexcel, plywood, planks or whatever. The design for each will be different. It is definitely not just an air gap. Transverse compression and tensile strength are important.

[pressuredrop]

Originally Posted by holmek

O.K. Let's start this debate all over again. The purpose of coring is to create a distance between the outer and inner skins of fibreglass to create structural stiffness. so. - - It matters little whether the core is balsa/foam or other hi-tech material, the core is not structural, the issue is the GAP. NOW, should that core material get saturated, what matter? Sure, there is a (slight) increase in boat mass, but even if the core should get totally removed at this stage, the GAP(even be it an AIR gap) is the strength. This gap will be maintained in both the saturated and removed scenarios since there are numerous points, thru hulls/deck joins/winch pads/keel floors etc to preserve the integrity of the distance between the outer and inner hull skins. What comments forumites?

[Randyonr3]

when Balsa is used as a core material, it is end cut and absorbs the epoxy through out, actually making it part of the epoxyed hull.. had the pleasure of installing a throu hul in a balsa cored boat.. the balsa was just as the hard as the glass applied to both sides.......

[gulfstar37]

You are right about the distance between the two layers; however one item you may not have thought about is the core material itself as a structural medium. For instance with any type of core when interfaced with the two layers, (they have to be bonded together) when a moment of force is applied between two points, such as when you walk on your deck, in all practices the center of the core material is in balance. The mass above the center is in compression and the mass below the center is in tension. This is what gives the cored material its strength. Now the differences in the material of core material may it be balsa, plastic, honeycomb, stringers etc. with give it the desired effect that may be weight of product, stiffness, ECT. The other item that is considered is the two outer layers and what that is made of. Remember when the two layers are in tension and compression, the outer layer has to be strong enough to hold it all together without separating. This is where some of the exotic DuPont, 3M, Dow, materials such as Kevlar and such comes into play. Again it depends on where you what the strength and how it is laid up.

The thing to remember is when a core losses its bond to the outer skins either by soggy wood, loss of adhesion, etc. it will loose its property and strength and fail. If the core rots out and all you have is air left between the skins it will fail.

Short physics 101.

[sdowney717]

yes core is structural. takes compressive forces and cant tear free from the glass or the boat will start falling apart. I have read from yacht survey.com about when the core breaks down and the boat pounding in the water turns it into dust.
Boat Hulls - Cores and Structural Issues: Online Articles by David Pascoe, Marine Surveyor

[cat man do]

Originally Posted by Randyonr3

when Balsa is used as a core material, it is end cut and absorbs the epoxy through out, actually making it part of the epoxyed hull.. had the pleasure of installing a throu hul in a balsa cored boat.. the balsa was just as the hard as the glass applied to both sides.......

The builders did something horribly wrong if thats the case

[Adelie]

The core carries shear between the inner and outer skins to they can act together to carry bending moment. Without the shear capacity you just have two independant skins.

25yr old Mechanics of Materials classes coming in handy.