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02-08-2011, 12:31
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#1
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Registered User
Join Date: Feb 2010
Location: Durban South Africa
Boat: L 34
Posts: 284
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Cored Hulls
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?
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02-08-2011, 12:40
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#2
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Senior Cruiser

Join Date: Oct 2007
Location: 29° 49.16’ N 82° 25.82’ W
Boat: Pearson 422
Posts: 16,307
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Re: Cored Hulls
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.
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02-08-2011, 12:44
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#3
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Registered User
Join Date: Dec 2009
Posts: 2,959
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Re: Cored Hulls
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.
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02-08-2011, 14:12
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#4
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Registered User
Join Date: Apr 2009
Location: West Palm Beach
Boat: Parkins Herreshoff 28
Posts: 956
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Re: Cored Hulls
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02-08-2011, 15:22
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#5
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Registered User
Join Date: Nov 2007
Boat: Beneteau FIRST 42
Posts: 1,836
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Re: Cored Hulls
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.......
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02-08-2011, 17:10
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#6
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Registered User
Join Date: Oct 2009
Posts: 174
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Re: Cored Hulls
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.
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03-08-2011, 08:13
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#8
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Registered User
Join Date: Aug 2006
Location: Brisbane Australia [until the boats launched]
Boat: 50ft powercat, light,long and low powered
Posts: 4,409
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Re: Cored Hulls
Quote:
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.......
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The builders did something horribly wrong if thats the case
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03-08-2011, 08:26
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#9
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Moderator
Join Date: Feb 2010
Location: La Ciudad de la Misión Didacus de Alcalá en Alta California, Virreinato de Nueva España
Boat: Cal 20
Posts: 21,616
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Re: Cored Hulls
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.
__________________
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