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thataway
Joined: 02 Nov 2003
Posts: 20803
City/Region: Pensacola
State or Province: FL
C-Dory Year: 2007
C-Dory Model: 25 Cruiser
Vessel Name: thataway
Photos: Thataway
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 Posted: Mon Nov 23, 2020 12:07 pm Post subject: Foam vs Balsa and repair of hull |
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We have had several threads on repair of wet/rotten core. When reading the recent Globe Vendee (solo race around the world) there were discussions about doing major repairs to cored structure when under way. The following Foam Materials in the Marine industry, by a structural engineer . If you are doing boat repairs on cored hulls, it is a must read. What applies to Foams, also applies to balsa. However The compression modulus of Balsa Wood is about 60 times that of M80 core--cell, and comressive strength is also 10X larger. Shear strength is also greater.
This information comes from forum "Boat Design.net". If you want to know about building and designing boats--a go to place:
| Quote: | The strength of the core IS important. It is the core which keeps the laminates apart. Balsa (end grain) is much stronger in compression and sheer than any of the foams. To achieve similar rigidity to a balsa core in a panel using a foam core will require a thicker core, and /or thicker glass laminates. The results are that a balsa cored panel can be lighter for a given stiffness than a foam cored one.
Compressive strength? That is precisely what is wrong with balsa in highly stressed areas it imparts the impact to the inside laminate! Hard to beat a “ductile foam” like airex or core cell ..Stiffness! Well that is great but taken to the point of yield a different story you have what ; failure better to give a bit and keep the panel intact!
Well that is great but taken to the point of yield a different story you have what ; failure better to give a bit and keep the panel intact!
There is a difference between stress and impact. In a stress loaded area it is important to keep the laminates apart, for rigidity. In an impact situation it really depends on how large the impact is : balsa will resist damage from small impacts better - it will support the outer laminate better than foam will. In large impacts it's true foam will transmit less of the force to the inner laminate, but the outer laminate and core will suffer greater damage.
Stiffness! Well that is great but taken to the point of yield a different story you have what ; failure better to give a bit and keep the panel intact!
The reason we use cores of any type is to increase stiffness, without adding weight. The desired stiffness of a panel could also be acheived by a solid layup, and often is, but at far greater weight than sandwich construction. So what we are looking for from a core material is the best possible panel stiffnes per weight. Panel stiffness comes from the cores ability to resist compression and sheer, and balsa is superior to foam in these properties. It is denser, but to get the same stiffness from a foam core requires a thicker laminate, or a thicker core, or both, and results in a heavier panel. ATL in Queensland make Epoxy/balsa/epoxy panels, as well as epoxy/foam/epoxy panels. They have comparitive data for them on their website. www.atlcomposites.com.au . |
Another must have for the person doing any glass work (mainly epoxy) is this PDF book from West Systems. This is a 397 page book on using epoxy in boats! Page 97 speaks to re-coring or coring a laminate. There are several pages dealing with specifics. I highly recommend reading the relevant parts of this book and keeping it as a download on one's computer.
Another subject we have not considered is the use of Vacuum bagging in the re-lamination of structures. Also the penetration of epoxy (which I always suggest for repairs which are not going to be gel coated to perfection) into balsa is addressed in this section.
_________________
Bob Austin
Thataway
Thataway (Ex Seaweed) 2007 25 C Dory May 2018 to Oct. 2021
Thisaway 2006 22' CDory November 2011 to May 2018
Caracal 18 140 Suzuki 2007 to present
Thataway TomCat 255 150 Suzukis June 2006 thru August 2011
C Pelican; 1992, 22 Cruiser, 2002 thru 2006
Frequent Sea; 2003 C D 25, 2007 thru 2009
KA6PKB
Home port: Pensacola FL |
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