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Thread: Cored or NOT cored????? Reply to Thread
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Topic Review (Newest First)
04-18-2007 09:27 PM
sailingdog Coldandhardy-

Thank you for your post. Although it may be a bit more technical than most of the users would appreciate, I am glad you posted it. If someone were looking to reinforce a fiberglass hull for additional abrasion and impact resistance, what fiber would you recommend be used? I've been looking at using some of the UHMW PE fibers for doing such a thing, and wanted your feedback on it.
04-18-2007 09:15 PM
coldandhardy
Kevlar in boats

This is an old thread and I am new to this forum (in fact it is my first post) but I believe that I have something of credibile interest that could be added here. To the non-technical this post will be tedious and boring!!!!

I recently retired from an "advanced composites" business and I am seriously considering the pruchase of a boat. In my business I used Kevlar (and Twaron) for several balistic applications and I used fiberglass a lot (I always spelled it that way)! I even used carbon fiber in many applications (in fact my spelling is generally bad and I rarely use spell check for blogs).

When it comes to boats I believe it is important that one considers carefully blending advanced materials like Kevlar and Twaron (aramid) and Carbon fibers with fiberglass and resins. From a physics and applications perspective one has to be particularly concerned about the aramid group of fibers.

These fibers typically have a wetting characteristic that repels the resin that is trying to bond to the fibers (it is of little consequence whether it is vacuum bagged or laid up by hand/this is a matter of physics/surface energy) . Sort of like oil and water! There will be those of U that may choose to challenge me on this one but I suggest that U tell that one of the larges aircraft manufacturers in the world that had to replace the leading wing edges of a popular model that they made with aramid fibers and resins to cleverly survive abrasive and impact failures in this area encountered on runways (they replaced them).

The leading edges of the wings failed from freeze-thaw cycling of absorbed moisture (not abrasion or impacts) and much sooner than their life expectancy. Simply stated water expands and contracts as it freezes and thaws. It turns out that the fibers repelled the resins during vacuum bagging (physics relating to surface tension mechanisms) and as the resins shrank on curing they pulled away minutely from the fibers. The resulting emply space provided a capillary space for moisture ingression at an atomic level. The freeze-thaw cycle thereafter can be a very destructive process (and was) to composites as the aircraft manufacturer discovered. Water expands when it freezes. This is a huge force. It can split steel barrels!! Resins and plastics have relativly little strength.

Aircraft parts experience multiple freeze-thaw cycles during take offs and landings. The effect was that the aramid fibers delaminated from the matrix. The laminate puffed up (a bit like pop corn) and took on more moisture (compounding the problem) and resulted in laminate failures.

I believe that some waterlines on boats experience this same cycling but at a slower pace albeit that there is more water present for obvious reasons (certainly not when the boats are parked where freezing never occurs). I would be very mindful of this potential problem in boats if re-inforced with aramid fibers!!! IMHO they will not be as strong as intended in a few years as they were engineered and tested to be when constructed!

Aramid and carbon also have another characteristic about them that is purely physics (that is to say that clever design cannot outsmart Einstein completly)! This is called CTE (coefficient of thermal expansion). Fiberglass is a very elastic material (sort of a perfect spring) - (it also intimately wets out and can bond elemently with the resins with no separation space (elemental or otherwise) between resin and fiber). Carbon and aramid are very strong and rigid when compared to the reisn that they are embeded in. In plain english these fibers have a relatively neutral CTE - close to 0 - (effectivley they do not expand and contract in a meaniningful way when compared to resins and fiberglass). What does this mean??

They can add a lot of stress to the internal matrix (the resin to fiber bond/if there is one at all) when compare to fiberglass when variable temperatures are encountered. This is probably not a huge effect in boats but internal stresses in the laminate do take away from the ultimate strength of the laminate during temperature fluctuations.

Polyester and vinylester resins shrink while they cure to varing degrees but the fibers do not. Epoxy, on the other hand, expands while curing and shrinks during the cool down after cure (the shrinkage is small and has very little effect because the bonds with the fibers are formed during the expansion pahase of the cure cycle. The effect is that epoxy bonds very well to the glass fibers (and other fibers as well) when compared to polyester and vinyl ester. The expansion of the epoxy during cure forces the resin at a molecular level into the fibers (in a mechanical way) such that it produces physical properties substantially greater (upon cure) than most other resins by far!

Finally carbon fibers are a galvanic material. That is to say that they contribute to galvanic corrosion. Most boaters already know a lot about galvanic corrosion so I won't embarrass myself by attempting a technical description of the pitfalls of such.

Coming back to aramid fibers in my summary I would personally be cautious. I do like their impact characteristis and their outstanding resistance to penetration failure. I simply don't know if they bring a bigger problem than they solve with them. The galvanic corrosion properties of carbon fibers can be engineered around but when it comes to aramid pure physics is a more dominant player that cannot easily be engineered around!
01-08-2004 01:46 PM
Jeff_H
Cored or NOT cored?????

Paul:

A couple of quick clarificaction points here:

-"They are lighter and stronger than any Glass boat."
Actually C&C''s and Tartans are glass boats, they just happen to use epoxy resin. While I love epoxy resin for many applications, Vinylester resins (as used in military and motorcycle helmets) actually produces a higher strength and more impact resistant laminate.

"Each boat is assigned one team of craftsman who build and supervise all aspects of a specific hull number from start to finish." That is how most Oriental boats were built in the early 1980''s and it is no guarentee of a high quality product. In fact having laminating specialists do the hull and carpentry specialists do the interior and electrician specialists do the wiring, and etc. generally results in a better quality overall at a lower price.

Hopefully, someday Tartan and C&C will follow the current ''best practice'' and go to Kevlar/ E-glass laminates with vinylester resins and eschew using non-directional fabrics. Then they really will have the are lightest, strongest, most durable boats for a lot less money.

Don''t believe everything that you believe in Tartan/C&C ads. 8^)

Respectfully,
Jeff
01-08-2004 10:09 AM
Bluesmoods
Cored or NOT cored?????

Look into Tartan and C&C Yachts. They are both built by Fairport Yachts in Ohio.

I strongly suggest you arrange a tour to see how these boats are built with Epoxy Hulls. They are lighter and stronger than any Glass boat. Each boat is assigned one team of craftsman who build and supervise all aspects of a specific hull number from start to finish.

I am sure that at some point in the future, other manufacturers will follow Fairport with the epoxy Hull and carbon fiber spars. I don''t own either C&C or Tartan but perhaps someday will be be a pround owner of a Tartan 4100!!
01-08-2004 02:31 AM
Jeff_H
Cored or NOT cored?????

I have owned over a dozen boats in my life and my family members have owned dozens more. Over the years I have sailed on, raced on, coached on, commissioned, done repairs or consulted on repairs on, rented, and delivered probably something between fity and a hundred different model boats. I try to provide commentary based on my experience on these boats but to write a list and analysis of each would be a full time job for a while. I already have one those running my architectural firm. 8^)

Regards,
Jeff
01-07-2004 11:13 PM
john232
Cored or NOT cored?????

Thanks for the information Jeff. I just got done reading "The Worlds Best Sailboats" The first book and then the 2nd volume. Ferenc Mate is the author who went around to some of the best Sailboat yards in the World in the late 80s and again in the early 2000s and wrote some pretty interesting info about how each individual yard produced and engineered their yachts. All the things you have been saying about the subject of laminating hulls has been discussed in His books and you seem to know alot about the subject. Thanks. Also I was wondering if you wouldn''t mind doing a little boat reveiw on some of the various yachts you are familure with and have sailed in the past years. It seems like it would be infomative to get reveiws from not only you but others also who own particular models of boats and can share there pros and cons on there experiances with different models of boats.. Just a thought. Later
01-07-2004 12:53 PM
Jeff_H
Cored or NOT cored?????

I am not sure where you came up with the idea that Cflex is lighter than conventional construction. Cflex generally ends up being significantly heavier and not as strong because of the large amount of fillers required to fair the hull. Voids at the rods are fairly common as well.

Cflex is but one approach to constructing a one off fiberglass hull but there are much easier and lighter ways to go. Fairing a CFlex hull is a real bear. While the stiffening rods are pultruded fiberglass, the actual fabric is wet out just like any hand laid up hull except that Cflex makes vaccuum bagging nearly imposible.

Is that what you could not put your finger on?

Jeff
01-07-2004 07:12 AM
jbarros
Cored or NOT cored?????

I hate to bring up amature methods, as I know that alot of amatures screw up perfectly good materials in the first place, but what about C-Flex planking? It''s lighter than conventional glassed hulls of similar thickness and is composed of unidirectional fibers properly infuesed, so in theory (and according to it''s fans) it should have most of the plusses with a bit less weight, and easy to use (as any boat building is "easy" ) for amature construction without need of any mold at all.

So... What''s horribly wrong with that picture that I missed? I''m sure there''s something, but for the life of me, I cant put my finger on it.

-- James
01-07-2004 01:54 AM
Jeff_H
Cored or NOT cored?????

The early 1970''s through early 1980''s was the worst period for glass work. The resins during this period went through a redesign and were brittle and blister prone. Manufacturers and designers alike had begun to lighten boats but most had not yet learned how to properly engineer these lighter craft. Better fabric and fabric handling techniques were becoming known, but most big manufacturers were still doing things the bad old way, lots of roving and non-directional fabrics, folding the cut fabric for storage, large volume useage of accellerators, manual resin mixing techniques, and so on.

Jeff
01-06-2004 10:18 PM
john232
Cored or NOT cored?????

WOW!! I like all the discussion on this subject. I guess I was really thinking about the older boats in the late 70s to about the late 80s as far as building materials concerning fiberglass lay-ups. I know I will probably never be able to buy a new 36 to 42 foot boat, brand new, for 300K, so I would want my used boat to be as trouble free as possible concerning spongy cores and osmosis problems. If I was buying a late model boat (or a new one) I would most surely want something like a SCRIMP process as its done at TPI for J Boats or the system they now use at Tartan/C&C, and kevlar sounds like a great idea also.
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