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Old 30-12-2015, 07:20   #841
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Re: Oyster Problems?

Quote:
Originally Posted by minaret View Post
Kevlar does not provide strength. It is the least stiff of all the fibers used in laminating. If you build a laminate of solid Kevlar, it will be floppy like a noodle. It is only good for point loading, ie you can't make holes in it. It's one of the more common misleading things I see printed in glossy brochures. It is also almost impossible to make repairs on. Not a good thing. The vast majority of boats I have worked on with Kevlar in the layup, it has been strictly a sales gimmick.
Minaret . . . you are usually much better than this post.

It is factually NOT the 'least stiff':

Material Young's Modulus (The standard measure of stiffness)
E Glass 30-40
Kevlar 70.5-112.4
Carbon Fiber 125-181

It factually DOES provide strength:

Strength-to-Weight
E Glass 564
Kevlar 993
Carbon Fiber 1013


And it is the most abrasion resistance of all the fibers - which is why it is used in bow sections, because they are frequently abraded (as with transom corners).

And the volvo 60's (just for one well know example) were kevlar hulls - so you can eat you hat now!

edit: and just fyi . . . . . . 'nomex honey comb', which is used in ALOT of boats, is in fact kevlar . . . . "PK2 KevlarŪ N636 para-aramid fiber honeycomb is an extremely lightweight, high strength, non-metallic honeycomb manufactured with para-aramid fiber paper (DuPont KevlarŪ N636 or equivalent) "
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Old 30-12-2015, 07:41   #842
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Re: Oyster Problems?

Quote:
Originally Posted by estarzinger View Post
Minaret . . . you are usually much better than this post.

It is factually NOT the 'least stiff':

Material Young's Modulus (The standard measure of stiffness)
E Glass 30-40
Kevlar 70.5-112.4
Carbon Fiber 125-181

It factually DOES provide strength:

Strength-to-Weight
E Glass 564
Kevlar 993
Carbon Fiber 1013


And it is the most abrasion resistance of all the fibers - which is why it is used in bow sections, because they are frequently abraded (as with transom corners).

And the volvo 60's (just for one well know example) were kevlar hulls - so you can eat you hat now!

edit: and just fyi . . . . . . 'nomex honey comb', which is used in ALOT of boats, is in fact kevlar . . . . "PK2 KevlarŪ N636 para-aramid fiber honeycomb is an extremely lightweight, high strength, non-metallic honeycomb manufactured with para-aramid fiber paper (DuPont KevlarŪ N636 or equivalent) "


A Volvo 60 is Kevlar ONLY? Doubt that very much. Throw some S glass into your numbers and you might start to understand what I'm saying. Try to run a grinder on Kevlar and you will definitely understand.
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Old 30-12-2015, 07:52   #843
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Re: Oyster Problems?

Quote:
Originally Posted by estarzinger View Post
Minaret . . . you are usually much better than this post.

It is factually NOT the 'least stiff':

MaterialYoung's Modulus (The standard measure of stiffness)
E Glass 30-40
Kevlar 70.5-112.4
Carbon Fiber125-181

It factually DOES provide strength:

Strength-to-Weight
E Glass 564
Kevlar 993
Carbon Fiber1013


And it is the most abrasion resistance of all the fibers - which is why it is used in bow sections, because they are frequently abraded (as with transom corners).

And the volvo 60's (just for one well know example) were kevlar hulls - so you can eat you hat now!

edit: and just fyi . . . . . . 'nomex honey comb', which is used in ALOT of boats, is in fact kevlar . . . . "PK2 KevlarŪ N636 para-aramid fiber honeycomb is an extremely lightweight, high strength, non-metallic honeycomb manufactured with para-aramid fiber paper (DuPont KevlarŪ N636 or equivalent) "
If I'm not mistaken, it's not a lack of stiffness, but lack.of compressive strength, which makes Kevlar "floppy", the way Minaret describes.

But you are right about abrasion and puncture resistance, and so Kevlar is not a "gimmick", and is found on more than a few really good boats, like German Frer's Hylas 54, which have aramid fabric in their skins (IIRC Twaron in the case of the Hylas)

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Old 30-12-2015, 07:55   #844
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Re: Oyster Problems?

Quote:
Originally Posted by minaret View Post
A Volvo 60 is Kevlar ONLY? Doubt that very much. Throw some S glass into your numbers and you might start to understand what I'm saying. Try to run a grinder on Kevlar and you will definitely understand.
Yes, the Volvo 60 skins used kevlar as the primary fiber (95%+).

S glass has good strength per cross section but is 'heavy', so less strength to weight ratio vs kevlar.

MATERIAL Strength to weight
Kevlar (ARAMID) 2514
S Glass Fibre 1906

And your argument about kevlar being hard to grind would seem to me to be strong support for the use of kevlar in bows and other abrasion likely spots, rather than an argument against it.
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Old 30-12-2015, 08:03   #845
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Re: Oyster Problems?

Quote:
Originally Posted by Dockhead View Post
If I'm not mistaken, it's not a lack of stiffness, but lack.of compressive strength, which makes Kevlar "floppy", the way Minaret describes.

But you are right about abrasion and puncture resistance, and so Kevlar is not a "gimmick", and is found on more than a few really good boats, like German Frer's Hylas 54, have aramid fabric in their skins (IIRC Twaron in the case of the Hylas)

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'floppy' is a rather non-technical term, usually meaning stiffness, more technically modulus of elasticity, which is measured by youngs modulus, and as I posted above Kevlar is not bad at it - better than e-glass, worse than carbon.

And yes, Kevlar is less strong in compression than in tension, which combined with its high abrasion resistance makes it better in the outer skin than in the inner skin of a cored construction.
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Old 30-12-2015, 08:09   #846
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Re: Oyster Problems?

just as an aside . . . . I happen to know something about composite fibers for three reasons . . . . (1) I was the CEO of a niche very high end composites supplier (Thin Ply Technology) - primarily extremely thin pre-pregs, but also other high end products, (2) related to that, I was involved in helping North's development of 3Di - primarily the machine technology to spread the fibers very thin (which we 'borrowed' from my company), and (3) I was a technical adviser to a friend who built a composite boat for high latitude (ice) work. And yes it in fact had a significant amount of kevlar in it (Reindeer).

and I might comment that it is the resin that is the 'weak point' on most technical dimensions in a composite hull. If you really want a stronger hull, you go to the better epoxy's with toughened nano fibers . . . . you can get a 10x improvement in resin performance (on most properties). But of course at a 20x price
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Old 30-12-2015, 08:20   #847
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Re: Oyster Problems?

Quote:
Originally Posted by estarzinger View Post

And your argument about kevlar being hard to grind would seem to me to be strong support for the use of kevlar in bows and other abrasion likely spots, rather than an argument against it.

Just trying to say I've never seen a scenario where the abrasion resistance of Kevlar came into play on a bow only laminate-other than when I had to grind it to repair impact damage. In which case, S glass or carbon would obviously be a better choice for less money. As a guy who repairs boats, making big expensive boats out of something impossible to repair is silly, unless it's something extreme like a Volvo 60. Taking a perfectly sound hull that is 99% easy to repair material and putting a 1% layer of impossible to repair material on the exterior layer is something worse than silly, IMHO. Especially if it's not likely to benefit you in any way.
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Old 30-12-2015, 08:33   #848
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Re: Oyster Problems?

^^ ok, I can generally agree with that.

Kevlar is in an odd material 'position' - it is generally lower performance than carbon, so if you are 'cost no objective' you generally definitely go carbon today. And it is both more expensive and harder to work with than e-glass, so generally the 'cost is an objective' guys go e-glass. You really only go kevlar if you are looking for serious abrasion resistance and/or fatigue resistance (not usually the case in hulls but is why you see it in the volvo spec 3di sails) and/or you need something with decent properties that electro radiation can see thru (there is kevlar deck panels in quite a few fancy boats in order to allow the sat coms to 'see thru the deck').
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Old 31-12-2015, 10:46   #849
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Re: Oyster Problems?

Quote:
Originally Posted by Dockhead View Post
If I'm not mistaken, it's not a lack of stiffness, but lack.of compressive strength, which makes Kevlar "floppy", the way Minaret describes.

But you are right about abrasion and puncture resistance, and so Kevlar is not a "gimmick", and is found on more than a few really good boats, like German Frer's Hylas 54, which have aramid fabric in their skins (IIRC Twaron in the case of the Hylas)
,,,
Yes, Kevlar has a low compressive strength but that kind of efforts can be very diminished with a clever design (meaning structural design), having the material a much bigger stiffness and strength than GRP. It was due to what could provide that it was extensively used on racing boats on the 90's. Farr designed a lot of kevlar racing boats, one of them, a 45ft won its division on this year's Sydney Hobart and yes, some of the VOR/whitbread 60 had a kevlar hull. Swedish Match (from the late 90's) had a kevlar hull (designed) by Farr. Heineken, from the early 90's, designed also by farr too. Possibly more.
Volvo Ocean Racer, "vo60", 60 feet, Ex. "Heineken"

Today composites have advanced a lot since the 90's and kevlar is almost never used alone but as part of engineered composites that take the best qualities of each material in a relation quality price.

Those composites are calculated with precision, its design involve a lot of calculations and are maximized for a given type of effort/use. Big NA firms have many times their own composite engineer, smaller firms outsource that type of service.

Many maxi yacht have hulls built with composites that use kevlar, for instance Mari-Cha III:

or several Southern Ocean, among them Liara:

Several Southern wind among them Cape Arrow:

and many other contemporary boats among them Kiwi Spirit designed by Farr for a fast solo circumnavigation:

Contemporary boats made of only Kevlar are more rare and even if Nomex, an Aramid as kevlar, is best know for the use as a top core, on boats like the Baltic Stig 72 GP, Nomex is used as main hull laminate and kevlar as core:

So I would say that Kevlar and other aramids have a much bigger use in what regards to be used as hull building materials than ".. only good for point loading,... It's one of the more common misleading things I see printed in glossy brochures.... Not a good thing, in fact it has many uses, some even top of the shelf ones, like Nomex for cores, enters the composites of many top boats besides the many other uses of Kevlar fibers in hull construction and kevlar is certainly a good thing among materials used on the marine industry otherwise it would not be so extensively used.

In fact I have to say I was surprised with the extension of that use.
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Old 31-12-2015, 11:31   #850
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Re: Oyster Problems?

According to a EGA survey referred on the book "Marine Composites" by Eric Green Associates, Eglass is used in 82% of the hulls, Kevlar on 23% of the hulls and Carbon on 15% of the hulls.

Those percentages don't refer of a hull made of a single material but of the use of the materials on hulls.

The book is from 1999 so I bet that the percentage of Carbon has increased greatly on the last years. Kevlar is still used as a cheaper alternative to carbon mostly on engineered composites, as a top abrasion material in what regards impact protection and the aramid derivate Nomex has an increased use as a top of the shelf material for hull cores.

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Old 31-12-2015, 12:13   #851
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Re: Oyster Problems?

Quote:
Originally Posted by Polux View Post
You are exaggerating.

There are many famous racing boats built in Kevlar or with kevlar as part of the used composites. Some circumnavigated and others are still racing after many years of hard use. Do you think they were built in Kevlar as a sales gimmick?

True that in what regards racing boats, or even top cruising boats today Carbon is superior to kevlar. but Kevlar is still used in GRP boats to increase resistance to impact on the forward sections.
No,, is not exaggerating, we sunk a 46 foot ocean racer made full of Kevlar 3 years agoo, he end in the rocks due a broken mooring and after we refloated and towed to the yard we discover is made of Kevlar, no one want to fix it, not me , not even the most experienced composite pro around here , we just towed back to 100 ft deep and sunk it... Kevlar jada jada,,,,,
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Old 31-12-2015, 12:35   #852
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Re: Oyster Problems?

Well, I'll bite on this one.

My boat is constructed of carbon fiber (with some glass below the waterline) with different core materials in different parts of the boat (Nomex and wood), but uses Kevlar in the bow, but on the inside of the laminate. This is for two reasons.

The first is that during panel testing, which, among other things, consisted of dropping heavy balls onto different constructions, it was found that Kevlar on the outside was in fact too brittle and would shatter upon impact. With the layer on the inside, while some of the laminate was breached, the panel did not lose it's watertight integrity. Below is an explanation from a well-known designer about the reasoning for the methodology.

As further protection from such a catastrophic situation a kevlar crash barrier
laminate was added in the center 30 inches of the hull forward of the
keel. This laminate is next to the core on both sides underneath the
carbon fiber. This is counter to present conventional thinking which would
put the kevlar on the outer laminate because of its abrasion resistance.
What was found in the panel testing at Guegon was that the kevlar in a
catasrophic hit would shatter on the outside skin because of its
brittleness and the stiffness of the underlying core and laminates and
even potentially holed. Dropping heavy metal balls on the laminate it was
found that while the glass and carbon skins were breached that the
laminate hung together because the kevlar as the last line of defense
works in tension to absorb the energy of the of the object and while
delamination occurred the laminate was not breached.


The other upside to this method of construction involving Kevlar is that moderate damage can be repaired without getting into the really difficult stuff, but I will confess that a collision involving this degree of damage will likely still be very difficult and expensive to repair.

Anyway, since we were on the topic, I thought one or two of you specialists might like to read a bit about the construction technique employed on our boat. Or not. Disregard at will.

Good sailing, TJ
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Old 31-12-2015, 13:48   #853
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Re: Oyster Problems?

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Originally Posted by Polux View Post
Last post of this year. I take the opportunity to wish to all a great passage and a better new year
Thanks Polux, and the same best wishes to you & yours.
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Old 31-12-2015, 17:33   #854
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Re: Oyster Problems?

Quote:
Originally Posted by TJ D View Post
Well, I'll bite on this one.

My boat is constructed of carbon fiber (with some glass below the waterline) with different core materials in different parts of the boat (Nomex and wood), but uses Kevlar in the bow, but on the inside of the laminate. This is for two reasons.

The first is that during panel testing, which, among other things, consisted of dropping heavy balls onto different constructions, it was found that Kevlar on the outside was in fact too brittle and would shatter upon impact. With the layer on the inside, while some of the laminate was breached, the panel did not lose it's watertight integrity. Below is an explanation from a well-known designer about the reasoning for the methodology.

As further protection from such a catastrophic situation a kevlar crash barrier
laminate was added in the center 30 inches of the hull forward of the
keel. This laminate is next to the core on both sides underneath the
carbon fiber. This is counter to present conventional thinking which would
put the kevlar on the outer laminate because of its abrasion resistance.
What was found in the panel testing at Guegon was that the kevlar in a
catasrophic hit would shatter on the outside skin because of its
brittleness and the stiffness of the underlying core and laminates and
even potentially holed. Dropping heavy metal balls on the laminate it was
found that while the glass and carbon skins were breached that the
laminate hung together because the kevlar as the last line of defense
works in tension to absorb the energy of the of the object and while
delamination occurred the laminate was not breached.


The other upside to this method of construction involving Kevlar is that moderate damage can be repaired without getting into the really difficult stuff, but I will confess that a collision involving this degree of damage will likely still be very difficult and expensive to repair.

Anyway, since we were on the topic, I thought one or two of you specialists might like to read a bit about the construction technique employed on our boat. Or not. Disregard at will.

Good sailing, TJ


This makes a hell of a lot more sense than the exterior laminate I see on production boats with this option.
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Old 31-12-2015, 17:52   #855
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Re: Oyster Problems?

Quote:
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Did a lot of Kevlar white water squirt boats and the like.

Ha! I'll bet you a box of doughnuts you can't fit in a squirt boat anymore.
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