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Old 29-12-2015, 15:39   #826
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Re: Oyster Problems?

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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.

Ah...another fad.
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Old 29-12-2015, 16:11   #827
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Re: Oyster Problems?

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Ah...another fad.



Worse. They put one or two layers of Kevlar in the bow only and then say something in the brochures about how the boat is Kevlar reinforced. The only effect it has, other than helping with marketing, is making one of the more likely areas of the boat to suffer damage much more difficult to repair. I've seen a couple of examples where there was a more substantial layup that made more sense, but only in the very top of the market. Talk to anyone who has had to do a repair on this stuff, and you will hear the tale of woe. This from a guy who has worn out many pairs of carbon/kevlar shears...
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Old 29-12-2015, 16:40   #828
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Re: Oyster Problems?

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Maybe someday some of these guys will catch on that racing has absolutely nothing to do with cruising. How likely are you to ground a boat during a Sydney-Hobart? Now compare that to how likely you are to ground a boat during a full seasons cruise in any popular cruising ground.
In 20+ years, I have NEVER been sorry to have 3/16" of steel between me and the ocean, minimum. Thicker at the keel root, with extra framing. I've grounded hard many times, with zero damage beyond the paint. Once I had a dent on the leading edge of the keel when it hit a rock ledge. The dent is still there, under the filler and new paint. Sure, I won't win a race, especially in light air, but for me, that's what the diesel is for. And when it gets rough, I sleep well. The ocean is not coming in unless I'm torpedoed or run down by a ship.
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Old 29-12-2015, 17:35   #829
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Re: Oyster Problems?

Ya...but just leave it in a hot harbor. I have seen the results of that 3 times.I had 2 steel's of my own. But I digress.
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Old 29-12-2015, 17:55   #830
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Re: Oyster Problems?

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My point exactly. All boats are "production" boats.
My boat is not a "production" boat.
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Old 29-12-2015, 18:44   #831
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Re: Oyster Problems?

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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.
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.
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Old 29-12-2015, 19:02   #832
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Re: Oyster Problems?

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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.


Sure, I'm exaggerating (). Have you ever conducted a repair on Kevlar?
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Old 29-12-2015, 19:16   #833
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Re: Oyster Problems?

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onono. Have you ever conducted a repair on Kevlar?
Well I know you can't cut Kevlar fabric with regular scissors and regular tools make a mess of an old Kevlar lay-up, so what else works?
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Old 29-12-2015, 20:23   #834
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Re: Oyster Problems?

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Sure, I'm exaggerating (). Have you ever conducted a repair on Kevlar?
Typically of you. I was referring to this quote, not to a partial part of it:

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.
Particularly to the more relevant parts that are in bold.
It is obvious that if kevlar did not provide a favorable Strength to weight ratio versus GRP many racing boats would not have been built with hulls in Kevlar.
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Old 29-12-2015, 21:44   #835
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Re: Oyster Problems?

Paolo,

You are arguing with the wrong guy. Kevlar offers resistance to puncture but it is not significantly "stronger" in terms of tensile or compression strength than more mundane fibers. Carbon fibers are many time stronger than Kevlar. Even glass fibers are about as "strong" as Kevlar. But resistance to puncture is a virtue and Kevlar helps there.

The proof is there are no Kevlar boats AFAIK. But there are carbon fiber boats.
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Old 30-12-2015, 01:06   #836
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Re: Oyster Problems?

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.
The proof is there are no Kevlar boats AFAIK. But there are carbon fiber boats.
Certailnly carbon is far better than Kevlar. Carbon used to be extremely expensive and it was prohibited by many racing rules to keep the costs down. Thus is was not used much. There were many Kevlar boats, since it was allowed and it was better than E-glass. Kevlar is considerably lighter and stiffer than E-glass. Lighter means thicker at the same weight making it even more stiffer. Stiffness is often more important than strength in boat building. Kevlar has also better strenth to weight ratio compared to E-glass. The higher the fiber to resin ratio the higher the difference, thus in high quality laminates with low amount of resin the weight difference is more significant.

Kevlar was/is quite expensive, thus it was used in high end racing boats and not in production boats just like carbon today. Even epoxy is very seldom used in production boat despite it is far better than polyester or vinylester.
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Old 30-12-2015, 03:13   #837
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Re: Oyster Problems?

Here is quite a lot of data about different fibers as epoxy prepreg: http://www.hexcel.com/Resources/Data...Technology.pdf

Note that modulus and strength are given as MPa thus per sectional area and 50% E-glass weights 1900 kg/m3 vs. Kevlar 1300 kg/m3. Thus at the same thickness it is about 30% lighter or you can make it about 50% thicker to reach the same weight.
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Old 30-12-2015, 07:12   #838
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Re: Oyster Problems?

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Originally Posted by Polux View Post
Typically of you. I was referring to this quote, not to a partial part of it:



Particularly to the more relevant parts that are in bold.
It is obvious that if kevlar did not provide a favorable Strength to weight ratio versus GRP many racing boats would not have been built with hulls in Kevlar.


Typically of you, you failed to answer the question.
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Old 30-12-2015, 07:14   #839
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Re: Oyster Problems?

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Well I know you can't cut Kevlar fabric with regular scissors and regular tools make a mess of an old Kevlar lay-up, so what else works?


Nothing. Thats the point. I have turned away major repair jobs in the past due to Kevlar in the laminate.
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Old 30-12-2015, 07:17   #840
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Re: Oyster Problems?

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Paolo,

You are arguing with the wrong guy. Kevlar offers resistance to puncture but it is not significantly "stronger" in terms of tensile or compression strength than more mundane fibers. Carbon fibers are many time stronger than Kevlar. Even glass fibers are about as "strong" as Kevlar. But resistance to puncture is a virtue and Kevlar helps there.

The proof is there are no Kevlar boats AFAIK. But there are carbon fiber boats.


Exactly. Show me a boat built only in Kevlar, no other fibers, and I'll eat my hat! Tells you all you need to know about Kevlar and strength. S glass is stronger and cheaper by far.
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