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Old 28 October 2015, 06:52   #21
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it is more specialist to work with and means the hull has to be epoxy not polyester.
Lol, is that right?? I'd say it's far easier to work with than fibreglass, and it works perfectly well with polyester resin, although most manufacturers use vinylester.
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Old 28 October 2015, 10:02   #22
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I've used kevlar fabric on High Power Rockets and it is nice to work with, stretchy compared to glass. The only issue I've come across is that it is best to cover it with a thin layer of glass, as it will fuzz out when sanded otherwise.
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Old 28 October 2015, 11:22   #23
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Lol, is that right?? I'd say it's far easier to work with than fibreglass, and it works perfectly well with polyester resin, although most manufacturers use vinylester.
You got there before me...

Digressing a bit, in the days when I made kayaks, Kevlar was often used. It has good tensile strength and little stretch. When a material bends it has to stretch on the outside of the bend so if it resists stretch it also resists bending, ie. it is more rigid. So, for lightweight kayaks they could be rigid and handle well. I guess it'll be similar for a performance boat hull. The downside of this for lamination is that once it's trapped inside a rigid medium (resin) it doesn't dissipate energy too well so it is fairly easy to puncture or cut, 'easy' being relative of course. The other disadvantage is that it's a slippery material and adhesion to its surface is poor so where bending does occur, it's more likely that internal delamination may also occur. It's a super material but it does have to be used intelligently. It's not sufficient to simply presume a hull will necessarily be superior because Kevlar is used in its construction.
In the kayak world, if the hull was to take a prolonged battering such as constantly being used in rocky rivers, my preferred choice was to use one piece of Kevlar in the high load area (under yer bum) to keep the boat's shape, and back that with full length Diolen which is a strong but flexible polyester cloth. It wasn't uncommon to have a well used boat with all over resin crazing and Kevlar delamination at impact sites but it all stayed together and was still usable.

In the rib world I've used Diolen with good results where there is a high stress area which needs a bit of flexibility to absorb the load, I'm thinking specifically where a transom joins into a deck area or cleats/u-bolts fasten through a hull or deck. It's common to see stress cracking here and often the thinking to deal with this is to make the area thicker and more rigid. Maybe not good thinking. Adding 5% flexible resin to the mix and use a forgiving reinforcement might be a better solution - I don't mean wobbly board, of course!

Anyway, 200 grand for an open rib does seem excessive but if there has been substantial RnD I guess those costs need to be recouped.
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Old 28 October 2015, 15:32   #24
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Its a great result for SARA and any other water borne life saving rescue organisation .the investment and development funding is extremely hard to find nowadays and will in time filter down to all rescue organisations and possibly upwards towards the RNLI .We should be complimenting SARA on their success with this project and the great work they do
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Old 28 October 2015, 16:50   #25
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Originally Posted by Dirk Diggler View Post
Lol, is that right?? I'd say it's far easier to work with than fibreglass, and it works perfectly well with polyester resin, although most manufacturers use vinylester.
Odd to use it with polyester or even vinylester resin that doesn't adhere to the fibres particularly well and therefore is weaker - given the driver for using it is strength (unless its actually just a marketing thing?).

It was quite some time ago but the guys who actually laid up the bits of aircraft I worked on moaned it didn't wet as well, and ruined their tools cutting it (both before and after curing I think). Those were structurally critical parts and small air bubbles etc were enough to cause rejection - perhaps hull lamination is not as demanding. Material cost mattered much less than trying to get the parts out the mould asap though. That might also be different in a low throughput boat building environment?
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Old 29 October 2015, 07:00   #26
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In the rib world I've used Diolen with good results
Diolen is lovely gear to work with, sorta poor mans Kevlar, similar properties, but much cheaper. And if you use the black Diolen, you can tell everybody its Carbon Fibre
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Anyway, 200 grand for an open rib does seem excessive but if there has been substantial RnD I guess those costs need to be recouped.
Yeh, you could almost buy a Scorpion for that
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Old 29 October 2015, 07:11   #27
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Odd to use it with polyester or even vinylester resin that doesn't adhere to the fibres particularly well and therefore is weaker - given the driver for using it is strength (unless its actually just a marketing thing?).
I think you'll find that for the job intended, Kevlar (almost certainly going to be Aramat) and Vinylester resin will be more than strong enough.

It's quite often a "marketing thing", I hear it all the time, builders saying we stick abit of Kevlar there for added strength, where a bit of thinking would solve the problem better.

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It was quite some time ago but the guys who actually laid up the bits of aircraft I worked on moaned it didn't wet as well, and ruined their tools cutting it (both before and after curing I think).
Lol, a bad workman always blames his tools. You need to try some, it "drapes" beautifully, goes round awkward cures much easier than glass, and cuts with basic tools. Off the roll a pair of Kevlar shears is easiest, (about 25 for a decent pair) and when green a stanley knife works well. Once cured, a 1mm cutting disc, or diamond if you're feeling flush, and sands ok, but as Dan F said it will go a bit fluffly.
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