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Old 07-12-2011, 15:38   #16
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Re: Hull Extensions for Catamaran

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Originally Posted by Southern Star View Post
One concern I have is that by attaching the battens to the existing hull and then installing the core directly to the battens, once I have layed up the exterior laminates over the core material I will end up with extensions that are wider than the hull to which they are attached. Or am I once again missing something?

Brad
How thick is 800gsm glass and paint?
A fraction of a mm?

If you threw several layers or woven rovings and choppie at at well then yes, you would have a problem.

Like minaret said, shave a mm or two off of the batten where it lays against the hull and make the step a fraction smaller than need be.
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Old 07-12-2011, 15:41   #17
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Re: Hull Extensions for Catamaran

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Originally Posted by minaret View Post
Look at the thickness of the laminate skin on that panel. It's about 1/16th, so thats easily a 20:1 scarf ratio. That is not intended to be a structural joint, it's just to join the panels together for positioning prior to glassing both sides of it.
I dont think that is correct
I think you will find the z joint is THE joint as you say, it has a 20:1 scarf which is more than adequate.

http://www.duflex.com.au/duflex2/bas...ing_the_panels

http://www.duflex.com.au/duflex2/pro...rformance_data


Quote:
Would you actually go to sea with a boat made of panels with 1/16th skins on either side?
yes, it is well proven
Thousand have been doing it for years.
Look to any duflex built cat
Strip planked cat
or custom foam sandwich multi for evidence
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Old 07-12-2011, 15:42   #18
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Re: Hull Extensions for Catamaran

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Originally Posted by cat man do View Post
But they will match each hull, believe it or not in the past I have worked on cats where there have been slight discrepancies in hull width and length, which is why the batten method is good as you end up with a step suited for that hull and no other.

Its also easy if you have an eye
If it looks right, invariably it is
I'm sure there are ALWAYS discrepencies in hull shape, after all it's a boat and the hulls are asymetrical, ie didn't come from the same mold. However, the transoms still need to be identical, or it will look wierd. Without a pattern for the transom, you could tweak the battens in opposite ways on each hull and end up with transoms that aren't that similar. Not a big deal on an extension only 2' long, with battens that are fairly stiff, but on most it's a must. I always use one even on a monohull just to make sure both sides come out mirror images of each other.
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Old 07-12-2011, 15:47   #19
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Re: Hull Extensions for Catamaran

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Originally Posted by cat man do View Post
A bad trait to have when building multihulls
Overbuilding is easy.
Building light with appropriate strength is more difficult.

One area you don't want excess weight is in the ends of a multi, especially if that weight is not related to the structural integrity of the vessel.

Thats why you want to back grind the external seam to the depth of the external laminate, so you can see and match the original laminate schedule. You want to tie each WR to WR, matt to matt, DB to DB, etc. You don't want your part to be stiffer or heavier than the original hull laminate. You want external and internal skins to be not only the same thickness as the originals but the same laminate schedule and resin make up. Proper scheduling back helps to do this, even if you already know the boats laminate schedule. It tells you how far to lap each layer on to the hull to match the original fiber orientation. Making a seam wider than recommended does not make it heavier by any means, you're not putting on more glass, just staggering the joints further apart for a stronger seam. Extra back grinding can actually help to make a stronger lighter part with a stiffer joint, not a heavier one.
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Old 07-12-2011, 15:49   #20
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Re: Hull Extensions for Catamaran

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Why oh why would it need limber holes and bilge pumps?
Read carefully, not both. If it doesn't have the one, it will need the other is what I said.
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Old 07-12-2011, 15:52   #21
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Re: Hull Extensions for Catamaran

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Originally Posted by cat man do View Post
How thick is 800gsm glass and paint?
A fraction of a mm?

If you threw several layers or woven rovings and choppie at at well then yes, you would have a problem.

Like minaret said, shave a mm or two off of the batten where it lays against the hull and make the step a fraction smaller than need be.
Not sure how much 800gsm lam would be, because we backwards Americans still use Imperial measurements and thats all Greek to me. But most cats that size have an external skin about 1/4" thick at the sheer and 3/8" at the keel, with something like an 1/8" internal skin. Could be more, could be less. But it certainly is not " a fraction of a mm". Don't know how many mm in a 1/4", but I'm sure there's one or two.
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Old 07-12-2011, 15:56   #22
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Re: Hull Extensions for Catamaran

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Originally Posted by cat man do View Post
I dont think that is correct
I think you will find the z joint is THE joint as you say, it has a 20:1 scarf which is more than adequate.

Composite Panels | Joining the Panels - DuFLEX by ATL Composites

Performance Data - DuFLEX by ATL Composites



yes, it is well proven
Thousand have been doing it for years.
Look to any duflex built cat
Strip planked cat
or custom foam sandwich multi for evidence
I work on a lot of cats and I've yet to see one with an external skin thickness anywhere near 1/16th of an inch. Maybe I'm just missing them though. I've built some well known boats by famous designers in duflex, although never a cat, and that's certainly not how we did it. I'd never join two panels with a 1/16' thick tape an inch wide and call it structurally sound all by itself.
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Old 07-12-2011, 16:03   #23
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Re: Hull Extensions for Catamaran

If you look carefully at the Z joint, you'll see that the skin is removed on the receving side of the lap joint. That means when you bond a Z joint together, you aren't bonding glass to glass, you are bonding the glass tab that sticks out to bare core. This gives you a flush fair joint ready for glassing, but it certainly isn't structural. You still have an exposed seam on both sides which must be taped, IMHO.
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Old 07-12-2011, 16:19   #24
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Re: Hull Extensions for Catamaran

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Originally Posted by minaret View Post
I work on a lot of cats and I've yet to see one with an external skin thickness anywhere near 1/16th of an inch. Maybe I'm just missing them though.
Probably

Production cats are notoriously overbuild using poor materials because they are cheap.
Cheap choppies, rovings and polyester resin is cheaper than stitched fabrics, thicker better quality core and better resins.

As an example, I worked on a 40ft "foamcored" production cat a while ago.
Its core was 10mm divinycell and probably 1/4 inch (6mm) of choppies and woven each side.
Needless to say the boat was an under-performing slugomaran.

Performance foam cored boats are as I described earlier
THICK core, lighter skins
Boats like this
and
are made from 20mm divinycel, 400gsm kevlar in and out and a 400gsm BIAX on the outside only for impact/abrasion resistance.
This would be a skin thickness of 1/16th or less

They have been well proven for decades of hard sailing

Schionning Designs - uses Duflex mainly
Again, thicker balsa core, thin skin, usualy around the 800gsm giving once again a 1/16th thickness

There is a good article here from one of the worlds greatest multihull designers, the late great Lock Crowther
LOCK CROWTHER DESIGN NOTES AS PROMISED
where he describes in detail how sandwich construction works.
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Old 07-12-2011, 16:24   #25
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Re: Hull Extensions for Catamaran

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Originally Posted by minaret View Post
If you look carefully at the Z joint, you'll see that the skin is removed on the receving side of the lap joint. That means when you bond a Z joint together, you aren't bonding glass to glass, you are bonding the glass tab that sticks out to bare core.
Wrong

Have you used the product?
Did you look at the information provided?

Why would you wast your time doing it?
As soon as you pick up a panel it would snap as it is only as strong as the end grain balsa core.
Easier and faster to just but the panel's, if, it is as you say which it is not.



Quote:
This gives you a flush fair joint ready for glassing, but it certainly isn't structural. You still have an exposed seam on both sides which must be taped, IMHO.
And if this is how it actually was done, I would agree.
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Old 07-12-2011, 16:44   #26
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Re: Hull Extensions for Catamaran

Yes, I have used it. Duflex makes panels with but joints, lap joints, scarf joints, and step joints. What you describe is a lap joint, what I describe is a step joint. The pic looks like a step joint to me. And I wouldn't use either by itself without glassing over it. Are you saying people build boats out of panels joined this way with no other glass over them? That's not what you describe in your example, you describe a boat with glass over the panels inside and out.
I can't imagine a biax and a layer of kevlar coming out at just 1/16", even vacuum bagged in epoxy, which I do a lot of. By the time you faired it and painted it as well it'd be at least an 1/8", probably 3/16th. I've built production sea kayaks with a heavier laminate schedule than that, I'd hesitate to go to sea in that...
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Old 07-12-2011, 16:49   #27
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Re: Hull Extensions for Catamaran

Have you got any pics of a hole saw plug with a ruler you can shoot me? I'd love to see a 50' cat hull with a 1/16" thick laminate schedule on either side of the core. How does it provide any puncture resistance when it's that thin? You'd think you'd be able to poke a hole through the outer skin with a screwdriver....
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Old 07-12-2011, 17:29   #28
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Re: Hull Extensions for Catamaran

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Originally Posted by minaret View Post
Yes, I have used it. Duflex makes panels with but joints, lap joints, scarf joints, and step joints. What you describe is a lap joint, what I describe is a step joint. The pic looks like a step joint to me. And I wouldn't use either by itself without glassing over it. Are you saying people build boats out of panels joined this way with no other glass over them?
You did look at the video and the product right?
The whole idea of using duflex panels is to eliminate work and fairing.
If you glass over the whole lot again you have to re-bog and re-fair the whole boat, so why bother using the system in the first place?

Quote:
That's not what you describe in your example, you describe a boat with glass over the panels inside and out.
I also described a foam sandwich boat, not a duflex one.


Quote:
I can't imagine a biax and a layer of kevlar coming out at just 1/16", even vacuum bagged in epoxy, which I do a lot of.
I can guarantee you, if done correctly, they come out at around this thickness

Quote:
By the time you faired it and painted it as well it'd be at least an 1/8", probably 3/16th.
Quite possibly, if your set-up is sloppy.
Then again I have seen boats go to hi build stage with glass showing clearly through the bog over the majority. Paint and primer film thickness are then measured in Microns.

Quote:
I've built production sea kayaks with a heavier laminate schedule than that, I'd hesitate to go to sea in that...
Are they a cored hull?
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Old 07-12-2011, 17:38   #29
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Re: Hull Extensions for Catamaran

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Originally Posted by minaret View Post
Have you got any pics of a hole saw plug with a ruler you can shoot me?
Not on hand, but rest assured, these methods have been used here for decades

Quote:
I'd love to see a 50' cat hull with a 1/16" thick laminate schedule on either side of the core.
No shortage of them out there.
Mine has a 16mm Kiri core with 600gsm DB in and out that equates to a 17 oz/yard?
How thick is that cloth (stitched)

Quote:
How does it provide any puncture resistance when it's that thin? You'd think you'd be able to poke a hole through the outer skin with a screwdriver....
Correct, but the sea does not pointload like a screwdriver does it.

Using a screwdriver or icepick is a recognised way to cut, hack an access/air hole into the hull of a capsized kevlar foam sandwiched multihull.
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Old 07-12-2011, 17:49   #30
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Re: Hull Extensions for Catamaran

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Originally Posted by minaret View Post
If you look carefully at the Z joint, you'll see that the skin is removed on the receving side of the lap joint. That means when you bond a Z joint together, you aren't bonding glass to glass, you are bonding the glass tab that sticks out to bare core. This gives you a flush fair joint ready for glassing, but it certainly isn't structural. You still have an exposed seam on both sides which must be taped, IMHO.
This is rubbish. The Z-join is actually more correctly called a Z-scarf. The skins on both sides are actually scarfed - tapered, so you're not just joinnig skin to core.

IIRC, ATL guarantee a minimum of 80% join strength compared to the panel.

Most Duflex skins are around 1mm or less. Even the 1200GSM quadraxial skins. This is because all excess resin has been pressed out.

And you certainly can't just stick a screwdriver through it.
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