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Old 06-01-2007, 10:15   #1
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Catamaran construction

Wondering about the construction of the common production catamaran, are they usually built with cored hulls or is it just the cheap ones? After reading about some delamination issues it got me wondering if this has to be done for weight reasons or is anyone building with solid glass and cored decks? Since there's a few homebuilding cats here are you coring below the waterline or not?
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Old 06-01-2007, 16:08   #2
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Not sure about later cats but my Prout is solid fiberglass from the water line down. Prior to buying this boat, I had a Privilege 39 fall out of survey for water incursion into the foam below the water line. Not a design or construction flaw, but a previous owner boner.

Where my Prout is cored, she is cored with plywood. Even at that, over time, you can end up with moisture in the core, but when you want strenght without excessive weight, coring makes sense.

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Old 06-01-2007, 16:25   #3
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I would have to say that a solid glass construction is a cheaper option for production builders.

Quality foam cores like Divynicell H80 , or Corecell don't come cheap and where production builders use them they then seem to load them up with excessive glass laminates in my opinion.

If stitched fabrics and epoxy resins were used they could save considerable weight again.

Timber cores in boats are fine eg: WRC, Kiri, Balsa and even ply, but it should be remembered that the only resin to use with timber is epoxy and any other resin system lacks strength and will fail eventualy on a timber core.

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Old 06-01-2007, 16:58   #4
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Lagoons are built with solid FRP under the waterline, foam core composite hulls above the waterline, and balsa core composite deck and bridgedeck, all using resin infusion. What they say (and, I'm no boatbuilder -- I would certainly defer to Dave's, and others, opinions) is that this combination is optimal for strength, weight distribution and expected loads in the respective places. Using the resin infusion process, they say they can closely monitor for proper saturation but keep the weight down by eliminating excess resin.

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Old 06-01-2007, 23:14   #5
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When there is talk of cores, how thick are these cores?
What is "FRP"?
How do you know when a core has become saturated?
When it's solid glass below the waterline.....does that mean the hull has no core and is thinner, or is it just as thick but all fiberglass? Sounds heavy.
A few pictures whould go a long way here.
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Old 07-01-2007, 01:18   #6
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G'day Limpet, it's really something you should google to understand but there is some info with picks on foam cored sandwich constuction here.

LOCK CROWTHER DESIGN NOTES AS PROMISED

Similar in timbercore, just use different resins.

FRP is fiber reinforced plastic, like most production boat's, swimming pool's and spa's. Polyester resin and choppergun rovings in a lot of cases. Heavy and weak.

I dont think a solid glass below the waterline would be as thick as a cored hull, but your right , it is heavy.

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Old 07-01-2007, 06:44   #7
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Most boats are built with vinylester resin these days, as opposed to polyester. Vinylester is slightly more expensive, but is stronger, fully cures, bonds to the core better, and is more impervious to water intrusion. Builder's can get away with building the hull with vinylester and the topsides in poly. I prefer epoxy if the budget calls for it.

Many boats are built with foam below the waterline...not my recommendation, but most one-off cored boats have little choice without a proper female mold. As for core thickness, that will depend on the over-all size of the boat, usage, and location or area on the boat core is used. For instance, the bottom of a 40' Sportfish will have up to a 2" core, the topsides may be 1", and interior panels may be 1/2". Usually the uncored areas of a boat will be only the thickness of the glass laminate that is used on the sides of the cored sections with some minor additional "beefing" up. Therefore, the non-cored laminate is considerably thinner than a cored portion.

By the way, if CSM (chopped strand mat) was used in a multihull, she is likely too heavy.
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Old 08-01-2007, 10:24   #8
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To perform cats should be kept light. To be as stiff as a cored panel, a solid glass panel would be at least twice as heavy, probably closer to 3 times. A lot of production boats are solid below the waterline, but this is more likely to do with the fact that they are intended for the charter market, where durability is more of a priority than performance.
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Old 08-01-2007, 11:02   #9
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The reason I asked the question was because most of the twin engine cats I see for sale under $150,000 are tired 10 year old charter boats,and even they seem to sell fast. I'm wondering how the bulk of them have held up to frequent bashing against the docks and other abuse. I suppose a good throrough tapping of the hull is a good first inspection step even before hiring a surveyer.
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