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Old 29-10-2003, 06:07   #31
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Hi CSYMan and all,

I checked out the link you provided to the "Yachtsurvey" site you suggested. Very interesting stuff. One paragraph in particular caught my attention:

"Since solid fiberglass hulls have been successfully built for over 40 years now, the question arises as to whether there is really any significant benefit to coring a fiberglass hull. Are cored hulls really stronger and lighter than solid cored hulls? We 've all heard the claim that cored hulls are lighter and stronger than solid laminates, but this is not exactly true. Cored laminates are stronger in flat panels, but are weaker when used with curved surfaces. My examination of hundreds of boat hulls damaged by recent hurricanes clearly shows that most cored hulls fared nowhere near as well as solid laminate hulls."

Reading the other information on the site makes many references to all the problems that have developed from water penetrating both wood. and foam type of cored hulls.I know my boats' hull and deck are hand laid up f/g. I have some annoying little leaks here in there that I keep working at resolving. By reading the Yachtsurvey info.,I can see where these types of small annoying leaks could turn into a real disaster with any type of cored hull/deck.What's everyone elses take on this issue?
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Old 29-10-2003, 19:36   #32
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Cored vs Solid

The problem I've seen with solid hulls is when they take a hit they'll crack into the interior of the boat. Off shore that could be fatal to the boat. With a cored hull it'll take some of that shock and usuall only puncture the outer hull. Sure it'll take on water between the hulls but not enough to sink the boat.
Also what I've seen is a solid hull will twist and flex more than a cored hull. The water that gets between the double hulls is fairly easy to get out, usually. I know, I've had to do it.

Let's say we have a 30' vessel. In order to have the same displacements the skin of a solid hull would be around 5/8" whereas a cored hull with a double skin of 1/4" each with 3/4-7/8" core making it around 1-1/8" thick. OK now, you hit something head on. What's going to happen to the hull???

Back to work, will continue later......................._/)
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Old 30-10-2003, 00:01   #33
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Roger on arguments for and against cored hulls, safety in a collision, etc.

Don't really have a strong opinion either way, but found it interesting a few months ago when I drilled through the hull of my CSY to install a new 1 1/2" thru-hull for an additional bilge pump.


The location was about 2 ' above the waterline and about 3 feet forward of the stern..

Found the hull to be 1 3/8" thick. The "plug" or round piece of fiberglass that came came out of the hole I drilled is now laying around the coffetable as a conversation piece. One can count 7 or 8 layers of fiberglass...

These boats were built back when resin was cheap and over-kill
was the name of the name.
The price one pays is HEAVY hulls and less than sparkling light-air performance.

As far as the modern boats with thinner hulls being built these days, see the above web-page yachtsurveyonline and read about the Carver motor yachts the author has surveyed and his comments.
He says the hulls are so thin they actually deform and buckle when the boats are hauled out and put on dry land.

He also slaughter Sea Rays and other boats....Quite juicy reading.
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Old 30-10-2003, 00:18   #34
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Post Part 2

Something has to give. With a cored hull the outer skin may puncture but the inner skin will most likely flex inward, being thin and flexable. Not so with a solid hull.
The main thing about the solid hull/fiberglass it that the corners need to be reinforced with a stringer like the transom, bow, doghouse or cockpit corners. This is where I have found most of the cracks on the boats I’ve worked on.
Whereas on cored hulls and decks the corner radiuses are larger, automatically making the corners generally larger and stronger. I have a neighbor here that hit a piling while trying to maneuver around a dock and crack the bow of his solid hull powerboat from the pulpit to the waterline. When you look inside it was obvious, there wasn’t any stringer. The manufacturer just popped it out of the mold and threw it together.
Most of the solid hull sailboats I’ve seen have a nice radius on the bows but the transom corners are still sharp.
If you get T-boned by a power boat or wash up on the rocks you’re probably going to have a 50-50% chance of puncture no matter what.
Back to water in the hull. When I got my boat I discovered water between the hulls when I put her on the hard for the winter. Water kept seeping down from the stern tube. I started poking around and found a pin hole on the edge of where the stern tube was laid in. So I went down to the lowest point inside of the cored part of the hull and drilled some holes and the water gushed up into the bulge. I vacuumed out about 10 gallons before it quit running. Then everyday after that for about 3 weeks I got a quart and less until there was nothing. Then I stuck in some wicks and a small fan for another two weeks until the wicks would stay dry. I glassed in some fiberglass pipe fittings in four of the holes and caped them. Then fixed the pin hole. After the next time I launched for the weekend I pulled the plugs and it was still dry.
Now a wood core that would be another story. I had to dry out my trailer boat too.
Personally, I would still prefer a cored hull with all the repairs I’ve had to do. They are more quite at night with the little creatures crackling away, are warmer in winter and can easily attach things inside without drilling all the way thru. New thruhull holes need to be sealed and the same with wood cores. But no big deal!

Foam or balsa cored decks are a pain!!! If you put in a winch, a block or any other deck fitting that requires strength it's better to cut out the core and install a more solid material. They'll crush under load and also allow water to get in. Ask me how I know!
My $.02 worth.
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Old 30-10-2003, 00:38   #35
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This hull doesn't flex

They don't build'm like they useta!


Everytime we go out, she goes for a ride on the titer toter.
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Old 30-10-2003, 04:20   #36
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Cored Hulls

I’m no Structural Engineer, so my opinions should be taken with “a grain of salt”; and perhaps someone else can better explain the concepts inherent in composite construction [Jeff ? ].


I generally respect David Pascoe’s opinions, but must take issue with the quoted statement, which I believe to be a broad condemnation - way beyond the evidence:

Stede quotes from "Yachtsurvey":
<< We 've all heard the claim that cored hulls are lighter and stronger than solid laminates, but this is not exactly true. Cored laminates are stronger in flat panels, but are weaker when used with curved surfaces. >>

There are several forces exerted upon any panel (solid or cored - curved or flat), including Tension, Compression, Shear, Cleavage, and Peal.

A Cored Panel can be described as an analogy for an “I-Beam”, where:
The Inner & Outer Skins of the Cored assembly represent the Beam’s “Flanges” and the Separating Core of the assembly represents the Beam’s “Webb”

I think it’s self evident that the composite strength of a Cored Panel is very dependant upon the Bonding Connection between the Interior Core and the (2) Outer Skins. Except in direct compression, the core is merely acting as a spacer between the Skins (the thicker the core the stonger the beam). A good bond (connection) at the Core - Skin interface will ameliorate Shear, Peel, & Hinging, insuring a strong beam - whether in a Flat or a Curved Panel.

David Pascoe (who has a LOT of experience & knowledge) notwithstanding, I’ll stand by my earlier statement that “Cored Panels make Light & Strong Hulls” - and I’m still leery of them, due to the difficulty (& rarity?) of doing them right.

OK, I too have oversimplified - sorry.

A full discusiion of Cored assemblies would be VERY useful. The understandings & concepts would apply to many allied topics, such as Mounting Deck Hardware, Repairing Damage, & so forth.

Anyone willing to start a thread?

OMO & FWIW

Regards,
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Old 30-10-2003, 05:34   #37
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Further to CSYMan, who wrote:
<< Found the hull to be 1 3/8" thick. The "plug" or round piece of fiberglass that came came out of the hole I drilled is now laying around the coffetable as a conversation piece. One can count 7 or 8 layers of fiberglass... >>

To achieve an aggregate thickness of 1-3/8" with only eight layers of ‘glass, each layer would have tio be 11/64" thick (<3/16" but > 5/32"). I find this counterintuitive - they seem to be pretty thick layers (to me). I don’t have any lay-up schedules, so perhaps someone could illuminate the subject.
Perhaps I'm full of it (again)?
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Old 30-10-2003, 06:20   #38
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I have started a thread called "A primer on fiberglass" as a point of departure on this and other FRP issues.

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Old 08-11-2003, 06:29   #39
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Design Article

Here's a link to a series of articles from "Ocean Navigator" on Ocean Voyaging Boat Design.

"Yacht Design"
http://www.oceannavigator.com/site/c...nt.asp?id=6676

And the current issue:
http://www.oceannavigator.com/site/csrv/bSource.asp
Ocean Navigator OnlineOcean Navigator Online

Interesting ...
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Old 08-11-2003, 07:12   #40
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CSY Dude:
Take your plug for "burn out test" in that test you will weigh the plug to the nearest .1 gram .Then put the plug into a furnace to burn off all the resin . , then re-weigh .The result will give you a resin to glass ratio. optiman is 50/50 .
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Old 08-11-2003, 13:33   #41
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Well, the ratio of glass fiber to resin on the CSY hulls are 45.2%

The tensile strengt is 21,400 psi and the flexural strength is 35,900 psi and the interlaminate sheer strength is 1800 psi.

The CSY boats were built pretty strong and it feels pretty comforting in rough conditions, no flexing of the hull and no loose bulk heads.
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Old 16-11-2003, 01:20   #42
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cored hulls

Three types of core material are commonly used, end grain balsa, rigid pvc (Klegecell, Dinvycell), and ductile pvc (Airex).

Balsa is the strongest core in compression and shear. The downside of balsa is that it's a non-durable species of wood and can rot quickly if water gets into the laminate. Also an impact load applied to the outer skin will be transmitted through the core to the inner skin possibly fracturing both.

Rigid PVC are non-biodegradable and closed cell so they will not absorb water. They come in differant densities depending on application. Under extreme loads great shearing forces develope between the inner and outer skins that can overcome the shear strength of the core, which would then fail with a bang. Granted most of us would never hope to subject the boat to these kind of loads. Rigid foam would have similar impact load transfer as balsa.

Ductile pvc foams look like rigid foam but are of a differant chemisty. They are somewhat rubbery and under high load will yield and deform without fracturing. Compared with balsa or rigid foam ductile foam can absorb a lot of energy without breaking the core or inner skin.

To me it would seem that ductile pvc would be best used in the hull and rigid pvc would be fine for the deck. I would not use balsa as we all know how hard it is to keep water out of the many holes we have in our decks. As far as which is preferable, a solid laminate or a cored hull it all comes down to preferances. To me a cored hull denotes a high ballast ratio performance boat. Proper engineering and construction is crucial. As far as impact resistance it probably does not matter either way with the exception that a solid hull would better survive grinding away on a reef if you were so unlucky as to put her on the bricks.

Cored hulls probably make more sense in multihulls or high performance monohulls where weight is more an issue.
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Old 16-11-2003, 10:31   #43
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Older, thick, resin-rich hulls needn't have a core to delaminate. The resin has very little strength. Its job is to transfer stresses between fibres, and it should only have to do this over the tiniest distance.

Preimpregnated glass, once used only to build fighter jets, is used with various cores in production panels for engine cowlings (cigarette boats, where the owner has to be able to lift it of those overpowered engines) and in some high performance sailboats. One boat of note is the Stiletto catamaran, originally put together by some out of work NASA types, I believe. The 27 foot long, 14 foot wide cat weighs 1200 pounds fully rigged out. A van hit mine at a 45 degree angle while it was on the trailer, pushing the trailer a couple of feet sideways. Stove in the fender and tore the "West Coast" mirror to shreds. The cat had a scratch on the rub rail, and required 20 minutes of rubbing to clean off the car paint from the epoxy gel coat. No soft spot.

The thing with prepreg, is that the stuff arrives from the manufacturer with the optimum resin already stuck to the glass, complete with an inhibitor to stop it from crosslinking until after layup. Typically layup includes vacuum or pressure, and cooking to drive off the inhibitor. The benefit is not just lower weight, but optimized strength; by ensuring there is no resin to unnecessarily increase the separation between fibres.

My point is that if your hull is thick and full of resin, it could delaminate along a layer of glass as if it was along the contact with a core.

I spent a fair bit of time fixing busted Hobies, and I think sandwich construction is the way to go. I believe military helicoptor blades are still made of fibre, core, fibre sandwiches. When it doesn't work right, it wasn't built right, regardless of solid or cored.

Certainly, I have to concede that the level of quality control necessary for good construction will vary depending on the construction method. I toured the Hobie catamaran plant in California twice, and saw two different levels of quality control. When they get it right, it is a wonderous thing.

I hope I am not annoying you folks by applying this experience to larger hulls.
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Old 16-11-2003, 17:15   #44
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I don't believe that there were any NASA folk involved in the Stiletto project. I knew some of the people involved in developing the Stiletto. In fact the canopy design came from a wise crack of mine. The initial owner of Stiletto was a friend of mine who had been the Hobie distributor for south Florida. I had known him in Miami when I worked commissioning boats and giving sailing lessons at a dealership.

. A few years later I was down at John Holmes boatyard in Nokomis and there was this fellow (I think his name was Bill Hughes) making a hull model for the Stiletto on a big ships bandsaw. He and I got to talking about his project. He told me that he was developing a 27-foot production catamaran. He was building the model to bring to the Miami Boat Show, which was about a month away. We kicked around a bunch of ideas. This was the first time I had heard of using honeycomb coring on a boat.

At that point they did not have a companionway design and Bill wanted to do something jazzy. John Holmes who owned the yard was building a stunt plane and was at the stage of building the canopy. The yard was littered with canopies that had not come out right and I wisecracked that I bet John had a few canopies that he could sell you cheaply. We discussed this idea for a while almost jokingly but Bill made a canopy shaped companionway for the model.

I ran into Bill after the show and he said that they had preliminary deposits on something like 50 boats.

It is my understanding that Stilettos (at least the early ones) were not epoxy but a thermoset polyester which was pre-pregged at Performance Sailcraft, as I believe the company was called. The cores on the early boats were Honeycomb. I don't know if the later ones were as well as the honeycomb tended to uptake moisture.

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Old 17-11-2003, 05:29   #45
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Thanks, Jeff; you could easily be right about the lack of a strong NASA connection. However, it was more than suggested by them. I bought my Stiletto in 1980, after doing research on it as an undergraduate in college studying strengths of materials. I am quite sure that it was epoxy, though, on two sides of a Nomex (Dupont) honeycomb core. I purchased spare exterior finishing (gelcoat?), and it definitely was epoxy. They also had been experimenting with aluminum foil honeycombing. The prepreg arrived in refrigerated vehicles.

Thanks also for the history of the hatch. Funny how ideas develop. It was truly a unique but functional system. I enjoyed the boat for 12 years before selling it. Lots of fun, and scarily fast and unstable; especially in the gusty winds of Atlantic Canada.

After 12 years, the boat was still in good shape, and if there was any moisture trouble, I certainly wasn't aware.

Sorry again for taking the thread off topic.
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