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Old 02-11-2015, 11:45   #46
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Re: Most boats have liners; which ones do it best?

Another excerpt:

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.

You payz your $oney and you sinks or swimms, as Zeehag might say.
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Old 02-11-2015, 12:28   #47
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Re: Most boats have liners; which ones do it best?

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And I am trained as a scientist, and therefore reluctant to form opinions based on other's anecdotal information especially without references or an explanation.

It's pretty easy to get into boat building, but pretty hard to do it well and actually make money at it. That is one reason there is a paucity of supervision over the less skilled laborers forming laminations.

So, in your personal experience, it is your opinion that cored hulls are no longer a problem, ie not controversial? Have you done any research into core materials? Have you done a large number of insurance surveys on damaged boats?
No fence Azul, but I'm away from my library and even if I were home I doupt that would have the time to look for references considering my opinion.
But for an explanation and as an answer to your question I can tell you that there are two ways of getting good bond between the outer skin and the core in mold build boats. First one is to use much smaller pieces of core material. Second one is to use vacuum bagging (and infusion even better). You being a scientist can propably figure out why they are better ways. What comes to one-offs you actually see the bond while laminating the skins, not quessing them. Unfortunately all cored boats aren't done these ways becouse it costs more, neither in the past nor today.
There's a lot of data about core materials so I don't need to do my own research just the engineering. And no, I don't do insurance surveys but I've done my part of prepairs.

BR Teddy
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Old 02-11-2015, 12:30   #48
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Re: Most boats have liners; which ones do it best?

Azul,
You can read more about cores here.
http://www.bpspecialprojects.com/PDF...20PROBLEMS.PDF
http://www.westlawn.edu/news/westlaw...d10_sept09.pdf see page 16
There are benefits and issues with Cores. The basic answer is it takes a little more skill or at least better training and QC with cores then solid laminates. Most decks have been cored for decades for good reason. There ares still failures in modern cored boats but it seems to much less then it used to be as best practices have permeated thru the industry.
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Old 02-11-2015, 12:39   #49
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Re: Most boats have liners; which ones do it best?

Quote:
Originally Posted by Azul View Post
Another excerpt:

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.

You payz your $oney and you sinks or swimms, as Zeehag might say.
And that conclusion is based on what?
Seriously, if you wan't to compare solid and cored hulls you must take account of the strength which they are build to. 40yrs ago nobody knew the real strength of the GRP so boats were 'overbuild'. If we build a cored boat to the same strength as those it would be as strong. In other words, 'The Safety Factor' has declined over the years.

BR Teddy
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Old 02-11-2015, 12:44   #50
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Re: Most boats have liners; which ones do it best?

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No fence Azul, but I'm away from my library and even if I were home I doupt that would have the time to look for references considering my opinion.
But for an explanation and as an answer to your question I can tell you that there are two ways of getting good bond between the outer skin and the core in mold build boats. First one is to use much smaller pieces of core material. Second one is to use vacuum bagging (and infusion even better). You being a scientist can propably figure out why they are better ways. What comes to one-offs you actually see the bond while laminating the skins, not quessing them. Unfortunately all cored boats aren't done these ways becouse it costs more, neither in the past nor today.
There's a lot of data about core materials so I don't need to do my own research just the engineering. And no, I don't do insurance surveys but I've done my part of prepairs.

BR Teddy
Likely those two techniques are better at forming a mechanical (but not a chemical) bond with the core material as the resin has more chance of being forced into the kerfs among other possibilities. You are then stating by default that in order to maximize the utility of using a cored hull, we are talking about a very expensive process which goes back to the principle of why bother unless cost is no object. I still have not seen any information (and I have looked) that a cored hull is safer than a solid hull.

Excerpt: FIBERGLASS BOATBUILDING: Cored Laminates

Most foam cores are made of polyvinyl chloride (PVC). These include Airex, a linear PVC foam that is resilient and easily springs back into shape after being crushed. Divinycell and Klegecell, which are cross-linked PVC foams, are less expensive and lighter, but are more likely to crack when crushed. The most expensive commonly used foam is Core-Cell, which is a styrene acrylonitrile (SAN) foam, and is reputed to be more impact resistant than its competitors.

The most exotic lightweight cores have a honeycomb structure and are made of various materials, including paper, aluminum, and plastic. Honeycomb cores are extremely stiff for their weight and, like balsa wood, are structurally efficient. The big problem with such cores is that their exterior lateral surface area is severely reduced. There is minimal contact between the body of the core and its laminar skins, which makes it difficult to achieve a strong bond. Vacuum bagging with epoxy resin is practically mandatory; as a result, this sort of construction is very expensive.

I remain unconvinced of the advantages of a cored hull and welcome more debate as I am sure others do. At present there appears to be an absolute lack of a near ideal core material that is affordable. Boatbuilders in particular have little credibility IMO, especially if they are selling cored hull boats.
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Old 02-11-2015, 12:52   #51
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Re: Most boats have liners; which ones do it best?

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And that conclusion is based on what?
Seriously, if you wan't to compare solid and cored hulls you must take account of the strength which they are build to. 40yrs ago nobody knew the real strength of the GRP so boats were 'overbuild'. If we build a cored boat to the same strength as those it would be as strong. In other words, 'The Safety Factor' has declined over the years.

BR Teddy
I agree with this bold statement. Therefore I would look for a new or used time-tested old school boat without a cored hull, but that is just me. As we say in NC, "IF bullfrogs had butterfly wings, they wouldn't bump their ass when they try to fly."
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Old 02-11-2015, 13:07   #52
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Re: Most boats have liners; which ones do it best?

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I agree with this bold statement. Therefore I would look for a new or used time-tested old school boat without a core, but that is just me. As we say in NC, "IF bullfrogs had butterfly wings, they wouldn't bump their ass when they try to fly."
That would have been my choice too if there were one to meet my other requirements.
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Old 02-11-2015, 13:48   #53
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Re: Most boats have liners; which ones do it best?

Slight thread drift from liners/grids to cored hulls but what's the current thinking on epoxy/plywood like RM is currently building? I've heard it argued that all the furniture is tabbed in and this adds to the structure and still gives full access to the hull while the plywood provides many of the benefits of cores (insulation, strength to weight, etc.) but is also easier to repair. Some are even sheathing these in glass/kevlar for Arctic adventures.

Just curious with all the talk of saturated cores how one of these would hold up in 30 years (asks the guy with a 50-year-old fiberglass boat w/ no liner)?
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Old 02-11-2015, 14:36   #54
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Re: Most boats have liners; which ones do it best?

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Originally Posted by vancouver25 View Post
Slight thread drift from liners/grids to cored hulls but what's the current thinking on epoxy/plywood like RM is currently building? I've heard it argued that all the furniture is tabbed in and this adds to the structure and still gives full access to the hull while the plywood provides many of the benefits of cores (insulation, strength to weight, etc.) but is also easier to repair. Some are even sheathing these in glass/kevlar for Arctic adventures.

Just curious with all the talk of saturated cores how one of these would hold up in 30 years (asks the guy with a 50-year-old fiberglass boat w/ no liner)?
I agree , a little thread drift.

But really , avb3, if you are looking for a new boat , have a look at a Bristol 41.1, 43.3 , 45.5, 47.7, 51.1 . I looked for a few years at everything under the sun , when a finally got around to looking at a Bristol my search ended . Not the fastest boat around the marks but what a great ride in a seaway and surprisingly dry . If you are looking for a comfortable cruiser with no liners or grid pans they are it . I know everyone is in love with there own boat but this is my 4th keel boat and so far my favorite , I would be another .

Regards
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Old 02-11-2015, 15:24   #55
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Re: Most boats have liners; which ones do it best?

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I agree with your premise that a boat without a liner is safer if involved in a collision for the reasons you state, however I think your statements about cored hulls are controversial. For example, coring a hull doesn't make it "stronger," rather it makes it stiffer. An uncored hull will absorb the same impact as an uncored hull, but it will deflect more which can actually be an advantage the same way an impact absorbing bumper on a car avoids damage in low speed accidents. Many purists would make the case that having a cored hull below the waterline is a potential disaster, and I have read numerous accounts lately of high end boats with horrible delamination problems from having a cored hull.

From FIBERGLASS BOATBUILDING: Cored Laminates

Depending on how thick the core of the sandwich is, the outer skins can be quite thin indeed. On a boat, however, there are practical limits, as the skins must at least be thick enough to resist ordinary impacts. As a general rule, as the lightweight core gets thicker, the relatively heavy skins can get thinner, and the whole structure gets stiffer and lighter. The larger the structure, the greater the weight savings relative to a solid structure of similar size and strength. For boat hulls the practical minimum limit is about 30 feet. On any boat shorter than this, the skin thickness needed to resist impacts is so great relative to the coreís thickness that there is no appreciable weight savings.

Another great benefit of cored hull laminate is that it provides great insulation. If you have ever cruised in cooler climates aboard a solid glass boat in the early spring or late fall and have awoken to rivers of condensation pouring off the overhead, you will appreciate the importance of this. A cored hull is always drier than a solid hull and is less likely to become a mildew farm. It is also warmer when itís cold outside, cooler when itís warm outside, and quieter as well.

Cored laminates do have some disadvantages and are in certain ways more fragile than solid laminates. There is no way to create a chemical bond between a core and its skins; instead the bond must be primarily adhesive. The best methods are either to lay up the core between two resin-rich layers of chopped-strand mat or glue the core in place with a resin-based adhesive putty. But even when a cored laminate is laid down with scrupulous care, it is more likely to delaminate than a solid one, particularly after suffering impact damage, whether the outer skin is punctured or not. In such cases, though there may only be a small area where damage is visible, the core will likely have separated from its exterior skin over a much larger area. Furthermore, the extent of delamination, however it occurs, can be difficult to ascertain if the core remains in close contact with its skin, as is often the case.

Cored laminates are also more susceptible to water damage. Any underwater core will eventually be invaded by some small amount of moisture migrating through its exterior laminar skin. Any puncture in the outer skin, no matter how small, will also readily admit moisture. If the puncture is underwater, even the speediest repair cannot prevent some saturation from taking place. This is why some builders insist on building only solid hulls and others will core a hull above the waterline, but never below it. Many cruisers believe no serious cruising boat should ever have cored laminate below its waterline; others believe a lighter hull is worth the risk of having to make more expensive repairs after a collision.
I think that discussion of all of the benefits and disadvantages of cored versus uncored hulls is way off topic here.

But to the narrow question of strength, which is what you were commenting on --

Cored hulls, pound for pound, are without the slightest doubt stronger than uncored ones, and not only stiffer. For the same reason that a steel I-beam is stronger than a flat steel bar -- it's simple geometry.

The main disadvantage of cored hulls is higher cost, and the other compromises are obvious enough that with the single exception of Oysters, all modern high end boats have fully cored hulls these days -- Swan, Contest, Wally, X-Yachts, Discovery, Hallberg-Rassy -- you name it. And Oyster is starting to go gradually to fully cored hulls, too, although massive uncored hulls (needing to be truly massive to make up for the loss of strength), and full skeg rudders, was their trademark for decades.

/thread drift
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Old 02-11-2015, 15:43   #56
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Re: Most boats have liners; which ones do it best?

Maybe liners in way of the keel aren't the problem, maybe high aspect ratio fin keels that put massive loads on the keel/hull interface when you smack rocks are the problem.
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Old 02-11-2015, 15:51   #57
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Re: Most boats have liners; which ones do it best?

Avb3,

Buy a used Oyster. No liner and you'll be able to get at absolutely everything. Wiring, hoses, mechanicals.... just take up the floorboards, lift the seating; to date, there hasn't been anything... except for the Yanmar impeller, but that's Yanmar's fault... that I didn't have easy access to.
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Old 02-11-2015, 18:07   #58
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Re: Most boats have liners; which ones do it best?

Quote:
Originally Posted by vancouver25 View Post
Slight thread drift from liners/grids to cored hulls but what's the current thinking on epoxy/plywood like RM is currently building? I've heard it argued that all the furniture is tabbed in and this adds to the structure and still gives full access to the hull while the plywood provides many of the benefits of cores (insulation, strength to weight, etc.) but is also easier to repair. Some are even sheathing these in glass/kevlar for Arctic adventures.

Just curious with all the talk of saturated cores how one of these would hold up in 30 years (asks the guy with a 50-year-old fiberglass boat w/ no liner)?
Wood is nature's own composite. Many types have a higher strength/mass ratio than titanium. Somewhat more maintenance, but IMHO a great material for boats.

I spent a couple of winters in Cowes berthed opposite a gorgeous schooner, Craftsman's Art, a multimillion pound superyacht. Made of -- mahogany. Encased in epoxy. Designed by Bill Dixon, who designed my boat.

It doesn't rot if you take care of it.

And wood in the core of a cored plastic boat doesn't rot, if it's properly made. With the balsa cut into end grain blocks, and resin infused in a vacuum bag. This is expensive, of course.
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Old 02-11-2015, 18:16   #59
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Re: Most boats have liners; which ones do it best?

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I think that discussion of all of the benefits and disadvantages of cored versus uncored hulls is way off topic here.

But to the narrow question of strength, which is what you were commenting on --

Cored hulls, pound for pound, are without the slightest doubt stronger than uncored ones, and not only stiffer. For the same reason that a steel I-beam is stronger than a flat steel bar -- it's simple geometry.

The main disadvantage of cored hulls is higher cost, and the other compromises are obvious enough that with the single exception of Oysters, all modern high end boats have fully cored hulls these days -- Swan, Contest, Wally, X-Yachts, Discovery, Hallberg-Rassy -- you name it. And Oyster is starting to go gradually to fully cored hulls, too, although massive uncored hulls (needing to be truly massive to make up for the loss of strength), and full skeg rudders, was their trademark for decades.

/thread drift
I've given you two sources which disagree with your statement. I'll give you another one, read about cores on Fiberglassics. I said it is controversial, but frankly I think you are just wrong. Stiffer, not stronger. It is the fiberglass not the core that absorbs the impact, and the core makes a laminate only negligibly stronger to resist a blow. At least that is what I read over and over and over again.

]
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Old 02-11-2015, 18:36   #60
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Re: Most boats have liners; which ones do it best?

Perhaps this disagreement stems from what concept of "strength" is being discussed.

Seems to me that cored construction can be designed to absorb the planned loads of a sailing vessel (rig loads, keel loads, impact from falling off waves... the normal stresses experienced by a sailing yacht or motor yacht for that matter). And that cored structure can be much lighter than an equivalantly"strong" solid layup. But that cored structure is not perhaps so good at resisting puncture loads, as in when hitting a pointed object, either afloat or fixed to the bottom. In such cases, the thinner skins over the core can be more easily compromised, and the resulting damage from both impact and water intrusion can be difficult to repair. There a solid layup can be superior in "strength".

Or perhaps I'm putting my words in other mouths, and one or the other of you are in fact correct!

Jim
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