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Old 02-11-2015, 20:03   #61
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Re: Most boats have liners; which ones do it best?

Jim, the puncture and impact loads are a function of the mass of the structure, so cored hulls should be tougher. What sticks in my mind is the images of the balsa-cored Sydney 38 Low Spped Chase almost intact high and dry on the rocks of the Farallon Islands. I also sailed on the balsa cored Olson 40 Fastidiots which was pulled intact off the beach after the Cabo Storm in 1983 where most other boats were destroyed.
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Old 02-11-2015, 20:32   #62
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Re: Most boats have liners; which ones do it best?

That Benetau 40.5 first which lost its keel had the keel bolted to a' liner'
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Old 02-11-2015, 23:03   #63
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Re: Most boats have liners; which ones do it best?

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Originally Posted by donradcliffe View Post
Jim, the puncture and impact loads are a function of the mass of the structure, so cored hulls should be tougher. What sticks in my mind is the images of the balsa-cored Sydney 38 Low Spped Chase almost intact high and dry on the rocks of the Farallon Islands. I also sailed on the balsa cored Olson 40 Fastidiots which was pulled intact off the beach after the Cabo Storm in 1983 where most other boats were destroyed.
Hi Don,

Yes, I well remember FAstIdiots being pulled off the beach in Cabo, while the crab crushers were rubble for the most part. Good old Santa Cruz workmanship and design!!

But being thrown up on a sandy beach isn't a puncture scenario for the most part,, so I'm not sure what to take from that incident. And the Sydney... was that boat retrieved, and did the hull have puncture wounds? Just sitting there on the rocks doesn't tell us much about the state of the skins, nor about the future usefulness of the hull.

But both instances do show that low mass helps survival, no doubt about that!

But my earlier comments were directed at the ideas that the two types of FRP construction were inherently more or less strong, and wondering what the respondents meant by "strong".

So, have you another boat in your future? Can't picture you as a land lubber, mate!

Jim
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Old 03-11-2015, 04:29   #64
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Re: Most boats have liners; which ones do it best?

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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.

]


Couldn't agree with you more. Cores are all about panel stiffness/weight. And they come with a lot of compromises.
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Old 03-11-2015, 04:57   #65
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Re: Most boats have liners; which ones do it best?

Just look at the delam on the back side of this cored panel which was struck by a pointy object. Whole side of the boat had to be rebuilt. Entry wound/exit wound is bad. Won't see anything like that in a solid hull. As someone who has rebuilt a whole lotta boats, I'd say a fair portion of the major structural problems I run into are core related. Probably a substantial majority. Many repairs are made much more difficult by the presence of core, and some would not even be necessary without it. Quite a few, actually. Cored construction was invented to allow the building of small fiberglass boats. Without it, hulls with the right scantlings in those size ranges oil can/flex too much. Same reason you don't see small steel boats. Single skin construction only starts making sense at a certain size range. For boats 40' and up, single skin absolutely does not have to be heavier than core for a desired strength. In this size range, coring is strictly a weight saving measure. It allows you to build the hull less strong, less flexible/able to withstand load cycles, but with the same panel stiffness. And using much less expensive material/weight to get there. Don't think there's no trade off there.
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Old 03-11-2015, 05:03   #66
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Re: Most boats have liners; which ones do it best?

Greetings and welcome aboard the CF, Geitz.
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Old 03-11-2015, 05:10   #67
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Re: Most boats have liners; which ones do it best?

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Originally Posted by El Pinguino View Post
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.

Yes! Liners are certainly a problem, but this one is right up there too. Combine a liner with a high aspect cast iron keel with no give to it, like lead, and you have a recipe for problems. The pursuit of a couple of extra knots of speed really really costs when it comes to construction trade offs.


IMHO, if you want faster, go longer; not lighter or wider.
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Old 03-11-2015, 05:18   #68
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Re: Most boats have liners; which ones do it best?

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Originally Posted by Azul View Post
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.

]
Well, the sources you quote do not say that uncored hulls are stronger. And they could not since it is not true. Ask an engineer.

It seems to me you are also not using the word "strength" in the engineering sense. Impact resistance is a different quality again. A composite structure with very thin layers of GRP might be less resistant to impact than a thick solid GRP structure -- it depends on the dimensions. "Strength", in the engineering sense, is how much load a structure can carry without being permanently deformed. This quality is highly relevant to boat design.

Kilo for kilo, or pound for pound, a cored composite structure is going to be much stronger than an uncored one. Not just "stronger", but many times stronger. This is a simple and uncontroversial fact; just ask any engineer.

In fact, there is a simple formula for it, used by aerospace engineers -- double the thickness by coring a composite structure, and you increase the strength 3.5x. Quadruple it and the strength is increased more than 9x. See:

Click image for larger version

Name:	Capture.PNG
Views:	88
Size:	121.9 KB
ID:	112148

Introduction to Aerospace Materials, Adrian Mouritz


Even more so if the core is balsa, rather than foam. Balsa has immense strength for its weight; its specific strength is 521 kN * m/kg, compared to 288 for titanium or 63 for stainless steel. See: https://en.wikipedia.org/wiki/Specific_strength. A balsa/GRP composite structure uses the GRP in tension and the balsa in compression for an immensely strong structure.

To get anything close to the strength of a cored structure, a solid GRP structure has to be made several times heavier.


Now we're talking here only about strength. There are other considerations, in choosing a material for a boat. than just strength. A cored structure is much more complex and more expensive to do right. If it's not done right, you can have problems with water infiltration and rot, delamination, and other problems. It's why inexpensive, mass produced boats are not cored below the waterline.
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Old 03-11-2015, 05:29   #69
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Re: Most boats have liners; which ones do it best?

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Well, the sources you quote do not say that uncored hulls are stronger. And they could not since it is not true. Ask an engineer.

It seems to me you are also not using the word "strength" in the engineering sense. Impact resistance is a different quality again. A composite structure with very thin layers of GRP might be less resistant to impact than a thick solid GRP structure -- it depends on the dimensions. "Strength", in the engineering sense, is how much load a structure can carry without being permanently deformed. This quality is highly relevant to boat design.

Kilo for kilo, or pound for pound, a cored composite structure is going to be much stronger than an uncored one. Not just "stronger", but many times stronger. This is a simple and uncontroversial fact; just ask any engineer.

In fact, there is a simple formula for it, used by aerospace engineers -- double the thickness by coring a composite structure, and you increase the strength 3.5x. Quadruple it and the strength is increased more than 9x. See:

Attachment 112148

Introduction to Aerospace Materials, Adrian Mouritz


Even more so if the core is balsa, rather than foam. Balsa has immense strength for its weight; its specific strength is 521 kN * m/kg, compared to 288 for titanium or 63 for stainless steel. See: https://en.wikipedia.org/wiki/Specific_strength. A balsa/GRP composite structure uses the GRP in tension and the balsa in compression for an immensely strong structure.

To get anything close to the strength of a cored structure, a solid GRP structure has to be made several times heavier.


Now we're talking here only about strength. There are other considerations, in choosing a material for a boat. than just strength. A cored structure is much more complex and more expensive to do right. If it's not done right, you can have problems with water infiltration and rot, delamination, and other problems. It's why inexpensive, mass produced boats are not cored below the waterline.



These numbers are for panel stiffness, not "strength".
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Old 03-11-2015, 05:36   #70
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Re: Most boats have liners; which ones do it best?

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Originally Posted by HappyMdRSailor View Post

....

A liner in an ocean going cruising boat gives me the serious willies... I mean by this particular liner example, one that's bonded to the hull without access to the interface... And by that I mean the bonded joints AND... hull skin from the inside...

If either the Potter or Chrysler that I have were simply increased dimensionally into a cruising size, it would scare the shiznit outta be not being able to get under the tub...
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...

I have just been through a collision in my boat. A fishing boat pranged me at anchor at night. The fisherman didn't have insurance.

If my boat had had a lightly built uncored hull, and had my boat had a liner, it would probably be totalled. Worse, the hull would probably have cracked, and the boat would have sunk, since you couldn't get to the inside of the hull with a linered boat.

As it was, I had instant access to the inside of the hull to check for a breach, and could have patched it if there had been one.
My point exactly!


And btw...
Serious drift to the "core" of this thread...
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Old 03-11-2015, 05:53   #71
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Re: Most boats have liners; which ones do it best?

minaret,
Lot's of us were waiting on you to chime in on liners.
I think there are liners that aren't necessarily structural, they exist to speed manufacturing, and then there are grids, or matrices or maybe some other names, they are purely structural and exist to add strength or stiffness I guess actually to the hull.

Are there actually two different things, and I assume any of that isn't optimal for repairs, but which manufacturers do better than other, any which should be avoided?

I assume Cat manufacturing is where you find the leading edge of cores, liners, grids whatever because that is the lightest way to achieve stiffness?
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Old 03-11-2015, 05:57   #72
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Re: Most boats have liners; which ones do it best?

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Just look at the delam on the back side of this cored panel which was struck by a pointy object. Whole side of the boat had to be rebuilt. Entry wound/exit wound is bad. Won't see anything like that in a solid hull. As someone who has rebuilt a whole lotta boats, I'd say a fair portion of the major structural problems I run into are core related. Probably a substantial majority. Many repairs are made much more difficult by the presence of core, and some would not even be necessary without it. Quite a few, actually. Cored construction was invented to allow the building of small fiberglass boats. Without it, hulls with the right scantlings in those size ranges oil can/flex too much. Same reason you don't see small steel boats. Single skin construction only starts making sense at a certain size range. For boats 40' and up, single skin absolutely does not have to be heavier than core for a desired strength. In this size range, coring is strictly a weight saving measure. It allows you to build the hull less strong, less flexible/able to withstand load cycles, but with the same panel stiffness. And using much less expensive material/weight to get there. Don't think there's no trade off there.
If a cored structure over-exploits the increased strength from coring, and has too thin an outer skin, then of course, it will be less resistant to impact (the engineering property is called "toughness", and this is related to but distinct from strength). It will be tempting for builders of less expensive boats to make cored structures with very thin skins in order to reduce cost -- and they will still be strong enough. They will carry the loads, but with very thin skins, they might lose resistance to impact. That's probably what was going on in Minaret's example. A non-cored structure of the same strength will be very heavy and highly resistant to impact. That's an ok compromise for some boats. But a cored structure which has not been made exaggeratedly thin will be both stronger and more impact resistant than the solid hull. Unfortunately it will be more expensive. So not all cored structures are specified that way.

When Minaret says that a boat over 40' can be "strong enough" without being heavier -- that does not mean that it will be as strong as a cored structure of the same weight. A cored structure of the same weight will be many times stronger than a solid one. Or, as is more often the case, it will be lighter and stronger. Half the weight and double the strength is easily achieved, using a cored structure. This is very desirable in sailing boats.


This: "In this size range, coring is strictly a weight saving measure. It allows you to build the hull less strong, less flexible/able to withstand load cycles, but with the same panel stiffness " however is false. Coring CAN be used that way, and is with some powerboats (Sea Ray is a glaring example). But I am not aware of a single modern sailing yacht which is built that way. Cheap sailboats with lightly built fully cored hulls disappeared 20 years ago. 100% of expensive ones, like Swans, Hallberg Rassys, Contests, Discoverys, and other high end sailing yachts, other than Oyster, have fully cored hulls, all the way down to the keel. They are specified to be lighter and much stronger than a solid hull would be. I doubt if Minaret has had to fix many of those.


I'm not talking about the topsides of mass produced boats. That's a different story. When you don't have to use vacuum infusion to eliminate the risk of water saturation, for structures which are not below the waterline, then it becomes much cheaper to use cored construction, and for sure some makers go way overboard exploiting the strength of cored structures to make them very thin and light. I've seen horrifying photographs of certain mass produced boats, whose topsides were penetrated and breached by light impacts. And one where an anchor chain just practically tore the bow off one boat. Everything is a tradeoff, which Minaret says correctly, and going too far making boats cheaper, will bite you. The moral of the story is to avoid the very cheapest boats, if you want a strong structure. The structure is one of the first things to get compromised, when boats get "value engineered" for mass production.
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Old 03-11-2015, 06:00   #73
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Re: Most boats have liners; which ones do it best?

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minaret,
Lot's of us were waiting on you to chime in on liners.
I think there are liners that aren't necessarily structural, they exist to speed manufacturing, and then there are grids, or matrices or maybe some other names, they are purely structural and exist to add strength or stiffness I guess actually to the hull.

Are there actually two different things, and I assume any of that isn't optimal for repairs, but which manufacturers do better than other, any which should be avoided?

I assume Cat manufacturing is where you find the leading edge of cores, liners, grids whatever because that is the lightest way to achieve stiffness?
You keep saying liners aren't structural, but almost by definition they are structural. Nautical architects bond liners and furniture and bulkheads to the inside of a hull to increase its strength, not just to hide stuff like wallpaper or to provide a cheap floorpan to a boat. It is not an airplane.
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Old 03-11-2015, 06:05   #74
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Re: Most boats have liners; which ones do it best?

[QUOTE=Dockhead;1952688]Well, the sources you quote do not say that uncored hulls are stronger. And they could not since it is not true. Ask an engineer.

From Fiberglassics (if you had googled Fiberglassics and core)

Stiffness vs. Strength

As you can see, adding a core between two fiberglass skins is a great benefit to stiffness. But stiffness and strength aren't the same thing. Confusing the two can be disastrous at sea.
In the previous example, 1” of core material was sandwiched between two ¼” fiberglass skins, resulting in stiffness 37 times greater than just ½” of fiberglass alone. But how much was the strength improved? The answer may shock you.
The increase in strength was hardly even noticeable. The overall increase was limited to the strength of ½” fiberglass plus the strength of the core material itself. It still takes a roughly equivalent amount of force to break through solid fiberglass as it does to break through cored fiberglass, the difference is that solid fiberglass is going to deflect, (bend), long before it fractures.

The references I have given were off the top of my head and didn't require research.

Apologies accepted. Thank you Minaret for chiming in. I will take the opportunity to thank you once again for taking the time to teach us a lot about how to take care of boats, and that you don't need to be an "expert" to do your own work well.
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Old 03-11-2015, 06:06   #75
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Re: Most boats have liners; which ones do it best?

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These numbers are for panel stiffness, not "strength".
Indeed not. Read the captions. The top line is stiffness. The second line is strength.

Minaret, I like everyone on here has immense respect for you as the best boatbuilder and fiberglass wizard in this community. But -- you are clearly not an engineer
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