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Old 05-05-2013, 03:06   #31
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Re: Cored Deck and Hull Stories?

My experiences with balsa core have given me a great deal of respect for it.

When we refitted my first boat after about ten years of hard use, we discovered that the builder had omitted to put a solid insert in the deck under the (deckstepped) mast step. There was cosmetic cracking in the gelcoat around the step when we peeled away the caulking compound, but no water in the balsa. Nor had the deck squashed appreciably.

It was only a small boat, but with a tall masthead rig of ample strength, and we'd had some good knockdowns, including one by a wave breaking into the mainsail half way up the first panel. To say I was impressed by the compression strength of end-grain balsa would be an understatement.


The other which springs to mind was at the other end of the scale: the strongest sailing hull I've been aboard in bad conditions was balsa cored. It had less reverberation than a concrete bunker when it fell off a wave, and it kept the rig (bearing in mind that the rig alone weighed as much as my first sailboat referred to above) tighter under those conditions than I would have thought possible: there was NO perceptible pumping of the rig, such as you generally get from the boat bending when the boat smacks into a big one and the rig wants to keep going.

The boat was built by Tim Gurr, a highly regarded builder of the time, in kevlar over encapsulated balsa blocks, except at the gunwhale (and of course in way of keel, winch drums, deck openings etc...) where it was solid kevlar, 50mm thick.

The last time I sailed on it, it was getting on for twenty years old and had done several circumnavigations, including one in a race, a number of other ocean races (including a Hobart which broke a couple of other maxis) and was still as stiff and uncompromising as ever.
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Old 05-05-2013, 08:38   #32
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50mm thick Kevlar will do that for ya. Wow, that boat will be here for eons.
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Old 05-05-2013, 09:23   #33
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Re: Cored Deck and Hull Stories?

Kevlar is actually the least stiff material of all the modern fabrics. Stellar abrasion resistance and resistance to point loading, really really low panel stiffness. That's why it's commonly used as a surface layer only, that's all you need to provide the desired effect. Any more is a waste of money, and will actually make the boat less strong, not more, when compared to other materials of the same weight.
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Old 05-05-2013, 11:00   #34
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Re: Cored Deck and Hull Stories?

Quote:
Originally Posted by Andrew Troup View Post
My experiences with balsa core have given me a great deal of respect for it.

When we refitted my first boat after about ten years of hard use, we discovered that the builder had omitted to put a solid insert in the deck under the (deckstepped) mast step. There was cosmetic cracking in the gelcoat around the step when we peeled away the caulking compound, but no water in the balsa. Nor had the deck squashed appreciably.

It was only a small boat, but with a tall masthead rig of ample strength, and we'd had some good knockdowns, including one by a wave breaking into the mainsail half way up the first panel. To say I was impressed by the compression strength of end-grain balsa would be an understatement.


The other which springs to mind was at the other end of the scale: the strongest sailing hull I've been aboard in bad conditions was balsa cored. It had less reverberation than a concrete bunker when it fell off a wave, and it kept the rig (bearing in mind that the rig alone weighed as much as my first sailboat referred to above) tighter under those conditions than I would have thought possible: there was NO perceptible pumping of the rig, such as you generally get from the boat bending when the boat smacks into a big one and the rig wants to keep going.

The boat was built by Tim Gurr, a highly regarded builder of the time, in kevlar over encapsulated balsa blocks, except at the gunwhale (and of course in way of keel, winch drums, deck openings etc...) where it was solid kevlar, 50mm thick.

The last time I sailed on it, it was getting on for twenty years old and had done several circumnavigations, including one in a race, a number of other ocean races (including a Hobart which broke a couple of other maxis) and was still as stiff and uncompromising as ever.
Quote:
Originally Posted by Andrew Troup View Post
My experiences with balsa core have given me a great deal of respect for it.

When we refitted my first boat after about ten years of hard use, we discovered that the builder had omitted to put a solid insert in the deck under the (deckstepped) mast step. There was cosmetic cracking in the gelcoat around the step when we peeled away the caulking compound, but no water in the balsa. Nor had the deck squashed appreciably.

It was only a small boat, but with a tall masthead rig of ample strength, and we'd had some good knockdowns, including one by a wave breaking into the mainsail half way up the first panel. To say I was impressed by the compression strength of end-grain balsa would be an understatement.


The other which springs to mind was at the other end of the scale: the strongest sailing hull I've been aboard in bad conditions was balsa cored. It had less reverberation than a concrete bunker when it fell off a wave, and it kept the rig (bearing in mind that the rig alone weighed as much as my first sailboat referred to above) tighter under those conditions than I would have thought possible: there was NO perceptible pumping of the rig, such as you generally get from the boat bending when the boat smacks into a big one and the rig wants to keep going.

The boat was built by Tim Gurr, a highly regarded builder of the time, in kevlar over encapsulated balsa blocks, except at the gunwhale (and of course in way of keel, winch drums, deck openings etc...) where it was solid kevlar, 50mm thick.

The last time I sailed on it, it was getting on for twenty years old and had done several circumnavigations, including one in a race, a number of other ocean races (including a Hobart which broke a couple of other maxis) and was still as stiff and uncompromising as ever.
We've had this conversation what -- three times in the last three months? I don't know why it has become such a hot topic all of sudden. A review of the archives would be valuable to anyone interested in it. I doubt that much really new can be said on the matter. No doubt all the usual participants will chime in with all their usual arguments.

My views are like Andrew's. Cored and non-cored hulls each have their own advantages and disadvantages, so everyone has to decide for himself. I think it can be pretty well summed up like this without wasting yet another flood of cyber-ink:

Advantages of cored hulls:

Stronger
Lighter
Better acoustic and thermal insulating properties

Disadvantages:

more complicated
more points of failure
more expensive
risk of water penetrating the balsa


The last is an acute disadvantage in cored hulls built to old technology -- before resin infusing and block encapsulating techniques were developed. But these techniques also substantially increased the cost of cored hulls, so this is also not any free lunch..

You pays your money, and takes your choice.
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Old 05-05-2013, 11:49   #35
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Re: Cored Deck and Hull Stories?

Quote:
Originally Posted by minaret View Post
Kevlar is actually the least stiff material of all the modern fabrics. Stellar abrasion resistance and resistance to point loading, really really low panel stiffness. That's why it's commonly used as a surface layer only, that's all you need to provide the desired effect. Any more is a waste of money, and will actually make the boat less strong, not more, when compared to other materials of the same weight.
I'm not an engineer, and I might be sadly misinformed, but I do not think that fabrics possess any such quality as "stiffness", which plays any significant role in making things with them. Their job is to work in tension, as far as I understand.

If talk about composites made with various types of fabric -- glass, aramids (like Kevlar), and carbon, for example -- Kevlar composites are stiffer than glass composites:

Carbon/polymer:
Density 1.551.6 g/cc.
Strength 35250 ksi.
Stiffness (E) 515 Msi (maybe 80+ Msi for exotic fibres).

Aramid/polymer:
Density 1.351.4 g/cc.
Strength 15150 ksi.
Stiffness 310 Msi.

Glass/polymer:
Density 1.92 g/cc.
Strength 35150 ksi.
Stiffness 2.57.5 Msi.


Composite engineering - Carbon & Kevlar fiber Mechanical Properties


I think it's pretty logical, if you think about it. Everyone knows that aramids like Kevlar have a higher modulus than almost any other fiber. Naturally, that would make a stiffer panel, as the fibers resist stretching as the composite panel is put under stress.

The reason why Kevlar composites are not that much used in building things is not because they are not stiff, but because they are unjustifiably (for recreational boating) expensive. Glass composites are already very good, at a fraction of the cost.

Kevlar composites are much used in aerospace. Parts of the Airbus A380 wings are made of them, as are the propeller blades of the giant A400M military transport (Airbus Military | A400M). Cost is not as much an issue in aerospace. Don't see much fiberglass there.
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Old 05-05-2013, 17:58   #36
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Re: Cored Deck and Hull Stories?

Quote:
Originally Posted by Dockhead View Post
I'm not an engineer, and I might be sadly misinformed, but I do not think that fabrics possess any such quality as "stiffness", which plays any significant role in making things with them. Their job is to work in tension, as far as I understand.

If talk about composites made with various types of fabric -- glass, aramids (like Kevlar), and carbon, for example -- Kevlar composites are stiffer than glass composites:

Carbon/polymer:
Density 1.551.6 g/cc.
Strength 35250 ksi.
Stiffness (E) 515 Msi (maybe 80+ Msi for exotic fibres).

Aramid/polymer:
Density 1.351.4 g/cc.
Strength 15150 ksi.
Stiffness 310 Msi.

Glass/polymer:
Density 1.92 g/cc.
Strength 35150 ksi.
Stiffness 2.57.5 Msi.


Composite engineering - Carbon & Kevlar fiber Mechanical Properties


I think it's pretty logical, if you think about it. Everyone knows that aramids like Kevlar have a higher modulus than almost any other fiber. Naturally, that would make a stiffer panel, as the fibers resist stretching as the composite panel is put under stress.

The reason why Kevlar composites are not that much used in building things is not because they are not stiff, but because they are unjustifiably (for recreational boating) expensive. Glass composites are already very good, at a fraction of the cost.

Kevlar composites are much used in aerospace. Parts of the Airbus A380 wings are made of them, as are the propeller blades of the giant A400M military transport (Airbus Military | A400M). Cost is not as much an issue in aerospace. Don't see much fiberglass there.


Try an experiment. Lay up three strips of glass of identical weight in the same resin using carbon, s glass, and Kevlar. When cured, you won't need any numbers or fancy testing. Just play around with them. The Kevlar will be like a wet noodle compared to the s glass and especially the carbon. I don't play around with chemical engineering manuals much anymore, but I do lay this sort of stuff up every day. Carbon is cheaper than Kevlar, and far stiffer. So why use Kevlar for anything but its intended purpose, abrasion resistance and point loading? It doesn't make sense. A hybrid lam of Kevlar skin under a single ply of s glass, followed by a structural lam made up of alternating layers of carbon and s glass would be both cheaper and vastly superior.
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Old 05-05-2013, 18:18   #37
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Re: Cored Deck and Hull Stories?

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Originally Posted by westsail374 View Post
Cored decks and hulls are often touted as giving great strength while saving weight. Different types of core materials are said to have different characteristics.

Some express concerns regarding core materials' tendency to soak up water and lose strength as well as gaining weight. Others tell stories of cores that separate from the fiberglass that surrounds them.

What experiences have CF members had with cored hulls and decks?

How serious are problems?

Are they limited to certain boats and/or manufacturers?

What solutions have been tried? With what success?

Should a buyer steer clear of cored hulls and/or decks?

The balsa core deck on my Ericson 25 is still stout, even after 38 years. Nevertheless, I did have some localized problems around the chain locker, where the balsa was little more than dust. I replaced the balsa with plywood, doing all the work from below (upside down), as I did not want to cut into the deck. I documented this entire project with pictures here:

The Ericson 25, a Trailerable Cruiser: Deck Core Repair, Chain Locker

Regards,
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Old 05-05-2013, 22:07   #38
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Nice post, Roscoe. How did you apply the glass to the underside of the deck without it falling off?
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Old 06-05-2013, 19:22   #39
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Re: Cored Deck and Hull Stories?

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Nice post, Roscoe. How did you apply the glass to the underside of the deck without it falling off?
Thanks SVNeko. I thickened up the epoxy a little bit with colloidial silica and I made sure to wet out the plywood with this thickened mixture prior to the application of the first layer of cloth. I followed the same pattern for each of the four layers of 10 oz cloth and then for the two layers of biax. The cloth never once acted like it didn't want to stay up there. As you can probably tell by the photos, by the time I got to the last layer of biax, I was thickening up the epoxy with lots of colloidal silica. It was like paste.

Roscoe
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Old 07-05-2013, 16:27   #40
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Re: Cored Deck and Hull Stories?

Quote:
Originally Posted by minaret View Post
Try an experiment. Lay up three strips of glass of identical weight in the same resin using carbon, s glass, and Kevlar. When cured, you won't need any numbers or fancy testing. Just play around with them. The Kevlar will be like a wet noodle compared to the s glass and especially the carbon. I don't play around with chemical engineering manuals much anymore, but I do lay this sort of stuff up every day. Carbon is cheaper than Kevlar, and far stiffer. So why use Kevlar for anything but its intended purpose, abrasion resistance and point loading? It doesn't make sense. A hybrid lam of Kevlar skin under a single ply of s glass, followed by a structural lam made up of alternating layers of carbon and s glass would be both cheaper and vastly superior.
Abrasion resistance and point loading are not the only uses of Kevlar. Kevlar fibers have an extremely high modulus of elasticity, which is why they are often used for high performance sails and ropes. I can't argue with your practical experience, as you have some and I have none. But they do make airplane wings out of the stuff. Perhaps laying it up in a composite requires a different technique than what you are using. The sources all say that Kevlar composites are stiffer than glass, which seems different than a "wet noodle".
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Old 07-05-2013, 19:16   #41
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Re: Cored Deck and Hull Stories?

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Abrasion resistance and point loading are not the only uses of Kevlar. Kevlar fibers have an extremely high modulus of elasticity, which is why they are often used for high performance sails and ropes. I can't argue with your practical experience, as you have some and I have none. But they do make airplane wings out of the stuff. Perhaps laying it up in a composite requires a different technique than what you are using. The sources all say that Kevlar composites are stiffer than glass, which seems different than a "wet noodle".


The numbers you are looking at are too simplified. Looking at the figures you posted earlier, the stiffness for Kevlar and "glass polymer" are actually in a similar, very wide, range. But what is "glass polymer"? There are dozens of different kinds of poly resin and even more different kinds of "glass" fibers and also different weaves to confuse the issue, as well as method of laminate, ie resin/fiber ratio. I think if we are going to compare Kevlar to "glass", we can safely compare it to the most high end glass, as that will still be cheaper than Kevlar. As would most forms of carbon. So lets compare it to a high end vacuum bagged S glass layup, done in alternating 0/90 + 45/45 of the same weight. I think you'll find if you make a 1/4" thick bar of Kevlar flat stock an inch wide and a foot long, you can bend it by hand to a good 2" or more of deflection. You'll have to try very hard to get 1/2" of deflection from the S glass lam. Especially if you throw in a uni or two. This is what I mean by "wet noodle".
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Old 24-06-2013, 20:58   #42
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Nidacore or Polypropylene Honeycomb Core

Reading thru this subject thread I was surprised to not see ANY mention of Nidacore or any other polypropylene honeycomb core materials used in decks?
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