SailNet Community

SailNet Community (http://www.sailnet.com/forums/)
-   Sailboat Design and Construction (http://www.sailnet.com/forums/sailboat-design-construction/)
-   -   Cored Hulls and Decks (http://www.sailnet.com/forums/sailboat-design-construction/69156-cored-hulls-decks.html)

tdw 10-17-2010 09:00 PM

Cored Hulls and Decks
 
Ok, so I realise that there have been many a thread on the subject but they tend to get spread and lost in the vast thread count that is SailNet.

I'm canvasing opinions. Not just on the materials but also the methods. Sure I would have to guess that a 30 year old Taiwanese with cored hull and/or decks could well be a nightmare but is that the case with a more modern, lets say non Asian, even non production boat ?

As the long and admittedly tedious search for the next Womboat continues, more and more I see cored boats. Sometimes just deck, sometimes, deck and hull to waterline, sometimes the whole shebang. There was a time I'd have ignored even a cored deck but am I merely behaving like a dynosaur ?

Drill into a core and fail to seal off the core then you are asking for trouble, but is that merely avoiding bad maintenance practices rather than a fault with the material itself, be that balsa or foam ?

My knowledge is limited to a belief that Balsa in stronger than foam but that it is subject to degredation from moisture. The same could well be said of other materials if not properly built or maintained. I would guess that is the case anyway. Some folk say that Balsa should never be used below the waterline, indeed some say no hull should be cored at all below the waterline.

So please...your opionions and experiences ?

Balsa - Above the waterline ? Below the waterline ? Avoid at all costs ?

Foam - Klegercell, Airex, Divnycell ? All essentially the same stuff ?

- Above the waterline ? Below the Waterline ? Avoid at all costs ?



I'd seriously appreciate the feedback.

sailingfool 10-18-2010 07:46 AM

Cored decks are all but universal on fiberglass hulls, there being a few early exceptions, but not enough to bother noting. Cored hulls are perhaps 50-50 in 80"s on boats, and are quite controversial. I personally have excluded cored hulls from from my personal purchase considerations as there are enough all glass hulls around that I still had good choices...but there are plenty of different opinions.

Cruisingdad 10-18-2010 08:23 AM

Quote:

Originally Posted by sailingfool (Post 655482)
Cored decks are all but universal on fiberglass hulls, there being a few early exceptions, but not enough to bother noting. Cored hulls are perhaps 50-50 in 80"s on boats, and are quite controversial. I personally have excluded cored hulls from from my personal purchase considerations as there are enough all glass hulls around that I still had good choices...but there are plenty of different opinions.

Pretty much my opinion too.

I would not buy a balsa cored hull for cruising if below the waterline. I would consider it if above the waterline. Not sure I have been on a deck that was not cored. I mentioned this is a previos thread, but Catalina decks are cored except where they know a 'through deck' is at. At those points they make the solid. I am not sure if that is true of all catalinas, but it is true of the newer ones and larger ones.

I had what I thought was a knowledgeable individual tell me thtat divnycell foam will rot. I do not know if that is true, but I might call around at the yards and see what they say about it. The benefit that I see of a cored hull, and the only benefit, is weight. You make a similarly sized boat with less glass but the same rigidity. But I would imagine that if that hull is ever cracked via an impact or stress or a poorly installed Thull, it would be a nightmare to fix.

It is funny because I had this very conversatin with Catalina on Friday. Their philosophy is not to core below the waterline for teh same reasons as mine. The new 445 is cored above the waterline, but to teh best of my knowlege, is the only boat. So I think somewhere in all of this you have to ask yourself what your comfort level is and use/purpose for the boat. It is not to say that a cored hull boat could not make a good cruising boat.. it just does not make the best choice for me. I would rather know I have solid glass at and below the water. Others no doubt will feel differently.

Brian

Faster 10-18-2010 08:56 AM

I think the key is the execution of the technique.. a properly built and engineered cored hull is going to be stiffer, stronger and lighter than solid glass. The problem is knowing if the boat was truly properly built.. this means good lamination techniques and properly isolating any through hull locations with solid material to avoid risk of leakage into any coring.

I tend to agree with the 'no core below the waterline' idea too. I feel that any blistering that might occur is a bigger problem with the relatively thin-skinned cored hull.

Of possible interest to you, td, our Caribbean sailing friends opted for a solid glass hull in light of the elevated temperatures in the tropics and how that might worsen any issues that may arise with time in a cored hull.

tommays 10-18-2010 09:12 AM

Well

The early Jboats (J24s and J30s) are cored hulls and there are plenty with hull issues beyond repiar that have been broken up for scrap

J24s dont even have anything holes below the water and the core still gets wet over time

And Jboats does a good build

Cruisingdad 10-18-2010 09:20 AM

Quote:

Originally Posted by Faster (Post 655507)
I think the key is the execution of the technique.. a properly built and engineered cored hull is going to be stiffer, stronger and lighter than solid glass. The problem is knowing if the boat was truly properly built.. this means good lamination techniques and properly isolating any through hull locations with solid material to avoid risk of leakage into any coring.

I tend to agree with the 'no core below the waterline' idea too. I feel that any blistering that might occur is a bigger problem with the relatively thin-skinned cored hull.

Of possible interest to you, td, our Caribbean sailing friends opted for a solid glass hull in light of the elevated temperatures in the tropics and how that might worsen any issues that may arise with time in a cored hull.

Agreed. I just don't see why you would take the 'risk' on it for a cruising or racing-cruising boat? Farr 40 circut, yes. Americas Cup? Yes. Non-cruising (not J42), yes. On others, why take the chance? Just core above the waterline. How much weight are you rela,ly saving? And if it is that critical, trim the pounds off of other areas like a CF mast and rigging, Removeable furniture, etc. I personally think there are a lot of ways to cut the weight out of a boat for racing while still keeping it 'safer' for cruising.

But again, these are my opinois and I realiz that there are many fine builders (Sabre just to name one) that use cored hulls.

Brian

T37Chef 10-18-2010 09:48 AM

Well let me see, my Tartan 37 is going on 30 years old, thats right 30 years old, there are no signs of moisture anywhere in the hull. It is cored below the waterline in a few areas, however its solid glass in any high stress area such as the keel & rudder. Now the deck, well I have two small spots, one near the hawse pipe for the anchor rode and the other is near the dodger, both due to poor after factory installations, most of the post 1980 T37's had solid deck where any factory fittings were placed.

So if a boat can last well into its 30's with out showing any signs of degradation or fatigue, I would happily buy another solid, well built, great sailing vessel if it were cored.

It feels pretty darn good to pass a newish Catalina 400 at full sail when I was was flying one reef in the main and one in the genoa...eat that CD!!! LOL :) BTW, he gave up and started to motor not long after we passed him coming out of Eastern Bay, not sure if he had somewhere to be or just couldn't stand getting an ass whooping :)

BTW, someone please show me some photos on any boat, cored or not, where the blisters have gotten into the structural FG/core?

PCP 10-18-2010 10:32 AM

Quote:

Originally Posted by Faster (Post 655507)
I think the key is the execution of the technique.. a properly built and engineered cored hull is going to be stiffer, stronger and lighter than solid glass. The problem is knowing if the boat was truly properly built.. this means good lamination techniques and properly isolating any through hull locations with solid material to avoid risk of leakage into any coring.

I tend to agree with the 'no core below the waterline' idea too. I feel that any blistering that might occur is a bigger problem with the relatively thin-skinned cored hull.

.....

I agree. Much more stiffer and lighter.

It is very important that you have a light boat over the waterline. This will provide with the same ballast a much better stability with relevance for the final stability, or safety stability, as it is sometimes called.

If you have a proper built cored hull with reinforcements where they should be, it would be as strong to impact as a solid fiberglass hull. Open 60's (and almost all race boats) use cored hulls and they are more stressed in some years of use than any cruising boat in a lifetime and there are several that are still around after 15 years of intensive use (and an incredible millage and many circumnavigations). They had their more than fair share of impacts, some so violent that had pulled their keels of, without breaking the hull.

The real problem is price. An underwater cored hull strong and reinforced is much more expensive than a solid fiberglass hull, so most boat builders just use cored hulls above waterline and on deck (where it does not need to be so strong and so expensive and where its use is more important) and use the less expensive solid fiberglass hull underwater. The solid fiberglassed hull will be, for the same impact resistance much cheaper than a strong cored hull.

That's why only really expensive boats use cored hulls all-around.

Regards

Paulo

sailingdog 10-18-2010 11:06 AM

Fuzzy—

I'd recommend you read the article I wrote on Cored Laminates, as it should give you a pretty good overview about core materials and such.

Properly designed and maintained, a cored-laminate construction boat is going to be stiffer, lighter and stronger than a solid-fiberglass boat. Fiberglass is a relatively flexible material, and to give it the rigidity it needs either requires laying it up fairly heavily or adding a grid of stringers and floor or installing a pan or interior liner of some sort. Many cored laminate constructed boats can do away with the need for a liner or interior grid, since the more rigid cored construction allows them to be stiff enough to not require it. This is quite often the case in many of the one-off or home-built multihulls.

Certain materials have distinct advantages over others.

Balsa is the strongest of the commonly used core materials in compression and tension, but has the disadvantage of rotting if exposed to water. It will tend to prevent water from migrating long distances and delaminating large areas, at least until it rots. :D

PVC foams can be broken down into two categories—rigid foams like Divinylcell or ductile ones like Airex. All PVC foams have the advantage of not rotting, but can allow small amounts of water to delaminate fairly large areas by migrating along the interface between the foam core and the laminate skin.

The ductile foams are better suited for use below the waterline IMHO. The rigid foams are excellent for the deck and cabintop. The weight of the foam can be varied depending on the requirements of the foam cored laminate's purpose. You might use a lighter foam for making locker covers than you would for the hull or deck.

The reason I like the ductile foams for below the waterline is that they can provide a hull with greater impact resistance, especially when coupled with a kevlar or spectra laminate skin. Unlike a rigid foam, the ductile foams will tend to compress under impact, rather than transmitting most of the force through the laminate to the inner skin. This combined with a kevlar/spectra laminate skin, which is far more tear resistant than fiberglass or carbon fiber-based laminates—often results in the inner skin maintaining its integrity in an impact, and preventing water intrusion into the boat from a collision.

Some early manufacturers used marine plywood as a core material. Plywood has several major problems as a core material. First, it is heavy compared to balsa or foam. Second, it has the worst characteristics of both foam and balsa—it rots like balsa, and it can allow water to migrate and delaminate large areas of the hull relatively quickly like a foam core can.

sailingfool 10-18-2010 11:30 AM

Quote:

Originally Posted by PCP (Post 655567)
...
If you have a proper built cored hull with reinforcements where they should be, it would be as strong to impact as a solid fiberglass hull. ...
The real problem is price. An underwater cored hull strong and reinforced is much more expensive than a solid fiberglass hull, so most boat builders just use cored hulls above waterline and on deck (where it does not need to be so strong and so expensive and where its use is more important) and use the less expensive solid fiberglass hull underwater. ...

Sure cored hulls have advantages, and are de rigueur in race boats, and are best when done "properly" by a high end builder. But I think you err if you believe that a "properly" built cored hull is immune to someday getting wet due to the vagaries and challenges of getting old. And if that someday comes to you as the owner, you face a repair bill that makes the cost of popping in a new engine look like chump change.


All times are GMT -4. The time now is 12:16 PM.

Powered by vBulletin® Version 3.8.7
Copyright ©2000 - 2014, vBulletin Solutions, Inc.
SEO by vBSEO 3.6.1
(c) Marine.com LLC 2000-2012