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bjung 04-24-2012 09:55 AM

Balsa cored hulls
 
I am wondering if todays materials and lamination techniques have eased the concerns about balsa cored hulls. Quite a few highly respected builders are still using balsa coring (Malo, Regina, Dehler just to name a few), and one would assume that they have been successful with this type of construction, or they would have abandonned the idea long ago...
A little research also yielded the same concerns with foam coring, which supposedly deteriorates between the laminates as well, and there are some concerns regarding adhesion of PVC and epoxy resins.
What are your thoughts?

sailingfool 04-24-2012 01:39 PM

Re: Balsa cored hulls
 
Quote:

Originally Posted by bjung (Post 862589)
I am wondering if todays materials and lamination techniques have eased the concerns about balsa cored hulls. Quite a few highly respected builders are still using balsa coring (Malo, Regina, Dehler just to name a few), and one would assume that they have been successful with this type of construction, or they would have abandonned the idea long ago...

Those builders may be successful in the sense that buyers still buy theie boats. How many of those boats, having been through several owners, end up with failed bottoms 20 years later probably doesn't lie back effectively to today's new buyers. Some, yes, many...hard to say, no one collects information like that.

My understanding is that one of the benefits of cored panels is they are cheaper than solid glass...cheaper has got its own attractiveness to a builder...

Jeff_H 04-24-2012 02:27 PM

Re: Balsa cored hulls
 
Actually, A cored hull is much more expensive to build, should retain a larger portion of its strength over time since there is less fatique inducing flexure, and for any given weight, starts out stronger and stiffer.

That is why boats from the higher quality boat builders are built with cored hulls while the cheaper boats are not.

The fatigue issue is a serious one. Actual testing of older boat panels by an insurance industry study showed that fatique dramatically reduced the strength in older panels over time. But, and this is a big but, cored hulls only retain a higher portion of their strength if there is little or no delamination.

Balsa coring is popular because structurally it has better stiffness and sheer resistance than a similar weight foam coring. Properly installed, Balsa coring develops a strong and rot resistent bond with the skin.

In terms of long term durability of foam, there are a pretty wide range of foam core materials with equally divergent properties which makes it very hard to make a broad generality about the relative durability of foam coring.

In terms of build method, these days most cored boats are constructed using resin infusion or some other form of 'vaccuum bagging'. This results in a better bond between the core and the skins and in theory results in better protection from core damage. The key to preventing rot in a cored boats is avoiding penetrating the skins into the core or sealing any penetrations to avoid water intrusion.

tommays 04-24-2012 02:52 PM

Re: Balsa cored hulls
 
http://i565.photobucket.com/albums/s...s/P5212467.jpg

J24 core sample

I have always wondered why the thin skin is outside keeping the water off the core BUT that is how J-Boats does it and there is not a lot of margin for error or making to the old boat hall of fame :)

http://i565.photobucket.com/albums/s...P5212468-1.jpg

This is the outer skin of a J160 that fell over in a yard and landed on a bunch of jackstands which took a bunch of core samples :eek:

Jeff_H 04-24-2012 04:10 PM

Re: Balsa cored hulls
 
The interior skin is in compression so is probably thicker to prevent buckling. The outer skind does have to withstand pucture though, so it does make you wonder....

PCP 04-24-2012 07:23 PM

Re: Balsa cored hulls
 
Quote:

Originally Posted by bjung (Post 862589)
I am wondering if todays materials and lamination techniques have eased the concerns about balsa cored hulls. Quite a few highly respected builders are still using balsa coring (Malo, Regina, Dehler just to name a few), and one would assume that they have been successful with this type of construction, or they would have abandonned the idea long ago...
A little research also yielded the same concerns with foam coring, which supposedly deteriorates between the laminates as well, and there are some concerns regarding adhesion of PVC and epoxy resins.
What are your thoughts?

The others you mentionI don't know but Dehler that had built balsa cored boats for many years is changing. I don't know why but building with balsa is cheaper than building with a high end foam and I am sure it is no better. I know particularly the Dehler 41 where the cheap version has balsa core and normal resins while the expensive version (lighter and stronger) is made with epoxy resins and top foam core.

I have observed that more traditional brands still use balsa and more modern and technical boats use almost all some kind of hi-tech foam and epoxy or vinilester resins. I guess they have a good reason to do that and it is not price for sure;).

Regards

Paulo

SloopJonB 04-25-2012 02:37 PM

Re: Balsa cored hulls
 
I would think that a bagged or infused cored hull would have little problem with the core getting wet over a large area. The core material is small blocks held on a scrim so it can drape easily over compound curves. With the pressure of bagging or infusing, surely the resin gets pulled into all the "interblock gaps", sealing them from one another?

Has anyone seen a piece of this type of hull cut up to test this?

Night_Sailor 05-27-2012 07:33 PM

Re: Balsa cored hulls
 
Quote:

Originally Posted by Jeff_H (Post 862694)
Actually, A cored hull is much more expensive to build, should retain a larger portion of its strength over time since there is less fatique inducing flexure, and for any given weight, starts out stronger and stiffer.

That is why boats from the higher quality boat builders are built with cored hulls while the cheaper boats are not.

The fatigue issue is a serious one. Actual testing of older boat panels by an insurance industry study showed that fatique dramatically reduced the strength in older panels over time. But, and this is a big but, cored hulls only retain a higher portion of their strength if there is little or no delamination.

Balsa coring is popular because structurally it has better stiffness and sheer resistance than a similar weight foam coring. Properly installed, Balsa coring develops a strong and rot resistent bond with the skin.

In terms of long term durability of foam, there are a pretty wide range of foam core materials with equally divergent properties which makes it very hard to make a broad generality about the relative durability of foam coring.

In terms of build method, these days most cored boats are constructed using resin infusion or some other form of 'vaccuum bagging'. This results in a better bond between the core and the skins and in theory results in better protection from core damage. The key to preventing rot in a cored boats is avoiding penetrating the skins into the core or sealing any penetrations to avoid water intrusion.


Well said: I'll add that older glass boats, prior to the mid 70's were grossly overbuilt. Builders were used to thick wooden hulls and make the glass layout particularly thick and strong. Some boats were built with balsa core in those days can be 2" thick or more. Amazingly tough boats that are literally bullet proof. Installing a new thru hull for example needs to be done properly to keep water out.

A moisture meter can be used to check for problems. However it can be somewhat fooled by properly done repairs. Pay particular attention to areas around thru hulls.

Night_Sailor 05-27-2012 07:47 PM

Re: Balsa cored hulls
 
Quote:

Originally Posted by SloopJonB (Post 863159)
I would think that a bagged or infused cored hull would have little problem with the core getting wet over a large area. The core material is small blocks held on a scrim so it can drape easily over compound curves. With the pressure of bagging or infusing, surely the resin gets pulled into all the "interblock gaps", sealing them from one another?

Has anyone seen a piece of this type of hull cut up to test this?

I have. Not on a hull but a balsa deck I re-cored.

I found some gaps where the balsa was not butted up close and the resin blocked water from getting into the adjoining area. I have lots of pictures from this job. I'm not sure if it shows this aspect. It was more obvious to me when it came time to scrape out the old core. Some of it peeled off and in other places I wanted straight line to lay new core so I had to remove some good core. There was rotten core--which was decomposed and wet with no bonding strength remaining, wet core which still had some bonding to the surrounding fiberglass, and dry core which was still 100% functional. I took off more than I needed to because I wanted to get rid of all the rotten and wet core.

As an aside point: One thing I did was replaced the core material wherever I mounted hardware with sections of sold glass core I cut out of 3/8" fiberglass panels. Overbuilt? Yes, but I don't have to worry about water ever getting into the core. I did not have to bed anything in as I am ok with water passing through. I might have some stainless corrosion from the fastner's at some point, but that is minor compared to making my deck core impervious to water.


Night_Sailor 05-27-2012 08:24 PM

Re: Balsa cored hulls
 
Quote:

Originally Posted by PCP (Post 862814)
The others you mentionI don't know but Dehler that had built balsa cored boats for many years is changing. I don't know why but building with balsa is cheaper than building with a high end foam and I am sure it is no better. I know particularly the Dehler 41 where the cheap version has balsa core and normal resins while the expensive version (lighter and stronger) is made with epoxy resins and top foam core.

I have observed that more traditional brands still use balsa and more modern and technical boats use almost all some kind of hi-tech foam and epoxy or vinilester resins. I guess they have a good reason to do that and it is not price for sure;).

Regards

Paulo

Balsa has much better adhesion than foam core. A stronger bond makes for a better laminate. But you have to keep it dry. Ask any boat builder and they will tell you the same thing. Personally I use balsa core mostly because I have lots of it around. It is cheap. I store it in the attic to keep it bone dry.

My preference is foam because it adds flotation. I would worry about it in lightweight boats that get bashed up on the ocean, like these Class 40's and Open 60's.

I use plywood for molds on things I don't care about.

I also like solid glass because it doesn't rot. I am debating building some cabinets, engine bay sound shields, and bulkheads on one boat out of solid glass just for rot prevention. Water gets in everywhere, and plywood seems to suck it up. My big boat is rather heavy. I don't think it will hurt much to use a lead sound shield around my engine. I like quiet diesels. A combination of solid glass rot-proof panels, insulating paint, glue on foam and lead sheeting is my plan.

Epoxy resins are heavier. I like them because they are stronger. I would always chose epoxy resins, and pay the premium for them. If they can make the boats lighter using epoxy then they are saving weight somewhere else. Switching from balsa to foam? A different layup? I suspect the epoxy is needed for better adhesion to the foam. I am no expert of foam, but I believe builders use a scored foam to increase adhesion. I'd like to see a cross section of both options. Hanse is selling an epoxy version. It is heavier, not lighter. I don't know what the core material is. So the layup and core are identical with the trade off being heavier epoxy for greater strength--something to think about when you are slamming brutally hard into a wave trough in the Gulf Stream every 45 seconds in a pitch black storm...I can tell you that from experience. I'll take the stronger hull any time.



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