Why is Balsa coring used anywhere on a boat instead of all closed cell foam?

[jimbunyard]

Originally Posted by wingssail

It’s all well and good to use terms like, “if it is done right…” or to describe the perfect technique for laminating a cored structure. The problem is that the builders don’t always have the skilled staff and the proper techniques aren’t always followed (the foreman was an alcoholic, the floor workers had sub HS education and their last job was McDonalds and the owner was busy trying to stave off bankruptcy). So most of us are stuck with boats that weren’t built perfectly. Deal with it. Talking about the benefits of various core materials is pretty theoretical compared to the real issues of quality control.

In my case a bunch of rednecks built my boat in a dirt floor barn in New Orleans, and it still has lasted well through 40 years of very hard usage. But I found uncured polyester resin 20 years after first launch and, Ok, the balsa core deck has rotted in several places and I’ve re-cored it. Not hard to do but I have not always had access to end grain balsa so I’ve used other softwoods, such as pine, and it has held up well. Most of my highly loaded deck fittings (what isn’t highly loaded on this boat?) now have big backing plates and solid wood cores, although they weren’t that way originally. It was a bit odd to watch genoa turning blocks move around on the deck as their bolts plowed through the skins and balsa. But look at this winch (photo), it is on balsa core with a ring backing plate and it withstands over 4000bs load, often. Been there for 40 years, no rot, no issues.

The hull was cored with C-Flex (no longer used) and it has been brilliant, strong, solid, and reasonably light. (my 43’ weighs 16,000lbs and 38% of that is keel, and then there is the motor, rig, winches, etc, so the hull is pretty light, about 5000lbs). I don’t have any foam core but I have seen foam core hulls where the outer skin has come loose from the foam and the whole structure flexes. Repairing that is a big job. Also on foam cores or any core with thin skins, the outside skin can be punctured, not good. My least favorite core material is foam because it may be difficult to bond properly to the skin (quality control, remember?).

Honeycomb panels: I have used nomex honeycomb panels (surplus from Boeing, but odd shapes!) for my entire interior build out since I first discovered the stuff. Light, strong, does not rot, and good compression strength; however it has to be tabbed since fasteners don’t hold although with big fender washers you can through bolt it. The polyester skins are best for glassing, but the carbon fiber skins are quite a bit stronger.

Bottom line: there are pluses and minuses to each hull construction method, but real world is that few boats were perfectly done, and saying, “if only it is done right, there is no problem...” is a bit disingenuous, and sort of self congratulatory. Any material can be used, and if done well it will serve. If not, you can repair it. Perfect is th enemy of good.

I'm somewhat confused by several points in your post.

Obviously nothing constructed can be considered 'perfect', if only because of the transient nature of time...(haha)

In the order of your post.

Why is it 'easy' to replace balsa core, yet replacing foam core is a 'big job'?

C-flex was a kind of construction material and method invented by Seeman, if memory serves, originally for use in coating wooden shrimp boats. It consisted of solid fiberglass rods in a woven fiberglass mesh that was stapled to a 5200-covered wooden hull that was then saturated with polyester resin, which was then faired, or fiberglassed and faired. Later it was used by itself for original constructions by being stapled to an either permanent or temporary mold, in a manner similar to cold molding or strip plank building.

I've never heard of C-flex 'planks' being used as core, but if it was then you have basically a solid fiberglass structure. If you have C-flex skin over a balsa (or other) core, then you are not comparing apples to apples, because C-flex itself, unlike flexible reinforcements, is considered a structural component of the skin (at least in one direction [until saturation,when it becomes structural in all three]). And C-flex skins are generally quite a bit thicker than those made from woven reinforcements because of the 'rods' from which C-flex is made...

I've used many different brands and types of plastic foams in many different modes. When coating with resin and fabric-type reinforcements, I've had only one instance of bonding failure, and that was with PVC foam that was contaminated on it's surface.

Which is not, of course, to say that stress induced failure is impossible with PVC or any other type core, but that proper care in construction technique goes a long way towards preventing, or at least lessening such failures.

Having dealt with honeycomb panels only twice (that I can remember), the only thing I can say is that in both instances, the plastic honeycomb did not seem to adhere to the polyester skin properly. That was at least 16 years ago, so I hope 'they've' got that issue worked out by now...

Perfect is the enemy of good enough...

Originally Posted by u4ea32

Well, of course someone who fixes boats gets called in when there are problems. I am not surprised you have seen many boats with problems.

What was the root cause of these issues? It obviously was not the balsa, as the many examples of old, dry balsa boats (such as mine) attest.

It is clear there are problems with some boats, including some expensive boats (Bertram's well documented problems that crushed that company 30 years ago).

There are also documented problems with delamination in un-cored fiberglass, especially when polyester is used. Polyester is quite porous to water vapor, so H2O molecules can migrate into any polyester laminate and then cause delamination because water does not compress, and ice expands.

There are examples of foam cored boats completely de-laminating, even those built with epoxy (no water vapor).

There are examples of metal boats being disasters, or perfectly fine and very long lived.

There are examples of wooden boats that have lasted a very long time with little problem -- those built by Gougeon are obvious examples. Yet clearly, most wooden boats do not last long.

So it is not simply the material.

To repeat myself and others.

Balsa core is a fine material, but it has a primary enemy, which is, of course, water, or more properly, water which persists long enough to enable, shall we say, negative consequences. Which makes it seem almost asinine to use it in boat construction, especially since there are now products that duplicate or exceed its' performance. Without the chance of rot-related failures...

Balsa was originally used because it was the only game in town, as competing materials became available, it was used because of it's real superior characteristics, then it's supposed superior characteristics, to now being used primarily (I think) because of its' (initial) lower cost.

As for 'root' or ultimate causes, it seems to me you have them backwards.
The ultimate cause of balsa core failure is that it is balsa, in other words, because it rots, or is more susceptible to rotting.

The proximate causes of failure are poor construction techniques, owner/operator errors, or perhaps more usually, a combination of both, which are the same for any core; indeed for almost any constructed thing.

I'm not really sure how the physical properties of water are changed by it being present in either balsa core or plastic foam. Seems to me that freezing or porosity problems would be the same, or mostly the same, regardless of core material, though the hydrophilic property of any wood, but especially balsa, would exacerbate those problems, especially when compared to the hydrophobic properties of the plastics from which foam cores are made.

The vagaries of life in the real world should put paid to any attestation of 'because my experience is 'this', therefore 'this' is 'correct' or 'the way it really is' ',

I have no doubt that yours and Wingssail's experiences are accurate, but I don't think they represent the norm. Both of your boats, by your own descriptions, seem to be custom built, or at least semi-custom built, and therefore, perhaps, out of the realm of what is more often under discussion in these pages, i.e. standard production boats.

For instance, in both of your boats, are either of you aware of what grade balsa core was used? Balsa, like plastics, is available in different densities, and those differences change its' water (and resin) absorption, as well as its' tensile and compression strengths; this crucial difference is often overlooked, essentially mooting any comparisons...

[Jim Cate]

For instance, in both of your boats, are either of you aware of what grade balsa core was used? Balsa, like plastics, is available in different densities, and those differences change its' water (and resin) absorption, as well as its' tensile and compression strengths, and this crucial difference is often overlooked, essentially mooting any comparisons...

That's interesting, and I was not aware of this aspect of balsa coring. Could you amplify on the subject a bit, please? What grades are typically used in what applications? Ie, is a different grade used in decks (where compression loads might be higher) than hulls or bulkheads? And is t here a cost differential between grades of balsa?

Our previous boat, built by Palmer Johnson, had air-x coring in the hull and balsa in the deck. We had minor delamination in part of the deck (without water intrusion) and minor rotting in way of the chainplate penetration. The hull had no coring issues when we sold the boat at age 30 years. I'll admit, though, that the skin thicknesses were hideously overbuilt by modern standards, and that may have contributed to its longevity!

Jim

[jimbunyard]

Originally Posted by Jim Cate

That's interesting, and I was not aware of this aspect of balsa coring. Could you amplify on the subject a bit, please? What grades are typically used in what applications? Ie, is a different grade used in decks (where compression loads might be higher) than hulls or bulkheads? And is t here a cost differential between grades of balsa?

Our previous boat, built by Palmer Johnson, had air-x coring in the hull and balsa in the deck. We had minor delamination in part of the deck (without water intrusion) and minor rotting in way of the chainplate penetration.

Jim

I can't, because I don't use it and I'm sure that major manufacturers specify those characteristics to their own idiosyncratic needs. I'm quite sure there is a price difference in densities, if only from (going way back) remembering the price difference relative to the hardness (for me it was quantified by my ability, or not, to cut it with a variously sharp exacto knife); the stuff I could cut easily was half as expensive as the 'good (spar quality) stuff'...

A quick search yields several different densities from 109 to 335 lbs/cu m.

A data sheet for one grade of coring supplied by Baltek, probably one or the largest manufacturers, is here

https://www.3accorematerials.com/upl...-SB-E_1106.pdf

Their website is here

https://www.3accorematerials.com/en/...s/baltek-balsa

...The hull had no coring issues when we sold the boat at age 30 years. I'll admit, though, that the skin thicknesses were hideously overbuilt by modern standards, and that may have contributed to its longevity!

I think that nicely sums up the underlying consensus...

[SV Windrush II]

I have personally seen whole sides of boats with their skin ripped off trying to take out all the rotten balsa core. For me, it is a risk I would not want to take. Making sure a owner never penetrates a sealed core area is highly unlikely, and that's not including any manufacturing flaws that no one would know about until its too late. There is all kinds of ways to stiffen a hull or deck than trusting balsa core to do it. Now since we have lower cost carbon fiber and better foam core material, there is better options.

[Chotu]

Everything you need to know about this subject is right in this chart containing all core materials.

Look at the "Compressive Strength" column and see what material is better for compression. WAY better. That's why it's in my deck. It's a far superior material.


https://www.corecomposites.com/filea...omparison2.pdf

Looking at that chart, given the size backing plates I used to spread the load of my deck cleats for instance, I could probably lift my boat up by a deck cleat. (It's a catamaran). TTY that with foam and it's going to crush and deform because it doesn't have the compressive strength.

Anyone can build a piece of junk in any core material. You can hand laminate foam or balsa poorly leading to delamination of either. In my case, all air was removed under vacuum inside a bag and then replaced with epoxy resin. I could have used sandwich bread for a core and it's not going anywhere. Because all air was evacuated and replaced with epoxy.

Foam is no better than balsa, just different. Right tools for the right job. When you need compressive strength, such as in a deck, nothing beats balsa. When you need ability of a core to spring back after moderate impacts, foam is your choice.

None is better than the other, they just have different properties suited for different uses.

Imagined faulty construction and boat owner misuse are not valid arguments. Don't buy poorly made boats and don't be an idiot and all cores can then be judged on their physical properties. Properties which are suitable to the use of the core, such as balsa being the absolute best core for a deck, where compresion and sheer are most important.

My boat is all foam. The hull, the dagger boards, the structural bulkheads, the connectives, the steps, the rudders, everything. Except the deck. I used the absolute best materials money can buy on the boat. And balsa is the best material money can buy when you are using it in a place where there are high loads on the laminate.

Right tools (materials) for the right job.

[dfelsent]

There are some better much pricier foams that are not on this chart, including Rohacell for example.
Compressive strength is not everything in our deck loading scenarios either. When bending a cored structure there are significant shear loads in the core. That number is certainly a more useful comparison for pure bending.
I’m not saying balsa isn’t the best bang for the buck when initially built.
And I won’t say foam doesn’t fail.
I have owned dry core balsa hulls. I have owned dry foam core hulls. Wood hulls. Pure glass hulls. Each has pluses and minuses.
Some are sensitivity to environmental factors, sensitivity to neglect, sensitivity to physical abuse. Structural weight. The list goes on.
There is no best material for all marine applications.

[Chotu]

Originally Posted by dfelsent

.
There is no best material for all marine applications.

This. Exactly. Each core has its properties and using the best one for the job is the way to go.

[Cheechako]

The problem I see with cored areas is not so much the material, but BOTH the end user and the manufacturer. In THEORY these products are fine. In REALITY they are installed in the hull by a guy who is the bottom of the totem pole at the boat maker. He's got mask, ventilator and goggles that he can't see well through, he's sticky all over, is high on fumes and that is his life for 8-12 hours a day. Thinking that he is going to get everything right, including saturating the coring (balsa or foam) with resin so it isn't so much like a sponge, or doesn't degrade just from vibration is a fools game. Much less relying on him to notice that the hardener injector is actually working properly and not over catalyzing or under catalyzing is very iffy. "It's end of day, and if I can just get this sprayed and finished without having to change that barrel my life will be easier".
It's like asking a drunk in a space suit to walk the white line perfectly.
There's a huge gap between material science possibility and real world ability.

[TeddyDiver]

Originally Posted by Chotu

This. Exactly. Each core has its properties and using the best one for the job is the way to go.

Including the competence and will to do the job right.
Most of the problems with delamination issues in the past were fails in bonding in mould laminated boats. Either failed infusion or no vacuum bagging at all were the most common ones. Such events have been unknown for one-offs. Wet and rotted balsa cores, besides the previous, are caused also of negligence in installation of hardware on decks without any responsibility or professionalism. The weight of the core has nothing do with any of it.

Teddy

[Cal31]

My 1980 Cal 31 was constructed with Klegecell foam in the deck. I know I have water intrusion but the decks are still solid. Given that an old boat, which might have had several owners, items added to the deck or just wear and tear on existing penetrations, my experience says the foam is a great option. Agree that Balsa has great qualities but can be compromised if penetrations are not meticulously constructed and maintained (hard to turn back the clock if a PO didn't do that). Really sad to see an otherwise great boat with spongy decks.

[Na Mara]

On aluminium boats its galvanic corrosion, steel boats its rust and electrolytic corrosion, solid GRP boats its osmosis and delamination, composite cored boats its delamination, etc. Every material has its advantages and disadvantages in boat building, and all need due care and maintenance if you want them to last a long time.

My boat is balsa cored to about 40 cm under the waterline but the manufacturer was careful to ensure that the core is eliminated anywhere something might attach to. This creates a superbly rigid and strong boat with great insulation properties that is reasonably easy to repair and upgrade. The negative side of this is that I just can't go around bolting things to my deck or poking holes through my hull willy nilly. I follow four rules stringently when attaching anything to my boat.

Rule 1. See if there is any existing attachment point that can be repurposed and use that.
Rule 2. If rule 1 can't be followed, then see if there is any nearby portion of the deck that is uncored and make the attachment there. The toe-rail is a favourite here as its two thick layers of laminate without core bolted and glued together with teak capping that is through bolted into place. Anything attached here with substantial washers is going nowhere.
Rule 3. If rule 1 and 2 can't be followed then reconsider if the supposed item is strictly necessary at all. I ruled out screwed down solar panels on my pilothouse roof on this basis.
Rule 4. If rule 1 and 2 can't be followed and the item is absolutely necessary, then, and only then, make an attachment to a cored section of the hull or deck using best practices (over drilling, filling with epoxy, redrilling, substantial backing plate or washers).

So far, in four years of ownership, I have never gotten passed rule 3. I have always been able to find a way of attaching what I need to without making a hole in a cored portion of the deck or hull.

Finally, I pay a premium each year to lift the boat and store it over the winter in a climatically controlled warehouse where the whole thing gets to dry out for 6-7 months. The boat has been cared for in this way since it was 3 years old.

The only fly in all of this is that Na Mara has a screwed down teak deck. Regina make their boats to best practice and it wouldn't surprise me at all if each and every screw was into solid epoxy but I don't know that. Indeed, it is mainly for this reason that I dry the boat out each year as a precaution against damage to the deck core from this quarter. When I have to replace this deck I will drill and fill all these holes and then glue down the next deck to avoid the never ending battle with replacing the teak plugs that fall out every year and for my own peace of mind.

Suman sumarum balsa cored hulls and decks are great features for structural rigidity, impact resistance and insulation characteristics but they are not features for those who ignore their boat except for when they actually want to go sailing, or for those who are drill-happy.

[Dockhead]

Originally Posted by Na Mara

On aluminium boats its galvanic corrosion, steel boats its rust and electrolytic corrosion, solid GRP boats its osmosis and delamination, composite cored boats its delamination, etc. Every material has its advantages and disadvantages in boat building, and all need due care and maintenance if you want them to last a long time.

My boat is balsa cored to about 40 cm under the waterline but the manufacturer was careful to ensure that the core is eliminated anywhere something might attach to. This creates a superbly rigid and strong boat with great insulation properties that is reasonably easy to repair and upgrade. The negative side of this is that I just can't go around bolting things to my deck or poking holes through my hull willy nilly. I follow four rules stringently when attaching anything to my boat.

Rule 1. See if there is any existing attachment point that can be repurposed and use that.
Rule 2. If rule 1 can't be followed, then see if there is any nearby portion of the deck that is uncored and make the attachment there. The toe-rail is a favourite here as its two thick layers of laminate without core bolted and glued together with teak capping that is through bolted into place. Anything attached here with substantial washers is going nowhere.
Rule 3. If rule 1 and 2 can't be followed then reconsider if the supposed item is strictly necessary at all. I ruled out screwed down solar panels on my pilothouse roof on this basis.
Rule 4. If rule 1 and 2 can't be followed and the item is absolutely necessary, then, and only then, make an attachment to a cored section of the hull or deck using best practices (over drilling, filling with epoxy, redrilling, substantial backing plate or washers).

So far, in four years of ownership, I have never gotten passed rule 3. I have always been able to find a way of attaching what I need to without making a hole in a cored portion of the deck or hull.

Finally, I pay a premium each year to lift the boat and store it over the winter in a climatically controlled warehouse where the whole thing gets to dry out for 6-7 months. The boat has been cared for in this way since it was 3 years old.

The only fly in all of this is that Na Mara has a screwed down teak deck. Regina make their boats to best practice and it wouldn't surprise me at all if each and every screw was into solid epoxy but I don't know that. Indeed, it is mainly for this reason that I dry the boat out each year as a precaution against damage to the deck core from this quarter. When I have to replace this deck I will drill and fill all these holes and then glue down the next deck to avoid the never ending battle with replacing the teak plugs that fall out every year and for my own peace of mind.

Suman sumarum balsa cored hulls and decks are great features for structural rigidity, impact resistance and insulation characteristics but they are not features for those who ignore their boat except for when they actually want to go sailing, or for those who are drill-happy.

Some people are not aware of all the progress in design and build practices for cored GRP hulls which took place after the first experiments in the 70's and 80's. I think all the good builders do exactly what you write, with solid places wherever a throughhull or fitting needed to go in. And the real revolution was in vacuum bagging and encapsulated blocks. Hulls built this way are pretty much bulletproof, and have all the advantages you mention -- immense strength, rigidity, quiet, insulation. Balsa wood has a higher specific strength than titanium -- a GRP-balsa sandwich laid up right is an immensely strong material.

I'm with you on the "rules". In 12+ years of ownership I have never drilled a hole into a cored part of my hull or deck, and killed a few projects because they would have required such holes. Now I think it can be done safely -- just cut back the core and epoxy in. With encapsulated blocks there is little risk. But I still just wouldn't do it unless I couldn't get around it.

Too bad you have a screwed on deck. Those are from the devil. The revolution in teak decks was the all glued down ones.


Regina is a beautiful boat, though -- like almost all the Scandi built cruising boats. The Swedes sure know how to build a boat

[Dpackie]

Thanks everyone that responded to this thread, it's very insightful.

I have seen only a couple references to solid laminate decks, and only that they are heavy as compared to cored decks. There is also a lot of talk about balsa being great for structural integrity of a deck.

My specific questions are:

Would replacing the core with solid fiberglass laminate provide the strength I need if the wet wood core was replaced by the same thickness of laminate?

How much weight would I really add in replacing the wood core with solid glass on a 30 foot boat.

I'm not worried about weight, my primary concerns are strength and never having to repeat the job.

Here is a photo tutorial of the total job, on a Morgan 30 that has been a god send to me as I have started work on our M30, for reference.
https://www.tommysolomon.com/photos/...174/large/#nav

Didn't Cal make some boats with solid fiberglass decks?

Dave

[Na Mara]

Originally Posted by Dockhead

Too bad you have a screwed on deck. Those are from the devil. The revolution in teak decks was the all glued down ones.


Regina is a beautiful boat, though -- like almost all the Scandi built cruising boats. The Swedes sure know how to build a boat

Year the teak deck is a bit of a PITA and it’s worse knowing that sometime in the not too distant future I have a massive bill due for replacing it. It just looks so damn good though. I’ll baby it along for as long as I can, before doing anything about it. As with anything, with TLC it lasts longer than most people credit.

There was recently a Regina 43 for sale that had striped the teak off and had simple painted nonslip. The thing looked weird. Sort of like catching your Dad in drag for the first time - nothing objectively wrong with the look, it just jars in the eyeballs. I won’t be going that route when the day comes.

[Dockhead]

Originally Posted by Dpackie

Thanks everyone that responded to this thread, it's very insightful.

I have seen only a couple references to solid laminate decks, and only that they are heavy as compared to cored decks. There is also a lot of talk about balsa being great for structural integrity of a deck.

My specific questions are:

Would replacing the core with solid fiberglass laminate provide the strength I need if the wet wood core was replaced by the same thickness of laminate?

How much weight would I really add in replacing the wood core with solid glass on a 30 foot boat.

I'm not worried about weight, my primary concerns are strength and never having to repeat the job.

Here is a photo tutorial of the total job, on a Morgan 30 that has been a god send to me as I have started work on our M30, for reference.
https://www.tommysolomon.com/photos/...174/large/#nav

Didn't Cal make some boats with solid fiberglass decks?

Dave

I think it's a terrible idea.


Solid fiberglass will be heavy and flexy. If you make it thick enough not to be flexy it will be so heavy as to disturb the boat's stability.


Decks need coring.