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

[Beau.Vrolyk]

The complaints about Balsa are all based on poor building technique or owner mods which were poorly done. It is a great building material, lighter with better bonding to resins. (As others have stated)

However, owners are a boat’s worst enemy.

If people will do the research to learn how to do things properly, they will find all sorts of problems disappear. Soggy cores in boat are just one of many problems brought on most often by poor DIY owners. Not all parts of a boat need to be amature-proof.

[Montanan]

BTW, please, please, please DO NOT USE fender washers as backing for any hardware. Use a proper backing plate.

Definitely utilize proper backing plates and fasten to such using thick fender washers so as to distribute force on the backing plate and to not cup the washer when put to high torque fastening compression load.

[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.

[ggordon1250]

Nobody uses balsa cores any longer. In fact most boat builders do not use any core below the waterline. But in late 80’s boat building they did use balsa

[stargate]

Balsa core is a great core material , if you don't allow any ingress of water. Any exposure to water will rot the balsa that is not saturated with resin. You cannot under any circumstances mount anything on a cored deck successfully unless you first remove the balsa around the place you drill the hole and fill it with resin based fill or epoxy based fill(not my 1st choice,merely because of compatibility) so you protect the core from water ingress and give the area some compression strength. There are no compromises , if you don't do this you will eventually destroy the surrounding area from water intrusion.
I have a boat with balsa core and it was built in 1973. There is no rot or de lamination anywhere in the core or the fiberglass over plywood deck. There has never been a hole drilled anywhere without doing it right.Just putting sikaflex or another caulking around it won't work long term

Any where you drill and put a screw into fiberglass you get a microscopic spider fracture in the fiberglass surrounding the hole, which allows moisture to enter.
Having built boats for 50 years and been a marine surveyor for 30, I have seen some Disasters. People have a perfectly good fiberglass deck , then put hundreds of screws through it to mount teak and later wonder why their deck rotted.
Balsa, like anything , if is used properly it is great,if you compromise it, it too will fail.

If your looking to purchase a boat, everyone will be different and have different problems. Get a good surveyor to check things out and make your self aware of just what your exposure to repair costs can be.

[wingssail]

Originally Posted by stargate

...You cannot under any circumstances mount anything on a cored deck successfully unless you first remove the balsa around the place you drill the hole and fill it with resin based fill or epoxy based fill(not my 1st choice,merely because of compatibility) so you protect the core from water ingress and give the area some compression strength. There are no compromises , if you don't do this you will eventually destroy the surrounding area from water intrusion...

Be careful of absolute statements senior stargate. My winches (8 of them currently, see photos above) were not installed per the spec you describe. Balsa core, not solid core, no epoxy filled holes, yet in 40 years they remain solid and dry. Not recommended, I know, but, well I didn't build it. I've had the bases off more than once, I see the condition, it's still OK. Other areas I've had problems, but it's not fair to tell someone that if it was ever done that way the deck will absolutely eventually be destroyed.

[Matt Johnson]

Originally Posted by wingssail

Be careful of absolute statements senior stargate. My winches (8 of them currently, see photos above) were not installed per the spec you describe. Balsa core, not solid core, no epoxy filled holes, yet in 40 years they remain solid and dry. Not recommended, I know, but, well I didn't build it. I've had the bases off more than once, I see the condition, it's still OK. Other areas I've had problems, but it's not fair to tell someone that if it was ever done that way the deck will absolutely eventually be destroyed.

Even worse is the statement in post number 64. How in the world does someone think that "they" don't use balsa anymore?

Matt

[u4ea32]

My Olson 40 is all Balsa Core, entire hull, entire deck. It was built from November 1983 and launched March 1984. Built by Pacific Boats in Santa Cruz.

Everyone knows about George Olson, who designed Grendel, big influence on the early Santa Cruz boats especially the SC27, and of course the Olson 30, 40 and so on.

What most people (including me until after I got the Olson) do not know, is that George's wife was an industrial engineer. She tested everything, documented everything, paid attention to the details. So the environment the boats were built in was not just a dirt floored barn or out in the open, like the norm. It was built in a building where they controlled for everything: temp, humidity, but also all preparation details of all materials, including of course the balsa. Skilled, trained labor that followed defined processes, not day workers.

My boat was raced hard and put away wet for its first 34 years. The original owners loved punching holes and installing stuff.

I stripped the boat completely, removed all the paint, inspected the entire structure.

We did have some issues with the core:

1) The depth sounder thru-hull was installed through the balsa cored bottom without ANY caulking or sealant of any kind, just barely more than hand tightened. It had been installed at least a decade earlier. The core around the transducer thru-hull was black goo, about the same consistency as peanut butter.

I immediately thought we should cut up the boat and send it to a landfill.

However, we cut the inside skin until we hit dry, tan balsa. That was about a HALF INCH! We just cut that small layer of goo out, put in several layers of solid glass, and re-drilled for the new thru-hull.

Besides the near heart attack when my finger first touched that black goo, the total time, labor, material, and cost to fix was not much worse than a dinner out.

2) On the port side deck near the shrouds, an inboard track was poorly installed. An area about a foot square needed to be re-cored: removed the inside skin, removed the rotten core, replaced the balsa with dry (we did the work outdoors, but under cover in dry Southern California air), and re-laminated the interior skin. Very little time, effort, material cost, labor, or money involved.

3) The vibration in the diesel drive train shook the hell out of the prop shaft strut. This cracked the filler around the strut, and allowed water into the core. Again, the water had propagated less than an inch around the strut. Again, fixed for almost nothing in time, labor, materials, or money.

In every other case where I have drilled holes to install anything, the balsa has been light tan, completely dry, and very well adhered to the skins.

OK, so all the above might be attributed to "build it carefully." But now consider this:

Amazingly, one deck hatch over the saloon has no sealant of any kind on the inside edge of the hole cut through the balsa cored deck. There is a plastic moulding that lines the hole for a visual finish, but the actual hole has no form of coating. Its been that way for 35 years. That easily seen unfinished edge of balsa is hard, dry, tan balsa. So even in the high humidity of being at sea level in the Pacific for 35 years, nothing degraded with that open edge of balsa in the deck, aft of the mast.

My conclusion: the danger of balsa is wildly over stated.

[Yak52]

Hi I Haven’t read all the reply’s but to keep this short and sweet the Balsa is mainly used as its compression strength, lightness and if your boat has a leak or say a small hole ie someone putting a fastening through and doesn’t seal it properly water will ingress and being end grain it will not spread.
Yes it will soak it up but won’t spread like other materials ...within reason.

[jimbunyard]

My conclusion: the danger of balsa is wildly over stated.

Maybe in an alternate universe...I've repaired (and not repaired) boats where virtually all the balsa core was rotted away. These pictures are from the internet though.

[Simi 60]

Originally Posted by ggordon1250

Nobody uses balsa cores any longer. In fact most boat builders do not use any core below the waterline. But in late 80’s boat building they did use balsa

You have no idea.

[u4ea32]

Originally Posted by jimbunyard

Maybe in an alternate universe...I've repaired (and not repaired) boats where virtually all the balsa core was rotted away. These pictures are from the internet though.

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.

[u4ea32]

A friend had a steel boat. He pointed out that the rust makes it easy to maintain: a rust stain makes it easy to focus on exactly the problem area and deal with it. Properly ground back and refinished, steel can last a very long time.

[stargate]

Originally Posted by wingssail

Be careful of absolute statements senior stargate. My winches (8 of them currently, see photos above) were not installed per the spec you describe. Balsa core, not solid core, no epoxy filled holes, yet in 40 years they remain solid and dry. Not recommended, I know, but, well I didn't build it. I've had the bases off more than once, I see the condition, it's still OK. Other areas I've had problems, but it's not fair to tell someone that if it was ever done that way the deck will absolutely eventually be destroyed.

I can only say you are one in a thousand . Fortunately for you the water has obviously not been able to get under your winches. Anyone can mount things how ever they like, the risk is yours, but if water gets in at all you will have issues, so why would you take a big risk by not doing it right. The percentage of problems after poor installation is in the top 90 percentile. But each to their own, good advise is always available, good listeners are not. Common sense is not a flower that grows in every garden. Good sailing

[Cheechako]

It's surprising nobody has done staggered coring squares in offset positions for decking. or other engineered alternatives. Each square isolated from the next, fully laminated over. But they probably dont really care if the boat lasts.
Ta Shing did a similar thing with small 3-4" squares but just shot slurry resin between. It worked pretty well. But I'm thinking 6" or bigger squares.
With the engineering in today's boats, this could be done well, and the indents used for locating bulkheads etc if done right. I doubt there would be any deck flex to speak of even if the laminated non cored "channels" were a 2-3 inches wide.