Balsa Core

[clockwork orange]

Originally Posted by chebba

I'm in San Carlos just starting to re-core my bow and decks. PO said decks were plenty strong what with three quarter marine ply. BS. Then he said the main shrouds were obviously attached to the hull. BS. Older Tawianese liked to bury chainplates in the deck. So I have a rotten deck at least to the cockpit and very questionable chainplates. I'll do the job. what to use for core. Every material is questionable and suspect with many obvious failures. If not installed correctly and maintained correctly any material will fail. Coosa board may be the exception. I cannot get it here. Balsa will infiltrate with water if there are minute channels left open, etc... Foam will compress and disintegrate if wet also. What's left that is easy to get, is already laminated, bonds well with fiberglass or either type? Plywood. Two three eights inch layers with staggered joints and all through decks solid fiberglass. Maintained it will last for the life of the hull.

Chebba, I don't know where you are getting your info but there is no issue with pvc foam, H80 Klegecell, Divinycell etc can get wet and nothing happens, if you dry it out it is as good as new, it does not disintegrate, period. There may have been problems with the older urethane foam but to be honest i don't think so. I actually own an old 1977 Lindenberg 26 with what i believe to be a polyurethane foam core in the deck and also the forward 2/3rd of the hull above the waterline. All the interior furniture is also cored. I bought this boat off ebay from boat angel and it had been sitting for months with standing water over the cabin sole, including a Midwest winter, the sole in the head was saturated because of dozens of screw holes where teak slats were screwed down without caulk. When I cut it out to create a liftout so i could install a transducer the jigsaw was throwing water in my face. I keep the cutout around to show people because it is now totally dry with absolutely zero degradation of any kind, no delamination from the skins and no disintegration of the foam, it is as good as new. The cockpit sole also had teak slats so i had to drill out the holes and pot them, again zero issues, solid as can be. I have seen the same situation with pvc foam cores, it can still get wet of course but it does not degrade or delam from the skins. The only structural issues i have ever heard of with foam can be traced to using too light or too thick a core for the job at hand as builders chase weight savings. If you want to use foam to replace your balsa core use at least an H80 foam (80kg/m3 or 5lbs/ft3) but you can go heavier of course, a typical balsa core is around 10lb/ft3 so you have plenty of room to work with. Of course Coosa would be a great choice if you can import it, i did my own tests for water absorbtion when it first came out and can substantiate the manufacturers claims. I never take manufacturers claims at face value without doing my own real world testing.

Steve.

[clockwork orange]

Originally Posted by Nicholson58

Lighter-yes, less expensive-yes, stiffer only by weight. 1 inch of solid GRP is stronger & stiffer than 1" balsa core.

I too have an older boat with wet decks. By the time I am done chasing out the wet balsa, I will have replaced nearly 320 square feet of wet balsa with syntactic foam (epoxy & microballoons drilled in & pumped in from the the cabin interior. If I do not, it is uninsurable and a structural mess. Balsa, at the time, was the best short-term solution to the need for ultralight and material savings rolled into one. In my opinion, it is inadequate for future construction of quality vessels. If you expect more than a 10 year life, no balsa should be in a boat. This is especially true of below water structure such as hulls & rudders in particular. There is probably not a rudder in the entire yard here in Muskegon that does not weep water after a winter freeze-thaw cycle, except mine. It is 6 inches thick X 3 feet X 6 feet, solid epoxy & microballoons with a carbon fiber skin vacuum bagged to the core.

You can see my repair photos for deck & the rudder in the photos gallery Member Galleries - Cruisers & Sailing Photo Gallery

The OP is looking for a heavy boat built before balsa-core or for a new-ish boat of quality epoxy or vinyl-ester construction with low density syntactic filler &/or light weight reinforcements. The heavy, solid core boats are out there. A yard mate found one on EBAY; 33 feet for 2200 dollars; well maintained & ready for a 10 Kilobuck, fair re-fit.

Unfortunatly what you are doing is not a good repair for several reasons, unless you are using a high elongation epoxy your microballoon mix will be quite brittle, not at all ductile like a real foam core, my guess is that since you are in Michigan you are using west epoxy which is quite low in elongation, in the region of polyester around 4% if i remember right, i don't use it because of this although G flex would be a lot better. The other problem with just drilling holes and injecting is that it is wet in there and full of composted balsa so the goo will not bond adequately to the fiberglass skins. It will feel solid but it is not a proper fix. A lot of amateurs go this route mistakenly believing that if they drill a lot of holes and let it sit for a while it will dry out in there, it does not, the only way unfortunately is to bite the bullet and cut it open and do a proper re core.
I would also disagree on balsa being a short term product, its never been a problem of the material, just incompetent builders and owners, but going forward things look good for balsa as more and more builders use vacuum infusion to build their boats and through threads like this more and more boatowners educate themselves on the correct ways to bed new hardware.

Steve.

[colemj]

I would put balsa in over plywood in a heartbeat. Plywood is your worse possible choice in every way except where compression strength is needed under fittings - and there it is a bad choice. Any concerns with balsa are magnified 10-fold with plywood - where water intrusion is guaranteed to run horizontally and thoroughly.

If you make all through fitting areas solid glass, there will be no way for water to get into the core.

Mark

[Seymore]

Originally Posted by Dockhead

And what's the relevance?

Cheeki Rafiki was a Beneteau First 40.7. In the second photo the balsa core and fiberglass sheath are clearly visible where the keel ripped off.

[colemj]

My eyes aren't that good, but I'm not sure I see what you describe seeing. I do see a layer of fiberglass peeled away, but I don't think I see balsa core (or any core at all).

Mark

[neilpride]

The first 40,7 dont use any kind of core around the keel área, not below the waterline if i remember well, solid glass....

[rmw2007]

Interesting post
Ross

[roverhi]

The primary job of a core is to keep the two sides of the laminate apart. A cored laminate gets its stiffness in a similar fashion as an I beam. It's the two skin sections acting in compression and extension against each other that makes them stiff. Anything that keeps them apart works though not as well if the skins are bonded to the core. Filling with epoxy and microballoons will do the job even if it's not bonded to either skin because of decomposed balsa core. What it is not is as light as a properly laid up balsa cored deck or hull. Resin is a heavy material, pick up a 5 gallon can of resin if you want to get a good idea.

Some foam cores have not worked out optimally over the long term. The bond between the core and the glass laminates can separate with the working of the hull. Still acts to keep the two sides of the laminate apart but the unadhered space can fill with water adding to the weight of the laminate. That defeats the purpose of a laminated hull, stiffness while still being light.

[colemj]

Originally Posted by roverhi

It's the two skin sections acting in compression and extension against each other that makes them stiff. Anything that keeps them apart works though not as well if the skins are bonded to the core.

That isn't true if the scantlings and engineering are designed for a thoroughly bonded core - which pretty much all cored boats are.

It may be true for a thought experiment carried to an extreme.

If the skins are moving independently from each other with a filled, but separated gap between them, it is not structurally sound.

Mark

[clockwork orange]

Originally Posted by roverhi

The primary job of a core is to keep the two sides of the laminate apart. A cored laminate gets its stiffness in a similar fashion as an I beam. It's the two skin sections acting in compression and extension against each other that makes them stiff. Anything that keeps them apart works though not as well if the skins are bonded to the core. Filling with epoxy and microballoons will do the job even if it's not bonded to either skin because of decomposed balsa core. What it is not is as light as a properly laid up balsa cored deck or hull. Resin is a heavy material, pick up a 5 gallon can of resin if you want to get a good idea.

Some foam cores have not worked out optimally over the long term. The bond between the core and the glass laminates can separate with the working of the hull. Still acts to keep the two sides of the laminate apart but the unadhered space can fill with water adding to the weight of the laminate. That defeats the purpose of a laminated hull, stiffness while still being light.

Actually it is not acting as an I beam at all unless the skins are indeed bonded to the core, just filling the void with goo that is not properly bonded to the skins, while it will certainly feel significantly stiffer than it was with the rotten core it is in no way structurally as sound as the deck originally was. The good news is that even a poorly repaired deck will be an improvement.
I have yet to see a foam core that has failed in any way in over 40 years in the trade, i have built a few and done repairs on a few that have been involved in race course collisions but have never seen a structural issue due to the foam core. As i said before, maybe with early polyurethane cores but nobody has used that in decades. I have heard of issued with some of the Whitbread 60s but i believe that was found to be due to choosing too light a core while chasing weight savings and was dealt with by specifying a minimum density in the rules. I have never heard of any problems with H80 or denser cores. I did a huge refit on a Choate 40 about 25 years ago, it was cored with H80 klegecell, (a pvc foam) It had a lot of wiring screwed to the inner skin and water was coming out of the core at the screws in various places, water was getting in somewhere and travelling all over the place by way of the scoring in the core, for years yet caused no problem other than an annoyance even in the frigid north. Even though balsa is structurally superior, any foam density from 5lbs/ft3 on up is more than adequate and it has the advantage of being available in many densities so if you do have shear issues its a screw up on the part of the designer or builder, not a fault of the foam.

Steve.

[minaret]

Originally Posted by clockwork orange

Actually it is not acting as an I beam at all unless the skins are indeed bonded to the core, just filling the void with goo that is not properly bonded to the skins, while it will certainly feel significantly stiffer than it was with the rotten core it is in no way structurally as sound as the deck originally was. The good news is that even a poorly repaired deck will be an improvement.
I have yet to see a foam core that has failed in any way in over 40 years in the trade, i have built a few and done repairs on a few that have been involved in race course collisions but have never seen a structural issue due to the foam core. As i said before, maybe with early polyurethane cores but nobody has used that in decades. I have heard of issued with some of the Whitbread 60s but i believe that was found to be due to choosing too light a core while chasing weight savings and was dealt with by specifying a minimum density in the rules. I have never heard of any problems with H80 or denser cores. I did a huge refit on a Choate 40 about 25 years ago, it was cored with H80 klegecell, (a pvc foam) It had a lot of wiring screwed to the inner skin and water was coming out of the core at the screws in various places, water was getting in somewhere and travelling all over the place by way of the scoring in the core, for years yet caused no problem other than an annoyance even in the frigid north. Even though balsa is structurally superior, any foam density from 5lbs/ft3 on up is more than adequate and it has the advantage of being available in many densities so if you do have shear issues its a screw up on the part of the designer or builder, not a fault of the foam.

Steve.

Have repaired several boats with saturated foam cores. One which was cored with 1" foam below the waterline weighed more than 1,000 lbs less after drying by Hotvac! No delam, no problems. Can't do that with balsa!

[colemj]

How does foam saturate? Isn't it suppose to be closed cell? That was 120 gallons of water you removed - more than could be expected from kerf channels alone.

Mark

[Dockhead]

Originally Posted by Seymore

Cheeki Rafiki was a Beneteau First 40.7. In the second photo the balsa core and fiberglass sheath are clearly visible where the keel ripped off.

That's not balsa which is visible, but one of the delaminated layers of glass. The Bene First 40.7 does not have balsa below the waterline, in fact, like most inexpensive production boats, it has no balsa in the hull at all. See: http://www.first407.org/TipsOgTriks/...ec%20sheet.pdf.

And even boats which have balsa below the waterline, do not have balsa at the hull-keel joint.

[minaret]

Originally Posted by colemj

How does foam saturate? Isn't it suppose to be closed cell? That was 120 gallons of water you removed - more than could be expected from kerf channels alone.

Mark

Just goes to show you how much volume is often in those kerf channels. I decored some substantial areas of the bottom for repairs too (large powerboat hit the rocks at speed) and it was evident that the core was saturated to a degree. Each individual block of foam was indeed dry in the middle, but there was a certain degree of moisture penetration into the foam as well. Required drilling holes through the external skin followed by application of Hotvac for about a month total. Saturation resulted from a (very) poor repair of the damage by a different yard, followed by more than a year in the water. This was a 55' approx. power boat.

[Dockhead]

Originally Posted by minaret

Have repaired several boats with saturated foam cores. One which was cored with 1" foam below the waterline weighed more than 1,000 lbs less after drying by Hotvac! No delam, no problems. Can't do that with balsa!

Yes, and modern foam coring materials are fine for hulls designed for it, but foam has the significant disadvantage of having very poor structural properties compared to balsa. Balsa is stronger than steel by weight and makes a tremendously strong sandwich with FRP. The key advantage of foam is that it is much cheaper and easier to work with than balsa. Constructing with balsa, using modern encapsulated blocks/resin infusion techniques, is laborious and very expensive, but yields the strongest, lightest structure by far.

I would bet that 100.0000% of the boats, in which you've replaced rotten balsa cores, were non-encapsulated, non-resin-infused balsa cores which have not been used, as far as I know, for decades by now. Just as obsolete as the crumbly raw polyurethene foam cores used in many boats in the '70's and '80's. Neither modern foam, nor modern balsa, should be confused with obsolete versions of the technology.