Balsa cores... Chocolate tea pots.... Discuss.

[NickRobinson]

I'm trying to imagine the design meeting where this construction method was first mooted.
'Right folks, we need a cheap, light substrate to put in our topsides in boats that may still be about in 30/40/ years...
This will be in walked on areas, with dozens of piercings, some highly stressed, in a challenging environment, mechanical, damp, temperature gradients etc.
Now, what could be good? Any model aircraft makers here?....'

Back street car dealers used to stuff rust holes with Daily Mirrors and glass over..... (I'm assuming the method is no longer used?)

 

[bigwow]

Brown paper was better!:encouragement:

 

[Fendant]

Well, Balsa orginally came from the aircraft industry. The Mosquito bomber is probably the best and finest example, faster the the German hunters and still able to carry a nice payload. Everything else with solid skins on a frame was much heavier and slower.

From there it made ist way into boatbuilding. Clear is that cored composites were a novel way, to combine stiffness , thermal insulation and weight saving. Unfortunately boats have never straight surfaces, so they came up with the idea to bond balsa cubes on a scrim closth ind order to coply with the curvatures. In the beginning builders were not competent enough to apply this technique correctly and a lot of delamination failures and water ingress into unfilled occurred. It was not until 10 - 15 years later that the leading builders learned that a balsa core had to be hermetically sealed to the outside and all voids properly filled, reducing the weight saving adavantage. Most of these hulls turned its blasa core into peat.
Sometimes this was not the fault of the original builder, but simply the result of a sloppy euqipment Installation/upgrade by small yards or DIY's.
Then came foamed cores, initially in racing yachts, replacing Balsa. Still Balsa had the highest compression strength, but foams evolved over time as THE preferred core material. Honeycomb cores , again derived from aerospace Technologies, were another dead end street. They were horribly expensive, and had a limited lifetime due to delaminations.
For a relatively flat deck Balsa is absolute ok, for hulls I would prefer a damage resistant foam like Airex R63 over any crosslinked Divynicell or Herex. Newer foams are now based on thermally formable PET polymers and are used increasingly in closed mold boat building techniques.

 

[Kelpie]

I remember reading a book on yacht design where the different core materials were presented with pros and cons for each.
Viewed in comparison to the alternatives, balsa actually has some very good qualities- stiff, strong, very lightweight, economical. If you are building a brand new racing yacht it would seem silly to ignore it. Of course what makes sense in a brand new racer may not make sense in a 30 year old cruiser...

 

[Owl]

Most materials have an expected lifetime - GRP is unusual in that the lifetime is often longer than the boat's first owners!

Balsa cores work extremely well as long as they are sealed - it is just that the sealing often doesn't last as long as we'd like. I think Don Casey remarks that balsa is longer lived than a plywood core, as water spreads along the layers of plywood whereas the end-grain balsa tends to keep the water localised for longer.

If you want a fast boat and don't care if it falls apart then a balsa core is a good idea. OTOH I'm very happy my boat is solid GRP throughout, but it is a very long way from new!

 

[mobeydick]

Plus, a lot of the foam alternatives go soft in raised temperatures (as in the Med/Carib where a lot of boats go).

MD

 

[Even Chance]

My E-Boat has Balsa core topsides and deck, and its just fine. I can see the topsides core through the clear interior layup. It is only the topsides though, and the bottom is a thicker layup.

 

[Fendant]

Plus, a lot of the foam alternatives go soft in raised temperatures (as in the Med/Carib where a lot of boats go).

MD

The foams soften > 90 Deg C. The softening of foam in warmer climate is a myth brought up by the Balsa guys when they started loosing market share.

 

[Ceirwan]

My old quarter tonner had balsa core on the hull above the waterline to the deck join, And then in the deck itself.
The stuff in the hull was perfect even though the boat was 40ish years old, the stuff in the deck was getting a bit creaky, but that's mainly down to bad practice.
If the manufacturer (and subsequent owners) correctly seal the core when drilling deck holes then its not an issue,

 

[ProDave]

I guess the question boils down to, did a 1970's or 80's boat builder expect their vessels to last 40 or 50 years?

 

[Ruffles]

I thought the point of end grain balsa was that epoxy soaks into the end grain and that damp tends to not spread across the grain.

 

[mobeydick]

What do current Hallberg Rassy's have as deck construction method? They seem to go out of their way to avoid saying. Hull is Divinycell, but deck is just a 'sandwich'.........

From their specification:
"Hand lay-up GRP hull, insulated with Divinycell closed cell PVC-foam
against heat and cold, except in the keel area and high load areas which
has solid laminate. .........Deck and coachroof areas
and also cockpit are of sandwich construction, solid in parts, laminated to
the hull. "

MD

 

[seaangler23]

Mine lasted 40years (74) but it's been spongy since we got the boat 4years ago so likely a while now and I guess once the seal is broken it will deteriate quick, I can understand the end grain soaking up epoxy but on mine there was very little but in the places where I have found it there was strength. I can see the benefits but on my boat where the front deck is 7 foot wide and just as long wth no bulkheads to support pit, it seems unsuitable I guess coat cutting back in the day

 

[Seajet]

My E-Boat has Balsa core topsides and deck, and its just fine. I can see the topsides core through the clear interior layup. It is only the topsides though, and the bottom is a thicker layup.

My boat's deck is balsa core, the hull is solid laminate.

The deck ( I saw mine being built ) is made up of small squares each resin soaked and sealed from the other; seems fine to me !

I am careful sealing deck fittings, but I would be with any material.

I would be concerned with foam sandwich construction though, especially hulls; I remember the accounts on of an early Whitbread boat ( Swan 65 ' Sayula II ' ??? ) where the load on the backstays had compressed the hull around the chainplates !

 

[rudolph_hart]

Wasn't it the Dufour Arpege that alerted us to the perils of water ingress in end grain balsa core back in the 80's, with 'springy deck syndrome'?

Yet reputable builders, such as Dehler still use it on deck & hull above waterline. I'm on my 3rd one in 20 years & none have developed a problem, probably because I have never tried to 'improve' the designer's deck layout (genoa trackc, organizers etc.) which risks leaks & laminate crushing through lack of moulded-in re-inforcing pads in the chosen 'better' location. I also regularly check through-bolted deck fittings & re-bed if in doubt.

Apart from 'self inflicted' failures, I do know of a balsa core failure on a racing yacht (approx a metre square in the bow topsides) probably through pounding close hauled. I was amazed how quickly & invisibly this was professionally repaired - don't know how much it cost, though.

 

[dom]

A friend of mine in a French boatyard recently told me of a renovation they are doing on a 1985 Jeanneau which includes the replacement of a 30 year old teak. The idea was to pull out the hundreds of screws, cut out any water affected areas and redo. Not one had leaked!

They were also repairing a Bavaria which needed several stanchions replacing and found that each bolthole had been properly plugged with a bonded plastic insert, something which apparently some of the top boat builders don't do.

I'm wondering if a proper survey of any prospective boat (including new ones where the work is subbed out) is not a much better idea than going around with all sorts of preconceptions in our heads, half of which may be sending us in entirely the wrong direction.

 

[jwilson]

Done properly and treated properly with care when attaching fittings balsa is a good core material, but I did once (in my foolish youth) buy a boat with a sodden balsa cored coachroof.

I couldn't face an overhead layup job for a perfect fix, so "semi-fixed" it by drilling a LOT of holes into the underside, some of which actually spurted water as I drilled. After about six summer months and in a hot dry spell I took off all the deck fittings from the top, enlarged the holes, injected some resin, refitted and sealed up the now probably still damp in places but not sodden core. Sold the boat a couple of years later.

 

[smert]

I would be concerned with foam sandwich construction though, especially hulls; I remember the accounts on of an early Whitbread boat ( Swan 65 ' Sayula II ' ??? ) where the load on the backstays had compressed the hull around the chainplates !

Foam sandwich is not great in compression across the sandwich, any holes that are going to accept a compression fitting should be done with some form of hardpoint (often just microballoons or similar filler). It is incredibly resistant to bending however.

I created a 8' x 4' sheet from 12mm Trident T4 foam and a single layer of CSM and a single layer of mat on each side. I was able to suspend it at the ends and stand in the middle. It bent slightly, but probably less than the equivalent thickness of ply would have, and it weighed fraction of the ply!

The big issue is ensuring the foam has taken up enough resin to ensure the skin is bonded.

 

[Ruffles]

The deck ( I saw mine being built ) is made up of small squares each resin soaked and sealed from the other; seems fine to me !

I hadn't realised there was resin between the blocks. I presumed it's supplied as a sheet? Are the blocks already sealed?

 

[oldgit]

I guess the question boils down to, did a 1970's or 80's boat builder expect their vessels to last 40 or 50 years?

Quite probably Yes.....
As for boats built in the 1990s to date.....?

Worth a look...
http://www.yachtsurvey.com/structuralissues.htm