Core that everyone thinks is solid

[marshmat]

We're in the process of buying a 1974 C&C 35 Mk.II. And an interesting question has come up.


The broker, various posters on owners' groups, Wikipedia, and the old reviews we've been able to dig up all claim that the C&C 35 Mk.I and Mk.II are solid fibreglass all through the hull, with balsa core in the deck.


But.... thanks to the Marine Museum of the Great Lakes, I was able to find copies of the original 1973 construction plans. And both the laminate schedule and the cross-section clearly show a 1/4" balsa core from centreline to waterline, plus a balsa-cored stringer halfway between the waterline and the sheer.


My first interpretation of the discrepancy is that the boat is indeed balsa-cored, but that C&C's layup and quality control were good enough that core flaws are basically unheard of over 300+ boats and 40+ years - so everyone now thinks it's solid.


An alternate interpretation is that the laminate schedule was changed between the drawing board and the shop floor, and the hull is in fact solid, but these plans don't reflect the change. There are other discrepancies between the plans and the actual boat, eg. the placement and shape of the "Liasil" floor laminates relative to the keel bolts, which suggest that at least a few things got changed between the drafting desk and the laminate shop.


We'll see what the surveyor finds, but I'm curious how the rest of you would interpret this?

[Breaking Waves]

You could almost certainly NDT determine this using one of the small ultrasound devices which surveyors use to check metal boat plating thickness.

1/4" would be an odd core thickness - enough to cause potential problems, but not much to really increase stiffness.

[tkeithlu]

The usual finding is that the builder cheated on the drawings to do something easier. Rather than thread very long rods, carrying the load through all layers, the builder of the Hyatt-Regency Hotel in Kansas City conveniently joined short rods to beams that supported the sky walks in the lobby. Killed 114 people. That's why you have "As Built" drawings.

[contrail]

A common opinion in the 1980's was that all C&C's were balsa cored boats, but beyond that, I don't know.

[GordMay]

Quoting from one ‘authority’ on C&C Yachts: “The construction of the two models is virtually identical; “solid fibreglass handlaid up hull alternating mat and roving, balsa core in decks only.”
➥ C&C 35 MKI
➥ http://www.cncphotoalbum.com/brochur...ot/35b2p04.jpg

[Sailor 99]

Originally Posted by marshmat

We're in the process of buying a 1974 C&C 35 Mk.II. And an interesting question has come up.


But.... thanks to the Marine Museum of the Great Lakes, I was able to find copies of the original 1973 construction plans. And both the laminate schedule and the cross-section clearly show a 1/4" balsa core from centreline to waterline, plus a balsa-cored stringer halfway between the waterline and the sheer.


We'll see what the surveyor finds, but I'm curious how the rest of you would interpret this?

Look yourself . There will be a clear change thickness in the inside of the hull where the core starts and stops.

[motion30]

Exactly what I was thinking

[marshmat]

Originally Posted by contrail

A common opinion in the 1980's was that all C&C's were balsa cored boats, but beyond that, I don't know.

That was definitely the case in the '80s, but likely not in the early to mid '70s, as far as I can tell.

Originally Posted by GordMay

Quoting from one ‘authority’ on C&C Yachts: “The construction of the two models is virtually identical; “solid fibreglass handlaid up hull alternating mat and roving, balsa core in decks only.”
➥ C&C 35 MKI
➥ http://www.cncphotoalbum.com/brochur...ot/35b2p04.jpg

Yes, that would agree with most of the "original" marketing materials and reviews that I've come across. I've never heard of a cored hull being mistaken for solid over so many boats and so many years before, which does point to the design being changed to all-solid somewhere between the drawing and the shop floor.

Originally Posted by Dark Horse

Look yourself . There will be a clear change thickness in the inside of the hull where the core starts and stops.

I'll do that the next time I'm able to go look at the boat (she's about a 5-hour drive away). The plans do show quite clearly where the core edge step would be, if it exists.

Originally Posted by Breaking Waves

1/4" would be an odd core thickness - enough to cause potential problems, but not much to really increase stiffness.

Yes.... I've only ever used quarter-inch core once, for the wheel fairing aeroshells of a solar car. It's a size that definitely doesn't make any sense in a boat hull, which makes me suspect that the plans I'm looking at were an early paper pass at the design - followed by a shop foreman saying "guys, what's with this, let's just make that solid instead, ok?"

[Michael Pope]

C&C yachts were quite a unique Company where the 5 major shareholders of the company were drawn together as one corporation and continued function and build the company. I was privileged to be invited to a trip back east to Niagara on the lake once a year to feed back reaction and problems from the west coast. George Hinterhoeler was the undisputed Head Boatbuilder he organized the production in the various plants and after polite consultation decided what materials and processes would be used. The design group would draw the boat. They would include layup schedules but they were youngsters
George would explain why the boat would not be built that way.
I would suspect that the 1/4 inch bottom balsa core was tried for a couple of yachts and then THE MAN would explain that it was cheaper and stronger to put a couple of more layers of 24 oz roving in that area and have no void and lump problems that required rework ---- Balsa Core is a pain in the but at all times ( George and I agreed on that completely) but particularly when laying it in loose and thin as the material is very fragile , The scrim breaks and drops out lumps of core. Cuthbertson the originator of the idea of Merging the Canadian Boatbuilders had made a considerable part of his fame with a Balsa cored yacht that won SORC . There was a lot of pressure from all sides to keep Balsa in the line. The more experienced boat builders resisted BUT.
Hinterhoeler is to be praised for succeeding in using it for deck core for so long with so few failures. To do so he learned many tricks but still found it necessary to use a resin rich layup schedule. No other core was available in quantity at near that price. J boats are still using it and it in my opinion Balsa should not be used in Yachts! period. Rob Mazza The curator of that Museum was a young junior and talented architect at C&C when I first met him .
Those were good years for Canada. Mike Pope

[Tillerjockey]

The earlier C+C's were solid fiberglass hulls, later they changed over to balsa core down to the waterline and solid glass below.

[Michael Pope]

Sorry I should have prefaced my remarks with "dealing only with the Canadian built Yachts. " Once the company went public The principals cashed out and lost control of the now American Company. I know nothing of what they did.! I think all the principals are now deceased! Michael Pope

[j.g.evans]

I have a 1981 C&C 36 that is balsa cored above the water line. You can see that the hull is thicker above the water line. I have had a few problems with water in the deck and have rebedded all fittings.