Balsa Cored Hulls

[d design]

Now Im hungry

[Chasing Summer]

This website has really good artiles:
Here's the main site: Boat Hulls - Cores and Structural Issues: Online Articles by David Pascoe, Marine Surveyor
This one is all about the history of GRP and the introduction of the core:
Structural Issues : Core Materials
The articles a rather lengthy, but lots of great information.

I still like my 1969 built solid GRP Spencer hull. I'll never have to worry about soft spots in the hull.

[Dockhead]

Originally Posted by Chasing Summer

This website has really good artiles:
Here's the main site: Boat Hulls - Cores and Structural Issues: Online Articles by David Pascoe, Marine Surveyor
This one is all about the history of GRP and the introduction of the core:
Structural Issues : Core Materials
The articles a rather lengthy, but lots of great information.

I still like my 1969 built solid GRP Spencer hull. I'll never have to worry about soft spots in the hull.

David Pascoe is well known for his strident position against cored hulls. But his experience is with Sea Ray power boats - not an example of good workmanship or proper technique.

He calls hull coring "hamburger helper" - nonsense. As has been mentioned, properly cored hulls are much more expensive than solid ones, plus much stronger, quieter, warmer, etc,

I have never heard of a core problem with a Swan, HR, or other high-end European sailboat.

However, this does not apply to early American cored boats like Irwins, made before proper techniques were invented (using end grain balsa, resin infusion, encapsulated blocks, vacuum bagging, etc.). Here, yes, you have to be careful.

Which is not to say that there is anything wrong with solid fiberglass boats, either -- it's just a question of priorities. Solid hulls are cheaper, simpler, and with nothing (almost) to go wrong -- a KISS approach. The designers of Oyster yachts think this is the way to go, for example. The tradeoff is that solid hulls will be much heavier to get similar strength, or will be less strong and rigid. Oysters, for example, are simply much heavier than similar Contests, Swans, etc., but they are strong. Beneteaus and Bavarias, which likewise use solid fiberglass hulls, take the other approach -- they are less rigid and less strong (while still being rigid and strong enough), than they would be if they had cored hulls.

[rognvald]

From an engineering perspective, cored hulls are stronger, lighter and better insulated than a solid glass hull. I would not used cored vs. solid as a criteria, but rather the reputation and integrity of the builder as others have mentioned. However, a poorly constructed core is prone to failure through delamination either by a poor wet out at layup or ingress of water. These can be major. A solid glass hull can also have issues due to improper saturation of the glass causing localized delamination in the layup and/or blistering. These are major as well. Bottom line: buy from a quality builder-- whether solid or cored. You only get what you pay for. Good luck and good sailing. P.S. I have a cored hull from the waterline to the keel on my Pearson 34-2 with no delamination or water after 24 years.

[Cheechako]

Originally Posted by d design

You want a end grain balsa core so theres no long grains to suck along water like a sponge.

I've never heard of any thing but endgrain being used. Does anyone even make coring that's not? At any rate, water moves great thru end grain coring... migrated about 12 ft from one fitting on a boat I had.
In a perfect world coring is great, unfortunately it's not a perfect world. all one has to do is read the many posts about deck coring problems and hull coring problems and realize that if you can avoid it, it's one more thing to go wrong... and catastrophically!
Would I buy a boat with coring?... if it ends well above the waterline yes if there are zero penetrations thru it, if below.. maybe if built by TPI.

[robert sailor]

Actually unless things have changed in the last number of years Swans were solid laminate but Baltics were cored hulls. Cored hulls by good builders have had excellent track records. If I was going to run into anything solid at sea a cored hull is much stronger.

[azsailor12]

Thanks for all the feedback!

[clockwork orange]

Guys, please stop this TPI love fest, I have recored more boats built by them than any other builder which is why I tried to stress in my last post that the only thing that matters is the actual individual boat in question, not the brand or the builder, if it surveys well and you trust the surveyor and you have been very clear to him that this is the most important part of the survey. TPI builds or has built boats for a lot of different brands and I have no doubt that they know how to build a cored boat correctly but they are apparently quite willing to do almost everything incorrectlt to please their client. They have built boats under the Pearson name, J boats, Freedom, Lagoon, Deerfoot and many others. I am fairly certain that there is a huge difference in the build quality of a Deerfoot and a J Boat. Believe it or not all the procedures for building a quality cored structure were well defined in the 1960s and are as valid today, TPI ignored almost all the correct procedures when building J Boats in the 80s. Several respondents to this thread have indicated they have dry hulls on Pearson and Freedom boats and yet ive seen horribly wet boats from both, so, its all about the individual boat.

Steve.

[clockwork orange]

Originally Posted by d design

You want a end grain balsa core so theres no long grains to suck along water like a sponge.

All balsa core is end grain, there is no such thing as face grain core and I don't believe there ever has been so this is not a problem but contrary to the belief of some, when water finds its way in it will travel sideways just fine in end grain cores and rot out large areas, I have often seen people claim that it does not but this is untrue. When you build with balsa in a female mold you use what is called contour core which is a lot of blocks bonded to a light fiberglass scrim on one side only which allows it to conform to the compound curvature, the gaps between those blocks open up leaving gaps, all those walls are supposed to be coated with resin when you are hot coating the core but in order to do that it requires draping the 2ft x4ft sheets of core over a curved table to open up the gaps and then rotate it 90 degrees and do it again before bedding it into either a wet layer of csm or a bonding putty, neither of which will migrate the thickness of a 3/4" core, even if vacuum bagged. Very few production builders take the time for this step so no surprise that cores rot out if water and oxygen get at it. Nowdays however with the advent of resin infusion its a whole different scenario, all those gaps get fully filled with resin and balsa becomes an excellent core. Those gaps consume a lot of resin but it is the same thing with foam or honeycomb when used in female tooling.

Steve.

[Island_Moose]

My Nonsuch 30U is balsa-cored to the keel, with solid glass at the through-hulls. George Hinterhoeller was one of the better builders, and a master at cored hulls/deck. It entirely comes down to the quality of the builder, but then so does everything. I've seen rudders with dry cloth just under the gelcoat!
A solid fibreglass hull would be idiot-proof, but it would have been fanastically-heavy to acheive the same rigidity.

[cal40john]

Well the Lightning I repaired decades ago didn't have endgrain balsa in it. I was going to fix one little poke that looked like the end of a spin pole had hit it on the inside (on the side, not the bottom) which resulted in me tearing out about a 6'x2' area of mush. Long time ago but IIRC the planks were about 3"x5". I believe it was a Clark boat. Of course it being a club boat I had no idea how long it had been there soaking up water before I noticed it and decided to do something about it.

Originally Posted by Cheechako

I've never heard of any thing but endgrain being used. Does anyone even make coring that's not? At any rate, water moves great thru end grain coring... migrated about 12 ft from one fitting on a boat I had.
In a perfect world coring is great, unfortunately it's not a perfect world. all one has to do is read the many posts about deck coring problems and hull coring problems and realize that if you can avoid it, it's one more thing to go wrong... and catastrophically!
Would I buy a boat with coring?... if it ends well above the waterline yes if there are zero penetrations thru it, if below.. maybe if built by TPI.

Originally Posted by clockwork orange

All balsa core is end grain, there is no such thing as face grain core and I don't believe there ever has been so this is not a problem but contrary to the belief of some, when water finds its way in it will travel sideways just fine in end grain cores and rot out large areas, I have often seen people claim that it does not but this is untrue. When you build with balsa in a female mold you use what is called contour core which is a lot of blocks bonded to a light fiberglass scrim on one side only which allows it to conform to the compound curvature, the gaps between those blocks open up leaving gaps, all those walls are supposed to be coated with resin when you are hot coating the core but in order to do that it requires draping the 2ft x4ft sheets of core over a curved table to open up the gaps and then rotate it 90 degrees and do it again before bedding it into either a wet layer of csm or a bonding putty, neither of which will migrate the thickness of a 3/4" core, even if vacuum bagged. Very few production builders take the time for this step so no surprise that cores rot out if water and oxygen get at it. Nowdays however with the advent of resin infusion its a whole different scenario, all those gaps get fully filled with resin and balsa becomes an excellent core. Those gaps consume a lot of resin but it is the same thing with foam or honeycomb when used in female tooling.

Steve.

[d design]

Ha! your wrong not all balsa core boats use end grain. Only most boats do. Ha!

[Andrew Troup]

Thanks Clockwork Orange for giving us the benefit of a thorough and (I think) authoritative PoV.

I personally didn't take his statement about endgrain as being likely to apply to EVERY boat ever built, but I would expect reputable builders to comply, along with most disreputable ones. Frankly one's topmast halyards would have to be a few feet short of a full hoist to contemplate using balsa sideways for anything bigger than a scale model, (or a Kon Tiki raft, perhaps, but for a short trip only).

My first contact with balsa was a sickening discovery, when we took apart our first boat in order to put it back together properly*

To our horror, the balsa-cored deck continued under the base of the deck-stepped mast. It had not been progressively substituted (ie by angled scarfs) with marine ply, as the design specified. The reason we were suspicious enough to drill a core inspection hole was that there was fine stress-cracking evident (although not worryingly so, under other circumstances) around the corners of the mast base.

This was a small boat, but with a tall masthead rig, intended for offshore work.

Furthermore we'd sailed extensively in very testing conditions, for ten wonderful years -- including one one occasion seas such as I have never encountered since, and never wish to.

When we cut out the offending outer skin, the balsa was still in textbook condition, and had not compressed relative to the unloaded core around it. (There was a mast support pillar, bearing against the underside, only about 2 1/2" square - worse than the mast base in terms of concentrated load - and that had only squashed the core less than 1/16".

We were mightily impressed. I'm not aware of any foam which would have lasted a year in this scenario (of course, we re-engineered the whole setup, nonetheless: It was a crazy omission)

* (lots of work, but well worth doing, because the hull and deck were very well designed and generally well moulded; it had then been put together in another yard, evidently by idiots)


I've since had the pleasure of doing several interesting trips on a racing maxi which was accidentally overstrong, having been launched about seven tons heavier than the designer intended (not the builder's fault: it was built exactly to drawings)

The basic structure was a 'boat within a boat'; the interior was notable for longitudinal walls, effectively girders spanning all the way from turn of the bilge to deckhead, of 2" thick kevlar over balsa (broken by regular round cornered rectangular openings for pipecots, stacked three high). These were curved only in one direction, and ran from the transom to the bow.

The hull proper was the same materials and scantlings. At the gunwhale, (and, as more usual, around the keel) the balsa feathered away to nothing, so for a considerable distance across the deck and down the topsides, the gunwhales were 2" solid kevlar.

This boat could fall off waves the size of small buildings all day every day, if required. It's the only boat I know which has no detectable reverberation from the hull under such conditions, but you couldn't help feeling a bit sorry for the ocean; at one point the crew nicknamed it the "Urban Wave Destroyer".

For comparison, the boat which beat it, in the round-the-world race it was built for, wasn't suitable for further duties after the end of the race, having developed a number of soft spots.

Under subsequent ownership it was used as a delivery vessel under charter to the NZ government to get scientists and significant amounts of deck cargo (eg building materials) to subantarctic islands, with the blessing of the insurance companies of both the government and the owner.

And thirty years on, the hull is still original, and as new.

So I fully concur with Clockwork Orange. It's not all about inherently good or bad materials, it's about inherently good or bad boats.

I would almost extend his claim and say that you could, given sufficient skill, ingenuity and time, build a strong durable boat out of almost any of the recognised boat materials

... and, (at the risk of losing anyone who has read this far without leaving the room) arguably not a few others (eg: papier mache would probably be feasible, given the right resins and some judicious fibre reinforcement. I seem to remember Bernard Moitessier making this claim, come to think of it - and he was a fan of steel)

[belizesailor]

Just cut a few nice big holes in my 14 year old end grain balsa cored decks to install a new windlass (and cut back core and filled of course). Core was in great shape.

However, I did find one spot where I had removed some thru bolted deck hardware a number of years ago and overlooked filling a hole in the inner skin (inside a locker and facing down). Even though this hole did not have any direct exposure to water, the moisture absorbed over the years was enough to damage a small area of core. Lesson learned...seal any holes (old or new) very well.