Balsa Core on Cat Hulls

[clockwork orange]

Nick, TPI did not invent the Scrimp process as you claim in post#12. Scrimp is the name given to Seaman Composites version of a process that has been around for years. Scrimp stands for something like Seaman Composites resin infusion manufacturing process, it is licenced to TPI and others. TPI was probably the first large builder to use it though.
Steve,

[minaret]

Originally Posted by jmarples

As well as a multihull designer for the last 30 years, I am also a degreed Mechanical Engineer and a NAMS certified Marine Surveyor. Much has been written about balsa cores by marine surveyors, in the last two decades and I merely paraphased the results. Like I said, SeaRay powerboats had significant problems with cored bottoms, resulting in law suits in the US. Many of the plastic production cats have avoided cored bottoms. The risk of deformation damage during haulouts is too great. Depending on the weight of the boat, insufficient blocking will probably cause some minor unseen damage. The boat bottom has greater contour shape to help support the skin than the flattish topsides, so coring is not needed as much. The risk of bumping a rock is little, but the consequences for damage in a cored bottom are great. It is not a good choice. The result is the same with all cores, unless they are very hard (and heavy). The high modulus skins, glued to low modulus cores will always shear (debond) at or near the glue line where the stress (due to deformation) is the greatest. That is an engineering principle in beam theory. The only way to avoid the problem is not to use cores. I knew this subject would be like kicking a hornets nest, but I think I am giving the subject fair assessment. Sorry if you are building with a cored bottom. The best I can suggest, is additional exterior skin thickness on the bottomsides to help prevent the problem.

Exactly why I spent four years shopping for a boat with no core in it anywhere...

[daddle]

Balsa core all the way down. 30 years old. No problem yet. Very fast. Silly argument.

[minaret]

Originally Posted by daddle

Balsa core all the way down. 30 years old. No problem yet. Very fast. Silly argument.

You've never had a problem, so therefore nobody can? Where's the logic...

[daddle]

Originally Posted by minaret

You've never had a problem, so therefore nobody can? Where's the logic...

Just saying that balsa cored boats, well built and maintained ones, have a long successful history. The performance advantages, for those who get a thrill out of sailing performance, are significant. Well worth the relative fragility.

I compare it to going barefoot. When one walks around barefoot one is much more careful where one steps. I am very often, almost always, barefoot. So far the advantages have far outweighed the burden of wearing shoes. I'm very careful about where I sail. And I'm very careful about hull maintenance.

Balsa core can make a great boat. No need to dismiss it out-of-hand.

[cat man do]

Originally Posted by jmarples

As well as a multihull designer for the last 30 years, I am also a degreed Mechanical Engineer and a NAMS certified Marine Surveyor.

Plenty of equally well qualified designers out there specifying cored hulls under the waterline

Much has been written about balsa cores by marine surveyors, in the last two decades and I merely paraphased the results. Like I said, SeaRay powerboats had significant problems with cored bottoms, resulting in law suits in the US.

Law suits, but not law changes, which is what I asked for, evidence.

The fact remains that SeaRays build large heavy planing hulls, that have large flat surfaces susceptible to repeated pounding

Multihulls and sailing yachts are a different beast altogether and the loads are no where near as high

Many of the plastic production cats have avoided cored bottoms.

and from that comment, I take it that many still do.

The risk of deformation damage during haulouts is too great. Depending on the weight of the boat, insufficient blocking will probably cause some minor unseen damage.

So if the owner, or the slipway staff lift in the wrong spot, damage can be done.

Simple solution is to lift in the right spot

The boat bottom has greater contour shape to help support the skin than the flattish topsides, so coring is not needed as much. The risk of bumping a rock is little, but the consequences for damage in a cored bottom are great.

Doesnt matter if a hull is solid glass or cored, if I hit the bricks at speed, I am coming out for repairs.

Best not to hit the bricks

It is not a good choice.

It is if you are interested in performance and weight saving.
Seems pretty stupid to save weight on 100 different components, using exotic materials only to give it all away making the bottom solid glass

The result is the same with all cores, unless they are very hard (and heavy). The high modulus skins, glued to low modulus cores will always shear (debond) at or near the glue line where the stress (due to deformation) is the greatest. That is an engineering principle in beam theory. The only way to avoid the problem is not to use cores.

And yet there are so many vessels getting around with cored hulls under the waterline with no issues whatsoever.
How is that?

I knew this subject would be like kicking a hornets nest, but I think I am giving the subject fair assessment.

Well, you are certainly showing one side, not sure if it is fair as there are so many different vessels out there used in so many different ways.
Solid hulls on many would be detrimental to their performance
Solid hulls on many could make them downright dangerous.

Sorry if you are building with a cored bottom.

Why are you sorry?
I'm not, though I would be sorry if I had solid glass, my cat would be a dog.

The best I can suggest, is additional exterior skin thickness on the bottomsides

Goes without saying and would, or at least should, be standard practice for any reputable builder

to help prevent the problem.

Its only a problem in your world, not mine.

[s/v Jedi]

I'm also pro-balsa core under the water line. So Sea-Ray frakked it up; I don't care, others have done equally stupid things at other areas like keels falling off or rudders etc.

I believe that 99% of the balsa core problems is caused by either manufacturing error or by after market upgrades gone wrong. Both easily avoidable.

I had expected much more talk about honeycomb cores here (like nida-core)...

ciao!
Nick.

[Factor]

I find myself generally in agreement with Dave - Cat Man do. Having said that, I cannot but carefully reflect on anything John Marples has to say. A living treasure of the multi hull world, not infallible, but absolutely worth listening to.

Thanks for dropping in John.

[clockwork orange]

Everything is a compromise, i love balsa, i make a lot of my income replacing it, that said it is never the fault of the balsa, as Nick pointed out it is mostly poor manufacturing or stupid owners who cause the problems, unfortunatly balsa is much less tolerant of this than foam, almost all problems with foam can be traced to the builder selecting too low a density for the job. I have had foam cored boats in the shop but have never had to recore one. I currently have 2 foam cored boats, one designed and built by me in 1987,zero issues ever,one a Lindenberg 26 built in 1977,decks and the topsides above the waterline from the bow back to about the middle of the salon,solid glass where there is compound curvature,again zero core issues except in the cabin sole in the head and the cockpit sole where there were lots of screw penetrations from teak slats.When i cut out the head sole to instal a liftout to instal transducers the jigsaw was throwing water in my face.It dried out and there is no evidence it was ever wet, balsa would have been actual compost. I should mention i bought this boat on e bay and it had been sitting with water over the sole. Sooo, my opinion for what its worth is that i personally have no problem with the use of balsa core anywhere in a boat as long it is built by boatbuilders, not just laminators, theres a big difference. Unfortunatly production boats are built by laminators with a shop manager who chances are has never built a boat but has an engineering degree.
Steve.

[minaret]

Originally Posted by clockwork orange

Everything is a compromise, i love balsa, i make a lot of my income replacing it, that said it is never the fault of the balsa, as Nick pointed out it is mostly poor manufacturing or stupid owners who cause the problems, unfortunatly balsa is much less tolerant of this than foam, almost all problems with foam can be traced to the builder selecting too low a density for the job. I have had foam cored boats in the shop but have never had to recore one. I currently have 2 foam cored boats, one designed and built by me in 1987,zero issues ever,one a Lindenberg 26 built in 1977,decks and the topsides above the waterline from the bow back to about the middle of the salon,solid glass where there is compound curvature,again zero core issues except in the cabin sole in the head and the cockpit sole where there were lots of screw penetrations from teak slats.When i cut out the head sole to instal a liftout to instal transducers the jigsaw was throwing water in my face.It dried out and there is no evidence it was ever wet, balsa would have been actual compost. I should mention i bought this boat on e bay and it had been sitting with water over the sole. Sooo, my opinion for what its worth is that i personally have no problem with the use of balsa core anywhere in a boat as long it is built by boatbuilders, not just laminators, theres a big difference. Unfortunatly production boats are built by laminators with a shop manager who chances are has never built a boat but has an engineering degree.
Steve.

Funny how those of us who actually make a living repairing boats all have the same opinion. I too love balsa because it gives me and my compatriots lots of work to do. Entertains me that people who own a boat with balsa core and don't have a problem (yet), refuse to believe proffesionals who have seen a vast quantity of rotten balsa core out there. They dont have a problem, therefore it's impossible that anyone else could have. I've worked in production boatbuilding, and I can tell you that most laminators could not care less about the finish quality of their product or boats in general. They are just there to get a paycheck. I am a laminator from way back myself, I know what goes on. I was in the molds laying up, not a project manager or any kind of non hands on guy. They use processes like SCRIMP and vacuum bagging now because it's the only way you can gaurantee your employees are doing a good job coring. Unfortunately the majority of boats out there were built before these processes were invented. I have re-cored some foam here and there, but nothing like the volume of balsa I've done. Given almost any balsa cored boat and a moisture meter, I can find some rot somewhere almost every time. Using the thermal imaging camera I can definitely find some every time. The pro's are not making it up. And I have done a lot of warranty work with Sea Ray as well as other major manufacterers. Sea Ray sent a crew from NZ to our yard to watch us re-core some rotten balsa, and then do the rest of the job themselves. I was shocked at how clueless they were, didn't even use decent safety gear. Those of you who think major manufacterers of production sailboats somehow care more than the powerboat companies are wrong. All they care about is selling you a boat that won't rot until after the warranty is over. And thats usually only a few years. Beneteau is the worst offender I've seen in this category. Of course newer boats that were built with modern processes are much less likely to have a problem, but most boats out there don't fall into that category.

[s/v Jedi]

Originally Posted by minaret

I've worked in production boatbuilding, and I can tell you that most laminators could not care less about the finish quality of their product or boats in general. They are just there to get a paycheck.
[...]
They use processes like SCRIMP and vacuum bagging now because it's the only way you can gaurantee your employees are doing a good job coring. Unfortunately the majority of boats out there were built before these processes were invented.

And this shows a new reason for not buying these boats: the frakkers who build and sell them don't deserve to survive.

Jedi was built with SCRIMP by TPI in '93 which is 18 years ago. I think most yards didn't start using the right techniques because they didn't care less, like you explained.

ciao!
Nick.

[minaret]

Originally Posted by s/v Jedi

And this shows a new reason for not buying these boats: the frakkers who build and sell them don't deserve to survive.

Jedi was built with SCRIMP by TPI in '93 which is 18 years ago. I think most yards didn't start using the right techniques because they didn't care less, like you explained.

ciao!
Nick.

Agreement at last! In my experience cats in general have a higher quality of coring work, I'm sure because they realize how important it is to the build. And any competent owner can buy a moisture meter and stay ahead of any problems. Good ones only cost a few hundred, much cheaper than any core repair. And before I get flamed by a bunch of beneteau owners, I am refering to the older ones when I mention seeing some core problems. The new boats I have ground into seem to be of a much better quality.
As far as thinking some of these companies don't deserve to survive, I can't tell you how many times I've been fully suited and grinding in a bilge or hot bubble tent and had the same thought....

[Cheechako]

The honeycomb core in many aerospace panels is nothing more than foil, you could easily mash it with your foot. However, once bonded with two thin outer skins...it is immensely strong and rigid.
The main problem I have seen with cored decks and hulls, whether foam, balsa or small plywood squares (passport used this method supposedly separated by resin), is implementation and engineers that dont understand that theory isnt reality. The voids between the squares are not filled with resin (takes too much resin!) and /or the fiberglass pulls away from the core when /or after curing. My 44 footer was cored above the waterline, when I cored a hole in the hull I discovered the outer hull was up to .1" away from the balsa core! The outer hull was nearly 3/8 thick up near the top of the hull, so it was quite strong anyway (thanks to the chinese over doing the glass work!) I guess it was more of an "insulated" hull! in theory I guess vacuum bagging should help this problem.... but does the layup pull away as it continues to cure over time....?

[clockwork orange]

Nick, TPI have produced as bad a boats as anybody, obviously they know how to build good boats so i put it down to the constraints they are put under by the company they are building for. Im guessing that each company has a project manager on site who drives the quality of their brand which would explain the wildly varying quality of product from the same builder. Interestingly, i have been told that TPI switched to scrimp part way through the production run of the J105, the scrimped ones turned out to be several hundred pounds heavier due to the resin finding its way into every nook and cranny in the core, obviously a better hull but making the earlier models that are not waterlogged very sought after.
Steve.

[clockwork orange]

Minaret,there are thousands of balsa cored boats sailing around where the owners have no idea they have rotten core, ignorance is bliss i guess. This is one area where i would have a lot more confidence in one of the kit catamarans built by the owner down under like the Orams, Shionnings etc than most production boats. The owner builders are much more fastidious in taking care of the details than the guys working for a paycheck at a production builder.
Cheechako, we did the entire interior fittout of an 81ft raceboat back in 1980/81 with F board (aluminum honeycomb) panels,it made sense for some parts,not so much for others, it was not as light as it would seem as all the edges had to be routed out and backfilled with epoxy filler to ensure that saltwater never got at it.
Steve.