Soundness of balsa cored hulls

[clockwork orange]

[QUOTE=smj;3897481]

Originally Posted by clockwork orange

Interesting in that there are different types of grid scoring, one being the type I’m talking about with the scrim supporting separate blocks.
https://fiberglasssupply.com/content/gsguidelines.pdf

Actually i missed one type which is "perforated" which refers to a plain, unscored sheet which is drilled full of small holes on even spacings. This is used in infusion primarily where you only need it to bend in one direction. There is a lot of confusion with the terminology of products in the composites world, not just cores and much of it is caused by different distributors calling products by different names. A prime example is the glass fabric stitched together at +/- 45 degrees. My earliest use of this was in 1980 or so when i believe it was introduced by a company named Knytex and their name for it was Double Bias so that is what i have always referred to it as. Somewhere along the line people started referring to the same product as Biaxial which would be fine except i have also seen 0/90 fabric referred to as Biaxial by distributors.

[clockwork orange]

Originally Posted by Dockhead

Correct. The blocks are entirely separate to eliminate any chance of water migrating from one to the other. In theory, vacuum bag infusion would be enough, so the separate blocks are a bit belt and suspenders, and there are structural drawbacks to blocks vs. planks, but this is considered the right way to do it and the way all quality builders do it.

The only reason for the completely separate blocks on a scrim, which we have always referred to as either Scrimmed or Contour balsa, is so it will lay into a female mold with compound curvature, with no tendency to spring back as long as it is laid in the right way around. This stuff has been used since the beginning of cored composites and because the vast majority of boats were NOT vacuum bagged they did NOT have full resin penetration between the blocks to stop water from migration from block to block. That only happens with infusion.

[GordMay]

A couple of interesting articles:

“Storm Damaged Boats Reveal the Limits of Fiberglass Hull Construction” ~ by Ralph Naranjo, Published in ‘Practical Sailor’: July 18, 2007, Updated: August 2, 2022
➥ https://www.practical-sailor.com/sails-rigging-deckgear/storm-damaged-boats-reveal-the-limits-of-fiberglass-hull-construction

“Storm-damaged boats offer a road map to key areas of concern in fiberglass boats. From downed rigging to sheared-off keels to gaping holes in the hull, these victims prove that fiberglass is far from indestructible and that no fiberglass boat is designed to withstand the point loading resulting from collisions, groundings, and poundings.

An inspection of storm-damaged boats reminded Practical Sailor editors that sailboats are designed, engineered, and built to handle sailing loads, and the point-loading that occurs during collisions or fetching up make all promises of ruggedness and survivability a tenuous crapshoot at best. Fiberglass hulls have many redeeming qualities, and it’s these positive traits as well as their limitations that boat owners need to understand. One thing’s for sure: It’s easier to become a proficient navigator, install a secure storm mooring, or transit to safe shelter before a storm, than it is to build or buy a catastrophe-proof vessel ...

... its impossible to state that any of the boats pictured here are “defective,” because the treatment each received was catastrophic, highly variable, and well outside the conditions they were designed for.

Nonetheless, understanding failure points can give us a better picture of common vulnerabilities and how builders might address them ...”

"Removing barriers to lightweighting ships with composites" ~ by Ginger Gardiner, Published in ‘CompositesWorld’, 12/3/2019
➥ https://www.compositesworld.com/arti...ith-composites

“Glass and carbon fiber-reinforced polymer (GFRP and CFRP) composites have been used to build marine vessels for decades, including numerous 40- to 60-meter minehunters and even larger vessels, including the Swedish Navy’s 72-meter-long Visby Class Corvette and the 75-meter-long sailing yacht Mirabella V. The 141-meter motor yacht Swift 141 (renamed Yas) is a Dutch steel frigate rebuilt using GFRP/CFRP in below-deck soles and three-deck superstructure (see “From frigate to luxury gigayacht”), including a composite-to-steel deck joint.

And yet, composites are rarely used in shipbuilding...”

[Dockhead]

Originally Posted by clockwork orange

The only reason for the completely separate blocks on a scrim, which we have always referred to as either Scrimmed or Contour balsa, is so it will lay into a female mold with compound curvature, with no tendency to spring back as long as it is laid in the right way around. This stuff has been used since the beginning of cored composites and because the vast majority of boats were NOT vacuum bagged they did NOT have full resin penetration between the blocks to stop water from migration from block to block. That only happens with infusion.

Not the only reason -- completely separate blocks also prevents water migration through the core. Provide, yes, that fully effective infusion takes place -- vacuum bagging.


All better quality boats, at least in Europe, have been done with vacuum bagging since the late 90's.

[smj]

Originally Posted by Dockhead

Not the only reason -- completely separate blocks also prevents water migration through the core. Provide, yes, that fully effective infusion takes place -- vacuum bagging.


All better quality boats, at least in Europe, have been done with vacuum bagging since the late 90's.

With infusion the gaps in the grid scored core acts as channels for the resin to travel and fully wet out the piece being infused. This I believe is the best method to try to fill all the gaps in the core, but can lead to a heavier structure.
With straight vacuum bagging, the gaps in the grid scored core I believe have to be manually filled with a resin based putty, maybe not as fool proof as infusing?
This is my understanding.

[clockwork orange]

Originally Posted by Dockhead

Not the only reason -- completely separate blocks also prevents water migration through the core. Provide, yes, that fully effective infusion takes place -- vacuum bagging.


All better quality boats, at least in Europe, have been done with vacuum bagging since the late 90's.

What you say is true but it is a by product,( a good one), not a reason for the way scrimmed/contour core is the way it is. and only happens with infusion, not with vacuum bagging and certainly not with hand layup. Contour core has been used for half a century or more and while vacuum bagging has been around since WW2 very, very few production builders used the technique. Vacuum bagging and infusion are very different processes. Infusion has been common only over the last couple of decades, largely to keep VOCs at bay.

[GordMay]

Here's a couple of good explanations of Vacuum Infusion & Vacuum Bagging:

“Vacuum Infusion Complete Guide”
➥ https://www.fibreglast.com/product/v...infusion-Guide

”The Vacuum Infusion Process (VIP) is a technique that uses vacuum pressure to drive resin into a laminate. Dry materials are laid into the mold and the vacuum is applied before resin is introduced. Once a complete vacuum is achieved, resin is literally sucked into the laminate via carefully placed tubing.

In a typical hand lay-up, reinforcements are laid into a mold and manually wet out using brushes, rollers, or through other means. An improvement on that method is to use a vacuum bag to suck excess resin out of the laminate. Vacuum bagging greatly improves the fiber-to-resin ratio, and results in a stronger and lighter product. If you are unfamiliar with vacuum bagging, we recommend reading our guide, “Vacuum Bagging Equipment and Techniques for Room-Temperature Applications”*, as VIP requires experience in this area and uses many of the same principles. Vacuum infusion builds upon these principles, providing further improvements to the lamination process ...” more

* “Vacuum Bagging Equipment and Techniques for Room-Temp Applications”
➥ https://www.fibreglast.com/product/v...p-applications

“Vacuum bagging is a technique employed to create mechanical pressure on a laminate during its cure cycle. Pressurizing a composite lamination serves several functions. First, it removes trapped air between layers. Second, it compacts the fiber layers for efficient force transmission among fiber bundles and prevents shifting of fiber orientation during cure. Third, it reduces humidity. Finally, and most important, the vacuum bagging technique optimizes the fiber-to-resin ratio in the composite part. These advantages have for years enabled aerospace and racing industries to maximize the physical properties of advanced composite materials such as carbon, aramid, and epoxy.

The reason that composites are used increasingly is the strength-to-weight advantages that they offer. The key to obtaining these advantages is maximizing the fiber-to-resin ratio...” more

[s/v Jedi]

Read up on SCRIMP: https://ocw.mit.edu/courses/2-996-sa...p_overview.pdf

Note that TPI is the builder in Newport RI that built the Sundeers, J’s and Lagoons (and undoubtedly others) using SCRIMP.

[clockwork orange]

Originally Posted by smj

With infusion the gaps in the grid scored core acts as channels for the resin to travel and fully wet out the piece being infused. This I believe is the best method to try to fill all the gaps in the core, but can lead to a heavier structure.
With straight vacuum bagging, the gaps in the grid scored core I believe have to be manually filled with a resin based putty, maybe not as fool proof as infusing?
This is my understanding.

This is all correct, with infusion you can control the fiber fraction well and it is pretty easy to achieve 70/30 fiber to resin ratio but you need to choose your core type carefully if you want to keep the weight down. Surrounding thousands of little blocks with resin will yield a heavier product. Some higher quality builders thermoform perforated sheet of foam to fit the mold without spring back so the only resin retained is in the perforations.
As far as vacuum bagging goes, you are still doing a hand layup but setting up a bag to compact it and hopefully pull some excess resin out into the bleeder, you are notsurrounding the blocks with resin.

[s/v Jedi]

Originally Posted by clockwork orange

This is all correct, with infusion you can control the fiber fraction well and it is pretty easy to achieve 70/30 fiber to resin ratio but you need to choose your core type carefully if you want to keep the weight down. Surrounding thousands of little blocks with resin will yield a heavier product. Some higher quality builders thermoform perforated sheet of foam to fit the mold without spring back so the only resin retained is in the perforations.
As far as vacuum bagging goes, you are still doing a hand layup but setting up a bag to compact it and hopefully pull some excess resin out into the bleeder, you are notsurrounding the blocks with resin.

I believe the balsa core, even with resin encapsulated blocks, is still the outperforming core material. It leads to lighter, stiffer hulls unless you go to space age aluminum or carbon honeycomb cores which is cool for racers.

[smj]

Originally Posted by s/v Jedi

I believe the balsa core, even with resin encapsulated blocks, is still the outperforming core material. It leads to lighter, stiffer hulls unless you go to space age aluminum or carbon honeycomb cores which is cool for racers.

From what I’ve read, Gunboats purposely left the resin out between the blocks as it is a huge weight savings. One guy said he would rather have some water running around in the core than the extra weight of the resin. Wouldn’t work on a balsa core but apparently ok with foam.

[clockwork orange]

Originally Posted by s/v Jedi

I believe the balsa core, even with resin encapsulated blocks, is still the outperforming core material. It leads to lighter, stiffer hulls unless you go to space age aluminum or carbon honeycomb cores which is cool for racers.

Balsa is a great core as long as it is not allowed to get wet, so,(only) with infusion that is reality. Its biggest advantage over foam is its compressive strength but it is also about twice the weight of the typical H80 PVC or SAN foams used in boat construction so is a lot heavier. The higher compressive strength is really not much of an advantage though as H80 is plenty adequate for most locations and if not you can substitute H100 or higher in localized areas. It should be mentioned though that there is a lighter version of balsa that is closer to H80 in weight. There are also balsa cores with factory coated faces to reduce resin uptake to reduce weight. Bottom line is that you can always build a lighter structure with foam with good engineering. Balsa is generally cheaper though and that's the main reason for its continued use.

[clockwork orange]

Originally Posted by smj

From what I’ve read, Gunboats purposely left the resin out between the blocks as it is a huge weight savings. One guy said he would rather have some water running around in the core than the extra weight of the resin. Wouldn’t work on a balsa core but apparently ok with foam.

Thats interesting, i have not read that. That would indicate that they were not infused, as with infusion all the spaces between the blocks would be filled up by default. I have worked on a Choate 40 with a kerfed klegecell foam core with water flowing all over the place through the kerfs with no apparent problems.

[s/v Jedi]

Hull stiffness is as important as weight for a racer. This is why they use honeycomb instead of foam.

With foam you can go lighter than balsa but at the cost of loosing hull stiffness. Even when at similar weight it can’t match the stiffness.

Yes, balsa is cheaper but it’s also superior. One of those cases where a cheaper natural material outperforms a man made more expensive material, but for wood man has never been able to match it’s characteristics.

Getting balsa core wet is a failed product. All materials have failure modes. I remember stories about foam filled rudders that had to be painted with light color antifouling to keep temperature down during haul outs or the foam would kind of melt/disform with big voids as the result (that filled with water).

[clockwork orange]

Originally Posted by s/v Jedi

Hull stiffness is as important as weight for a racer. This is why they use honeycomb instead of foam.

With foam you can go lighter than balsa but at the cost of loosing hull stiffness. Even when at similar weight it can’t match the stiffness.

Yes, balsa is cheaper but it’s also superior. One of those cases where a cheaper natural material outperforms a man made more expensive material, but for wood man has never been able to match it’s characteristics.

Getting balsa core wet is a failed product. All materials have failure modes. I remember stories about foam filled rudders that had to be painted with light color antifouling to keep temperature down during haul outs or the foam would kind of melt/disform with big voids as the result (that filled with water).

Stiffness is probably the most important property of any panel on any boat, not just racers but it has more to do with the spacing of the skins and the laminate architecture than what the core is made out of. People get a bit carried away with "this has more compressive strength" or "that has more shear strength" when the truth is that it just needs to be adequate for the task at hand, no more, no less. Balsa is not superior or inferior to foam, they both perform well unless of course the balsa gets wet, which it does all too often unfortunately. Balsa cored boats really require an extra level of awareness on the part of those building it and subsequent owners to keep water out of the core, it is simply not at all tolerant of water intrusion where foam is to a larger degree. Obviously you still should keep the water out.
Some of the worst balsa cored construction i have seen has been from older J boats, built by TPI btw, same as Jedi. They followed none of the established procedures for working with cores the were learned back in the 1960s that have already been discussed in this thread. Examples of poor build practices we have seen on several J35s have been the hull core running all the way up under the deck flange instead of closing out short, the decks on these boats starts in from the hull sheer and on one boat something with a sharp corner or edge had been dropped on the edge breaking through the glass, not a big hole so they squirted some silicone in and completed the season. When we got to it several seasons later, water had penetrated anyway and because the core wasn't closed out a large are of core was rotten or saturated. One two boats the transoms were rotted out because the builder did not replace the core where the backstay chainplate was bolted on so the bolts went straight through balsa and to make matters worse the core was not closed out at the edge of the transom, so the transom core met up with the bottom core so the bottom rotted out too. The decks were a disaster as everything was bolted straight through balsa. Really shocking how bad these boats were built by a builder with a good reputation. These boats were prior to them using infusion. Tartan Tens and some C&C models had the cores rotted out in the bottom because they did not use enough glass on the inside and only flow coated where you could see when you lifted up a floorboard. they rotted out from the inside. This is the problem with balsa, its a great core and its never its fault, its poor building practices and ignorant owner. Foam is more tolerant of this.