Balsa Core on Cat Hulls

[cat man do]

Originally Posted by s/v Jedi

Nick.

I seriously doubt you could push your thumb INTO a composite panel

[s/v Jedi]

Originally Posted by cat man do

I seriously doubt you could push your thumb INTO a composite panel

I agree... but he might have meant just the core, as in without a skin that can spread the thumb-force over a wider area

ciao!
Nick.

[djambalawa]

Just a newby question sorry - apart from balsa vs foam is there any negatives of having solid under the waterline? I guess weight would be one (although the low weight might be stabling?)?

[44'cruisingcat]

Originally Posted by s/v Jedi

I agree... but he might have meant just the core, as in without a skin that can spread the thumb-force over a wider area

ciao!
Nick.

I seriously doubt the person making these claims has had any experience actually handling these materials.

There's absolutely no way you could push your thumb into an end grain balsa core. You'd be able to make an impression with your thumbnail, that's about it.

Even foam, which is much softer, is too stiff to go poking your fingers into it. You could dent it slightly with a thumb.

the core makes a huge contribution to the panel stiffness. Take away the core and you have two thin glass laminates, probably less stiff than the same thickness of plywood. Seperate them with a core and you have an immensely stiff panel. To say the core doesn't contribute is ridiculous.

[s/v Jedi]

Yes... but when you epoxy a single layer of cloth onto each side of a paper honeycomb core you also get a very stiff panel while the core is just air and some paper that can be squashed with a thumb...

I still don't know what the best core material is... I have seen balsa rot but also foam rot and melt. There's so many brands and types of foam...

I'm very happy with my balsa core. I have found 1 thru-hull fitting above waterline where the core wasn't replaced with solid glass/epoxy and the balsa was wet for about 1/2" around the hole... after 17 years. Not so bad and quickly fixed.

ciao!
Nick.

[minaret]

Originally Posted by 44'cruisingcat

So there were thousands of litres of water, tucked away somewhere between the "totally impervious to water" core and the correctly bonded laminates???? Just where do you think it was?

If it was any of the good brands of balsa, it simply couldn't have happened. They don't claim balsa is waterproof, so they take steps to prevent water from migrating through the core. The balsa is cut into 3-4 inch blocks, and there is an epoxy glue line between the blocks. On the core I have experience with there is also a much thicker glue line every couple of feet. So a damaged laminate or incorrectly done hull penetration would only result in a small area being wet.

Obviously the water was in the countless little voids in the seams between the foam blocks. This was a very expensive yacht from a well known and reputable yard, and wasn't the first time I've seen this sort of thing by any means, it was just the only case of a satured cored HULL that I've managed to dry out. I've seen exactly the same situation in balsa cored hulls small and large over the years, and believe me it was MUCH worse. Like sometimes the balsa looked like oatmeal. Or all that was left inside was white fungal fuzz. The problem is that unless you use vacuum bagging or infusion, voids in the core are almost inevitable. And I've laid lots of core. Acres of core, really. They built a lot of boats before these techniques were even invented. And so there are a lot of boats out there with voids in the core. Most of them didnt use epoxy for bonding in the core. And the problem with using thickened epoxy at all for core is that it is heavy, unless you use 407 or 410s to thicken it, and then its brittle instead of flexible cuz those are made for fairing. Most production boats were built using core-bond or a similar polyester product for that purpose, because its very liteweight and slightly malleable. Most of the balsa I've worked with up to 1" thick was in 1" blocks. These blocks are attached to a fiberglass cloth on one side. If you look carefully you will see that the blocks are aligned on the cloth with a little gap all the way around them. This allows the blocks to flex on the side opposite the glass backing, so you can core on a curved surface, which is obviously the point of using blocks. Otherwise you would just use a flat panel with holes drilled in it for squeezeout. These gaps around the blocks of balsa mean that by nature there will be a lot of fillage. I've done it by hand countless times, and you can be very thorough and liberal with the resin and corebond, but if you cut your panel with a saw later you WILL find voids, be they ever so small. One of the problems is that you generally are putting the core down with the glass backing up, so you'll have a nice surface to scratch up and laminate the inside skin to after coring. This means you cant see the gaps between the blocks anywhere except the edges, and it's hard to judge the squeezeout. It can be plenty around the edges but not enough in the middle, etc. Plus in an industrial setting you are usually working so fast to get core down before the corebond kicks, you dont have time to be sure. Hence vacuum bagging. Oh and water weighs 8.345 lbs. per US gallon, meaning about 375 gallons would weigh about 3000 lbs. Not all that much in the core of a 50' powerboat. And I can definitely stick MY thumb into core, balsa or foam, at least 1/4". Unless you back it up with something. Or unless it's 10lb. foam, then good luck. 15 lb you can't dent with a hammer. 25 lb. you can't cut with a saw unless you use carbide abrasive blades.

[minaret]

I've cut into older balsa cored hulls built in Taiwan, and elsewhere, and found all the filler in the core, which was often made of "shingle", was made out of leftover shop resin mixed with the sawdust swept off the shop floor, including lots of cigarrette butts and God knows what else.
One local PNW builder was notorious for this as well...

[44'cruisingcat]

Originally Posted by minaret

I've cut into older balsa cored hulls built in Taiwan, and elsewhere, and found all the filler in the core, which was often made of "shingle", was made out of leftover shop resin mixed with the sawdust swept off the shop floor, including lots of cigarrette butts and God knows what else.
One local PNW builder was notorious for this as well...

Which is poor workmanship. And I can't see any reason that foam would be immune from that either.

In reality though, you are talking about practises (in your previous post) that are completely out of date today, except for some very poor and shoddy builders. The balsa cores you buy today do use epoxy to join the blocks.

[44'cruisingcat]

Originally Posted by s/v Jedi

I'm very happy with my balsa core. I have found 1 thru-hull fitting above waterline where the core wasn't replaced with solid glass/epoxy and the balsa was wet for about 1/2" around the hole... after 17 years. Not so bad and quickly fixed.

ciao!
Nick.

Exactly the point. The foam salesmen will tell you that the instant water touches balsa it will completely turn to mush, and your whole boat will be ruined. Reality is different.

[44'cruisingcat]

Originally Posted by djambalawa

Just a newby question sorry - apart from balsa vs foam is there any negatives of having solid under the waterline? I guess weight would be one (although the low weight might be stabling?)?

Weight is the biggie. The boat weighs more, so you either sacrifice payload or gain displacement. Hulls need to be fatter to carry the extra weight, so they need bigger rig, sails and engines to drive them, which needs a stronger structure to handle the loads, which raises weight even more.....

[Event_Horizon]

Originally Posted by 44'cruisingcat

Incorrect. The core has to keep the laminates apart. When you bend the panel the laminates try to pinch together. They also try to slide relative to each other.

Yay TRUE!

If you imagine a panel using a soft latex foam as a core, bending it would be very easy. The core would offer no support, so you'd be just bending 2 thin laminates.

Boo FALSE! Despite the weakness of the core in compression, it could still be very stiff (depending on thickness). However, it would not be very STRONG. So once you bent it (which could take considerable force) it would buckle and fail quite quickly.

The higher the compression strength and sheer strength, the stiffer the panel given the same laminate thickness.

Originally Posted by bewitched

Shear strength yes, compression strength no. Take a look at the spec sheets, compession strength is less than 1% that of 0.5mm of fiberglass. You could easily push your thumb into it.
When a wave acts on a cored hull the exterior laminates are put in compression, the interior laminates are put in tension and the core, where the neutral axis lies, experiences some amount of shear.

Remember bewitched, we are NOT talking about someone laying a panel on the ground and jumping on it. We are only concerned with the panel under bending forces. The compression strength of the core is very important because it determines the STRENGTH (stress that can be applied before yield/failure) of the panel.

Oh also, there is never any shear force on the neutral axis. You can tell because the neutral axis is located on the neutral axis...which is defined as the axis subjected to pure bending forces...which doesn't include any shear forces.

[GordMay]

Originally Posted by 44'cruisingcat

Incorrect. The core has to keep the laminates apart ...

Indeed. The core, in a laminate sandwich panel, functions primarily as a spacer and connecting unit between the outer & inner skins.

Naval designers specify laminated sandwich construction for much the same reason architects use I-BEAMS and trusses: to increase stiffness and strength while at the same time decreasing weight. The core in a sandwich laminate acts much the same as the web in the I-BEAM, by connecting the load bearing skins. The increase in stiffness is directly related to the height of the web (or thickness of the core). It is generally recognized that the stiffness and strength of a sandwich panel under bending, end loading, and torsion(macroscopic loading) are sensitive to the thickness of the core and to the in-plane properties of the face sheets.

The main function of the core material is to distribute local loads and stresses over large areas. Local stresses applied to one side of the sandwich have only a reduced local effect, because the exposed skin and the core will distribute the loads to a larger area of sandwich. Because of this fact, a sandwich structure generally exhibits superior behaviour under bending, torsion, impact and compression, parallel or perpendicular to the skins.

See ➥ Core Failure Modes - Fig.2 - Cruisers & Sailing Photo Gallery

And ➥ Cored Laminates - Fig. 1 - I-Beam Analogy - 1. I-Beam Analogy Cruisers & Sailing Photo Gallery

[jmarples]

Balsa core is not recommended below waterline on any boat. There were class-action law suits filed against Sea-Ray for that. Sure the balsa is better in compression, but the problem is the glue line between skins and core. The deformation of impact causes a shear failure of the skin to core bond, with either type of core. The damage usually extends far from the impact and is difficult to assess. Removal of the skins is usually recommended for a good repair, which is expensive. Water intrusion from any damage will travel down the kerf lines of contour balsa or foam, allowing water to soak a much larger area than the local impact zone. Rot will only occur if the soaked zone is allowed to periodically dry out - like in the deck. Foam core will not rot, but it has lower shear strength, so the damaged area from any impact will be larger. jmarples

[cat man do]

Originally Posted by jmarples

Balsa core is not recommended below waterline on any boat.

Can you point to any sort of studies, laws or legislation that has been bought in to support your view?

Do your opinions only apply to Balsa cores or are other cores immune?

[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.