different core types (foam pvc, divinicell vs balsa)

[fhrussell]

Originally Posted by Pblais

I'm not clear if you can make pentrations in a synthetic core and get off with no end problems even if the material in the core won't rot.

Just to answer this.... There will be problems eventually. The balsa obviously will rot. Synthetic cores become powder via a pulverization. This happens when the two skins have delaminated from the core. As the boat works and experiences compression and tension, the skins begin to chafe at the core. Once it begins, it gets worse, quick. As a rough visual, imagine putting a small piece of cake between your two flat hands. Start rubbing together and that is what's going on with the synthetic core.

Again, this all starts with proper construction...or lack of...

By the way, you see balsa in decks and not foam because the foam will crush under the loads of mounted hardware. Balsa will take those loads, but again, you have to be careful when mounting the hardware as always.

[Canibul]

So, would the ideal hull be totally stiff, with no deflection or movement at all?

[hellosailor]

"Just to answer this.... There will be problems eventually."
EVERYTHING has problems eventually. One day the sun will go supernova and destroy us all, including the boats. The pyramids were pilfered, the marble cladding on them was stripped off. The Acropolis was shelled by artillery and then torn apart by iron reinforcements rusting out.

"Again, this all starts with proper construction...or lack of..."
And more to the point--with proper engineering, construction, use and maintenance, any of these materials will outlast the life of the owner. With improper any of those...it can fall apart in a year.

So?

Canibul-
"So, would the ideal hull be totally stiff, with no deflection or movement at all?" Define ideal.<G> I imagine that costs, performance, your definition of "totally" and "long enough" would all affect it.<G>

With the usual exceptions and learning curves (i.e., there was a filler material sort of like the 3M "Scrunge" fiber pads for a while, it went out of use after it was found to simply not hold up in use) an awful lot of the FRP boats ever made are still out there, still performing "good enough". With the exception of the ones that developed leaks, went unmaintained, and rotted out. Sure. How close to all of them? I don't think anyone knows. The ones that have rotted out usually died from owner neglect, haven't they?

Like any other mass market (if you can call production boats mass market<G>) product if it was built "as good as we can" then no one would ante up the price of buying it.

[cat man do]

Hi fh russell
I'd still stick to my point that the main reason you see Balsa core in prdouction boat is that is cheap. I have worked on a lot of totally foam boats and what we do is route out the core where a fitting goes eg: winch with a 150mm hase, take out 250mm of core and fill with "hairy bog" epoxy and glue powder with short glass strands in it.

No rot, no compression.

Also Kiri and Western red cedar don't rot like balsa and don't powder like foam. Not that I think thats something I'd worry too much about, I have not witnessed it happening.

Dave

[GordMay]

Pblais said “... A cored surface is just a truss ...”

I believe a composite sandwich panel is more analogous to an “infinite I-Beam”, than a truss.
The distinguishing characteristic of a truss is the triangular interconnection of it's two chords by a series of diagonal web members.

There’s a simple primer on the mechanics of composite sandwich panels at: http://www.oneoceankayaks.com/Sandcore.htm

And see also:

"Core Failure Modes" http://www.cruisersforum.com/gallery...age.php?i=1327

And

“Replacing Damaged Balsa Core” ~ By Bruce Niederer (EXPOXYWORKS Magazine)
http://westsystem.com/ewmag/19/Replacing_Core.html

[schoonerdog]

just one quick question and then I swear I'll let this drop! I noticed that our hatches and probably deck fittings don't have the "scooped out section" filled with epoxy and filler, instead I'm looking at the divinycell foam. I'm planning on carving out the foam from the outer edges and filling it in with epoxy and filler. Do most boat builders cut corners on like this and think just because they've sikaflexed the hatch down it's good enough?

[fhrussell]

Hellosailor, Dave, and all,
I was specifically replying to what Paul had questioned...whether synthetic cores can tolerate penetrations. As for balsa use, yes, I agree and know firsthand why balsa is used. We never use it in anything anymore, except to re-inforce molds because it is inexpensive. Actually, it is still very strong stuff and can be used to great success if done properly. I wouldn't hesitate using it in my own boat...I just wouldn't use it under or around any elective penetration of the hull or deck, just for stiffening in large areas, keeping the through-hull areas solid or backed with ply or aluminum. To add to my response on core penetrations, the core doesn't immediately begin to deteriorate. It's a long process. Up here in the northeast US, if there is water penetration between the skin and core, water fills in, freezes in the winter, and then the breakdown really kicks into gear the following year.

I'm putting a transom door in a boat now that had some penetration in the transom area due to a poorly installed ladder. We had to wet-vac the foam out!

Another core material we like these days is NidaCore. It's been nice for decks and bulkheads. Anyone have any experience with it?

By the way, Dave, my 43 year old boat had rot all over when I rebuilt her in 2004, but the western red cedar stringers appeared to be like new throughout the whole boat!

Here's an interesting read on the synthetic core topic here...

http://marinesurvey.com/yacht/material.htm

[cat man do]

Hi guys , I personally don't have a problem with balsa either if done correctly. In my opinion this means epoxy and routed out and filled core wherever the skin is compramised. I always seem to use a lot less epoxy than poly anyway so the dollars work out similar,........... if you get your epoxy at the right price.

I have looked at Nida-plast/Nida-core a few times and while I would not use it for a hull,yes I know it is supposed to have Lloyds cert, and have seen 1 powercat where it was used that had a pretty print through pattern after some time in the sun, I could never figure out how to get those temporary screws to bight into air.
Great for anything without too much shape though.But obviously my technique needs working on 'cause it can get formed into good shapes.

The extra longitudenal strength you get from strip plank is what gets me off in hull construction. It means you can use less glass and resin on your skins if you realize that longitudanal planks equals a shitpile of unidirectional in that direction. eg:in my 50ft hulls we've got away with 16mm Kiri core and 600gsm double bias in and out with extra wher req. On foam i'd go almost twice as much on a 20 mm core.

And yes , I have seen some builders not filling in the core cutouts on foam. the argument being a weight saving, but I know some of these guys are just slack. Obviously if your hatches have been in for ages your probably ok, but if your pulling them out, i'd get rid of the core and back fill with hairy bog. without the hairy bits you may find your filler cracks.

Again these are my opinions only, that i've got from first hand experience. And that is an exellent article, but I still could'nt imagine a solid glass cat. Quality cores and good layups still have their place.

Have fun

Dave

[fhrussell]

Dave, you hit it on the head ...'longitudinal strength'. I am a huge fan of cold-molding for that reason. The skin itself has so much more stability in all directions than foam at a very little weight penalty in comparison. Has anyone experience in Jim Brown's Constant Camber? Kurt Hughes has also gone in this direction with his wingmasts, putting more thickness in the mast wall and less stringers and bulkheads in.
Having faired more than my share of foam one-offs, I can attest to the wonders of wood! As a life-long multihuller, I cringe everytime we sloth on another layer of fairing compound....all that weight!

[cat man do]

Indeed, cant believe the amount of GURU built cats out of foam/ kevlar and similar that think nothing about 1/2 plus of bog. I freak out at 3mm.
Dave

[BigCat]

I think well of balsa because a sailboat I finished out is hitting the 50,000 mile mark, with its balsa decks still in good shape. As has been said by others above, you must drill out larger than your bolt needs and fill with epoxy putty so as to keep water out of the core, and a conservative builder (me, for example!) won't use it in the hull below the waterline. Some fairly high-quality, spendy boats are built with vinylester and balsa, like the "J" sailboats.

Gregor Tarjan, a catamaran authority, rates vinylester at a 9.7 with epoxy a 10 for strength-it's much better than polyester, and cheaper and less toxic than epoxy. I'm using this combo in my next project. I should mention that I think users of balsa should use pre-coated balsa, such as Baltek Balsa gold or ProBalsa Plus. The pre-coating will prevent the core from sucking out the resin from the laminate before it sets up. Pre-coating also keeps the panel light by keeping the core from sucking up a bunch of resin.

(No connection between me and Baltek or Diab.)

[Joli]

We've owned both balsa and foam boats. I have no problems with either provided the boat is built and maintained correctly. Some of the early problems with balsa were caused by builders not using end grain, one small leak turned into a huge problem quickly since the water could and did wick through the structure.

Hinckley made the switch to foam years ago but is now back to balsa. The reason? They had core failure in shear and ended up having to skin the outside of the boat, re core with balsa, re-skin the boats, and refinish. Not an inexpensive proposition.

The bottom line is you can have a well built boat with any core material but understand they all have strengths and weakness's.

[BigCat]

Freedoms seem to be prone to problems with balsa in their hulls. I wonder what they did wrong? The balsa we used in the 70s had little gaps between the squares, and the little gaps filled with resin. Water and rot couldn't spread without there being delamination first because of that.

It occurs to me that balsa is going to be a better match for a hull with little curvature that a really well-rounded hull with lots of compound curves. My radius chine building project is perfect for balsa, because there is so little curvature in any area of the boat that has balsa. I am not coring the chine area, which has a lot of curvature, in part because it is below or near the waterline. My topside panels have no curvature at all in the vertical dimension, and only 1 1/2' of curvature fore and aft in 65' of length. That is only 2.3% of curve, almost indistinguishable from being completely flat--barely more than 1/4" per foot.

"The bottom line is you can have a well built boat with any core material but understand they all have strengths and weakness's."

[Alex Pereira]

Hi,

I am not a sailor (yet), but i'm involved with core materials and FRP.
Do you know about a new natural core material with a outstanding behaviour in water ??

Agglomerated cork sheets, specially designed for FRP called NL-10, produced by Amorim.
It is being used in Europe (France and Portugal) with good results.

Everybody knows the sealing properties of the wine corks, so the agglomerated cork sheets used has a core material, keep the same properties.

Think about it., a complementary core material.

[Robertcateran]

Most cores work pretty well if done properly. I plan on using polyprop honeycomb for the resilience. I am considering using pre made panels for much of the boat where there is little curvature. I do not want to get into the vinylester/epoxy argument, except to say I wouldn't use anything but epoxy for sticking to wood and I am two minds about what to use on synthetic cores. I want to see how well they wet out carbon and basalt.
Not sure about the cork. Maybe it is sufficiently stiff at a good price.