Opinions about Cored Hulls ...

[minaret]

Originally Posted by DerekKelsall

In 48 years of concentrating on foam core structures as designer and builder, the materials have proved to exceptionally easy to use, totally reliable/durable, economical and exceedingly versatile. Nothing else compares. No, I do not sell foam. My choice is based on my own direct experience. Obviously I see and have used alternatives. The materials spec has changed little over the years. The technique to handling the materials has changed totally. All of my standard catamarans start as a kit of flat panels, infused on a mold table, from which it gets a smooth finish and ensured fair lines.

It is unfortunate that sandwich structures as a whole have suffered from a fair amount of poor choice of materials, poor choice of technique and some strange beliefs on behalf of a few professionals. For example, in US a lot of surveyors condemn any core below wl. All my boats have foam below wl. Most designers still specify balsa for decks. Experience tells us that they will almost certainly rot at some time. Some very simple rules will ensure that decks of foam sandwich are totally reliable. Claims are made by suppliers of different cores point to a list of failings of PVC and Corecell foam. I can say positively, that none of these apply if done correctly.

Mention was made of the bond between skin and foam. The surface of the foam is cut cells. When done correctly, each cell is filled with resin, providing a 100% contact between the foam and the skin.

Honeycomb does not have the track record of foam and it is no where near as versatile. Remember we are dealing with v. thin skins. Skins too thin to guarantee that water will not penetrate. A fissure in the skin reaching the foam is of no consequence. A fissure in a honeycomb or balsa core is not good news.

I can point to many boats I built in the 60's and 70's which are passing survey today. I know of none which have been scrapped. A few wrecked. A few tris capsized but no sandwich failures.

The topic is covered in Catamarans - Kelsall Catamarans - Boat Designs

Happy boating,

Derek.
FRINA

Couldn't agree more with this assessment of core materials.

[Cheechako]

While I'd like to accept cored hulls below the waterline, to me, it's not what the Japanese would call a "robust" design. Sure it'll work in a perfect world, but the world is not perfect. It's a little like saying " I'm just going to drive my motorcycle on the freeway everyday at high speed without a helmet... I'm a perfect driver so nothing will happen to me." With most cored hulls it seems to be only a matter of time.... not to mention, if a cored hull get's blisters (without core saturation) is it thick enough to peel and leave enough material? It's kinda like the double whammy regarding risk.... of course, JMHO.

[DerekKelsall]

Originally Posted by Cheechako

While I'd like to accept cored hulls below the waterline, to me, it's not what the Japanese would call a "robust" design. Sure it'll work in a perfect world, but the world is not perfect. It's a little like saying " I'm just going to drive my motorcycle on the freeway everyday at high speed without a helmet... I'm a perfect driver so nothing will happen to me." With most cored hulls it seems to be only a matter of time.... not to mention, if a cored hull get's blisters (without core saturation) is it thick enough to peel and leave enough material? It's kinda like the double whammy regarding risk.... of course, JMHO.

PVC foam does not take up water, so no fear of getting waterlogged. In 1969 I build a concrete slipway in Sandwich, in Kent, using offcuts of PVC foam as spacers. I took pictures about 8 years ago, showing the foam with square cut edges still above the concrete.

I argue that any pressure on the boat to drive water through the skins is higher on the decks than below water level. The variation of temperature between sunshine and a shower will tend to suck water through any fissures. The more constant temperature below wl is under less pressure. I have run tests of various small panels by alternating between the hot and the cold tap.
This is my argument that balsa in decks is the worst scenario.

On impact resistance - the foam sandwich wins over solid and if this is a serious concern, use the linear PVC below the wl.

Blistering is a separate issue. Nearly all of my boats got a very good paint system applied all over, including below wl. A none issue in my experience of our design.

Happy boating,

Derek.

[beiland]

I realize this subject thread is dealing with "cored hulls", so my observation here on cored hulls is that they should be (and very often are not) properly marked as to where these vessels can be proper chocked when out of the water for storage or repair . The chocks themselves are of such a relatively small area that they can easily 'point load' the hull, particularly a cored hull. if the vessel is built so strong as to accept this 'point loading' then you might consider it overbuilt.

Derek is very much sold on foam as the best core material. While I see his arguments there is a lingering thought in my mind,....over many years I have always heard many reservations about using foam in the decks of vessels, particularly those that will see a lot of tropical exposure. In fact that was a sales point that the balsa people always stressed....balsa was better in the decks of tropical boats.

So I would be interested to hear from some of the experienced surveyors here about the use of foam cores in decks.

[scoobert]

I can add a positive on a foam core.
my tayana is foam cored. it leaked like a sieve. then left to freeze with water in it.
how much more abuse can you give a core?
still after slowly filling the voids with resin, and checking the core by hand in the worst spot, its solid. no give whatsoever.
if it was bulsa, the boat would be parked next to the wood trawler over under the tree of death.

[beiland]

BIG BANG #1, Lion Whelp's impact test

These two brothers bought an older 65' Alden schooner, and went about restoring and upgrading her. They had a MAJOR concern with hull integrity,

“When you're doing 9 or 10 knots at night there's no way you can see a log or container just under water ahead of you. It's a probability issue," Phin says.

He points out that right after a hurricane in 2003 a whole load of telephone poles came off a container ship in the vicinity of Bermuda. The poles were 120 feet long and 3 feet thick, plenty big enough to sink a yacht. There was also a sea buoy loose in the area and that buoy was 10 feet in diameter”.

So they built some test panels to do a little investigating. One panel was built to the same layup as the original hull, and another two panels were backed up with 3” thick NidaCore PP. Laying around their boatyard they had some old pieces of railroad track (pretty tough stuff). They picked out a 6 foot long, 157 lb section, and proceeded to drop it onto the test panels, not flat sided, but end on,.....first from 9 feet, then 16',..... and finally 30 Feet......WHAM !!


.....excerpted from an article that appeared in Soundings mag,
Building a Bullet Proof Boat
http://www.soundingsonline.com/boat-shop/on-powerboats/266332-building-a-bulletproof-boat

.....Have a look at some of the photos and the damage here:
Testing the NidaCore
http://www.portlandyacht.com/lionswhelp/whelp.html
(you have to click on the word Construction, then the drop-down menu, click on Design/Innovations)


Brian's Note: Not a very 'professional test', but impressive in its own right. Sure speaks to the ductile nature of the PP honeycomb core material. This ductility is a good match for the ductile nature of steel itself.
**********************************************

…...from that same article,

The problem with a conventional plank-on-frame hull is that you have the planking on the outside of the ribs and the ceiling on the inside, with a void that's the thickness of the ribs between them, Phin says. If you puncture the hull planking, you can't get to the damage to stop the leak because the ceiling is in the way.

They bonded 3-inch-thick Nida-Core - the same thickness as the ribs - to the planking with WEST System epoxy. "Then we glassed over the Nida-Core from the inside with epoxy 10-08* glass creating a smooth surface on the entire inside of the hull, from sheer to sheer," Phin says. "Next, we epoxied a 5/8-inch fir tongue-and-groove ceiling over everything and painted that. We created a monocoque [one piece] structure, a continuous I-beam that added a great deal of strength and impact resistance to the hull, as well as thermal and acoustical insulation.

...hmmm, I wonder if we could eliminate some of the numerous smaller ribs and stringers of the conventional welded-up steel hull and substitute some PP core material bonded to the steel skin??....might save a lot of welding time, and hull plate distortion. IDEA??

[minaret]

Perhaps it'd be smarter to bond an aluminum honeycomb to an aluminum hull, with a thin alu or composite inner skin? Many similar (or exactly the same) things have already been tried.




Aluminum Honeycomb Core - Aluminum Honeycomb Cores Products

[beiland]

BTW, what would you propose we use to bond the alum honeycomb to the outer alum skin?

What really bonds absolutely to an alum sheet?

The reason I bring this up is I was trying to think of what I might use to absolutely bond to a steel sheet / plate? ...rather than 'welding' a piece of metal?

[minaret]

Originally Posted by beiland

BTW, what would you propose we use to bond the alum honeycomb to the outer alum skin?

What really bonds absolutely to an alum sheet?

The reason I bring this up is I was trying to think of what I might use to absolutely bond to a steel sheet / plate? ...rather than 'welding' a piece of metal?

Smart Adhesives M2 Methacrylate would be the obvious choice. Technique would be the stumbling block, most methacrylate open times are short. Totally doable though. Would be interesting to play with.


Methacrylate | Adhesive.com - Research, Compare, Buy

[model 10]

Originally Posted by minaret

Smart Adhesives M2 Methacrylate would be the obvious choice. Technique would be the stumbling block, most methacrylate open times are short. Totally doable though. Would be interesting to play with.


Methacrylate | Adhesive.com - Research, Compare, Buy

I believe that Nomex honeycomb is the perfered material. Al. honeycomb is usually seen in high temp apps. I wonder about super glue (methacrylate) used for bonding a critical structure. As usual, some sort of epoxy is what you see. Honeycomb is hard to bond to compound curved surfaces which is why someone would be tempted to use superglue. Vaccum bagging is probably a better way to go.

[minaret]

Originally Posted by Guy

I believe that Nomex honeycomb is the perfered material. Al. honeycomb is usually seen in high temp apps. I wonder about super glue (methacrylate) used for bonding a critical structure. As usual, some sort of epoxy is what you see. Honeycomb is hard to bond to compound curved surfaces which is why someone would be tempted to use superglue. Vaccum bagging is probably a better way to go.

Super glue is a cyanoacrylate. Methacrylates are more and more commonly used by major marine manufacturers to bond together structural members. Much better than epoxy. Most hull liners are bonded in with methacrylate, for instance. One would obviously vacuum bag with it and a honeycomb core.

[perchance]

Originally Posted by beiland

BTW, what would you propose we use to bond the alum honeycomb to the outer alum skin?

What really bonds absolutely to an alum sheet?

The reason I bring this up is I was trying to think of what I might use to absolutely bond to a steel sheet / plate? ...rather than 'welding' a piece of metal?

5200?

[colemj]

Originally Posted by perchance

5200?

No, they are discussing structural adhesives like Plexus.

Mark