sandwich vs single skin hulls

Hi RNB and welcome to YachtForums!

When you say performance, it can be several things, but for trawlers and long range sailing boats a solid GRP can give a smoother ride without slamming and also extra strength if you collide with a hard object. But for yachts where you want less hull weight, the infusion with a core material is to prefer. (Swan has it on the new 42 New York Yacht Club racer).

Sound insulation may be improved on slowgoing boats, but on high speed boats (35-45 knots cruising) we experience almost the opposite. A light cored construction is creating more noise from the sea than a solid more heavy. We now have to add dampening material and weight inside the vacuum cored hull...

To build with infusion is more time consuming (and expensive) but gives a better working environment, why I think more builders will end up doing it.

If you are planning a performance sailer, you probably mean faster than a cruiser and perhaps mainly sailing in daylight, which points towards a composite hull, like the boats from Green Marine. When it comes to strength, there are different things you like to achieve, torsion or structural strength where cored hulls are superior. Also impact strength from the elements, but impacts from hard and sharp objects is a weaker point. The outer skin is much thinner on boats built with core and infusion and once you crack it open, it is almost like opening a can. Nothing you would like to experience on a night crossing with a small crew. Or ever...

Maybe Alustar is a good compromise of weight saving and impact strength after all...

If it were my boat and I wanted composite construction, I would go cored. I am also assuming that this is a one-off, not a production boat. The differences in building in a female mold for production are significantly different than the process of building a one-off over a male framework.

My favorite core for this application is Core-Cell, an SAN foam that comes in different densities and cuts, including strip-planking. The core is strip-planked and glued together, and the outside skin is laid up. Impregnators to get the resin/fiber ratio right, and vacuum bagging provides very good laminate characteristics. After removing the hull with core and outside skin from the framework, the inside skin is laid up. Therefore, you elimate the core bonding layer which eliminates any risk in a poor core-to-skin bond. In production female tooling, it is essential that the core be bonded to the outside skin with either an infusion process or into Core-Bond under vacuum.

Any builder who knows what he is doing can build you an appropriately strong and stiff hull, be it solid or cored. Whatever weight difference there may be are taken account of the the design process. There is no secret here, it is just a matter of keeping weights and centers under control as one would do in any normal design process. There is no reason why a cored hull can't be built robustly that will last for ages. Choice of materials and skill in the processes consistent with those materials are what are key.

One of my favorite designs that I happened to see built was the 50' round-the-world sailing yacht Airco Distributor, designed by Rodger Martin for Mike Plant who built it himself. The hull was built in Newport, RI, in 1984-85 with Airex cored fiberglass and Dow Derakane 8084 vinylester resin. This yacht has been around the world three times in the Around Alone Race (formerly the BOC) and has crossed the Atlantic 15 times. It still competes from time to time. This is a great testament to longevity in cored construction.

In my current design, the Globetrotter 45, I am designing a one-off version for cored composite version with Core-Cell core and vinylester resin. If it goes to production, the basic production laminates will be close to the one-off laminates because of these choices of core and fabrics. I do not know the builder yet, but certainly the builder will be well versed in these materials, and that's not too hard to find.

I agree totally. The only real drawback with cored is when there the outer skin has been breached and it has not been dealt with quickly. I know the various manufactures of cores say their products do not absorb water, I beg to differ. After you have seen someone drill hundreds of small holes to drain out moisture in the core material prior to repair you begin to wonder if it would be better to have a thicker single skin. I have seen it happen a couple of times, the initial damage looked more like a bruise than a gash but generally many square feet of core material was saturated with water.

Yes, that happens more often than it should. Mostly I would bet that you could trace a soaked core back to a poor bond between the core and the skins, and also to the "slots between the blocks"--contoured core--which are natural pathways for water to get in. A well made boat, even with contour core, will have all the slots between the blocks filled with putty, and the skins 100% bonded to the core.

This is one of the reasons that a one-off hull with strip-planked core has such a much higher degree of success than a female molded boat. In the one-off case, the skins are always going onto the core surface in full view both sides--you can see and take care of any voids--and if there are slots between the blocks, you can fill those before you laminate. There is no excuse for voids. In the female molded case, you are setting the core into the outside skin blind--you cannot see the joint to the outer sking at all, and you cannot see 100% how the slots between the blocks are filling. Best success comes with vacuum bagging the core into a putty like Core-Bond, and the core should also have bleed holes in it to allow some of the putty to come all the way through to the side that you can see under the vacuum bag.

An interesting alternative I saw at METS is a new core-material called 3D-core. It's kind of middle of the road between cored composites and single skin.

The core material is composed of hexagonal foam cells. Infusion is to be done with inside and outside skins together and creates a resin-filled cross connection between the skins. The end-result is a composite with honeycomb-like foam cells in it.
Because of the cross-connection between inner and outer skin, it's impact resistance is a lot better than with a conventional cored composite. For the same thickness you end up stronger. For the same strenght, I don't know if it's lighter or heavier.

One other advantage is that the 3D-core material can be bent at will, making it easier to create complex shapes (see attached pic).

As far as I know, the material is not yet approved by any class society.
The not-very-informative website is at http://www.escomposite.de/.

Bruno

Thanks Innomare,

This material seems easy to work with, but I am afraid that with vacuum infusion you will have to build a thicker outside skin to avoid visible patterns on the surface..?

On our 40' powerboat we are using precut Divinycell which has an even outside and still we can see hints of a structure through the gelcoat, which has to be rubbed and polished for a perfect finish.

For areas around the keel we have solid GRP and the transom has Divinycell with higher density for strength. Lamination is with vinylester and all stringers have carbon fibre reinforcements.

Sandwich vs

To RNB
I hasten to state, that I am not "religious" in this matter.
My observations are based on real world, hands on experience.

To give you, perhaps, a clearer view of theory vs actual
perfomance, we submit the following:
- First, the theory of a cored hull being lighter than a solid hull,
is based on a very basic engineering rule:
- " That a beams strength goes up - by the square of the depth."
i.e.
Two 2 x 4's nailed together - side by side - will be twice as
strong as a single 2 x 4 when resisting a downforce.
Whereas; a SINGLE 2 x 6 is ALSO twice as stong as a single 2 x 4.
Resulting in also doubling the strength - while only increasing the
weight - equivilent to a 2 x 2.
(The reason why this so - is quite fascinating - but requiires a
longer explanation than you might have patience for today.)
- So, it has been suggested by others;
that the core functions as many little I - beams carrying load from
the outside skin to the inside skin.
- It follows: that a wide core, sandwiched between two skins can be
lighter than a thinner, single skin.
All the above is well known and can be demonstrated in the lab.

HOWEVER........theres always a however......
In said lab - inconvenient realities surface.
Observing the results of HIGH density core samples - tested to
destruction - I noted an interesting phenomenon.
The fractures often resulted in an elegant parabolic line moving from
one skin to mid center. Then becoming hyperbolic, as the force moves
on to the other skin . Like so: ~

- Just as is shown in diagrams in engineering text books !
Practice and theory, actualy supporting each other. Amazing.....
( Note: the I'm using the word " elegant " as used by engineers.
As in....using the least amount of material for a given result.)

After this, however things become less sure.....
With LOWER density cores, shear lines were much less elegent.
Fracture lines were more jagged and varied.

These are the conclusions I made fom this:
First, designers and engineers are lead astray by said, text books,
that define the force at mid point of the core, as ZERO.
It well might be. For a milli second. As 8000 lbs of force travel
through the core, from one side to the other.
But everybody got excited. Making the PRESUMPTION that zero
force, for a moment, means that you can get away with VERY
skinny, little I - beams. Like honeycomb cores.
Where AIR is the main component!

This does work. In theory and practice. For the first couple of
excursions offshore.....
However, it can only work , if stability of the skin(s) and core
are maintained.
That is; the little honeycomb I - beams must stay at 90 degrees to
the skin(s) surface.
Note: A loss of only 2 or 5 degrees from verticle, of a post or
I - beam results in loss of strength of 90 % or more.

This brings us, to perhaps the least appreciated element at work here.
Let us PRESUME that all is well and good - and your low density foams
and honeycombs are performing as effectivly as they do in the lab.
Transmitting force, seamlessly, from one side of the hull to 'tother.
However, as I've stated, these I - beams must maintain said,
verticle alignment.
MAINTANCE OF THE CONNECTION OF THE CORE MATERIAL TO THE
SKINS IS ESSENTIAL.
As delamination of only one skin, can result in going from 200%
greater strength over a solid material to 25% less strength than
said, solid material.
The key is: THE SHEAR VALUE BETWEEN CORE AND SKIN(S).

Going back to the " elegant" diagram.
If the total resistance of the composite is a value of 12,000
And even if the skins can sustain this load alone - it does not follow,
that resistance can be maintained where the skin and core connect.

As in the case of a hull, one inch thick, at mid point we may have
almost zero force. However as we follow the break line... At 1/16"
from the mid point - it has climbed to 1000 or more - and at a 1/4"
from mid point - the force has jumped quickly to 6000. As we
continue along the break line - within a 1/8" of the surface we can
now be close to 10,000.
Note: We have not yet reached the breaking strength of the
TOTAL composite.
However; if the core can not maintain a force of 10,000........
It will fail BEFORE the skin will. Shear will take place where skin
and core used to be connected. ( Where you cannot see it.)

It follows: that, in my experience, resistance to delamination is
DIRECTLY related to the shear strength of the core
- NOT the skin material.
The actual bonding of core to skin - is a bit of a red herring.
Not that I'm overwhelmed, with how well disimilar materials
can be stuck together. At the best of times.
Even if you have a good bond. If the core has failed in IT"S SELF
- you will have failure in the structural integrity of the total
composite anyway.

Therefore; as density is usualy a function of overall strength,
denser ( heavier ) cores will be less prone to delamination or
internal failure.
It also follows; that the closer the core is to the density of
the skin the less likely that delamination will occur.

It also, unfortunatly, follows that the combined weight of the
denser core and skin - are know approaching that of a solid skin!

So why bother.......?!!

In my own situation I will happily spec out honeycomb laminates
for INTERIOR structures.
For offshore, expedition yachts - given the above, it will be
solid skin below the waterline.

Hope....all above helps -and has not served to confuse
things further!
I think, by now, you now see that this topic is a lot more
complicated than might be expected.

Cheers !

Getting to the core of it

But, rolled aluminum, welded to tabbed aluminum, welded to aluminum, with a vacuum in the middle, how might this hollow material differ? I know the patent holder, and best of all I sure like the stated 'R90' rating, in his 48mm profile!

Honeycomb structures for interior applications makes the most sense to me anyway you look at it. MDF is abysmal when drenched. I like this all wood product called multi-core. (Assuming it's a real marine glue.) And of course, lots of top woodworkers have turned to vacuum-forming for their curvilinear veneered stuff.

Battleships aside, I still don't get why a steel boat with a well thought lead keel can't brush up against the occassional reef. Are there any specs to persuade one that this is possible?

core infos

one known producer is Nida-Core. there you will find a lot of information about core technologies.

http://www.nida-core.com/english/index.htm

Nida-core

I found them last year via a woodworker's link, and thought 'dumb site.' Note their flat panels say laminating with formica, aluminum, corian- 'to be advised.'

Very well, back to hulls. You guys have numerous issues at play. Acoustics and insulation still top my list, whatever the substrate. Epoxys and vinylized polymers are clearly advantageous, and quite case specific. Will we ever get around to bouyancy? Overall scale, and thickness?, or am I just delusional to preclude that laminates (over, say 8m) are not seaworthy in the least?

Tri-star makes some valid points, but he seems to present it in a way that suggests engineers "forget" about the shear loads within cores. This is not the case. We engineers deal with both the face stresses and the core shear stresses all the time when engineering cored laminates. Core shear is not to be ignored, and in fact the ideal sandwich laminate will fail in the core and in the skins at the same time. In reality, one or the other (core or skins) will fail first, and the key is to achieve as much factor of safety in the design that is reasonable and consistent with weight, cost, and ease of lay-up.

Regarding MaxResolution's remark about steel hulls for this hypothetical sailboat (20-25 meters), certainly that can be done. All it takes is to engineer the boat to the material at hand--it's done every day. I am consulting right now on the rig for steel schooner of just that length, and boat designs can be found around the world from a number of notable designers.

CORED hulls - Part TWO

To RNB:
This reply will be shorter - I promise.
( My daughters tell me, I some times pontificate a little long.....)

Yes, I agree with you - epoxy has merit. Being more resistant
to water than many materials.
People will say - but it costs twice as much.
Perhaps.........because it's TWICE as good ?

Here is a hull layup of a vessel designed AND built by us,
that has stood the test of time.
- X.layers of high grade F. Glass. ( On the water side.)
- One layer of Western Red Cedar.
- One layer of Fir.
- (2) X. layers of F. Glass. (On the inside.)
All held together with aircraft grade epoxy.

Note: In this layup; the core is - difinatively - HIGH density !

Note: It's common practice for boatyards to put more glass on
the outer, waterside of the core, than on the inside. Mostly for
cosmetic reasons.
However, our layup is ONLY concerned about structural integrity.
With ours - there is more glass on the INSIDE.
Because: Very high load impacts on the water side will more than
likely, cause failure to start on the INSIDE......
- As the rock just impacted, is creating a COMPRESSION loading
on the outside of the hull.
- And a TENSION load internaly.
- So often: failure starts INSIDE the vessel.
- As glass and carbon fiber are there to take tension loads:
It seems logical, to me - to beef up the area that most
needs to resist tension - with more tension resisting materials,
Than to do so, on the outside of the hull.

Cheers !

Ship Yard Experts

To RNB

This reply will be shorter still........... I promise !
You indicated, RNB that you would like conclusive
scientific evidence re: cored hulls. Don't we all !

Some well engineered and well crafted vessels with
sandwich construction HAVE made many ocean
crossings. No debate about it.

You have had the benifit of disussing the pros and
cons, with various experts and manufacturer's
representatives. All of us - including myself - might
have vested interest in these concerns. Now, I'd
suggest, you conclud your research with a simple, low
cost procedure.

Go down to some boat yards. Close to quiting time.
Walk past the front office. Avoid anyone wearing a tie!
Talk to the guy with the grungyest overalls.
Take HIM for a beer.........
Ask HIM, what boats he dreads showing up again in the
yard for repairs.

To be clear. We HAVE produced cored boats - and will
do so in the future. Also, our vessels seldom show up for
repairs. Even the ones built 25 plus years ago.
So, presumeably, we know what we are about.

Regards.