[RNB]

From the research I've done there seems to be a compelling case for cored hulls for sailing yachts in terms of the advantages of lower weight, improved stiffness and impact strength, thermal and acoustic insulation. But there remains significant differences in build techniques between different yards. For sail boats of 20 to 25 meters it does not even seem to be an issue of cost. Swan for example build (mainly) single skin hulls as do Oyster. Baltic, HR, and many other yards swear by cored hulls. There is a parallel discussion here about resin infusion, and other build methods but what is the view as to why we dont see more cored hulls with the overall performance advantages they offer?

[AMG]

Originally Posted by RNB

... why we dont see more cored hulls with the overall performance advantages they offer?

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.

[RNB]

Thanks for your comments. I have been researching a good deal on this subject with a view to having a 22 - 25 metre performance cruising boat built. I keep thinking I am quite well informed until I learn something new -which is often! This area of materials technology in boat building is changing and clearly relatively complex. I may have been seduced by research I have seen on stiffness, rigidity, and impact strength shown to me during a meeting with SP Systems (now Gurit) which I have to say was compelling (esp the carbon/kevlar, e-glass, corecell epoxy sandwich - which admittedly is a v. high quality solution). But it also seems to me there are "religious" views about cored vs single skin hulls as it is difficult to get a consistency of view from either naval architects and boat builders. There are builders who have stated to me they will never, ever build a cored hull (incl one well known brand in the UK). I know it is only quite recently that core materials have become available in forms and shapes which make it simpler/more practical to include in the lamination or infusion process, producing no resin voids and good bonding between core and skin. Also the fact that the price of carbon and kevlar is coming down, and computer aided design and plug development is bringing economies and a degree of accuracy in build processes not seen before (which has allowed Shipman to produce their 50 and 63 ft performance cruisers for example). So I try to keep an open mind on the subject. Its interesting to note that all the RNLI boats built by Green Marine in the UK have been built with cored hulls for the last several years - and you cannot get a more demanding build spec than life boats. There are of course those who believe I should build in alustar or other marine grade aluminium alloy - but thats another debate! What I want is a conclusive scientific answer but of course as with most things to do with designing and building sail boats, it ends up being a compromise between competing forces - principally use and purpose, cost and design. We continue exploring! Best. RNB

[AMG]

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...

[Eric Sponberg]

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.

Eric

[cranky]

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.

[Eric Sponberg]

Originally Posted by cranky

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.

Eric

[RNB]

Thanks for this perspective. Certainly it would seem that contoured core availability has made a significant contribution to improvements in skin/core bonding. The danger of core penetration is I guess why you see some boats with a single skin below the water line and/or kevlar reinforcement and a sacrificial bow to cope with dangerous impacts. These comments have helped - so thanks. RNB.

[Innomare]

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

[AMG]

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.

[RNB]

Thanks Innomare. I have to say that the www.escomposite.de web site is possibly one of the worst I have ever seen. Nevertheless they seem to have an interesting product but possibly from my (risk management) perspective a little too new for me to take on board at this stage. I need proven materials technnology which means I go with what is known and welll understood I think.
Thanks for your input. RNB

[tri - star]

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 !

[MaxResolution]

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?

[RNB]

tri-star. Yes your analysis makes alot of sense and in fact I have received a similar argument (in much briefer form) from Oyster in the UK. Their emphatic statement was that since they wanted to keep their customers afloat, they would always build single skin hulls! Cored hulls in their view are simply unable to withstand penetrating impacts (though I seem to remember that SP Technologies (Gurit as they are now called) did some work for them with their SPRINT system 18 months or more ago). The only other advantage of a high density cored hull I feel, even if it does get close to the weight of a single skin hull, is the thermal and sound insulation properties it provides, which are important on a cruising boat. Also, more recently German Frers said in an email to me that (most) new Swan's are now going to be built with cored hulls, because of the recent development of core shapes which can be produced specifically to follow hull lines, delivering a significantly better bond between skin and core, and enabling a much simpler (lower cost) build process for production boats (though the current Swan web site does not support this it seems). There is also the design/engineering dimension of having a sacrificial bow, with a "pram" bow remaining, in case of mid ocean collison with a container. The other element of this debate is the resin used where it is clear to me that epoxy is far and away the best to use, given its bonding properties at various temperatures and its much greater resistance to water penetartion than alternatives. But this remains a complex area and I appreciate your views and thoughts (and hope I'm not transgressing by mentioning names in this forum but it serves to demonstrate the differences of opinion here from some respected parties). Best. RNB

[Ju52]

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