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...

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.

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 !

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?

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 !