Shocked by hull material catamaran

[Just Another Sa]

Originally Posted by Stumble

There could be, but it would be a weird design. Panel stiffness is dictated by the loads and by the distance between frames and bulkheads. So the wider the spacing of the internal framing the thicker the panel needs to be. It is not impossible to imagine designing a boat with no internal support structure that needed 1" thick panels to get sufficient stiffness. It would be counter to all modern design, but it's possible.


What i think gets lost here is that skin thickness is all about strength. It does nothing else for a boat in normal conditions. And frankly boats don't need much strength. They just aren't loaded much that way and fiberglass is really incredibly strong. What boats generally need, is stiffness, and stiff and strong are very differently things.

Dyneema rope has bubkis for stiffness, but a huge amount of strength. They are not the same thing at all.


Fiberglass is actually pretty amazing stuff. An epoxy and S glass laminate is actually stronger (tensile strength) than epoxy and carbon fiber by about 50%. But nothing, or almost nothing in a hull is ever placed in tension. It is almost always stiffness that matters for a boat. So why lug around thousands of pounds of fiberglass laminate that adds nothing to the failure mode of the boat?

Even in this damage, the strength of fiberglass was never at issue, it was the abrasion resistance of it. And frankly that's a whole different kettle of fish.


The reality is that for most boats the required stiffness is a couple of orders of magnitude more important than its strength. And the best way to get the required stiffness is to add thickness. Within reason it doesn't even matter what adds that thickness, more roving fiberglass, a core, CSM, carbon fiber, concrete, etc. none of it matters, the important question is how thick is it.

So why is carbon fiber so desirable when it is weaker than fiberglass? Because it weighs about 1/3 less and has three times the stiffness than S-glass. But if you just needed strength you would use glass every time.

Note -all comparisons are general in nature I took some liberty in the rounding.

There is practically speaking never any need for 25 mm thick skins for boat hull panels optimized for bending stiffness / weight ratio.
If the density of fiberglass skin is 1800 kg/m^3, and that of the core is 80 kg/m^3, the optimum panel thickness is 89 times that for the skin.
1 inch skins would result if the core is 87 inches thick to replace 18 inch thick solid laminate with the same stiffness. Those kind of panels are not found in any vessel I would call a boat instead of a ship.
Hence panels with 1 inch skins are not optimized for minimum weight for a given bending stiffness.

Further more I don't know why you bring properties of S-glass into this, as it is seldom used in cruising boats if ever. It is used in some extremely fast powerboats. E-glass is the fiber most fiberglass is made of.

You wrote: "An epoxy and S glass laminate is actually stronger (tensile strength) than epoxy and carbon fiber by about 50%."

Properties: S-Glass Fibre
Tensile Strength for S-glass is between 4700-4800 MPa.
http://www.toraycfa.com/pdfs/T1000GDataSheet.pdf
Tensile Strength for that particular grade of carbon fibre is 6,370 MPa.
In that comparison S-glass is not 50% stronger, but weaker.
Of course that is the strongest carbon I'm aware of, and the differencies between different carbon fibre grades are substantial. Such 50% statement is therefore not useful for any purpose and depend a lot on the brand/grade of epoxy and curing cycle as well. Most epoxy breaks with less stretch than S-glass, limiting the laminate properties in tension.

[jimbunyard]

Originally Posted by Dockhead

What money Groupe Beneteau save by building cheap, they mostly pass on to the buyers. Buyers have a voracious desire to have bigger and bigger boats for less and less money -- GB are only giving people what they want.


I admire them for building so efficiently -- every boat they make, is a lot of boat for the money.


But they do seem to go too far sometimes, especially what concerns structure. The problem here is not the balsa core, but the very thin GRP layup. This is doubly tempting to cat builders because weight is such an issue, but I would not want any boat with such a thin outer skin. Yikes.

Well...as somewhat usual for me, coming in late at night will allow me a certain--looseness--of tongue...

Certainly GB can offer a 'lot of boat for the money' as long as one knows what that phrase means, and (haha) qualifies it accordingly...

If by 'a lot', you mean creature comforts and 'status quo' accoutrements designed to sell boats to a specific group of (possibly) undiscerning individuals uninitiated in the vagaries, whims and uncertainties associated with unsupported open ocean travel, well, you're right.

But---given that a large percentage of these 'adequate' boats will never see any, or very little, extreme conditions (and given the proven shortcomings of these 'mass production' techniques), you may be right...if whatever thousandth of a percentage of the population is exposed to the uncertain exhilarations of sailing, even if only Wednesday night club racing, rather than 'X' (insert whatever your current cultural abomination at this point is), well it might be postulated that GB has a positive effect.

But, again, the idea that a market is designed to be exploited is false on it's face and in the very idea of a market; a market only exists if people can pay. If an 'industry' invents a market by lowering the standard to enable the participation of those who really can't afford to participate normally, then whose responsibility are the casualties (in whatever form) incurred by this lowering of standards?

Without going (at this juncture) into the finer points that need (again) to be made in this discussion, I append the video below, of a very brief description of the production of a larger Hatteras yacht. At 1:37 in the video, have a look at what the producer of the video (and by association, Hatteras), is extolling as a virtue. For anyone who has any experience in strength of materials, red flags should be going off at that example. Precisely where one wants the strongest layup, the chine, with an at least reasonable amount of fiber to resin ratio, what do we have? Pure resin with poorly fitted pvc foam filler blocks. And, again, this is held up as an example of a higher standard!

https://www.youtube.com/watch?v=Xp7SfPQYvBA

[Just Another Sa]

For those who are actually interested in sandwich panel optimization, this link will contain the required math:
https://en.wikipedia.org/wiki/Sandwich_theory
For the best stiffness for a given weight, the core weight = 2/3 times panel weight, and both skins are 1/6 of panel weight. Said panel weight should not include weight of the glue film between the core and skin laminates. For example halve of a millimeter thick skins with 44 mm thick core. In reality I have never seen any hull panels dimensioned like that, all skins have been heavier and core thinner.
In reality panels are dimensioned as a compromise between bending strength of panel and point loading properties requiring somewhat thicker skins. Core density selected for required compressive properties for foams.

To replace solid laminate with a cored panel 2 times as thick and with the same weight results 6,85 (not 12) times the stiffness with 80 kg/m^3 foam and 1800 kg/m^3 laminates.
4 times as thick as a solid laminate results 33 (not 48) times stiffer with the same weight.
Both calculated using the math in the link provided above.

[Just Another Sa]

Originally Posted by robert sailor

I can certainly see that build mindset on a racing cat as weight is the enemy but a slow heavy condo cat it doesn't make sense other than as a money saver.

So how heavy would you want it to be, if money saving would not be an issue?
So heavy the bridgedeck clearance is negative and the boat becomes a monohull?
Wouldn't it make more sense to buy a monohull instead of a cat, if that is the requirement for normal people to whom cost does indeed make a difference.

[Just Another Sa]

Originally Posted by Dockhead

The problem here is not the balsa core, but the very thin GRP layup. This is doubly tempting to cat builders because weight is such an issue, but I would not want any boat with such a thin outer skin. Yikes.

So how thick would you want the skin laminate to be?
How heavy, slow and costly would you expect a cat could be and still sell?

[jimbunyard]

And, by the way, the 'keel' sections of the hull in the boat in question, as well as a certain percentage of the lowest part of the boat, are not cored, as seen in the red-encircled areas shown in the attached pictures. That is, of course, not to say that the thickness of the skin, in either the cored or uncored portion, is in any way adequate for the application...

[Just Another Sa]

Originally Posted by minaret

You guys are all hung up on sailboats. I work on large power boats on a regular basis which have scantlings which far exceed the numbers we are discussing here. 1" core skins are not so rare at all in that world. I have after all posted pics here of a 64' Nordhavn bottom with solid glass over 4" thick. Took me 126 plies of laminate to rebuild.

So you are saying powerboats require heavier hull panels than commercial freighter or naval ships using typically 10 mm or 3/8 inch thick steel?
4" = 101.6 mm thick solid fiberglass weights the almost the same as 15/16 inch steel.

[Just Another Sa]

Originally Posted by jimbunyard

And, by the way, the 'keel' sections of the hull in the boat in question, as well as a certain percentage of the lowest part of the boat, are not cored, as seen in the red-encircled areas shown in the attached pictures. That is, of course, not to say that the thickness of the skin, in either the cored or uncored portion, is in any way adequate for the application...

There is clearly cores shown in your red-encircled areas in pics 2 and 3.
There is still laminate on it in pic 1 preventing it from showing.
There is no evidence in pics you showed that there is any solid laminate used in the hulls (not the minikeels).

[Just Another Sa]

Originally Posted by minaret

In other words, if you need a 1" external skin with a 3/4" internal skin to meet design strength requirements, but 1 3/4" solid panel has sufficient stiffness, what does the core do for you but complicate construction and introduce more failure modes?

If a 1" external skin with a 3/4" internal skin cored laminate is required to meet design strength requirements, 1 3/4" solid panel will not meet the same requirements for strength.
4" ... 8" solid laminate is more likely for the strength requirements, depending on the core thickness of the other choice.

[jimbunyard]

Originally Posted by Just Another Sa

There is clearly cores shown in your red-encircled areas in pics 2 and 3.
There is still laminate on it in pic 1 preventing it from showing.
There is no evidence in pics you showed that there is any solid laminate used in the hulls (not the minikeels).

You don't understand what you're looking at then...

[GordMay]

Originally Posted by 44'cruisingcat

But is 80 mm enough? Why not 800mm? Who gets to decide how much is enough? ...

Engineers & NA's?

[a64pilot]

Originally Posted by GordMay

Engineers & NA's?

Hopefully yes, although from my experience in Business and manufacturing it's often the greed of the owner / board and accounting that makes the determination

[nortonscove]

Originally Posted by weavis

Improving the structural beefiness: The problem is that whilst the better materials are cheap, the labour cost to do it is usually more because of techniques involved....otherwise weight vs cost vs labour would favour the better materials involved. In the end, it always seems the cheapest method wins out... and they can charge a premium for it that appears a better deal than a better made product..

I have learned through your boat building expedition that a custom designed vessel made to a general design gives the customer EXACTLY what he wants. Im in favour of this as the customer specifies what is important to him.

Yes but the owner dictating can easily fall victim to lack of foresight or idiocy. This can be found to have run awoke in the fishing industry here in Newfoundland. You end up where designers and builders shoulder the blame for a vessel that doesn't work or can't be sold due to its tunnel vision design.

[valhalla360]

Originally Posted by a64pilot

Hopefully yes, although from my experience in Business and manufacturing it's often the greed of the owner / board and accounting that makes the determination

Makes for good movies but in reality, the engineers almost always make the determination because they are the ones held responsible if something happens.

That doesn't mean the engineers have free reign to build a $10million dollar 30' boat.

Physicists work in the world of "exact" and "truth". Engineers work in the real world and know the costs and trade offs to their choices is a critical part of their job. If you've ever taken the PE exam for licensing, there is a section on engineering economics.

If the design brief included ice breaking, I would agree it was insufficient but that's not what this boat was designed for.

[valhalla360]

Originally Posted by senormechanico

I'm old enough to remember back in the early '70's when crash bumpers began to appear on cars.
Al Gore tried to pass legislation to require cars to enable passengers to survive a 70 mph crash into an immovable concrete wall.
It would have caused cars to be massive and have fuel mileage in the low single digits.
Thankfully, the '73 oil embargo nixed that stupid idea !

Good example.

I've messed with people telling them I could eliminate better than 95% of serious injury car crashes...and I'm telling the honest truth. When they bite, I tell them the solution...add a governor to all cars limiting the top speed to 10mph.

Go out to an empty parking lot some time an try rolling a car without exceeding 10mph. Heck, take a buddy and try a head on crash and odds are good you both walk away uninjured.

The problem is a 10mph cap on speed comes at a cost.

It's the same thing here. You can build a 3" thick solid fiberglass hull but it will be heavy, slow and have reduced cargo capacity (discussion of 100'+ mega yachts work similarly but you just confuse the non-technical observer talking about 25m skins)