Minimum Skin fibre mass requirements

Hi,

I am in the design of a 28 footer pleasure boat. I use the standard ISO 12215-5 (unfortunalty I only have the validation version) to calculate the laminate requirement.

I would like to use sandwich construction for the plating of the bottom hull and througth the optimisation process I found this laminate schedule (note that the boat top speed is 40 knots and I use polyester Resin)

- 1 Layer of Roving-Mat 2408
- Corecell 1/2 inch thick ( M100 )
- 1 Layer of Roving-Mat 2408

According to the standard, it is enough for my application. But I am nervous about the puncture resistant of that lamina. The standard gives this formula to calculate the minimum skin fibre mass requirements :

(Mass/m^2 of outer skin) = fw*k4*k5*k6*(0.1*lwl+0.15)

To simplify it is

(Mass/m^2 of outer skin) = 0.675 * (0.1*Lwl+0.15)

This give me a minimum mass requirement of the outer skin of about 450 GSM or about 14oz/sq.yard. My outer skin have a fibre mass of 32oz/sq.yard so according to the standard I am more than 2 times overdesign on that value.

Does anyone have an opinion on those values? Do you think it make sense?

 

I don't know about the standards or how to work the formula. Those layups might work for a shower stall, but I would be nervous about the double 14oz one.

You are designing a boat for yourself or something?

 

i would LOVE to hear the experts on this one

no way - its WAY TOO LIGHT
unless there are battens every 100mm = 4"

cant see it AT ALL

 

The 'puncture' resistance is totally different to the 'pressure' loadings.

The ISO, and indeed all prescriptive rules, can only give a generalisation of expected global and local loads.

If your design maybe subjected to puncture loads, how does the rules differential between a slamming load and a puncture load??...it doesn't, because it can't. To do so would require the rules to know what you boat is and what kind of puncture load.

That is where you, as the designer, must establish a puncture load, since over what area, and what load to apply?....so, prescriptive rules only work to give a generalisation of what is required. The operation of the vessel, its duty, that is down to the designer to ascertain.

So, if you think the resulting scantlings are "thin", prove it..by establishing a load scenario of a puncture. Then design to that. And this will supersede any other load, if greater than established by "rules".

 

I think you are mistaking the use of ISO.

The ISO is just the minimum legal requirement to be able to sell a boat on the EU market.

If you take a car analogy, all cars sold in europe do pass the legal safety requirements. But when these cars pass a user safety test, the user safety appreciation goes from nill to 5 stars.

So, if you want users rate your boat 5 stars, you should use other rules.

 

I've worked out similar sized boats using Dave Gerr's method ( Elements Of Boat Strengths) and by a couple other methods and they always come out lighter than I'm comfortable with. You didn't define the unsupported square ( space between transverse and longitudinal members) and I haven't worked with ISO 12215-5 but that laminate sounds thinner than any method that I have used would suggest and most certainly much thinner than I would consider.
Gerald

 

Although it does not apply to power boats, you should read this:

http://www.icomia.com/technical-info/document.asp?TI_ID=31

Page 36 paragraphe B5

"Since the aim of this standard is to promote safe structural arrangements for this critical component of a sailing craft, users should not attempt to aim for minimal compliance and should always make reference to previous experience"

This give you a hint on how ISO should be used.

 

I am surprised of people saying it is too light when they do not know the loaded weight of the boat nor the size of the panel.

 

We build a 29 foot yacht and have a lighter outside skin than what you are proposing. Our outside skin is one layer of 1.5 oz mat with a ply of 18 oz knit e-glass against the core. This is infused so we do not use an 1808. We have absolutely no problem with the strength, stiffness or impact strength for this boat. In fact, the structural strength and stiffness of this boat is very high. It feels very solid sailing upwind in big wind and big waves. My opinion is that at a 2408 is over kill for a 28 foot sailing yacht.

 

You do not use mat because it is infused, or because you use epoxy resin :?:

Btw, 2408 woven roving + mat is not biaxial (woven, not knitted)

What is strange, is that given the high price of corecell , there is very little financial gain to use polyester + roving instead of epoxy + biaxial w/o mat.

 

We don't use the mat in the plies of our infused laminates because our testing showed the mat adding very little in strenght and stiffness, but did add weight and cost to the laminate. We infuse with vinylester and use balsa core. Our testing also shows that in an infused laminate, balsa has lowest resin uptake for any of the contourable cores. While it is possible to thermo-form a PVC or SAN core, and thus reduce the resin uptake in the core, thermo-forming is not cost efficient in our production boats.

 

What is sort of surprising is folks wanting answers to such technical questions without supplying any crucial information, such as shape, structure, loaded weight or weight of load, how or where it's to be used.

He says it's a pleasure boat, so I assume it's not for ultralight competition. So it gets loaded for pleasure and some creature comforts. That would seem to be average loaded weight or a little less.

Mainly though, it's a 28 foot power boat with a top speed of 40 knots/46 mph/74 kph. Loaded weight possibly becomes secondary to operating loads such as slamming into waves, etc.

But, like I say, I don't understand the math.

 

Well, he's not building a sailing yacht, the structural loads must be different. The layup you describe is probably stiff, but structural strength doesn't rely only on stiffness. A lightbulb is pretty stiff, up to a point. I wonder what a point load would do for your layup. A direct hit against a sharp rock or dock corner seems like it would easily puncture a hole in such a light layup that has no flexibility, allowing water into the balsa core. I assume the whole hull is balsa cored, even below the waterline.? Aside from a direct puncture, a long scrape along a sharp object seems like it could easily go completely through or at the least weaken the integrity of the outer skin, which would weaken the integrity of the whole cored system in that area. But once again, I'm not an engineer and I don't understand the math.

What brand of boats do you produce?

 

More specifications

Thks for all those answers but it more confuses me than it helps me...

First, this is the first boat I design so I can't refer to any experience at all so I don't really have the choice to refer to a standard.

I think the idea of doing my own standard for the puncture resistance is a great idea but as I design it for a company I don't really have the time to spend weeks on a valid model. If someone have a great idea on how to do it fast... it would be very apreciate. Then I think the best way will be to do destructive test on sample when the lamina will be chosen.

Some people ask for me precision of the situation. Here it is!

The maximum dimension of a totaly suported panel is : 875 mm x 660 mm
The shape is totally flat for the bottom panel.
The full loaded weight of the boat is : 3400 kg
It is design for category C meaning significant wave heights up to 2m and a typical steady wind force of Beaufort Force 6 or less (an inland boat... not offshore.)
The hull is vacuum bagged.

I don't understand why Greybarn say he doesn't use 1808 because it is infuse. What is the problem of infusing 1808?

If I don't go with sandwich lamina I obtain this lamination schedule with the ISO standard :

- Roving 18 oz
- Mat-roving 1808
- Mat 2 oz
- Mat 2 oz
- Bulking material 2mm
- Mat 2 oz
- Mat 2 oz
- Mat-roving 1808
- Roving 18 oz

This lamination needs very more labour works than the sandwich laminae proposed before. As we oriented our conception on production i'm not sure this a great choice.

What do you guys think about that?

 

Oh I forgot,

The pressure on the panel is about 36 KN / m^2

Thks