Minimum Skin fibre mass requirements

[JimmyJames]

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?

[SamSam]

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?

[Manie B]

i would LOVE to hear the experts on this one

28 footer pleasure boat
boat top speed is 40 knots
polyester Resin
- 1 Layer of Roving-Mat 2408
- Corecell 1/2 inch thick ( M100 )
- 1 Layer of Roving-Mat 2408


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

cant see it AT ALL

[Ad Hoc]

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".

[fcfc]

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.

[Commuter Boats]

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

[fcfc]

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.

[fcfc]

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.

[Greybarn]

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.

[fcfc]

... 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. ...

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)


... I use polyester Resin)

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



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.

[Greybarn]

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.

[SamSam]

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.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.

[SamSam]

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.
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?

[JimmyJames]

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?

[JimmyJames]

Oh I forgot,

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

Thks

[fcfc]

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



I do not think there is a problem. He just said that with vinylester resin and cored, the 08 mat layer is useless, just soak resin and add weight, but do not add strengh.

With a lower bonding characteristic resin, the mat may be mandatory, because it help bonding layer to core.

Have you checked the price of corecell sheets, the man hour to put the flat corecell sheets in the mold, and cost of the quality control to insure the skin between the mold and the core is correctly bonded on the core ?

[fcfc]

BTW, if you are using roving directly in you mold with polyester resin, (as in your monolithic layup) you should read something called print through.

[terhohalme]

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?

No, it does not. I think, you probably didn't check the second moment and the section modulus of the panel. You also calculated the absolute minimum fiber weight, not the needed thickness. Needed glass weight to the panel skins is about 1 kg/m2 (0/90 MX) and needed core thickness is more than 12.5 mm. I would ask you to study the standard thoroughly.

[JimmyJames]

After reviewing my calculus I obtain this laminate schedule :

- 1 Layer mat-roving 1808
- 1 Layer of mat 1.5 oz
- 1 Corecell M100 1/2 po
- 1 Layer of mat 1.5 oz
- 1 Layer mat-roving 1808

This lamina have a second moment of inertia 1.75 times over the standard. It also have a section modulus 1.14 times over the standard (both direction). The stress in each ply is always under the standard. Those security factor are in addition to the security factors included in the standard.

I think the 1808 would eliminate the print-through problem (am I right?) as the mat layer will face the mold. The 1.5oz mat layer would also make a good adhesion layer the core.

But after all, I still have the same problem : the outer and inner skin of the sandwich laminae is only 1.36 mm thick. That seems very thin in case of an impact...

Does anyone have a solution to that problem? (Sorry Terhohalme but I didn't understand what do you mean by calculating the needed thickness instead of the minimum mass requirement)

[Greybarn]

If this is hand lay up in a female mold, then our standard (based on the previous lam sched) would be Gelcoat, 1.5 oz mat (let this cure), outside structural layer(s), core vacuum bagged to outer skin with corebond, inside structural layer(s). Putting the mat on the to surface against the core helps to eliminate some of the rough edges from the hand lay up process.

[JimmyJames]

We plan to vacuum bagged (infusion) the whole laminate in only one operation. That's why I'm trying to lower the number of skin that have to be dry laminated.

I'm not sure why you laminate 1 layer of 1.5 oz mat and let it dry before laminating the structural layer. I thougth one goal of the vaccuum bagging process was to reduce the amount of void in the laminate then reducing the chance of osmosis and giving a stronger laminate (due to resin/glass ratio). By laminating this first layer don't you raise the chance of osmosis problem?

[terhohalme]

(Sorry Terhohalme but I didn't understand what do you mean by calculating the needed thickness instead of the minimum mass requirement)

Minimum fibre mass is determined in 10.5.6. Other requirements (like skin thickness) are from 10.5.1 to 10.5.5 and sandwich calculations in annex D.2.

Please, use SI system with ISO standards.

[War Whoop]

To stay out of trouble and avoid any potential Buckling use 1708 then 1808 ,Corebond 5/8” Corecell and 1808 then1708 inside laminate.

[JimmyJames]

Ah ok Terhohalme! Now I understand! I only have the validation version of the 12215-5 standard and this version stops at section 9. I will order the official version this week! Where is the cheapest website to purchase it. We are not a big boatbuilding company yet so we try to keep the budget as low as possible.

Which section do you think would be necessary for a powerboat?

[Commuter Boats]

If you're going to purchase additional technical support, I would suggest Dave Gerr's " Elements Of Boat Strengths" as being very easy-to-use and conservative. It's generally accepted that Gerr's method will develop a sound structure although a lighter structure is possible.