Minimum Skin fibre mass requirements

[fcfc]

Originally Posted by JimmyJames

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]

Originally Posted by 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?

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]

Originally Posted by JimmyJames

(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.