# Hull thickness with corecell

Discussion in 'Fiberglass and Composite Boat Building' started by Jussi_2, Jan 12, 2014.

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### Jussi_2Junior Member

Hi! How thick laminate skin U are using over corecell in boat hulls? I dont know how much hitting energy the vawes gives, so I don't know how to calculate it.. ofcourse it depends the structure of the hull, how many supports there is, but average? I was suprised that airplanes rear wing has 0.6mm skin. Also one professional composite person was thinking that if its skinned with carbon fiber, he believed that it could be under 1mm thick per side.. But it sounds so incredible thin.. Example for 10 meter long catamaran hulls....

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### rxcompositeSenior Member

There is no comparison of pressure between a boat hull and an airplane wing. Water is denser than air and boats are measured in tons, not in pounds.

If you mean by rear wing, you mean the horizontal stabilizer? Federal Aviation Rules part 23 for a light aircraft requires that it must carry a load of 38 psf. So for a tail area of 24" x 68", sans the equation, it will require a skin thickness of 0.007" of fiberglass skin for a maximum thickness core of 1.33". If a high tensile carbon fiber is used for the skin, it will be less.

Not so for boats.

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### Jussi_2Junior Member

Do U have any idea about the skin thickness for boat hull?

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### rxcompositeSenior Member

For a 16 footer boat we are currently discussing on another thread, it needs a minimum of 0.0.065" (1.6 mm.) outer skin thickness for a 2'x 3.5' panel minimum with 3/4" (20mm) core. and that is for a 2 person boat. Boat regulations requires that it be 0.20" (5.4 mm.) thick, min.

For a 10m cat hull, more info is required like breadth of one hull, subdivisions, displacement, speed, area of operation.

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### Eric SponbergSenior Member

Jussi_2, there is no correct answer here. The thickness of the fiberglass skin will vary from boat to boat, design to design. There are no good averages. The bigger the boat and the faster it goes, the thicker will be both the skins and the core of the hull laminates. The specifications are usually determined by engineering of the laminate.

If you would like to at least get a grasp of what is involved, you should pick up a copy of Dave Gerr's book, "Elements of Boat Strength." That will get you to a basic understanding of engineering boat structures in composites (a well as other materials). For more detailed composites information, I recommend that you download the "Design Guide for Marine Applications of Composites" which you can get here:

http://www.shipstructure.org/pdf/403.pdf

Ultimately, if you have a good understanding of boat engineering, you can use the ISO 12215-5 structural standard which you can purchase from the ISO standards organization:

International Standards Organization (ISO) www.iso.org
Scroll to bottom, under ISO Store,
Click on: Publications and e-products
New Window: In search window type “12215”
Individual standards appear, ready to purchase on-line with credit card. It costs 210 Swiss Francs (if prices have not gone up).

The ISO standard is very complicated for the non-engineer. For composites structures, it is even more complicated than the ABS Classifiation Society structural standards for composites. However, if you purchase the standard, it is also possible to download The Self-Certifier spreadsheet program that is available and actually fairly easy to use. With proof of purchase of the ISO-12215-5 standard, you can order the Self-Certifier program from IMCI in Belgium:

http://www.imci.org
Click on “Boats and Components”
In left margin, click on IMCI Software
Open “this application”
Fill out the application, and email it with proof of purchase to Uli Heinemann at info@imci.org
Proof of purchase is receipt from ISO or ANSI and cover page of your downloaded version of the 12215-5.
Uli Heinemann will send you a NAVIGATION KEY which will get you started using the spreadsheet program.

I hope that helps.

Eric

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### rxcompositeSenior Member

That (Eric's) would have been my next reply. I guess Eric summed it up and lead you into the water.

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### Jussi_2Junior Member

Thanks Eric! I will check these books. I have just read: Gregor Tarjan: Catamarans. The complete guide for cruising sailors.

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### Jussi_2Junior Member

I was at boat fares yesterday, and talked with many boat seller.. I asked about the structure of the hulls, and many of them told me that they are using just thick fiberglass under the waterline.. no sandwich structure.. Big motor boats as 10-12meter long, they told there is about 12mm thickness.. Can that be a possible? Sellers not always know the technical informations so good, so i was just starting to think that did they just throw something from the hat.. or can it really be just 12mm thick fiberglass.. Sunseeker manhattan 55 were build with using carbonfiber tubes as a frame, but the seller didn't know the thickness of the hull.

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### Eric SponbergSenior Member

Jussi_2, you probably heard correctly. Many boats, power and sail, use solid fiberglass in the bottom without any core. The reason is to be very sure that water migration, should it ever occur through the laminate (polyester laminates are slightly porous), does not get to any core, which can be really bad structurally. So they don't take any chances.

Also, the bottoms of boats are usually heavily built with longitudinal and transverse framing that has to be there anyway to support the bottom panels as well as the engines, equipment, and interior structure. The whole bottom structure is easier to build and makes for stronger structural details without the core in the bottom.

Finally, boats have through-hull fittings for various reasons, some more than others, and not always in the same places from boat to boat if the boats are semi-custom or custom, with features tailored to the owner. So putting through-hull fittings through solid laminate is easier than through cored laminate, where the core has to be specially milled and treated at each through-hull opening. In a semi-custom or custom situation, it is difficult to pre-plan, sometimes, where all the through-hull fittings are going to go. This is often done after the hull is already molded. If the bottom laminate is solid, this job of placing and installing through-hull fittings is a lot eaiser than if the bottom is cored.

It is not unusual to see bottom panel thicknesses on the order or 12 mm thick or greater. Depending on the structure, I have seen many bottom panel thicknesses much greater than that.

I hope that helps.

Eric

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