fiberglass hull thickness

Discussion in 'Boat Design' started by Gianf1041, Dec 18, 2019.

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Gianf1041Junior Member

Luckily you are there, I was starting to build "the house from the roof" ........ however I am working in the direction you are suggesting to me.
Thank you all.

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CacciatoreJunior Member

Ciao Gianf1041 pleae look Iso 12215-5 to have an overview about boat scantling.

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Gianf1041Junior Member

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fallguySenior Member

I ran some numbers from reading Gerr's book very quickly and without a great effort. Here are some numbers I came up with..

Assumptions:
L = 8.5M
Beam = 3M
Depth of hull = 3M ? pretty deep assumption
Speed = 10knots maximum
Planing hull = NO

Sn number = 2.7
FRP Calculations
Basic Shell = 8.84mm (6.35*Sn^1/3)
Bottom = 10.17mm (115%)
Keel region = 13.27mm (150%)
Upper topsides, deck, cabin = 7.52mm (85%)

Core 19.46mm (20mm) (2.2*basic shell)
Outer skin 3.54mm (40% of basic shell)
Inner skin 2.65mm (30% of basic shell)
Total skins 6.19mm

Probably some issues here with some of my assumptions. The depth of the hull might be an awful lot at 3M. A 2M depth changes things quite a bit and core would probably drop to 15mm

I really only spent about 30 minutes on the reading and the writing of the post here, so do be kind.

I must admit, I would have expected the inner skin to be thicker (intuitively for whatever my intuition is worth!). ~~~gelcoat perhaps?

Last edited: Feb 11, 2020
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TANSLSenior Member

In the calculation of the scantlings of the panels of a PRF hull it is necessary to verify the fulfillment of three things:
• stress distribution across the thickness of the laminate, so that the admissible sigma is not exceeded in any of the layers.
• minimum dry fiber mass of the laminate.
• dry fiber thickness.
The third study is the least important but it is the only one taken into account by approximate formulas that circulate around the amateur world. And it is not even known if the thickness values they give us refer to dry fiber or fiber plus resin.
This means that, in my opinion, the OP would do well, as he has allready been advised, to also consult the ISO standards applicable in his country.

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rxcompositeSenior Member

ISO and LR almost have the same formula for arriving at LAMINATE thickness based on the glass content, fiber weight of the laminate. It is accurate even if compared to "melted glass" and resin technique.

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TANSLSenior Member

It is not easy to know where the ISO formulas come from but it is easy to venture that they have been based on the existing CS Regulations. Some think that ISO is a copy of ABS Rules
I believe that a certain thickness can be achieved in many different ways. Therefore, the thickness itself does not serve as a guarantee of compliance. What must be ensured is that the fiber content is adequate and that the load on each layer does not exceed the maximum allowable value.

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Gianf1041Junior Member

I apologize if I was wrong in the approach to the problem of my idea.
I'm trying to translate what you write, probably the topics you deal with are very technical for me, .......... as I posted my experience is only of navigation, only now I'm taking a look at the design problem of a boat.

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Gianf1041Junior Member

Sorry, "depth of hull" ......... is this.

Attached Files:

• depth of hull.pdf
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fallguySenior Member

I know the definition, but not the precise number for your drawing. I was unable to determine it.

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CacciatoreJunior Member

3 m could be H+Hk ??

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Gianf1041Junior Member

Definition of "depth of hull" by Gerr.
Slowly, slowly the hull takes shape thanks to your suggestions.

Attached Files:

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• depth of hull-Dave Gerr.pdf
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374.1 KB
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Ilan VoyagerSenior Member

Mein Gott. What a thread...I see that a lot are discovering that naval engineering and structural design is not really for uneducated amateurs as that work needs knowledge and experience. It's very funny but I've never seen forums of amateur surgeons or amateur dentists, but there are plenty of guys guessing wildly the thickness of a GRP hull.
First you must have a complete first project relatively precise as the final scantlings will depend of numerous factors specially the choice of the type of reinforcement structure. For example, most of the furniture can become structural reinforcement. For a "slow" summer boat the scantlings can be pretty light as the stresses are finally very moderate. The period of sailing, the kind of navigation thus the elements you have to affront are determinant. You do not build a simple panga for coastal navigation in the Caribbean in light and fine weather like an Alaskan all weather fishing boat.
There is no need of wild guessing, nor divination. There plenty of examples of successful similar designs plus a full big bucket of national and international rules. And some are totally mandatory at least if you want to sell later the chef d'oeuvre, the master opera. Or at least to be capable to get an insurance for it.
The Gerr Book is a good start. It's very conservative -normal in the USA as you do not want to be ruined by some responsibility trial, so you won't be innovative-, truly really very conservative but for small boats in the heavy side it's fairly good and indecently simple tool to use. At the lone condition that you feed the formulas with the good ciphers as close as possible to the reality.
Beginners have a tendency to overbuild as they feel unsure and they have no idea of the real stresses and how to manage it.
So first draw a first project rather complete. Make an estimation of displacement, you can take the displacement of similar boats for a first calculation. Use the fixed furniture as the benches and the floors as structural elements. Decide the type of the structural elements, there several choices and where they are placed.
So, having all the measures, specially the size of the panels between supports, you can use the Gerr book easily with realistic results. No need of a 3/4'" foam, and even less a useless 1/4''. The reality must be somewhere with a 12 mm 90 kg/m3 foam with 3 to 5 mm skins if you use good quality glass, cloth, mat and eventually UD and a good polyester resin like the Reichhold Polylite 480.
A last suggestion. Making laminates is a hard, horribly smelling and itchy work. Do not make your life more miserable with cheap materials of unknown origin from Ebay or some Internet shadow shop.
Use a good resin that wets well like an isophtalic Reichhold. Use good quality glass, mat and cloth, not too heavy.
Use UD if possible, UDs are very easy to wet without air bubbles, and need less mat so you get a better ratio glass/resin.

Last edited: Feb 11, 2020

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TANSLSenior Member

Maine tante,@Gianf1041, so I will apologize for trying to complicate your life, when the thing is very simple: put in your hull a "conservative" thickness and when you try to get the project for the "CE" marking approved by the competent authority of your country, if they ask you for your justification, you say that "you have been very conservative."
Fortunately in Europe, even the amateur designer must project complying with certain standards and, as far as I know, there is no way around them. I repeat, fortunately.

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