# Stringer laminate thickness per Gerr

Discussion in 'Boat Design' started by makobuilders, Aug 22, 2017.

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### makobuildersMember

I'm using Gerr Elements of Boat Strength as a guideline. Reference to the laminate thicknesses specified on stringers and engine beds, it is specifically referring to those that are formed over foam or balsa cores. Therefore the fiberglass needs the calculated thickness to support the loads.

However I cannot find in the book any reference to laminate thickness when applied over solid hardwood cores, which are a structural element themselves.

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### gonzoSenior Member

His method works well and is simple to use. Why are you changing the stringers to solid hardwood core? The beauty of Gerr's book is that you don't need any major engineering calculations. You can simply treat the stringers as beams and calculate equivalent values.

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### TANSLSenior Member

I do not think this comparison is very adequate (I do not say it is incorrect). Could you explain in more detail why this similarity ?.

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### gonzoSenior Member

Sure. Firstly, though, do you understand basic beam and truss theory?

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### TANSLSenior Member

You mean the theory of continuous beams, the theory of the three moments or what theory of the beams you mean?. As for "truss theory", the truth is that I do not know what that might be.
The first thing I must clarify is that when I speak of a beam I mean the transverse element that supports a deck.
But in addition to the theory (whatever the theory you are thinking about), you have to take into account how each element is loaded and, in general, the way of working of a beam is different from that of a stringer. It is very important, regardless of the theory you are talking about, to know how each element of the structure works and the charges to which it is subjected. Therefore, among other things, a beam is not a stringer and vice versa.

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### gonzoSenior Member

No, that is not beam theory. Beam theory is the area of engineering that studies beams. Trusses are another basic engineering structure.

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### TANSLSenior Member

It is possible that an area of engineering is based on the theory of beams but to say that "Beam theory is the area of engineering that studies beams", in my opinion, is totally incorrect.
Frankly, I do not think there is a "trusses theory" but I would be delighted if you would indicate where I can find information about it. It's never late to learn.

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### KailaniSenior Member

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### vkstratisNaval Architect

As a general rule of thumb you can assume 0.25mm for every 100 grams of CSM and 0.16mm of thickness for every 100 grams of roving. This is quite valid for laminates with 35% glass content. This is what ABS rules suggest. If you use the wood "core" as a structural element through you need to calculate the required scantlings (cross section dimensions) of the beam differently than those of a FRP beam. Best is to refer to ISO 12215 or class rules

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### TANSLSenior Member

Hi Kailani, thanks for the links, What I'm trying to explain to Gonzo is that a theory, whether Euler's or Bernouilli's, is not by itself an area of engineering, although there are areas of engineering that study them and use them in their calculation procedures. Do not confuse the whole with the parts.

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### makobuildersMember

The builder produces these vessels from older plans and the designer has passed away year ago. They rely on hardwood stringers as the structural elements which are covered with 2 layers of glass (about 1.4 mm thick) to encapsulate it. I suppose I was just researching to see if it would be of any great benefit to specify 3 or even 4 layers of laminate. I realize this is arbitrary in nature.

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