# fiberglass hull thickness

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

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### Gianf1041Junior Member

For the bottom up to the waterline which sequence of fabrics I recommend you apply starting from the gel coat to laminate the immersed part of the hull, I calculated a thickness of 18 mm animated with 7 mm foam core.

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### fallguySenior Member

That is intuitively too thick. How are you arriving at such high thickness and who makes a 7mm core?

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### Gianf1041Junior Member

Traditional simplified rules .......... "The elements of boat's strength" - David Gerr - ..... a boat scantling number is identified ....... simple formulas for immediate reading, example : the calculation of the average thickness of the hull plating, ........ 15% less than the average thickness in the upper part, 15% more than in the sides in the immersed part .......... ... formula: t = (3,28 x Loa) 0,5 + (Boa x 3,28 x 1,58) ....... t = average thickness in monolithic fiberglass. ?????

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### fallguySenior Member

You are proposing more glass than core!!!

No way is that right.

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### gonzoSenior Member

You can't start a design by determining a laminate schedule. Otherwise, there is no justification for any of the dimensions. The laminate is not the basic starting point, but rather one of the finishing points.

bajansailor and fallguy like this.
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### Gianf1041Junior Member

Sorry but, ...... I probably don't explain well with the English language; with the proposed formula I only calculate the average thickness of the hull regardless of the type of laminate .......... my idea is to evaluate whether it is more convenient to think about the bottom of the animated hull or just grp / resin fabric. I hope I have translated my idea correctly.

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### Gianf1041Junior Member

It is certainly not the starting point but it is one of the points for calculating the weight of the hull ........, if the hull is monolithic or with a soul.
animated laminate ............ the weights are different. ??

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### gonzoSenior Member

You can't calculate the total weight of the boat starting by the lamination. The lamination will be designed based on loads. The static loads will be determined by the interior appurtenances, machinery, tankage, etc. The dynamic loads are determined largely by speed and area of operation. Further, framing or lack of it also has a huge impact on the laminate schedule. It is not a language problem, but a method problem. Start at the beginning: What and where are you using the boat for?

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### Gianf1041Junior Member

This is what I calculated following Dave Gerr's suggestions; I'm not sure I have correctly interpreted what he wrote about the areas of the hull, ......... we accept advice, suggestions and graphic indications.

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### Gianf1041Junior Member

Sorry........................!!!!!

#### Attached Files:

• ###### upper:lower topsides extend.pdf
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### fallguySenior Member

You still have not explained the boat.

ocean or lake?
Horsepower?
Engine type?
Passengers?
Displacement?
Beam?
Length?

I am not a Gerr student, but I know the chicken and egg story here.

The use and vessel size defines the basic requirements.

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### Gianf1041Junior Member

As I have already indicated in my previous posts, ......... the boat that I have in mind is a small motorsailer of about 8.50 meters (a crew of four people) for navigation in the western Mediterranean, with a ratio SA / D minimum 9 (40% sail, 60% power) and a D / WL 180/250 average displacement ratio, an average Lwl / B ratio 2.5-3, diesel engine with sufficient power to reach the speed of the hull approximately 10 knots and reserve of strong wind power.

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### bajansailorMarine Surveyor

Re the attachment that you posted showing the body plan - the chines are very unfair, and hence the frames will be as well.
I think you also need to work on producing a properly faired lines plan (unless the apparent unfairness is a result of how it was printed?)

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### fallguySenior Member

I can tell you on a gut hunch you will need cores between 12-24mm. I am not a naval architect; but experienced enough to know core thicknesses used...I would revisit Gerr using foam cores of 12,15,20 mm and see what you get for scantlings and report is back here.

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### TANSLSenior Member

Gianf10041,
Here you have a scheme of laminates for a fishing vessel of 24 m in length, with the corresponding fiber weights. If you need something similar for a smaller ship, you just have to say it. But I want to warn you that the thicknesses without more, as you are trying to find out, will not help you at all. The thickness, I would almost say, is the last thing to consider. What must be taken care of is that the minimum dry fiber mass is adequate and, very importantly, that the layers of each laminate support the loads to which each one is subjected. And then, although once the above has been verified, it would not be necessary, check the thickness.
Probably all this will seem very complicated and you are right because the work with PRF is very complicated and the tricks used 40 years ago are no longer useful, nor are they allowed.

NOTE : T 1200 means "fabric" 1200 grames weight

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