fiberglass hull thickness

[fallguy]

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.

the only reason I bothered to coin in is noone was pushing back on 11-12mm skins and a 7mm core

I agree the 20mm foam seems a bit much and I agree that this is amateur hour.

But noone else gave him a wild suggestion on skins n core.

I would never design a hull. But I recently got a tender plan and I intend to drop the numbers into Gerr to compare my plan with Gerr. If nothing else; it makes good reading and is a well written text.

 

[rxcomposite]

the only reason I bothered to coin in is noone was pushing back on 11-12mm skins and a 7mm core

I agree the 20mm foam seems a bit much and I agree that this is amateur hour.

But noone else gave him a wild suggestion on skins n core.

We will get to that. I agree this is very much an amateur hour.

It is already 7 pages long and we are still in the definition of Depth.

 

[Gianf1041]

In small steps without exaggerating but only to learn I hope to get there too ..........; I understand that many of you are patient professionals.

 

[gonzo]

To answer the question you started the thread with, give the approximate width, depth and general shape. On the other hand, if you are looking at designing a hull, start at the beginning of the process: state what the boat is expected to accomplish and how it should do it.

 

[TANSL]

In post # 11 the OP uses a language that seems quite professional.

let's say 7mm, + two sides respectively 6mm + 6mm x 8-10 layers of impregnated biaxial fabric with a thickness of about 20mm.

I believed that, from there, we could talk about and give him, as he demanded, technical support. I never thought we were in the phase of knowing how the dimensions of a ship are measured. I am somewhat confused, really.
He wonders about the weight, which is one of the most difficult and important issues to determine on a ship, before knowing how its dimensions are taken. Curious and amazing.

 

[Ad Hoc]

....It is already 7 pages long and we are still in the definition of Depth.

 

[Ilan Voyager]

I repeat, fortunately

Oh yes fortunately. You can get the European rules for small boats. Search for that, rather easy to find by Google. The Gerr book being very conservative will probably comply with the European rules but to check all the points. Evil is in the details. There are also rules of security detailing all the security material required following your category of navigation, even including the mandatory mooring equipment (don't forget it in the calculus of displacement). You'll have a lot of pages to read and understand.
You have been warned: NA and NE are professions with rather long and hard studies asking after the diploma obtained with a lot of sweat and headaches a few years of experience are needed even for small yachts. Rookies have hard times at the beginning specially in commercial boats.

 

[TANSL]

It is not that Gerr's book solutions are more or less conservative but that the current procedures for scantling the hulls with composite materials require a series of calculations that are not even mentioned in that book.

 

[rxcomposite]

Ok Gianf, forget about the Depth for a moment because that is complicated. Find the displacement of your boat. The formula is Length waterline x Breadth Waterline x Draught x Cb. Cb or Block Coefficient determines how "fat" your boat is. Barges and Bulk carrier will have about 0.85 while skinny and fast boats with elliptical hull or sharp V hulls will have 0.35 to 0.4. Yours is just a regular sailboat. Use 0.5 for the Cb and tell us what you get. We can adjust it later. At this point, we are interested only in underwater lines so we can figure out the pressures.

 

[Gianf1041]

Ok Gianf, forget about the Depth for a moment because that is complicated. Find the displacement of your boat. The formula is Length waterline x Breadth Waterline x Draught x Cb. Cb or Block Coefficient determines how "fat" your boat is. Barges and Bulk carrier will have about 0.85 while skinny and fast boats with elliptical hull or sharp V hulls will have 0.35 to 0.4. Yours is just a regular sailboat. Use 0.5 for the Cb and tell us what you get. We can adjust it later. At this point, we are interested only in underwater lines so we can figure out the pressures.

I apologize to everyone for complicating the problem, as an amateur I have tried to understand how to start a boat project; as I said in a previous post my experience is only of navigation so my design knowledge is limited, perhaps I have crossed over my border, ....... I hope to have translated correctly;
I have calculated the block coefficient by dividing the hull volume in m3 with the product of the three main dimensions: Lwl x Bwl x hull immersion. The result of the calculation is: Cb = 0.35

 

[Ilan Voyager]

It is not that Gerr's book solutions are more or less conservative but that the current procedures for scantling the hulls with composite materials require a series of calculations that are not even mentioned in that book.

The Gerr book permits to make very crude scantlings on some kinds of GRP hulls with such big security margins that the verification of "local" conditions is not necessary.
Scantlings on sandwiches, a fortiori advanced composites, were not even covered correctly by the A1 Lloyds, nor the Veritas. I remember that only the DNV had some rules resembling to a reality. I do not know how the classification societies cover the topic of advanced composites now as I have not made scantlings since years. Surely they have good books as the composites are very common now.
Composites can be specially tricky specially with the foams and delamination is the constant danger. Corrosion of honeycombs was another problem but nowadays nobody use aluminium honeycombs in marine use, the lesson has been learned.
But the miseries happened when nobody had rules of thumb and a solid empiric basis plus fine calculation methods (and powerful computers). That was the case at the beginning specially in racing boats. Even for a prototype of mine hunter 52m 550 tons the engineers of the French Navy suffered beginning of the 70ies. The ship was strong enough but suffered of a lack of rigidity which killed regularly the drive shafts, plus some local delaminations. Now there are no problem of lack of rigidity nor delamination on mine hunters or big composite boats.
Nowadays for a lot of types of boats there is enough data and cooking recipes for designing and building safely with no excessive weight. The technique is fully mature so lots of yards build composite boats as fishing boats until 50 meters long in sandwich and surely bigger for yachts.
But very careful engineering is needed when you go for boats like a 33 meters trimaran ocean sail racer weighting complete only 13 metric tons...

 

[rxcomposite]

I have calculated the block coefficient by dividing the hull volume in m3 with the product of the three main dimensions: Lwl x Bwl x hull immersion. The result of the calculation is: Cb = 0.35

Still does not tell us anything. What is the L, B, D you used?

 

[Gianf1041]

Still does not tell us anything. What is the L, B, D you used?

Sorry, but I don't understand the meaning of the translated question ............. = I calculated the block coefficient Cb and then calculate the displacement, as you say: "how fat the boat is" ..... .... the displacement calculated according to your formula is ...... Lwl x Bwl x D x Cb = mc 2,312 .......... the Cb = 0.35.
I also calculated the hull volume with the sum of the areas of the two corresponding sections; their average is multiplied by the distance between them example: area sect 2 + area sect 3 divided 2 x distance between the two sect = mc, adding the volume of all the sums I obtained the volume in mc = 2,350... then the displacement . ? I hope I have explained and translated correctly.

 

[rxcomposite]

Sorry, but I don't understand the meaning of the translated question ............. = I calculated the block coefficient Cb and then calculate the displacement, as you say: "how fat the boat is" ..... .... the displacement calculated according to your formula is ...... Lwl x Bwl x D x Cb = mc 2,312 .......... the Cb = 0.35.
I also calculated the hull volume with the sum of the areas of the two corresponding sections; their average is multiplied by the distance between them example: area sect 2 + area sect 3 divided 2 x distance between the two sect = mc, adding the volume of all the sums I obtained the volume in mc = 2,350... then the displacement . ? I hope I have explained and translated correctly.

What is the dimension(distance) of the Length waterline, Breadth waterline, and draught you used to get to Cb of 0.35? If I don't know the draught, I won't be able to calculate bottom pressure.

 

[Gianf1041]

What is the dimension(distance) of the Length waterline, Breadth waterline, and draught you used to get to Cb of 0.35? If I don't know the draught, I won't be able to calculate bottom pressure.

6.81 x 2.23 x 0.44