# Density of fiberglass used in boat construction?

Discussion in 'Fiberglass and Composite Boat Building' started by a25004, Apr 3, 2013.

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### a25004Junior Member

Hello all!
I'm currently working on an engineering project for my university.

My team want's to build a "catamaran" style boat that collects garbage.

I have some questions, and I hope you can help, I think I came to the right place!

The problem:

It's going to be fairly large, aprox: 4m x 3m x1m
For weight calculations, and to be on the safe side, let's consider it "cube" like.
If you consider 1cm thick on each plaque, it results on a total of 0.76m3 volume.

The first material that was instantly brought up was FiberGlass.

1st question:
what should we use?
Fiberglass cloth (multiple layers) + Resine?
Polyurethane + Fiberglass?

2nd question:
What is the density of fiberglass cloth+ resine?
I have came across a large spectrum, from 64 kg/m3 to 1800kg/m3, and as you know, that is going to give different values, it is either extra light or extra heavy...

3rd question:
I mentioned a 1cm thick.
Is that enough, or is it too little?
For a 64kg/m3 density, 2cm or more we can manage, because it's going to give approx: 48kg.

We wan't to calculate the expected weight of the boat, so we can calculate the motors and batteries.
But we don't know the density of fiberglass/type of fiberglass to use.

Project on SolidWorks:

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### tomasSenior Member

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### a25004Junior Member

Yes, that link is correct, thanks!
Concerning the fiberglass, any suggestions?

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### TANSLSenior Member

My opinion, but surely there will be many different opinions and many of them will be correct:
1st question: Fiberglass cloth (multiple layers) + Resin (don`t forget gel coat)
2nd question: let's say 500 kg/m3 for fiberglass+ resin (which may be the same and can be more if the job is not well done)
3rd question: about 7 mm thick (probably less)

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### gonzoSenior Member

Laminates are engineered for their application. The density would vary considerably with different fabrics and resins used. The methods used-hand laid, infused, vacuum bagged, etc.- also have a large influence on the final density. The shape of the hull and the type and size of reinforcements also influence the laminates. The size and thicknesses, called scantlings on marine design, are determined by all aspects of the vessel. If you are going to build something that looks like a cube or any prismatic shape, sheet metal or plywood is a better choice.

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### dinoaSenior Member

Glass-polyester composite density can range from 1.2 to 2.0 depending on glass resin ratio, weave and polyester brand. A 70% glass resin ratio of glass tow will top out at 2.0 and a light polyester resin with no glass at 1.2.

Dino

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### TANSLSenior Member

Laminates are engineered for their application. I don´t know what this means.
The density would vary considerably with different fabrics and resins used. True
The methods used-hand laid, infused, vacuum bagged, etc.- also have a large influence on the final density. Infused and vacuum … use to be the same method. Infusión without vacuum is very dificult. More than the method, which influences is the quantity of resin is used which, effectively, with some methods may be lower than with others.
The shape of the hull and the type and size of reinforcements also influence the laminates. Not at all. Influence of course, the separation between reinforcements not its type. Your "ship", probably will not need stringers.
The size and thicknesses, called scantlings on marine design, are determined by all aspects of the vessel. Scantlings are much more things that just thicknesses and not all aspects of the ship, far from that, are influencing them
If you are going to build something that looks like a cube or any prismatic shape, sheet metal or plywood is a better choice. Absolutly No. Please use the material you like better.

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### a25004Junior Member

Wow! Thanks so much for all the answers!

My team is aiming at the lowest weight possible.

By our calculations (solar panel, batteries, motors, etc...)
It's weighing 350kg, with some safety factors.

I will keep this thread updated in case anyone's interested.

Thank you all once again

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### gonzoSenior Member

TANSL:I understand from this and other posts that you struggle with English and marine design. However, please don't confuse people. Scantlings are the dimensions of parts; that is the definition of the word. Also, using the material "you like better" works for interior design, but is terrible engineering.

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### FMSSenior Member

A naval architect will specify the type, weight, and sequence of the FRP laminate schedule as part of the design. The required strength/stiffness/characteristics of each element are determined by dimensions, speed, operating requirements, any weight requirements imposed by the SOR, and budget.
The SOR in the first post
eliminates concern for engineering a lighter weight laminate, such as cored or exotic carbon laminates. The word "large" combined with 4m is confusing to me for a garbage collection vessel. Answering question 1 requires other questions such as the vessel's lifecycle, type of garbage, hauling capacity, and more to be answered.

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### a25004Junior Member

Sorry for not specifying.

4x3x1m is large comparing to the other "teams" in the university.
(They have vessels with 1x0.5x0.3 for example.)

The garbage is going to be collected in a 4x3x0.3m cage (aluminum casing with nylon net)

This project is "paper" only, it's never going to be built because we lack the funds.

We just want to know the density, etc... to have values for project purposes only.

Thank you

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### gonzoSenior Member

Is this something that will be towed between two sponsons? The rules and limitations of the design requirements will determine most of the answers to your questions. One of the most important things in engineering, is that you have to give sufficient, relevant information to get a usable answer.

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### a25004Junior Member

https://www.dropbox.com/s/5xi0v9v97aapc1p/tcharam.png

We thought of that design, but we don't know anything about boat construction.
Can't this be done with fiberglass + resin applied over a one piece mould?

Also, this won't be built, it's a paper only project!
So, we just need to know the materials, density, etc... to calculate the boat weight to calculate the batteries and motors to be used.
It's not required for us to know how it's going to be made (Fabrication processes, etc...)

Given the dimensions and general design, can I get an estimated thickness+density to calculate the overall estimated weight?

Also:
vessel's lifecycle:
We want this to have a long life, 10years +?
Type of garbage: the kind of garbage you see on shores, like PET water bottles, coke cans, plastic bottle caps, etc...
Hauling capacity: aprox 2400L
We estimated the full garbage weight at apron 80kg.

This is a great forum, it's full of expert people
I apologize for not having as much knowledge...
I want to know if this is do-able on paper.

Thank you once again!

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### TANSLSenior Member

a25004, I have no doubt that our discussions on the meaning of "to engineering laminates" does not interest you at all. To make up for the patience you had with us, I have engineered laminates (that sounds so wrong) for your boat and I got the results I attached.
I had to assume several data not indicated by you, but I think it may be a good guide for your calculations. I placed, to lower somewhat the thickness, a frame every 1000 mm and, on the sides, two horizontal stringers, separated 350 mm.
Thereby obtain a thickness of 6.527 mm dry fiber with a weight of 10.318 kg/m2. That should add a similar amount for the weight of the resin.
I hope I have helped. If there are any mistakes, I hope you know excuse my clumsiness in "engineering laminates"

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### a25004Junior Member

Thank you so much for your help!

What kind of information do you need for a more complete answer?

I expected that giving an approximated volume, and having fiberglass+resin density was enough to calculate the weight, but it seems it isn't that simple

Density is given by = weight / volume

If we know the thickness and the density, we know the overall weight.

I think fiberglass cloth + resin has a lower density than fiberglass laminates, or is it the same thing in the end?

What do you think it would be the best?

Do you think we can get a max weight of 100kg with these dimentions(they are over dimentioned, to have safety factors) and that thickness?

So, in the end, we have three variables: weight, density and thickness.

I don't know if I am being to "bold".

I never had contact with boat construction, and after some research, fiberglass is proven to be somewhat resistant, so not much thickness is needed.
Is this right?

This boat that will never be built, is not projected to be "hitting" on stuff.
It's going to be remotely controlled, so the person that is controlling the boat knows what he's doing, and not going to crash on things.

I thought that 2cm was too little for thickness, but I see your calculations returned "6.7mm", which is even lower.
Is fiberglass that "good"?

I am sincerely sorry for not being able to express myself (lack of knowledge in this area/I'm not a native English speaker), it's holding you all back in proving a definite answer, but ask me questions and I will try and answer them to have a more defined problem.

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