Fibreglass layup advise

[wet feet]

I suggest a small trial panel,bagged over a shaped former.The cost of the former for a real hull might be a significant addition to the budget for the whole project,as might the cost of the vacuum equipment.Then there is the significant challenge of getting it all leakproof.A successful test piece would go some way to validating the proposed technique.

 

[gonzo]

There is no need for a trial panel. All the material characteristics are available. It is only a matter of engineering a structure with known materials.

 

[TANSL]

In a construction with PRF it is always prudent to test and have the most correct data possible based on the experience of the builder that, I see no other way, is obtained based on the test panel. There are several variables that intervene in determining the properties of a laminate (I call them variables because they vary considerably according to the good practice of the builder) so the designer who designs the structure should take into account the real resistance that, with the uses from the builder, they get on their laminates.
Of course, more or less standard values can be assumed, but it is advisable to take into account the reality of the builder (the new ISO 12215-5:2019, for example, requires taking it into account).

 

[fallguy]

There is no need for a trial panel. All the material characteristics are available. It is only a matter of engineering a structure with known materials.

The mystery is the stiffness. We've established it cannot be as stiff as plywood for the same weight, so he would need to build a test panel to determine a stiffness that is acceptable vs the ply.

He can follow something like Gerr's method, but ultimately; he needs to be satisfied with less stiffness.

Whether it be three or four layers of combi mat is the likely decision. Using Gerr, the bottom may be different than the sides as well.

So, the best way to go is to run the numbers in Gerr, build a test panel to see if the system is sufficiently stiff, then build.

He can certainly use Gerr or some other method, but the result my not be to his liking because things like stringers, for example, may need to be more to meet Gerr's method.

As for the rest of us speculating, not much else we are able to do because we have no data on the boat atm.

Also, if four layers of combi vs three; the hull weight varies quite a bit, so the decision is a big one..

 

[gonzo]

Stiffness is not a mystery. There is more than enough published data on material properties. Gerr's book is based on published data.

 

[Rumars]

I for one don't understand is the desire for a solid FRP panel as a substitute for plywood. The UK market offers a variety of relatively inexpensive hardwoods wich work well with epoxy, so in my opinion cold molding or strip planking should be the logical choice if plywood is to expensive.

A solid FRP boat should be designed from the start for the material, not converted from ply.

 

[fallguy]

Stiffness is not a mystery. There is more than enough published data on material properties. Gerr's book is based on published data.

If you have a source of data for flexural rigidity of frp panels of various types of glass and resin; please share it. I looked awhile back and came up with nothing. Even for something as simple as mould making; it was more a test and decide if it is stiff enough than any available data.

The reasons for this are quite simple. Each boat gets a laminate schedule based on need, not generalizations.

Building a plywood hull from frp is close to reinventing the wheel. Not impossible, but borders on silly.

FRP is not pos buoyant even, so if the designer specified even buoyancy foam or didn't; it will be incorrect either way.

And, like I said earlier, how many stringers for less rigid hull?

Despite the track record between you and the Spanish fellow; he is not wrong here. Making a test panel is really much wiser than laying up the entire hull; parting the former and then deciding it is too flimsy. Agree?

Even the use of combi mat may not be ideal.

In Gerr's text; he provides a calc for the thickness in FRP. Just because the OP wants to use glass A does not mean it is best, or that 3 layers of it meet Gerr. Could be he needs 3 layers of combi and a layer of something more to get needed stiffness.

People post here all the time expecting immediate, simple answers. And I am basically defending the need for engineering design and testing. So it seems a bit odd you would be against.

 

[wet feet]

There is no need for a trial panel. All the material characteristics are available. It is only a matter of engineering a structure with known materials.

Quite true if one wishes to work from material characteristics and first principles and has the figures that need to be achieved.Which won't yield too much insight into the amount of work in making a shaped former and subsequently applying enough vacuum for good consolidation.

 

[gonzo]

You are referring to workmanship and quality of raw materials. In that aspect, it is up to the builder to control them. This is an area where amateurs often fail in. Years ago I read the advise by Bruce Roberts to first build the dinghy for the dream yacht. It is some of the best advice I've heard. Building a small boat, with similar materials, gives an opportunity to learn and practice without a major investment. Also, the errors made while learning do not result in structural flaws that may later be significant.

 

[fallguy]

Actually, I would still like to see a flexural rigidity chart for any given laminate or series of laminates.

The problem is noone is willing to produce them because yes, workmanship, methods, and even materials vary greatly.

Someone like rxcomposite may have his own private database of such info, but it seems to be closely held based on my own searches trying to build solid frp angle iron replacements; for one. I came up empty and around this time, I decided boat building was as much art as science; unfortunately.

So, while we disagree, in part, we agree in part. And, ultimately the Spaniard is correct.

 

[gonzo]

The problem is noone is willing to produce them because yes, workmanship, methods, and even materials vary greatly.

Material Properties https://fibrolux.com/grp/knowledge/properties/

 

[fallguy]

???

link is for grp profiles, not hull laminates

 

[gonzo]

They have sheet fiberglass too. It is just one example of how you can get E values to estimate stiffness. However, they have controlled environments and processes.

 

[fallguy]

They have sheet fiberglass too. It is just one example of how you can get E values to estimate stiffness. However, they have controlled environments and processes.

about the only resource I could find was contributor @rxcomposite here on the forum..I dod plenty of googling

 

[rxcomposite]

Hi all
I am looking into replacing the plywood skin of a design with fibreglass .
I would use the plywood bulkheads and stringer's as designed but am thinking about a glass skin (cost of plywood is crazy)
The skin would be 9mm in Ply.
This is on a wharram cat 42 feet.
Constructive input please

You just can't arbitraly choose a 9 mm laminate ply over a plywood skin who knows what thickness it is. That is guessing.

If you intend to use the original bulkhead and stiffener spacing, post the dimensions so that experienced members could give you an idea of how thick the new fglass laminate will be. Right now, I will tell you it is going to be heavier. It will be "strong" but not "stiff". You must satisfy both compressive/tensile strength and deflection.