fiberglass over cold-molded

[Hernandiz]

Hi,
I'm working on the scantling of a 40ft sailboat using ABS offshore Racing Yachts. Construction will be of plywood-epoxy. I read on a thred that it should be applied as cold-molded. if I do so a outer fiberglass skin is not included as structural...
Is it corect?

[rasorinc]

A single layer of light fiberglass over wood is meant to waterproof it. It is not structural.

[apex1]

Hi,
I'm working on the scantling of a 40ft sailboat using ABS offshore Racing Yachts. Construction will be of plywood-epoxy. I read on a thred that it should be applied as cold-molded. if I do so a outer fiberglass skin is not included as structural...
Is it corect?

That is not a clear advice "should be"?! What does your designer / NA specify? Do you try to make the calculations yourself?
Why should a member of the hull structure be excluded from structural calculation? Especially if it is of noticeable weight and strength? So, to your question.... NO
rasorinc... he did´nt say single layer. And even if, see my statement above.
Regards
Richard

[Hernandiz]

Thanks rasorinc.

Richard... who told you I would exclude a member from structural calculation?

I'm talking about about an aditional fiberglass skin on the exterior side of a plywood hull.
By the way I'm the designer...

[apex1]

Thanks rasorinc.

Richard... who told you I would exclude a member from structural calculation?

I'm talking about about an aditional fiberglass skin on the exterior side of a plywood hull.
By the way I'm the designer...

You.... told me, you did´nt say you would include the outer layer in your calculation Mr. designer!

Regards
Richard

[yachty4000]

My understanding is that ABS treats glass sheeved boat as a normal boat with a high density core so you end up with a very strong over engineered boat. The worse thing you can do is one layer of high tech laminate on wooden structure as you effectively end up with a the laminate taking the load. Sheaving with one layer of chop strand mat to bond a wooden structure seems sensible (SP Systems have some good fact sheets on this) I have done this on strip planked restoration projects. I am not brave enough to interpret ABS for you and if it is a racing yacht I would start looking at ISO12215

Isn;t this what Gudeon Brothers (West Epoxy) book is all about cold moulded construction could be worth a read?

[Steve W]

So are we talking about a cold molded boat or a plywood boat? either way the typical glass, dynel or polypropylene/epoxy sheathing as rasorinc said should be considered as non structural, it is mainly there as a waterproof membrane and to give a uniform surface for the high grade paint surface that everyone thinks they need. If you are designing for cold molding the Gougeon book is a good rescource as is Modern wooden yacht construction by John Guzzwell.
By the way,you would not sheath with chopped strand mat ever as csm is held together with a binder which is designed to be dissolved by the styrene in polyester resin and under no circumstances would you use polyester to sheath a wood boat,you must use epoxy for this and while it will wet out csm it does a poor job of it as it has no solvents,also you would end up with a poor resin to glass ratio,actually the best fabric for this purpose is probably Dynel or polypropylene.
Steve.
Steve

[Landlubber]

Can only agree with you Steve W....if the skin is for waterproofing purposes, use epoxy and dynel....it is tenacious with plywood.

I helped build a boat in 1972 that way, she is still alive and kicking well.

[apex1]

So are we talking about a cold molded boat or a plywood boat? either way the typical glass, dynel or polypropylene/epoxy sheathing as rasorinc said should be considered as non structural,
Steve

Sorry Steve, if we are talking about a cold moulded boat the outer layers (plural), are often applied, and calculated, as part of the structure (in our boats at the bottom only). Ply is a different subject of course.
I agree with your statement about mat and abt Epoxy.
But what was the question? Cold moulded with Ply? Outer skin ?
Hi,
Construction will be of plywood-epoxy. I read on a thred that it should be applied as cold-molded. if I do so a outer fiberglass skin is not included as structural...
Is it corect?

And who did ask?

By the way I'm the designer...

Regards
Richard

[Steve W]

Apex, my confusion stems from the thread being titled "fiberglass over cold molded" but the ops post states "construction will be plywood epoxy".on re reading it i think hes going to cold- mold but with plywood strips instead of veneers. Ive built or been on the build team of more than a dozen cold molded boats back in the 70s and almost all were sheathed with 1 layer only of dynel which is only about 4.2oz,certainly not calculated as a structural member.A few used glass cloth but only 4 or 6oz.
Regards,Steve.

[Hernandiz]

http://s425.photobucket.com/albums/pp335/guyher/?action=view&current=2007_0610DomRep0050.jpg
http://s425.photobucket.com/albums/pp335/guyher/?action=view&current=2006_0811DomRep0007.jpg

[PAR]

Dynel or Xynole for that matter are not going to impart strength to the hull shell, though they will add considerable weight, water proofing and most importantly, abrasion resistance. The abrasion resistance comes from the high resin to fiber ratio.

Your original question about sheathing lacks definition. Basically a thin sheath (as has been stated) will impart little strength, unless a high modulus fabric, only providing abrasion thickness. This assumes the wooden, molded hull shell, withstands the loads and the sheathing just offers protection.

On the other hand a thicker sheathing or a hull shell engineered with the molded potion, being part of a laminate (sandwich or other wise) is a different matter entirely. In these cases, yes the sheathing does impart some to quite a bit of the hull shell stiffness, depending on laminate engineering.

Secondly, mat is a waste of resin in an epoxy lay up. Mat is a bulking material used in relatively low modulus resins, such as polyester and completely unnecessary with epoxy. The mat will give up and break fibers long before the epoxy, making it useless in an epoxy laminate. The use of mat will dramatically increase laminate weight, thickness, brittleness and not significantly contribute to the strength of the laminate.

To answer specifics about scantlings, I'd need to know considerably more about the structure you're working with.

Plywood can be applied in a number of ways. Molding plywood is a fairly common method to get compound curves and one I employ often. Hull shell stiffness can be accomplished through a few different techniques, depending on the design brief and ultimate goals interested in attaining.

[apex1]

Apex, my confusion stems from the thread being titled "fiberglass over cold molded" but the ops post states "construction will be plywood epoxy".on re reading it i think hes going to cold- mold but with plywood strips instead of veneers. Ive built or been on the build team of more than a dozen cold molded boats back in the 70s and almost all were sheathed with 1 layer only of dynel which is only about 4.2oz,certainly not calculated as a structural member.A few used glass cloth but only 4 or 6oz.
Regards,Steve.

Steve, I understand the confusion, naturally, the description of the choosen method was insufficient (and still is).
As long as we are not equipped with a serious information about the building method and the purpose, the "outer skin" is meant to provide, we can throw a dice to answer the original question yes or no.
On our larger designs (50´to 103´) we apply 300 gsm glass / Epoxy in two layers, on the whole hull, one additional of 220 gsm underwater and finally one layer of Polyethylen of 120 gsm on the bottom. And that is very much within our structural calculation.
The enquirer told us, that he is "working on the scantlings... to ABS rules". If I take that as a fact, my answer to his question is NO!
I do not assume, someone who is able to calculate scantlings to classification rules, is asking for the influence of 60 gsm strandmat on structural strength!
But now, that we know the enquirer is the designer, we can expect some specific particulars about the design.
Regards
Richard

[Hernandiz]

Look a the pics of my last post. this is a similar construction.

[apex1]

Look a the pics of my last post. this is a similar construction.

I did, but could not find any data of the construction method, or the way (layers) the boat is sheathed.:?:
But to be a nice guy, I will give a proper answer. The boat shown, obviously is a simple Ply sheet construction with some outer glass / EP skin to protect the wood (abrasion and water intrusion). As far as I can see from the picture, it seems to be a bit more than only a thin tissue, but I assume it is not calculated as a load carrying member of the hull. The multi chine ply would carry the loads without the assistance of the skin. Although even if you apply 3 layers of Ep to prevent the wood from water intrusion, you add noticeable strength, more naturally, with the inforcement of fiber. But as mentioned, this is not taken into account in the scantling calculation.
So, now my reply to your first question is YES.
Regards
Richard

[Hernandiz]

Thank you for that answer Richard.
You all were right my question was't clear. English is not my first language and I'm not always clear enough.
In fact I'm familiar with steel corstruction, not wood and at first glance I tried to applied the formulas for sandwish construction. But 2 years ago I participate to the construction of the boat you saw on last pics.This is a mini transat design by Jean-Pierre Magnan. The hull were 1/2 oukoume with 12oz fiberglass roven outside only. If there is no fibreglass inside it can't be sandwish but 12oz look oversized to be un-structural.
So I try to find how to apply this kind of design with ABS rules.

[Steve W]

Apex ,i can see why with total of 600gsm on the topsides and 940gsm on the bottom you would include it in the structure,thats a lot heavier than a typical sheathing although its on a larger boat than i was thinking of.I agree 100% with your last post.
Hernandiz,even though the fabric skin is not structural you should do your own test samples with the various fabrics,say, glass, dynel and polypropylene on a piece of plywood and then pound them with a ball pein hammer. My preference is for dynel for the following reasons
1/ it drapes really well,better than glass.
2/ it consumes more resin which would be a bad thing if it were a structural skin but is exactly what you want in a waterproofing skin.
3/it absorbs impact better than glass.
4/its more abrasion resistant than glass.
5/contrary to what some have said its not heavier than a glass skin in a typical application as dynel is about 4.2 oz whereas in a similar application you would likely use 6oz glass so while it uses more resin its about the same weight.
I have not used polypropylene cloth but i believe its similar?
Steve.

[apex1]

Thank you for that answer Richard.
You all were right my question was't clear. English is not my first language and I'm not always clear enough.
In fact I'm familiar with steel corstruction, not wood and at first glance I tried to applied the formulas for sandwish construction. But 2 years ago I participate to the construction of the boat you saw on last pics.This is a mini transat design by Jean-Pierre Magnan. The hull were 1/2 oukoume with 12oz fiberglass roven outside only. If there is no fibreglass inside it can't be sandwish but 12oz look oversized to be un-structural.
So I try to find how to apply this kind of design with ABS rules.

You are right, 12oz IS adding sufficient strength to be noticed in the calculation (at least as a safety factor).
And do´nt mix "design" and "diseno", design is "construccion", "edificio".


Regards
Richard

[apex1]

Apex ,i can see why with total of 600gsm on the topsides and 940gsm on the bottom you would include it in the structure,thats a lot heavier than a typical sheathing although its on a larger boat than i was thinking of.I agree 100% with your last post.
Thanks
I have not used polypropylene cloth but i believe its similar?
Steve.

Steve, nono it is 300gsm total in two layers, than add the rest! Alltogether it sums up to 640 at the bottom (fabric only).
Dynel is a acrylic fibre (and a good choice for the intended use), whereas I pointed to PE (Polyethylene or "Spectra", "Dyneema" etc.), wich is the strongest fibre available in composit construction. PP (Polypropylene) is a different stuff, and as far as I know, the Aussies $ is made of it (which holds me back from a deeper investigation about the application in yacht building).:D
Regards
Richard

[Hernandiz]

Thanks for the advice!
Dynel is about the same final weight but what about the final cost:?:
(compare to fibreglass)

[PAR]

Dynel and Xynole will absorb two to three times the resin of a regular weave cloth of the same weight, possibly more. Given the two, Xynole is the better choice. My tests show Dynel to be around 3 times more abrasion resistant the cloth of the same weight. Xynole about 6 times better then cloth of the same weight. Again, much of this is the resin to fiber content. Both of these materials drape over compound curves much better then regular weave cloth, which is a real benefit, that must be traded with the additional weight and the fact that they will not contribute to the strength of the laminate (too stretchy).

Dynel costs slightly more then good quality regular weave cloth. I haven't priced Xynole recently, but it was about twice that of regular cloth.

[Steve W]

Par, thats interesting about the xynole, ive got to try it sometime,thats a polyester i think. you know,we all say that these various fabrics dont add strength but while they may not add structually they sure add significantly to the impact resistance in small plywood craft,this was shown in a rather dramatic way when my son and i were building ourselves a couple of stitch and tape kayaks,we reduced the plywood to 3mm instead of 4mm and then instead of glass taping the outside seams we sheathed the whole outside with dynel.When we were not working on them we hoisted them to the ceiling of the shop to get them out of the way with a tackle system.when taking one down i dropped it from 20ft on to concrete,it did no damage at all,i was suitably impressed,if it had just been sealed with a few coats of epoxy it would have sustained considerable damage,in this instance i had treated the dynel as a structural part and reduced the scantlings accordingly.The finished boats at 17ft 10 with doubled bulkheads so we can break them down into 3 pieces for storage weigh in at 42 pounds.
Steve.

[tom28571]

I have fairly extensive data on testing of various sheathing materials but can't get the table to post in a readable fashion. Suffice to say that Xynole is ahead of the other synthetics and way ahead of fiberglass on abrasion resistance. No single layer of material other than biaxial non wovens add significant stiffness to a wood panel.

The fabric is much more determinant for abrasion resistance than the epoxy.

The abrasion tests are in the September/October 2002 issue of Boatbuilder magazine.

[yellow cat]

check this www.multihulldesigns.com kurt hughes comments is my greatest inspiration so far, i ordered his vhs video but a dvd can probably be purchased.
i saw combo (wood/fiberglassed ) boats in the past, delamination due to dissimilar structural stresses was a problem . Epoxy seems to be doing well, it's holding on some small projects. I would use fiberglass for impact resistance but not stress added resistance, delamination could occur. Fire retardant mesh can be added inside the epoxy. I picked up some roxul mineral insulation and will saturate it partially with epoxy, could be an option to fiberglass with many advantages. a mesh is also mentionned in west-system web site, its for airplanes also .

[apex1]

i saw combo (wood/fiberglassed ) boats in the past, delamination due to dissimilar structural stresses was a problem .
Epoxy seems to be doing well, it's holding on some small projects. I would use fiberglass for impact resistance but not stress added resistance, delamination could occur. Fire retardant mesh can be added inside the epoxy. I picked up some roxul mineral insulation and will saturate it partially with epoxy, could be an option to fiberglass with many advantages. a mesh is also mentionned in west-system web site, its for airplanes also .

Sorry yellow cat, I am shure you are a bit besides topic.
What please is "combo"?
Delamination is absolutely no problem! It just does´nt happen.
Epoxy does´nt "hold on some small projects" only, it holds on every size of hull. And makes it stronger, to which extend so ever. Have a look at my Gallery there are some "small projects" shown.
And we are talking about Epoxy / glass application only in this thread!
The advantage of the glass / Epoxy application is mainly the better stress resistance, secondly the better impact protection! And how would you like to apply a glass layer for impact resistance but not for stress resistance? Does your layup "know" what was your intention?
What is a "fire retardant mesh" good for, below the waterline? Just to weaken the matrix?
Sorry but your post was total crap!

Regards
Richard

[yellow cat]

sorry i was not clear. The delaminations occured with polyester resin and fiberglass, not epoxy, at least so far. The fire retardant is for inside engine room fuel storage areas and kitchen. If a mineral fiber is combined with epoxy on one side of a sandwich panel and fiberglass is on the otherside i will not be surprised for structural gain.

[apex1]

sorry i was not clear. The delaminations occured with polyester resin and fiberglass, not epoxy, at least so far. The fire retardant is for inside engine room fuel storage areas and kitchen. If a mineral fiber is combined with epoxy on one side of a sandwich panel and fiberglass is on the otherside i will not be surprised for structural gain.

If you read the thread, you will notice that we have been asked for a outside sheathing of a plywood hull with glass / EP.

And "at least so far" is nonsense. Delamination does not happen. period
The fire retardant you promote, is (sorry) nonsense too. There are well proven systems and even Paints available, light, cheap, and easy to apply.
A partly soaked mineral fibre would reduce the positive properties of both materials, being expensive and heavy.

Regards
Richard