Laminate Moisture Content

[Pete]

I hope to start a discussion on the subject and limit the subject to the effects of moisture content and the properties and process of removing the moisture. I know there are many opinions on blisters and what if anything to do about them. I would like to avoid that discussion in this thread.

Let me set up the questions. I have a 1976 48 LRC. In 3/07 I hauled it for a bottom job and found that the barrier coat on the boat was coming loose. Reason is the bottom had excess antifouling paint left on the gelcoat when the barrier coat was applied. Not sure when it was done or who did it, but shame on them. The bottom also had a good one hundred blisters, from one to a few as big as 3 inches in daimeter. The delamination was between the mat layer and the structural layer in all cases. My decision was to sand the bottom to the original gel coat (what a job), open up every blister and remove everything necessary to have nothing but good glass left. These spots were then relaminated with West epoxy and 4 to six layers of glass cloth depending on the depth of grinding. The bottom was then faired using West epoxy to the point that no indication of work could be detected when wet down and viewed at any angle. Next 14 gallons of epoxy based Sea Hawk Tuff Stuff barrier coating was applied in 6 coats plus one layer of bottom paint all in one day (two people). This was 3 times the recommended coats, but Sea Hawk said the more (within reason) the better if you were not concerned with adding weight. Next day applied two more coats of bottom paint, let the boat sit for 6 days on land and then it was launched.

During the process my moisture meter continued to show excessive moisture in the laminate, but after six weeks of dry and windy weather I decided to do the barrier coat. Next point is the bilge in the boat is absolutely dry. Some areas are painted for appearance and some accessible areas are not. Recently, I measured some of these interior exposed laminate locations with the same moisture meter and got much better results, much the same as I see on the hull topsides.

After all that, my question is should I infer that the lower reading are due to moisture evaporating out of the laminate due to its porosity? Anyone have results of reading a "wet" hull on the outside and results from the inside at the same time to give me a clue? What if any difference is there in the strength of a structural laminate correlated with moisture content?

One last item, I have run two dehumidifers in the boat for the last 3 months circulating air throughout the inner hull surface as best I can. One side benefit of this has been the absolute elimination of any hint of a "Hatteras Smell'.

Thanks

Pete

 

[krush]

I have always wondered if there is scientific peer reviewed analysis on moisture in composites. Everything I've seen or heard has not been from engineers (not saying an engineer is required). I realize what causes blisters, but is there confirmation that XX% increase in moisture on your meter really leads to weakening?

If I have some time this week, I hit the library and see what I can find.

 

[Boss Lady]

Here is an excellent read on blisters.

http://www.zahnisers.com/repair/blister/blister1.htm

 

[58yfsb]

I'm more of a reader than a poster since I'm relatively new to the Hatteras community. But, since I'm one of those dang composites engineers by day, I'll add my $0.02.

First, moisture enters the laminate by a process called Fickian diffusion, which is essentially, a reversible process. That is, it takes the same time to "move out" as it does to "get in". So, if the boat has been in the water for 10 years to get to some weight % of moisture content, it will take 10 years to get back to zero (sorry you're not going to wait it out). But, laminates absorb moisture at fairly slow rates and it takes many-many years to fully saturate a laminate on the order of 1 inch thick. I know what "many-many" is for resins used today (vinyl esters and epoxies) but I'm not sure what the number is for resins of yester-year.

The good news is that most composites engineers are well aware of strength reductions associated with moisture absorption and laminates are designed accordingly (especially those designed by reputable yacht builders such as Hatteras, Bertram and others). So, I wouldn't worry at all about the strength of your "wet" hull. Balsa cored areas are another story when it comes to moisture intrusion but not applicable to the hulls in our boats (at least to my knowledge).

Hope this helps.

 

[krush]

Tony,

Could you recomend a book or publications that I can snag from the library (interlibrary loan if needed) on the subject?

I would like to pin down a number for strength reduction...or is there a rule of thumb?

 

[Pete]

Tony, let me describe my wishful thinking and get your reaction. The wishful thinking goes like this: Since the owners of the boat have been located in Cape May, NJ, Philadelphia, PA, Annapolis, MD, and now New Bern, NC, I am assuming that it has been in the water for 32 years except for short haul outs for maintenance, painting and such. In 3/07 it spent seven weeks on the hard doing the bottom with a heavy application of Sea Hawk Tuff Stuff epoxy barrier coat. I would estimate that there is between 50 to 75 mils of barrier coat. Next step was to hold the relative humidity around 40% on the interior over the winter while circulating the air around the interior hull surface. My thinking is that the laminate is somewhat permeable, otherwise the moisture would not be in it in the first place. So, the wish is: seal the outside, hold the inside at low humidity, and hope that the moisture will slowly migrate through the laminate to the inside low RH air. Your expert opinion would be most appreciated.

Pete

 

[58yfsb]

Not sure I'm an expert at anything - most of what follows you probably already know ...

Water diffuses through (most) all polymer materials - even the highest quality aerospace grade laminates absorb water. It is also true that poor quality hand layup laminates (high void contents) will absorb more water than pre-pregs, infused, or well fabbed hand layup laminates fabricated from the same resin. Surface porosity at gel-coat to laminate interfaces is the likely "initiator" of the blisters you describe. Note that barrier coats are typically epoxy-based because, even though they will eventually absorb more water in total weight %, they do so at a much slower rate.

Now for the opinion part - It's important to get the moisture out to ensure good adhesion of material being added; the longer the area is dry (drier) prior to the "repair" the better. Although I obviously can't say for sure I'd be very surprised if the inherent strength of the laminate is a problem.

Two primer type references you may want to have a look at ...

Old-School:
Marine Design Manual for Fiberglass Reinforced Plastics
Gibbs and Cox, Inc. - Naval Architects and Marine Engineers
McGraw-Hill Book Company, 1960
(Yep 1960 - so the Rockwell Guys surely had this one on their desks).
Pg. 5-37 lists some "typical wet knockdowns" for 25 oz/yd^2 WR of 20-35% although these are probably conservative.

More Recent:
Marine Composites
Eric Green Associates, Inc.
www-dot-marinecomposites-dot-com

 

[krush]

I've read:

Author Scott, Robert J., 1938-
Title Fiberglass boat design and construction / Robert J. Scott.
Publication Info. Jersey City, N.J. : Society of Naval Architects and Marine Engineers, c1996.
Edition 2nd ed.
Call number VM321 .S414 1996

But it only had design constraints and rules of thumb, nothing about moisture.

 

[SeaEric]

It will be a cold day in H - E - double hockeysticks before I let some goof ball buyer and his *ahem* surveyor, drill a hole in and perform a "laminate profile" on the hull of my fiberglass boat. Period.