Osmosis: 3 questions to boost my knowledge

[Dave100456]

1) Without a gelcoat or flowcoat top layer, is GRP more susceptible to osmosis?
2) If a GRP bilge is left with water (fresh or saline) in it for months/years can Osmosis start?
3) If the bilge is dried out, will osmosis stop (if it ever started)?

Thanks

 

[vyv_cox]

1) Without a gelcoat or flowcoat top layer, is GRP more susceptible to osmosis?
2) If a GRP bilge is left with water (fresh or saline) in it for months/years can Osmosis start?
3) If the bilge is dried out, will osmosis stop (if it ever started)?

Thanks

No.2 yes. There are osmosis bubbles in my grp heads sink and in my engine bilge.
No.3. osmosis will not proceed further if dried out but of course the bubbles will still be present.

 

[Tranona]

Osmosis process and as GRP is water permeable liquid will flow. Gel coat and flow coat (essentially the same thing) are generally less permeable than the laminate so will resist the flow so you No2 is correct.

However there is a distinction between the process of osmosis and the negative effects in some GRP laminates as it is the uncured styrenes in the laminate that start the process of creating blister in the outer coating. So, if your bilge is dry that only reduces the water getting into the laminate. The liquids that create the blisters will still be there unless the outer layers has been removed and the laminate cleaned and dried out.

 

[Refueler]

1) Without a gelcoat or flowcoat top layer, is GRP more susceptible to osmosis?

Not necessarily. It depends if there is uncured or poorly resined laminate. The likelihood is 'wicking' where moisture follows glass strands.

2) If a GRP bilge is left with water (fresh or saline) in it for months/years can Osmosis start?

Answer 1 applies again as you are generally referring to a non-Gelcoat surface. But if Gelcoated as many more modern boats are - then Osmosis can of course occur.

3) If the bilge is dried out, will osmosis stop (if it ever started)?

Osmosis stop ? The mechanics of the blistering is such that only breaking the blisters and repeated washing out can in any way start to reduce the action. Moisture gets in between Gel and laminate or via glass strands. The moisture then increases density as it absorbs uncured resins, this in turn creates pressure and the blister develops. This process cannot be stopped - only reduced.

Its actually unusual to see blistering in bilges unless of a later build with poorly applied gel. Most boats have good final resin layers in bilges / internal surfaces. Older boats - it was common for the remaining resin to be 'spread about' ....
It is also unusual for moisture to penetrate deep into laminate unless operator has been bad at his job and left poorly resined areas. Blistering usually is between Gel and top one or two laminate layers ...

 

[slawosz]

I really recommend this book: Fibreglass Boats: Construction, Gel Coat, Stressing, Blistering, Repair, Maintenance: Amazon.co.uk: Hugo Du Plessis: 9781408122747: Books
You will learn everything you need to know!

 

[Praxinoscope]

I really recommend this book: Fibreglass Boats: Construction, Gel Coat, Stressing, Blistering, Repair, Maintenance: Amazon.co.uk: Hugo Du Plessis: 9781408122747: Books
You will learn everything you need to know!

Sad to remember that he died in 2018, real master of GRP technology. But his book is a must for anyone who mucks about with boats and GRP.

 

[Dave100456]

Thank you all for the responses. Informative and to the point, great!
The issue relates to a boat I looked at that had probably mainly rain water (from the taste) sitting in the bilge for a considerable time.
Cheers

 

[Refueler]

Thank you all for the responses. Informative and to the point, great!
The issue relates to a boat I looked at that had probably mainly rain water (from the taste) sitting in the bilge for a considerable time.
Cheers

Usually its not such a problem when its bilges ... the advantage is that if kept dry - and any blisters / actioned areas are opened up - the strata can dry out and stay reasonable. Its unusual for damage to be deep.

Generally I would not walk away from a boat such as ..... but of course I would wonder at the 'owners' neglect and what else is not looked after.

 

[Refueler]

There are various books / papers out there on this subject ..

Hugo de Plessis
Nigel Clegg

are two good examples.

Plus of course many pages online ...............

 

[vyv_cox]

Osmosis stop ? The mechanics of the blistering is such that only breaking the blisters and repeated washing out can in any way start to reduce the action. Moisture gets in between Gel and laminate or via glass strands. The moisture then increases density as it absorbs uncured resins, this in turn creates pressure and the blister develops. This process cannot be stopped - only reduced.

Not sure that is the mechanism. Osmosis is driven by plasmolysis, in which a dilute solution passes through a semi-permeable membrane to dilute a more concentrated solution. Thus osmotic pressure is generated, expanding the concentrated solution and blowing out blisters in the gel coat. Removal of the dilute solution will halt plasmolysis. The blisters will still be there, containing concentrated solution.

Complete eradication requires opening of the blisters and washing out the solution.

 

[Refueler]

Not sure that is the mechanism. Osmosis is driven by plasmolysis, in which a dilute solution passes through a semi-permeable membrane to dilute a more concentrated solution. Thus osmotic pressure is generated, expanding the concentrated solution and blowing out blisters in the gel coat. Removal of the dilute solution will halt plasmolysis. The blisters will still be there, containing concentrated solution.

Complete eradication requires opening of the blisters and washing out the solution.

With respect ........ please go back and read my post.

Passing through strata
Diluting / absorbing
Pressure increase
Blisters

Break open blisters
Wash out ...

Those are ALL in my post ... simply put ...

 

[vyv_cox]

With respect ........ please go back and read my post.

Passing through strata
Diluting / absorbing
Pressure increase
Blisters

Break open blisters
Wash out ...

Those are ALL in my post ... simply put ...

Density????

 

[Refueler]

Density????

Ever broken open a blister ... not noted the increased density / viscosity ??

Its precisely because of that - the fluid cannot permeate back out ...

 

[VicS]

Ever broken open a blister ... not noted the increased density / viscosity ??

Its precisely because of that - the fluid cannot permeate back out ...

Not caused by density or viscosity, although changes in either may be a result. Osmosis is driven by differences in concentration of a solute. Water will pass through a semipermeable membrane from a lower to a higher concentration.

.

 

[Refueler]

Not caused by density or viscosity, although changes in either may be a result. Osmosis is driven by differences in concentration of a solute. Water will pass through a semipermeable membrane from a lower to a higher concentration.

.

Please show where I say different ..... my comment was basically moisture passing through - disssolving / absorbing and increased density / viscosity ... which then prevents its release unless blister is broken open and then washed ...

I have NOT described what Osmosis is .... unless the English Language has suddenly changed !!

In fact I even mention WICKING .... which is another form of moisture transit ....

Lets see ...

The mechanics of the blistering is such that only breaking the blisters and repeated washing out can in any way start to reduce the action. Moisture gets in between Gel and laminate or via glass strands. The moisture then increases density as it absorbs uncured resins, this in turn creates pressure and the blister develops. This process cannot be stopped - only reduced.

Mechanics word is used there for the breaking and washing to try and reduce the action ... NOT as assumed to be description of Osmosis. Not my fault - you and others decide to misinterpret.

 

[eilerts]

SHOTGUN SHOOT OUT! I'm in!
In physics osmosis well defined and understood. This physical process is the last part of the more complex boat related phenomenon that inherited the name "osmosis". osmosis builds the blisters, but that is just the last phase. The nasty "blister juice" is the cause of the physical process osmosis, but the juice itself is caused by another process called hydrolysis, or simply reaction with water. One of the things that reacts with water is the polyester. It is thankfully a slow reaction, but nevertheless real. The reaction products are alcohol and organic acid, both soluble in water, and contributing to blister building. This reaction is speeded up by the presence of acid, so it is likely self catalyses ( self accelerated). I.e. two good reasons to get rid of the "brister juice", and probably more important than drying out the hull. Another component that reacts with water is the acetate glue that holds glass strands together to form a mat. It breaks down to acetic acid, another powerful solute.
The presence of styrene itself does not contribute to the physical process osmosis, as it is not water soluble, it is not a solute. Perhaps it does, if it is oxidized. The presence of styrene suggests that the polyester is undercured, and undercured polyester is more vulnerable to the reaction with water.
GRP, that is not covered with gelcoat or paint, does not get blisters, as there is no semi permeable membrane present, but the slow reaction with water is still going on.
A dry bilge is preferable, for the reason that, if water can ender the laminate from the outside, through the gelcoat, it can also leave at the inside. This is probably the reason why rudders get blisters earlier than the hull.
If your boat is full of blisters, and you have no time or money to treat it, it would probably be a good strategy to just puncture the blisters and put the boat in the water, as the sea water is far less aggressive that the blister juice.

Shoot!

 

[vyv_cox]

SHOTGUN SHOOT OUT! I'm in!
In physics osmosis well defined and understood. This physical process is the last part of the more complex boat related phenomenon that inherited the name "osmosis". osmosis builds the blisters, but that is just the last phase. The nasty "blister juice" is the cause of the physical process osmosis, but the juice itself is caused by another process called hydrolysis, or simply reaction with water. One of the things that reacts with water is the polyester. It is thankfully a slow reaction, but nevertheless real. The reaction products are alcohol and organic acid, both soluble in water, and contributing to blister building. This reaction is speeded up by the presence of acid, so it is likely self catalyses ( self accelerated). I.e. two good reasons to get rid of the "brister juice", and probably more important than drying out the hull. Another component that reacts with water is the acetate glue that holds glass strands together to form a mat. It breaks down to acetic acid, another powerful solute.
The presence of styrene itself does not contribute to the physical process osmosis, as it is not water soluble, it is not a solute. Perhaps it does, if it is oxidized. The presence of styrene suggests that the polyester is undercured, and undercured polyester is more vulnerable to the reaction with water.
GRP, that is not covered with gelcoat or paint, does not get blisters, as there is no semi permeable membrane present, but the slow reaction with water is still going on.
A dry bilge is preferable, for the reason that, if water can ender the laminate from the outside, through the gelcoat, it can also leave at the inside. This is probably the reason why rudders get blisters earlier than the hull.
If your boat is full of blisters, and you have no time or money to treat it, it would probably be a good strategy to just puncture the blisters and put the boat in the water, as the sea water is far less aggressive that the blister juice.

Shoot!

Are you TNLI under alias??

 

[eilerts]

TNLI? Peviously perforated member, not seen or heard since last shoot out?
New to me

 

[STOL71]

Osmosis is as much a problem from the outside as it is from the inside (bad ventilation, wet bilges).
It will restart after the boat has dried out when the boat gets wet again, unless treated.

 

[bignick]

Nigel Clegg’s guide to Osmosis is available on his website (for free)…

https://www.passionforpaint.co.uk/product_uploads/15574838191.pdf