Fresh Water and Osmosis

[Lizzie_B]

Your opinions and expertise please (though they're not always the same thing)! Is a fibreglass boat that has spent its entire life in fresh water more or less likely to suffer from osmosis compared to those that have spent their life in salt water? What are the causes of any difference (if there is any)?

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[webcraft]

More likely if in fresh water. It's all to do with relative osmotic pressures - no doubt someone here will be able to give you a more technical explanation.

- Nick

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[alahol2]

Definitely fresh is a killer.
Dinghy left on (saltwater) mooring 3 months, no problem.
Dinghy left on grass 3 months (fresh water dew/rain), absolutely covered in pimples.

<hr width=100% size=1>I really would rather be sailing...

 

[Ric]

Osmosis occurs when there is a different solution strength across a semi-permeable membrane, and the solvent crosses the semipermeable membrane into the more concentrated solution.

This is not really the case when fibreglass deteriorates when immersed in water, because there is no water in fibreglass to start with. A better description of what goes wrong with fibreglass boats would be that they just go soggy.

Where osmosis will occur (and this may be relevant to you) is where a boat has been in seawater for a long time and the fibreglass has gone a bit soggy. If you put it into freshwater, it will rapidly get a lot soggier as it will suck up lots of freshwater.

If the boat has spent all its life in freshwater, it won't be much different than if it has spent all its life in salt water.

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[silverseal]

I am a sceptic about so called osmosis. Early fibreglass boats built in the 1960's had two gel coats, and most never have suffered from so called osmosis. Having said that, if you get a blister and burst it, it may have an acetic acid smell - like vinegar. I personally believe that a duff gel coat causes capillary action down the stands of glass, leaching out certain water soluble chemical which gather in the blister. I dont think it make any difference whether salt or fresh water, and frankly no one has ever recorded a lost boat due to structural failure due to "osmosis". Unless you have actual delamination, dont lose any sleep!


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[johnlilley]

All to do with the density of the water that the boat is in. Very cold seawater, Arctic almost, the process is very slow because it is dense water, warm Med water, still saline but less dense the process is slightly quicker, right up to the fastest and least dense which is warm fresh water. Hope that gives some indication. The propensity of the laminate to develop blistering makes a difference of course, but two boats with exaxtly the same risk will differ according to the above conditions about where they are kept afloat and the length of time they are afloat as against on dry land. Hope that helps
John Lilley

<hr width=100% size=1>John Lilley, John Lilley & Associates, Web site www.seasurveys.co.uk for osmosis /timber problems

 

[johnlilley]

All to do with the density of the water that the boat is in. Very cold seawater, Arctic almost, the process is very slow because it is dense water, warm Med water, still saline but less dense the process is slightly quicker, right up to the fastest and least dense which is warm fresh water. Hope that gives some indication. The propensity of the laminate to develop blistering makes a difference of course, but two boats with exaxtly the same risk will differ according to the above conditions about where they are kept afloat and the length of time they are afloat as against on dry land. Hope that helps
John Lilley

<hr width=100% size=1>John Lilley, John Lilley & Associates, Web site www.seasurveys.co.uk for osmosis /timber problems

 

[Avocet]

Isn't the temperature a more important factor than density? I'm no expert on this but lots of chemical processes are catalysed by heat. I think water is at its most dense at 4 degrees C. either side of that the sensity goes down..... I think!

BTW, live in the North West and I've heard lots of people saying that boats that have lived on Windermere are more likely to have osmosis than similar boats kept on salt water round here. I've no idea if its true though.

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[BrendanS]

Osmosis is by definition a movement of water between different levels of solutes (things dissolved in water) hence the reference to density, which should really more accurately be about the 'saltiness' of the water. Water which is very salty will have less osmotic effect than freshwater, because salt water has higher levels of solutes, so less diffferential to the fibreglass which has a high level of solutes.

Heat plays a part, but osmosis cannot occur unless there is a difference in levels of concentration of solutes.

Confused?

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[Avocet]

That all makes sense to me but doesn't density vary with temperature too? - not just with salinity?

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[Mollari]

There are a number of factors to take into account
1 Are there any voids between the gelcoat and glass as older boats did not have the tech that they do today.

2 Hydralisation or water ingress into the hull. Has the boat been allowed to dry

3 get the Ph levels of the blister juice checked, rather than just relying on the nose test

4 Purchase "fibreglass Boats by Hugo du Plessis" it has become my bible, any issues with my hull have been resolved with this book.

Best of luck, I hope this info is of some, if only a tiny amount, to you

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[BrendanS]

Density is irrelevant. It's the salinity and temperature which are important. Density varies as a function of temperature, salinity and pressure, so is effectively an idependant variable

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