Jcorstorphine
Well-known member
If I was looking to purchase say a 30 year old boat which had been in freshwatef for say 50% of its life, would it be more prone to Osmosis.
Are boats more prone to Osmosis in freshwater
- Thread starter Jcorstorphine
- Start date
aquaplane
Active member
If it was prone to osmosis, the osmosis would happen faster because the fresh water would be trying harder to dilute the blister than salt water. Fresh water wouldn't cause the blister to form initially any worse than sea water.
jwilson
Well-known member
In general, yes, freshwater worse than seawater, and the warmer the water the more likely 'osmosis' is. However some boats get blisters and some just don't seem to. Looked at a 30+ year old boat at the weekend and it had a very clean hull and very low hull moisture meter readings - virtually new boat levels, despite 30+ years of normal UK usage.
coopec
N/A
I agree that osmosis is more likely when the yacht has spent most of its' life in fresh water: I don't think any knowledgeable person would dispute that. (There are a lot of threads where that opinion is supported)
You might find this article helpful.
http://www.yachtsnet.co.uk/osmosis.htm
You might find this article helpful.
http://www.yachtsnet.co.uk/osmosis.htm
johnlilley
Active member
Basically the denser the water is the less likely to suffer from the same degree of osmosis over a given time. Hence warm freshwater (low density) more risky than cold seawater (higher density).
John Lilley
John Lilley
johnalison
Well-known member
That doesn't seem to tally with my 60yr old chemistry memories. I thought that osmotic pressure was dependant upon the difference in molecular concentration of solutes each side of a membrane. I assumed that the effect of higher temperature was just to speed up the process, as with most chemical and physical processes.
eilerts
Member
The boat phenomenon "osmosis" is a result of several coupled processes with different time frame. The very first process is diffusion of water molecules into the initial dry GRP hull, which starts when the boat is launched for the first time. This goes on until the GRP is saturated. The main driving force is the undersaturation of the hull. The small difference in water concentration between seawater and freshwater makes little difference.
As water concentration rises in the GRP a slow chemical reaction starts called hydrolysis. This breaks down polyester to smaller water soluble molecules. The speed of this process is dependent on water content in the GRP, temperature, and materials quality. The saltness of the water outside the hull is not relevant here, but freshwater may be warmer in general, and that can make a difference.
As the amount of water soluble components enriches, another process starts - osmosis (in the chemical sense). It is this process that builds the blisters. It works as long as the concentration of water solubles are greater inside the GRP than outside. The driving force is this difference in concentration. Here salt or fresh water makes a big difference. This is because the critical level for start up for this process is lower in freshwater and is reached earlier. The blister growth is also stronger as it takes more water to dilute the stuff inside the GRP to the outside level.
I should mention that in this process it does not matter if dissolved stuff outside is salt and something else inside the blisters.
The two first processes builds the fundament for the third. If the boat has been in freshwater, but is fee of blisters it may not be worse than a similar boat that has been in salt water. A blisterfree boat that has been in saltwater may soon develop blisters when moved to freshwater, if its level of water soluble hydrolysis products is just below the level of salt in seawater.
My answer will be, it is more likely to have osmosis, but not necessarily more likely to get it.
Jan
As water concentration rises in the GRP a slow chemical reaction starts called hydrolysis. This breaks down polyester to smaller water soluble molecules. The speed of this process is dependent on water content in the GRP, temperature, and materials quality. The saltness of the water outside the hull is not relevant here, but freshwater may be warmer in general, and that can make a difference.
As the amount of water soluble components enriches, another process starts - osmosis (in the chemical sense). It is this process that builds the blisters. It works as long as the concentration of water solubles are greater inside the GRP than outside. The driving force is this difference in concentration. Here salt or fresh water makes a big difference. This is because the critical level for start up for this process is lower in freshwater and is reached earlier. The blister growth is also stronger as it takes more water to dilute the stuff inside the GRP to the outside level.
I should mention that in this process it does not matter if dissolved stuff outside is salt and something else inside the blisters.
The two first processes builds the fundament for the third. If the boat has been in freshwater, but is fee of blisters it may not be worse than a similar boat that has been in salt water. A blisterfree boat that has been in saltwater may soon develop blisters when moved to freshwater, if its level of water soluble hydrolysis products is just below the level of salt in seawater.
My answer will be, it is more likely to have osmosis, but not necessarily more likely to get it.
Jan
Last edited:
PCUK
Well-known member
Also depends if it had an anti-osmosis treatment at some time or even from new.
lpdsn
New member
Does that speed it up?
