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  1. #1
    Join Date
    Nov 2017
    Posts
    30

    Default Osmosis - freshwater vs. seawater

    Hi all,
    Does anyone know if osmosis is more likely to develop on a boat moored in freshwater or seawater? It's a bit of an academic question but interested in your thoughts.
    Thanks

  2. #2
    Join Date
    May 2001
    Location
    UK East Coast
    Posts
    35,444

    Default Re: Osmosis - freshwater vs. seawater

    If the boat is going to get osmosis, it will happen faster in freshwater than in seawater. But lots of boats aren't prone to osmosis.

  3. #3
    Join Date
    Jul 2006
    Posts
    4,695

    Default Re: Osmosis - freshwater vs. seawater

    Quote Originally Posted by pvb View Post
    If the boat is going to get osmosis, it will happen faster in freshwater than in seawater. But lots of boats aren't prone to osmosis.
    Freshwater worse than seawater. Warm water much worse. The freshwater thing makes me smile when I see fair-sized yachts advertised as ".... only used in fresh water" as if it's a big plus. though to be fair absence of salt does reduce some corrosion in alloy fittings etc.

    I don't think there are any GRP yachts that are not prone to osmosis - eventually. I have heard a surveyor say that he's seeing modern gelcoat resin boats with osmosis as owners think they're immune and thus keep boats full-time afloat. Overall though even with eventually some osmosis GRP is a pretty good material. And a few blisters doesn't necessarily mean huge bills.

  4. #4
    Join Date
    Aug 2011
    Location
    On the Celtic Fringe
    Posts
    13,857

    Default Re: Osmosis - freshwater vs. seawater

    Quote Originally Posted by jwilson View Post
    Freshwater worse than seawater. Warm water much worse. The freshwater thing makes me smile when I see fair-sized yachts advertised as ".... only used in fresh water" as if it's a big plus.
    Just think of the lack of anti-foul on its bottom.
    Cynical Scottish almost retired engineer.

  5. #5
    Join Date
    Jul 2008
    Location
    East coast UK. Mostly. Sometimes the Philippines
    Posts
    8,520

    Default Re: Osmosis - freshwater vs. seawater

    I am not any sort of expert beyond reading Hugo du Plessis masterwork and being present at a couple of surveys of GRP boats. I don't think that visible blisters are a reliable indication; the problem is high moisture levels in the grp layup which eventually reduce its strength. What you don't want to see are white looking fibres. Certainly grp boats benefit from an opportunity to lose some moisture, annually.
    Last edited by Minn; 02-04-19 at 12:04.

  6. #6
    Join Date
    Nov 2017
    Posts
    30

    Default Re: Osmosis - freshwater vs. seawater

    Hi, thanks for the feedback. Very useful, and I guess the exacerbation in freshwater is down to chemistry. Our boat is in freshwater (Cardiff Bay) and fairly newly acquired so we've not lifted her out yet.

    Netx question - how long should a boat be out of the water for each year to make it worthwhile with respect to drying out a bit?

  7. #7
    Join Date
    May 2001
    Location
    UK East Coast
    Posts
    35,444

    Default Re: Osmosis - freshwater vs. seawater

    Quote Originally Posted by jrussill View Post

    Netx question - how long should a boat be out of the water for each year to make it worthwhile with respect to drying out a bit?
    I've never been convinced that it's necessary to allow the hull to "dry out" ashore, and I certainly don't believe any significant drying is likely to occur over a typical British winter. So if you really want to give it chance to "dry out", you'd need to leave it ashore for a summer at least.

    Thousands and thousands of boats are afloat all year round without major problems. If a boat is destined to get osmosis, it'll happen sooner or later, regardless of whether it gets lifted out for periods.

  8. #8
    Join Date
    Mar 2013
    Location
    London
    Posts
    5,912

    Default Re: Osmosis - freshwater vs. seawater

    Quote Originally Posted by jrussill View Post
    Hi, thanks for the feedback. Very useful, and I guess the exacerbation in freshwater is down to chemistry. Our boat is in freshwater (Cardiff Bay) and fairly newly acquired so we've not lifted her out yet.

    Netx question - how long should a boat be out of the water for each year to make it worthwhile with respect to drying out a bit?
    Is it an old boat worth not a great deal? If so I wouldnt worry about it and just go sailing. If its a newer expensive boat then theres lots to worry about,osmosis probably being well down the list.

  9. #9
    Join Date
    Nov 2017
    Posts
    30

    Default Re: Osmosis - freshwater vs. seawater

    Yes, it's an old boat (1980ish Virgo Voyager) and not worth a great deal in the grand scheme of things. It's mainly for my interest. But we are looking at a larger Colvic Countess 28 that has probably been in the water for years too (and also not worth much), so I'm trying get a feel of how much to worry without paying to have her lifted.

  10. #10
    Join Date
    May 2001
    Location
    North from the Nab about 10 miles
    Posts
    8,773

    Default Re: Osmosis - freshwater vs. seawater

    Dont forget that if rainwater has been allowed to gather in the bilges, you will get osmosis inside as well! Seen it,
    Is Conservation for wildlife or conservationists?
    http://boatownersresponse.org.uk

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