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Old 01-02-2015, 06:50   #31
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Re: Salt vs Fresh water ?

As long as were posting actual events, my friend and slip neighbor had to replace his engine this winter because the manifold and risers failed and let seawater into the engine when it was running. Water doesn't compress like air so the engine was ruined.

He blamed this on using aftermarket parts, not originals. They failed in three years.
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Old 01-02-2015, 07:26   #32
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Re: Salt vs Fresh water ?

It is 304, not 314 stainless, and it has less corrosion resistance than 316, but higher strength. 304 is not a bad choice at all in fresh water applications - more strength, less expensive, and the corrosion issue is not as important.

Do any of the common small auxiliary engine manufacturers even make a raw water cooled engine anymore? If they are more prevalent in fresh water, it is because they are very old models and their salt water brethren all died years ago. Certainly, there is no boat built with an engine installed specifically for fresh water anymore. I don't even think there was in the past.

Osmosis can be more prevalent in fresh water - particularly warm fresh water - but not because of the size or density of water molecules! It is due to the differences in osmotic pressure of the two. Gelcoat and fiberglass are semi-permeable membranes so, accordingly, warm water causes osmosis more than cold, fresh more than salt.

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Old 02-02-2015, 10:29   #33
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Re: Salt vs Fresh water ?

As to freeze thaw damaging rigging.... Nope, doesn't happen. As has said by another it'd be the same in Maine as in Michigan. The comparatively small amount of water that gets into the fittings doesn't due any damage when it freezes. To the contrary fresh water rigging lasts many many times longer than saltwater. I know if we are looking at cyclic loading ect of rigging it wouldn't last any longer. However usually it's the crevice corrosion and pitting that does in rigging.


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Old 02-02-2015, 10:32   #34
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Re: Salt vs Fresh water ?

There are also many older boats with atomic 4 raw water cooled engines still going strong. Many older Volvo diesels too. Raw water cooling isn't a problem in fresh water. Of course we only get half the use each year too so...


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Old 06-02-2015, 00:28   #35
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Re: Salt vs Fresh water ?

Quote:
Originally Posted by Jim Cate View Post
WTF? A water molecule is the same size no matter what other ions are associated with it. Your statement makes no sense whatsoever.



Jim

You may be right Jim, but I seem to remember from physical chemistry 101a thing called "the hydration shell". This is like a cloud of water molecules that surrounds a cation such as Na. Don't know if this would have any significance for GRP.


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Old 06-02-2015, 01:46   #36
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Re: Salt vs Fresh water ?

I knew I had seen it someplace.
It's not just a story.
It's quite a comprehensive write up but towards the bottom there is a list of situations in which osmosis can be expected to be worse.
http://www.yachtsnet.co.uk/osmosis.htm


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Old 06-02-2015, 04:02   #37
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Re: Salt vs Fresh water ?

The fact remains that in practice some yachts ‘get osmosis’ - ie blisters, and some don’t. It is known that several factors increase the likelihood of blistering. These are



Long periods afloat without layups



Warm tropical waters



Fresh water is worse than salt water



Coloured resins (including white - the most common) are worse than clear resins

Historically, there have been some batches of boats that have suffered severely from blistering. Often this was due to changes in layup specification, and use of new materials. For example, it is now known that the use of PVA emulsion bound glass mats is bad practice. Emulsion bound mats were introduced in the 1960s as an improvement, and accepted by Lloyds and other classification societies for standards of hull construction for almost 20 years. Emulsion bound mats are now regarded as bad, as the PVA is water soluble, and tends to encourage wicking. Mats used now should be powder bound, especially in the outer layers of a laminate. Nevertheless, plenty of yacht builders still use emulsion bound glass mat in inner layers of hull layups.

Whilst some builders, including those who produce some very expensive boats, have had runs of boats prone to blistering, they have also turned out apparently identical boats that have not blistered. Current thinking is that cleanliness, temperature and humidity control in the moulding shop, and precision of the mix of resins, are the key to building boats that will not blister. Nevertheless I know of one builder who worked in a dirty corrugated iron shed where the climate control was dependent on whether an easterly wind blows through the gaps in the door. His boats actually had a rather good reputation for ‘not getting osmosis’.



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