Osmosis and Moisture Meters on Wet Hulls?

CaptainBob

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Couple of questions for you lot...

1. If a 20 year old GRP vessel got a clean bill of health from a surveyor in terms of no signs of whicking, blistering or de-lamination, and a low moisture meter reading, 3 years ago, and spent the subsequent time in the water (some sea, and 1 month fresh) apart from 6 months on the hard (so a total of 29 months in sea water and 1 month in fresh water).... would you think it could have started to develop worrying osmosis related issues by now?

2. Same vessel, currently in the water at the end of the summer season in the water, with 6 month old AF and some green growth just starting to come. If it was hoisted for a few hours, and then jetwashed, so a surveyor could stick a moisture meter on it (after scraping back the AF here and there) - would that yield useful information? Or would the fact that the hull will blatantly be really quite damp mean this would be a near pointless exercise?

Thank you!
 
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My comments are based entirely on book reading as an interested amateur who owns an old GRP boat.

Point 1: Probably not. The conditions for Hydrolysis / Osmosis are established at time of GRP lay up. If it has not developed after 20 years, then it will not likely develop rapidly because the lay up was of a good enough standard. That does not mean that it will develop later.

point 2: The readings will be of limited value in establishing if the risk of Osmosis is present. It will give you a good indication if the hull has a high water content if used properly taking into account calibration and deep and shallow readings. If the sensor is just stuck on the hull and a reading taken (after suitable surface preparation) then that by itself is insufficient. As I understand it a reading that is indicative of high water content (relative) is still a true reflection of the water content. However, if the level was found to have fallen away to low reading after a winter lay up, then that would be a good sign.

Read through this it may illuminate your problem a bit more: http://www.osmosisinfo.com/handb9a.pdf

Unfortunately this is all from memory as my books are at home and I am at work (abroad). Hopefully you will get a succinct and better answer than this (maybe even corrected).

What I do know is that you need to use a surveyor who understands the limitations of the moisture meter and readings taken just after haul out and the range of interpretations that can be made. Not all Surveyors with moisture meters apparently can do this. Of course talk to a few Surveyors in your area and ask their opinions.

I hope at least this will raise your post to the top and prompt someone with a clearer understanding to comment.
 
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minkysailing

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Couple of questions for you lot...

1. If a 20 year old GRP vessel got a clean bill of health from a surveyor in terms of no signs of whicking, blistering or de-lamination, and a low moisture meter reading, 3 years ago, and spent the subsequent time in the water (some sea, and 1 month fresh) apart from 6 months on the hard (so a total of 29 months in sea water and 1 month in fresh water).... would you think it could have started to develop worrying osmosis related issues by now?

2. Same vessel, currently in the water at the end of the summer season in the water, with 6 month old AF and some green growth just starting to come. If it was hoisted for a few hours, and then jetwashed, so a surveyor could stick a moisture meter on it (after scraping back the AF here and there) - would that yield useful information? Or would the fact that the hull will blatantly be really quite damp mean this would be a near pointless exercise?

Thank you!

1. A 20 year old GRP vessel with no signs of surface abnormalities and low moisture readings to suddenly develop a serious osmosis problem with high hull moisture readings in 3 years is unlikely. This would appear to indicate that either the assessment 3 years ago was incorrect or the recent readings/survey are flawed.

2. A vessel lifted and assessed for moisture content/osmosis within 12 hrs in my opinion is flawed and has limited use for determining the actual hull moisture content and likely implications. However a good surveyor does not rely solely on surface hull moisture readings when assessing a hull for condition.
 
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SHUG

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I gather that these days , surveyors use sophisticated moisture meters which give a depth profile of the water content . This can also be used to detect "wicking".
There will be surveyor along in a moment to explain all this but the bottom line is that simple moisture meter readings can be very confusing.
 

Crabman

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Simply taking moisure meter readings will not work if the yacht in question has just been lifted . When taking moisture meter readings I used a surveyor who took the readings inside and out 24 hours after the boat had been lifted .He also used thermal imaging and ultrasound . The survey was on a 44 ft yacht that I purchased .The bottom line is the dryer the boat when having a survey done ,the better and if you can attend I would ,I learnt a lot from the surveyor.
 

pappaecho

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During my recent survey, high moisture readings were recorded, even though the 30 year ols boat has no sign of wicking, blistering or osmosis.
On questioning him more closely he admitted that the meter "was not reliable" when read copper based antifoul.
Guess what - I used E45 antifoul last year it is 45% copper.
In short the meter readings were meaningless. The fact there is no sign of osmosis or delamination is enough for me
 

Crabman

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During my recent survey, high moisture readings were recorded, even though the 30 year ols boat has no sign of wicking, blistering or osmosis.
On questioning him more closely he admitted that the meter "was not reliable" when read copper based antifoul.
Guess what - I used E45 antifoul last year it is 45% copper.
In short the meter readings were meaningless. The fact there is no sign of osmosis or delamination is enough for me

Would meter readings be unreliable if the hull was epoxied? I would not be to happy if scrapings were taken if copper or epoxy was already applied .
 

Spyro

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On the other hand I've seen boats that have been sitting out of the water for over a year up for sale then you see a surveyor has been along scraped some A/F and taken moisture readings.
Does that give any useful information?
 

GrahamM376

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Would meter readings be unreliable if the hull was epoxied? I would not be to happy if scrapings were taken if copper or epoxy was already applied .

Had some meter readings taken before epoxying my hull last winter. According to the surveyor, the new meters will read accurately through epoxy so no need to scrape through it. Having had various boats checked over the years, it seems the meter is only as good as the bloke using it and many are covering their backsides with comments like "this boat may develop osmosis in the future".

If there are no blisters then don't worry, high moisture content isn't osmosis.
 

Csail

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Just ignore it, we spent a fortune (6k) and no difference and all further surveyors said ignore it. I think its just a plan for boat yards to make money.Ask about and i bet you won't find anyone who has sunk a boat due to osmosis.
 

pappaecho

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In reply to the question whether moisture meters would be affected by epoxy coatings, my answer would be that the meter is measuring conductivity of moisture, so the more moisture the higher the reading. Copper specially at 45% is a very good conductor, and so could completely negate the readings attributed to moisture.
Epoxy is an insulator, and unless it is filled with a metallic filler, it should have no effect on the conductivity and the reading of "moisture"
 
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