Hull moisture readings question

[richsharp]

Ive been told that some surveyors require a GRP boat to be brought ashore for up to 5 days prior to taking moisture readings on the hull and others do it whilst the boat spends an hour hanging in the slings having just been lifted out of the water. Can anyone advise if these two approaches produce dramatically different results? Which is the most/least meaningful? Obviously for the bloke footing the bill for the survey there may be a significant cost implication.

 

[johnlilley]

Hi There,
yes it is a dilemma sometimes, but some (although quite few) can be lifted from the water and after surface drying, moisture meter readings might indicate low readings, in which case a reasonable assumption could be made that the readings would be even lower after a few days, however, the larger majority in my experience, will have moderate or higher readings immediately on lifting and surface drying. It would be unfair to the boat to note these as high readings at that stage, because there is a likelihood that the readings would continue to drop for a few more days, the largest drop taking place within the following 3 or 4 days with a continual but ever decreasing degree of drying for a much longer time.
In the case of a vessel with higher residual readings, the readings would not drop to any great consequence after the passage of 4 or 5 days, but as you can see, one vessel might result in much lower readings than another after a few days, despite them both having high/er readings on the first day out of water. My web site may help with some further understanding of moisture meter readings, hope that helps.
John Lilley

 

[richsharp]

Thank you for your explanation, from which it would seem that the prudent approach is to take readings after 3 or 4 days drying time. Would wet weather in that drying period also confound matters?

 

[johnlilley]

Continual rain obviously affects localised areas where it runs down, but it is usually possible to find "dry"areas" Heavy rain can be inconvenient, but atmospheric humidity does have a minimal affect, particularly if you require absolute accuracy but, in my experience, not excessively so, but like all things, You have to be there on the day...However condensation on the surface is something that should not be overlooked by your surveyor
John Lilley

 

[owen]

your answer makes complete sense. however given that many surveyors are pushed by clients to do the survey betwen tides do you think this makes a diagnosis of high moisture content more likely .?

 

[johnlilley]

Probably, although when I survey under these circumstances, I try to make it very clear that there is almost no point in taking moisture readings and certainly no point in making any assumptions about chemical activity within the laminate, unless of course, it indicates very low readings at that time, which is only the case occasionally. It is impossible to predict if high readings at lift out will change after a few days. By the same token it is unfair for a surveyor to automatically frighten a purchaser or owner by not using guidance about the circumstances of taking the moisture readings and, in my opinion, no boat should be unfairly criticised just on the basis of high/er readings without other indicators and full explanation of causes of high readings, including lack of time out of the water. A day or two out of the water is usually enough to indicate if there will be a residual high reading, but it is helpful if a degree of common sense is observed by the surveyor and the client with full explanations instead of scare tactics.
John Lilley

 

[Guest]

I agree generally with Johns answers but am surprised that a couple pf points are overlooked :

a) The moisture that will normally reduce by boat standing ashore is actually not the hull moisture - but that contained / trapped in coatings - ie antifoul etc.
b) Despite Meter manufacturers claims that readings can be obtained through antifoul etc. - that is something I would never do. I would always scrape back to gel or base to obtain dryest possible surface to check - whether stood ashore for 1 hour or 1 year ...

With regard to experiments that I conducted on my own boat many years ago to confirm my experience / knowledge I presumed I gained over years ... I tested my boat as follows :

1) Lifted ... wipe of area of antifoul and test ...
2) Same area immediately after 1) - scraped of antifoul and tested. Reduced reading by significant amount.
3) Same area later after standing in yard and prior to antifoul application. Little change from 2)
4) After antifoul tested again ... reading increased small amount !

Not conclusive but came someway towards confirming it can be done on lifting a boat - but with due care and attention. I would also say that it depends on the meter used - the Old die-hard Soveriegn Meter as I have experienced does not show so well the differences in the 4 instances above as for example the Tramex ...

Finally as I have found - who's paying the bill doesn't want to pay storage charges waiting to do a test !

 

[johnlilley]

Hi Nigel
Yes I agree, it goes without saying that if high readings are present, removal of the antifoul coatings can sometimes show a reduced further reading, however, I do think that in some cases water related moisture can remain within the thickness of the gelcoat close to the surface without being of any structural consequence at that time, but this moisture (as against chemical build up in the laminate which causes blistering and high readings) does dry out subsequent to lifting and can alter any first impressions about hull activity. Some gel coats are more porous than others and it is only fair to give a boat with immediately high readings on lifting a chance to physically dry for a few hours. I am sure opinions will always be different in this area and that can only help eventually in increase in knowledge.


John Lilley

 

[Chris_Robb]

This was an extract from the survey on my Oceanlord, to show you a practicle example. The Surveyor did comment that it was unusually dry - especially for a boat that stayed in the water over winter sonce 1991!

EXTRACT
F. Areas of antifouling were scraped away to inspect the gel below the waterline and in these areas, all was found visually to be in good condition.
G. Moisture readings were taken using a Sovereign Moisture Meter both above and below the waterline.
H. Moisture readings above the waterline were found to range between three and four.
I. Moisture readings below the waterline were found to range between four and five.
J. These readings are good and indicate that the vessel was moulded in the correct conditions and that she has absorbed little or no moisture since the day she was moulded.
K. Moisture readings of up to fifteen are acceptable.

 

[chiefeng]

Are there any reasons why the readings cannot be taken from inside the hull if accessable, to give some kind of comparison?
Rgds
Bob

 

[damo]

I have read that a significant amount of moisture in a hull can originate from the inside of the boat eg bilges, rainwater, condensation. So even if the outside is allowed to dry out moisture can still keep entering the grp.

Anyone know the story there?