rivonia
Active member
Why do some surveyors actually SCRAPE big patches all over below the water line before doing damp tests and tapping with a hammer and other surveyors do not scrape at all ??
Fair winds
Peter
Fair winds
Peter
Surveyor Scrape
- Thread starter rivonia
- Start date
D
Deleted member 36384
Guest
Increases The Attenuation and Eliminates False Positives Caused By Bond
From the book I have read on surveying the interface between the paint and GRP i.e. the bond security can give a false positive test result. So by removing the paint you get a positive test result.
I would have more confidence in a survey where the hull paint was scraped back than in one that was not.
The above opinion is not based on any practical experience, just what I have read.
However, I was involved in wall thickness surveys of metal pressure vessels and pipe work. In this case the ultrasonic sensor had to be coupled directly to the parent metal otherwise it would give a false reading. The signal would reflect back off the paint / steel interface causing a false reading.
In both cases direct measurement on the parent material will increase the attenuation through the material. Paint is relatively soft and will reduce the attenuation i.e. damp out the signal.
All the best,
BlowingOldBoots
From the book I have read on surveying the interface between the paint and GRP i.e. the bond security can give a false positive test result. So by removing the paint you get a positive test result.
I would have more confidence in a survey where the hull paint was scraped back than in one that was not.
The above opinion is not based on any practical experience, just what I have read.
However, I was involved in wall thickness surveys of metal pressure vessels and pipe work. In this case the ultrasonic sensor had to be coupled directly to the parent metal otherwise it would give a false reading. The signal would reflect back off the paint / steel interface causing a false reading.
In both cases direct measurement on the parent material will increase the attenuation through the material. Paint is relatively soft and will reduce the attenuation i.e. damp out the signal.
All the best,
BlowingOldBoots
Woodlouse
New member
The antifoul on the hull will hold moisture and throw any moisture meter readings to the point of being useless.
I think the answer to your question is probably the surveyors performing different types of survey. An insurance survey for example won't be thorough enough to require testing the moisture content of the hull as all the insurance company wants to know is whether the boat is likely to sink any time soon. A pre-purchase survey however will test the moisture content as the surveyor will be looking for any potential problems whether present, or likely in the future.
Oh, and also a boat with an epoxy sheath under the waterline is less likely to require a moisture reading as the epoxy resin is impermeable to water, where as polyester resin is porous.
I think the answer to your question is probably the surveyors performing different types of survey. An insurance survey for example won't be thorough enough to require testing the moisture content of the hull as all the insurance company wants to know is whether the boat is likely to sink any time soon. A pre-purchase survey however will test the moisture content as the surveyor will be looking for any potential problems whether present, or likely in the future.
Oh, and also a boat with an epoxy sheath under the waterline is less likely to require a moisture reading as the epoxy resin is impermeable to water, where as polyester resin is porous.
johnlilley
Active member
Moisture meter readings can be taken directly through anti foul coatings and, if the readings are low then, with regard to moisture meter readings, no further action is needed however, if readings are high, the removal of anti foul coatings can make a difference. The prime benefit of removing the antifoul coatings is to be able to inspect the surface of the gelcoat or epoxy for signs of disturbance caused by wicking or any blistering that, in its early stages, cannot usually be seen by the covering of thick antifoul.
The other important area where it is important to check the gelcoat surface is at points of high stress where experience shows that the stress cracking could be a possibility. Very often a stress crack will show through the antifoul coatings although one has to look quite hard to discover it however on any newly applied antifoul coatings any underlying cracks cannot usually be seen until the antifoul hardens and becomes brittle.
John Lilley
The other important area where it is important to check the gelcoat surface is at points of high stress where experience shows that the stress cracking could be a possibility. Very often a stress crack will show through the antifoul coatings although one has to look quite hard to discover it however on any newly applied antifoul coatings any underlying cracks cannot usually be seen until the antifoul hardens and becomes brittle.
John Lilley
Spyro
Well-known member
So is there much point taking a moisture reading on a boat that has just been lifted out of the water? Antifoul scraped or not.
yacht surveyor
Member
When just out of the water all moisture readings will be higher, whether with or without anti-fouling. In a lot of cases for pre-purchase surveys the prospective owner will only want to pay for a lift and hold.
A good surveyor who knows his/her meter will take into account the higher readings in his/her interpretation of the results and his/her report.
Epoxy coatings give significantly higher readings and can take a longer time to dry than the hull. This is because the original hull gelcoat surface is smooth from the mould, and the epoxy is rolled on later. This means that there is a larger surface area.
Now to why we scrape. Moisture readings are part of it but not the full story. By comparing the moisture content in the scraped areas to the unscraped areas we get an idea of what moisture content is trapped in the paint. This allows us to quickly sweep areas (especially for cored boats) with the moisture meter to determine if the core could be wet.
The main reason for scraping antifouling is to identify defects in the gelcoat underneath the antifouling, such as cracking, wicking and the start of osmosis and to see the state of the underlying gelcoat. You can get hints that there might be something going on from the gelcoat, but you do need to get underneath to be sure.
Get yourself a copy of Paul Steven's book on surveying yachts. There is a lot of interesting and useful information in there. I think everyone should read it. (No business connection with Paul by the way - I just think it is a good book)
Hope that is useful.
A good surveyor who knows his/her meter will take into account the higher readings in his/her interpretation of the results and his/her report.
Epoxy coatings give significantly higher readings and can take a longer time to dry than the hull. This is because the original hull gelcoat surface is smooth from the mould, and the epoxy is rolled on later. This means that there is a larger surface area.
Now to why we scrape. Moisture readings are part of it but not the full story. By comparing the moisture content in the scraped areas to the unscraped areas we get an idea of what moisture content is trapped in the paint. This allows us to quickly sweep areas (especially for cored boats) with the moisture meter to determine if the core could be wet.
The main reason for scraping antifouling is to identify defects in the gelcoat underneath the antifouling, such as cracking, wicking and the start of osmosis and to see the state of the underlying gelcoat. You can get hints that there might be something going on from the gelcoat, but you do need to get underneath to be sure.
Get yourself a copy of Paul Steven's book on surveying yachts. There is a lot of interesting and useful information in there. I think everyone should read it. (No business connection with Paul by the way - I just think it is a good book)
Hope that is useful.
Bosun Higgs
N/A
I once bought a boat with osmosis, discovered on the survey. It was in the early stages but I was astounded when the surveyor told me it had the pox. I went back to the yard to have a look since I was convinced it was OK. But it wasnt - and the way I could see the very very shallow blisters was as quarter moon shadows where the scraper had lifted as it passed over a blister and not therefore removed every last bit of antifoul.
moisture meter readings alone are not enough. visual examination is necessary.
moisture meter readings alone are not enough. visual examination is necessary.
Clyde_Wanderer
New member
Do you have the full details of Paul Steven's book on surveying yachts, ie correct title, publisher, ISBN No?
Thanks.
C_W
[2068]
...
