Osmosis: bottom peeled but not drying?

[lenseman]

I'm "in the trade" . . . . Furthermore, the fact he told you it was "5%" would be cause for concern, as the manual specifically states that the meter does not give a percentage moisture content.

There is a possibility that the person who gave a measurement was using another type of Tramex meter, "Tramex Moisture Encounter" (TEX-MEP) which is designed to be used for buildings, roofs and walls. This gives results in percentages?

Apparently, there is a different type of Tramex specifically for use on GRP and plastic yachts, the "Tramex Skipper Plus" (TX-SMP)

 

[dombuckley]

But OP specifically said that a Sovereign meter had been used. In this application, the words "chocolate" and "fireguard" spring to mind....!

I'm with you: the Tramex Skipper is my own personal "weapon of choice".

 

[Manuel]

I'm with you: the Tramex Skipper is my own personal "weapon of choice".

Moisture meters of these kinds do not sense moisture. They sense the dielectric properties of the hull. A totally dry, non-conducting plastic will have an almost zero 'loss angle'. If something within the field of measurement conducts electricity then the 'loss angle' increases. The assumption, with these instruments, is that the loss must be due to moisture in the hull. That need not be true. Condensation on the inside of the hull, in the bilge can (and often does) give a significant reading. If carbon fibres were used in the mat, they might also conduct, giving a reading. Any fibrous material such as balsa will possibly have, inherently, a greater loss angle than pure plastic. Gelcoat is not impermeable and does become wet. This tends to dry out after a few weeks but can be very significant. Any antifoul is likely to cause a loss, and give a false reading. Not all mat is totally dry when the hull is made.

It is vital to realise that whatever your instrument might say on the dial, it is not actually reading 'moisture'. It is reading 'tan delta', the loss angle. In many cases tan delta might be a reliable indicator of moisture but the instrument must be used with caution.

A man sitting in the Ritz with a magnum of Bollinger, wearing a DJ, might look like a rich toff. Or he might be dead broke and drowning his sorrows.

Appearances can be deceptive.

 

[bluedragon]

Is that what they actually measure! That's interesting...

Many years go when I was working on a project that related to the heating of plastics in RF or microwave fields, the effects of certain lossy fillers in a low-loss polymer was massive. It makes me wonder therefore what gel coat fillers or pigments might have been used in hulls in the past, and to what extent they have raised the "base" level reading?? I've never seen that discussed anywhere.

 

[oldsaltoz]

I suspect it would make little difference as it's so thin and represents a small percentage of the total.

Avagoodweekend......

 

[Manuel]

I suspect it would make little difference as it's so thin and represents a small percentage of the total.

It's also the part in contact with sea water all the time. Water or conductive salts will cause loss and appear to be moisture. The instrument can only give the total loss figure, it cannot tell you which layer the loss is in or what is causing the loss. Normally surveyors scrape off the antifoul in the areas to be tested ("patches") but some hull treatments (epoxy shields, etc.) and the gelcoat itself can retain moisture. A next step is to remove the gelcoat down to the mat but if there is good evidence of a problem with the core then a hole is going to be the only definitive way. It's not a big problem. Many yachts are holed below the waterline and are repaired to as-new condition so none of this is going to be a problem for a decent yard. But it all costs money.

 

[bluedragon]

I suspect it would make little difference as it's so thin and represents a small percentage of the total.

Avagoodweekend......

But it's also the part of the hull where the meter contacts, so it may have a disproportionate effect. I've noticed a quite a difference in reading when measured from the outside or inside of a (non-cored) hull in the same area, indicating that the concentration of "lossy" material is on the outside, not in the lay-up. I don't think surveyors often check from inside, do they?

 

[Manuel]

I don't think surveyors often check from inside, do they?

They must check that the compartment on the other side is not damp or filled with damp, or metal materials. Otherwise the test is pointless. The procedure is to empty and dry the bilge sections you want to test, use a hammer to locate the sections and scrape off and dry the points of test.

 

[Bilgediver]

After six months it still has a moisture reading above and below the waterline of 20 (sovereign meter) as opposed to the required 5. We suspect the balsa core is wet and it has been suggested that we put on another layer of glass followed by epoxy rather than GRP. Does anyone have any experience of this method? Any other ideas?

My first though as no doubt others may have stated is that the readings seem very suspicious.

With readings like this I d have held back doing anything until the readings were confirmed as good. The fact you are getting the same readings above the waterline as below would suggest that maybe hings are fine and the calibration of the meter needs checking. Certainly not a reason to put on the butchers apron and dig out the shaver and gritblaster!.

Did you notice a pronounced smell of vinegar after shaving the hull and has the smell diminished?

You must realise that the drying out process actually needs a little help if there is a problem. You need to wash the hull twice a day with warm water to speed the process up. It is not water you are removing but the products of osmosis which cake up on the exposed surface and slow down the action. Therefore the need to we the hull. The so called drying is not complete till the smell of vinegar has long since departed. If no smell of vinegar at day 1 then sack your surveyor!!

You could have removed a substantial amount of the boats strength if the shaving was over enthusiastic so be carefull of what you do next to ensure no further strength is lost.

The suggestion you have received seems OK however cannot proceed till the substrat is clear of the products of osmosis or you will just trap it in.

I would be very suspicious of a set of readings from the gunwale to the keel that gave the same consistantly high readings as it is usual for the hul to appear quite dry as you get further from the waterline.

Taking cored samples of the balsa is a good idea and will confirm if this is creating the rogue readings.