Osmosis and moisture readings

First of all, what is osmosis? All laminates in a marine environment, including epoxy, will allow water molecules to pass through them. During manufacture small voids are often created between the gelcoat and first ply of laminate, where the water will condense out and then start to break down (hydrolyse) components in the laminate. These include the ester linkages in the polyester as well as trapped dirt and debris. Breakdown products include a variety of acids, alcohols and metallic compounds. In older boats they include acetic and hydrochloric acids from the emulsion binder used in the manufacture of glass reinforcing cloth, which give blisters their characteristic 'vinegary' smell. Glycols can also be released from the resin, these are hygroscopic, attracting further water and give blisters their 'greasy' consistency.

So we now have tiny pockets of concentrated solution under the gelcoat and this is where osmosis begins. Osmosis is the process whereby water molecules pass through the gelcoat (a semi-permeable membrane) to dilute the more concentrated solution. The water increases the fluid pressure in the cell which can eventually distort or burst the laminate or gelcoat.


To fully assess the condition of the laminate a section would need to be ground out and chemically analysed, a process that is clearly not acceptable to the owner as part of the survey. During survey the condition of the gelcoat is assessed by removing the antifouling at several test patches around the underbody. This is done using a sharp, flat edge creating a planing effect that leaves the gelcoat intact but reveals any high spots, often the first stage of blistering. This information is combined with moisture readings taken using a Sovereign Quantum capacitance-type moisture meter. Moisture readings have to be considered in conjunction with the period the vessel has been ashore and the type of resin used. For example, orthophthalic resins were used up to the early/mid 1990s and tend to absorb and retain moisture; these yachts can be expected to show ' high' readings for at least a week or two after lifting out, even where the laminate is sound. However since the mid-1990s isophthalic and vinylester gelcoat resins have been widely used which can show satisfactory moisture readings within an hour or so of lifting out. Furthermore, if ashore for less than two weeks in summer or four weeks in winter (UK), readings below the waterline will be affected by surface moisture and can be expected to fall. Where an epoxy coating has been applied (usually with a brush or roller) the surface is full of micro fissures and voids which take up moisture and can double the length of time to dry out. It should also be noted that where temperatures are close to dew point and humidity levels are high, accuracy will be reduced.

Due to the above it should be stressed that there is no direct correlation between moisture content and laminate condition. It is not uncommon for a well laid-up hull using good quality resin to have high moisture content and no visibly detectable moisture-related defects. Subject to these factors, readings for FRP solid laminates using a Sovereign Quantum moisture meter can be interpreted as follows:

0-15: can be considered dry for all practical purposes.

16-20: some moisture present but of no significance.

21-30: considered medium but at the top of this range approaching the point where the risk of moisture-related defects developing is becoming significant.

31-45: considered high and at a level where the risk of moisture-related defects being present but not yet physically detectable is significant.

46-60: very high and usually accompanied by physically detectable signs. Likely to be accompanied by a significant increase when switched to deep mode.

>61: extremely high and indicative of possible laminate damage in addition to osmotic blistering. Likely to be accompanied by a significant increase when switched to deep mode.

It is recommended that the vessel is laid up ashore for 2 to 4 months each winter and that an annual inspection is made of underwater sections in relation to defects relating to osmosis.

What safety equipment should I carry?


Merchant Shipping Regulations require vessels over 13.7m. to carry specific fire fighting equipment, as below.

Fire fighting appliances required on craft over 13.7m and under 24m

The Boat Safety Scheme also contains sensible advice.

Life-Saving Appliances

Merchant Shipping Regulations require vessels over 13.7m. to carry specific life-saving appliances, as follows. Reference is as for fire-fighting equipment above.

Life-Saving Appliances required on craft over 13.7m but less than 24m in length

GRP Repairs - Some helpful tips.

Firstly confirm the extent of the damage, it may be necessary to remove deck hardwear to gain full access. Ensure correct personal protective equipment is worn when carrying out repairs.

Surface (Gelcoat) Damage only

Where not extended through the gelcoat into the laminate, for cracks v-out the crack. Otherwise remove loose gelcoat and chamfer edges. Clean with acetone or styrene then fill with colour-matched gelcoat paste, letting it bulge slightly. When it begins to cure seal the surface with plastic or PVA. Fair and polish when cured.

For deck voids, break away the cracked gelcoat then use a rotary grinding point to grind the interior surface of the cavity. Chamfer the gelcoat edges. Clean with acetone or wipe with styrene (better), fill cavity with a putty of polyester (laminating not epoxy) resin and chopped glass. When hard, fill with gelcoat, overfilling slightly. Roll a piece of plastic into the repair and seal edges with tape. Sand flush and buff when cured.

Laminate damage

Shallow gouge - use a scraper to open up the damage putting a smooth chamfer on each side of the gouge. Wipe the V with styrene to reactivate the surface then fill the V with polyester resin thickened with chopped glass. When the resin kicks fill the remaining depression with colour-matched gelcoat paste, letting it bulge slightly. When it begins to gel seal it with plastic or a coat of PVA. Fair and polish when cured.

Deep gouge (deeper than upper two or three layers) - grind the damaged area into a depression with a 12-1 chamfer. Wipe with styrene and coat with polyester resin. Position and wet out increasingly large areas alternating mat (first layer) and cloth to the bottom of the gelcoat layer. When resin kicks brush on (or spray) 500 microns gelcoat paste. Fair and polish when cured. Consider applying additional laminate on the inside with epoxy to provide further strength.

Rules and regulations for pleasure craft

The Merchant Shipping Regulations apply to all vessels under the UK flag and vessels in UK waters or operating from UK ports. These include COLREGS, MARPOL. Further information can be found at the following link:

Information on regulations that apply to commercial vessels can be found at:

Seacocks - are they safe?

Fittings used are often made from forged brass. Whilst these valves are in common use, ordinary brass such as this is subject to de-zincification in sea water. The ISO standard relating to metallic valves and skin fittings below the waterline, ISO 9093-1, only requires the valves and associated fittings to have a service life of 5 years in terms of corrosion resistance. The valves and fittings can often pass all the tests described above but consideration should nevertheless be given to replacing them with DZR (de-zincification resistant brass) or bronze, both of which have a much longer potential life.

Petrol Containers/LPG Cylinder Stowage

The following is taken from the Boat Safety Scheme (

Spare petrol containers must be stored in the open where any leaked petrol would flow overboard unimpeded, or in a suitable locker. Any locker used to store spare petrol must be:

· drained to the outside from the bottom; and,

· secure and constructed of a material of the required thickness, in good condition; and,

· free from objects that could block the drain, damage the petrol container or cause petrol vapour to ignite; and,

· fuel-tight to an equal or greater height that the top of the cap for the petrol container; and,

· self-draining and the drain hole must have a minimum internal

· diameter of 12mm (1/2 in) and must not be blocked; and,

· the locker must not open into any engine, battery or electrical equipment space; and,

· the drain line material including connections must be complete and in good condition.

Seaworthiness and Security

The Financial Ombudsman website

provides useful information. It should be noted that most insurance policies exclude losses caused by 'unseaworthiness'. This is defined under the Marine insurance Act of 1906: 'she is reasonably fit in all respects to encounter the ordinary perils of the adventure insured'. In looking at claims rejected on this basis the FOS will look for evidence as to whether the vessel was regularly and well maintained by a professional marine engineer - and whether the servicing and maintenance was carried out in line with manufacturer and industry recommendations. In addition there are basic checks and precautions that FOS believe reasonable to carry out include the following:

§ checking the bilges (inside the bottom of the vessel), to ensure that there is no water in them - as this may be evidence of a leak;

§ inserting bungs into the drain holes at the back of the vessel, if it is being launched from a trailer;

§ closing all seacocks, to prevent water getting back into the vessel; and checking all safety equipment.

The FOS website also provides important information on security; that insurance policies often include additional security requirements e.g. a particular type of lock, dinghies being named, outboard motor serial numbers recorded etc. It is important to read the policy in detail to ensure the requirements are met.

Buying a boat - the sale process