GLASS-REINFORCED PLASTIC (GRP)
Published: 25 April 18
First of all, I would like to thank Hugo for his superb presentation on surveying keel matrices. I think many surveyors will take a much more careful look and not be afraid of lifting the boards.
It is only April and twice this year I have heard of repairs associated with possible groundings failing after about 18-30 months. All are immediately forward of the keel/hull attachment.
This is one of the most highly stressed areas of a boat. Immediately aft is the front of the keel, a heavy weight hanging off subject to severe vibration and possible impact shock: immediately forward is the base of the mast or compression post also subject to significant, but different vibrations and the fulcrum between the fore and aft stays and the shrouds. As far as I can make out, as the two reports are from different ends/sides of the country they are not related.
Both involve repairs which after the application of anti-fouling are essentially invisible to the naked eye externally and probably not detectable via a hammer tap. A good internal examination may reveal the presence of a repair which has not been reported to the surveyor.
As the surveyor, you have no idea how the repair was carried out. Was the mast un-stepped? Was the keel removed? How was the hull supported? Was it carried out under controlled conditions or in the boatyard? What experience has the repairer and what materials did they use? All of which have a bearing on the quality of the repair and the stresses which may be set up in the repair and surrounding hull area.
Then, of course, the keel is re-attached and the mast stepped which will change the shape of the vessel. If the owner or rigger then decides to “tune” the rig the hull shape may change yet again.
I have always thought that craning is a violent act. The boat is supported by two strops (probably similar in area to eight prop pads) moved, swung about and placed in the water where it is supported by the whole underwater area, once more changing the hull shape. I wonder how many boat builders re-align the engine at this stage?
The boat is then used for a number of months in unknown conditions; possibly touches the putty; and is sold. Craned out and back in again for a survey or maybe more. The surveyor gives it a clean bill of health; the new owner maybe moves it to another part of the country by sail or possibly on transport which places a whole new set of stresses on the structure. The new owner uses it for several months, placing it under strain, takes it out for the winter and discovers some cracks in the hull forward of the keel. Something the surveyor must have missed!
To quote an old cop show “Let’s be careful out there” (Hill Street Blues)
Now to something a bit more mundane
After reading a number of reports for upgrading, I thought it would be a good idea to go back to some basic stuff. This is also brought on by my observance over a number of years of “surveyors” surveying GRP vessels without the use of a hammer. I've even came across one “surveying” using a claw hammer.
Construction methods and materials have changed over the years, sometimes for the better and sometimes not. Many older vessels will have a much thicker hull shell than many modern boats. Those built in a certain period in the late 1980s had “soft” porous gel-coats which cost a resin
manufacturer a considerable sum of money.
In my personal opinion, there are a number of mass produced boats, usually at the economic end of the market where the shell thickness, internal structural support and the connection between the two is pared down to the bare minimum. This is OK when new but, after several years of use and flexing, the shell can begin to suffer. If we add in the stresses and strains imposed by either a rig and ballast keel or constant hammering at 25 knots across a moderate sea state, then close examination of the hull is imperative.
The examination of a grp/frp hull and deck must use all three basic tools to provide full information as to that hull's structural condition.
Mark 1 eyeball; to verify any distortion; any blistering; to determine size and frequency of blistering; any cracking or discolouration; or any other possible anomalies.
The first two stages of blistering are not necessarily osmotic, they are degradation of the gel-coat. In other materials they would be blisters in the paint coating and should be considered in the same way. In some cases, they are blisters between two gel-coats, or between polyester and epoxy, or in the
primer for anti-fouling.
Photo courtesy of Sam West, YDSA Accredited Surveyor.
Stage 2 blisters should have a sample batch broken open and possible pH measured, or at minimum sniffed. A check can be made on whether the outer plate laminate is being degraded by the acid.
Stage 3, damaging osmotic blistering, where the plate laminate is being forced apart by osmotic pressures, is often invisible to the naked eye as the size of the blisters, equivalent to a dinner plate or bigger, blend in with the shape of the boat. This is where the hammer comes into its own.
Hammer - I consider this the most important tool to detect any possible voiding inbuilt as a manufacturing fault or repair, de-lamination caused by blistering or damage, the extent or size of delamination, whether it is an isolated area or prevalent in several areas. The hammer will also locate
internal frames and longitudinals. Their position may affect moisture meter readings and will certainly be necessary to know when monitoring drying-out for treatment.
Close hammer testing also forces the surveyor to visually examine the whole hull for visible problems.
Moisture meter - To determine the possible uptake of moisture into the laminate and the variations between sections of the vessel and comparisons with the areas above wind/waterline. These comparisons are essential as, generally, they provide a baseline against which the bottom measurements can be gauged. Having said that, I have come across a number of vessels where areas above the wind/waterline were higher than those below. Further investigation found either repaired damage or a moulding fault. In addition, internal structure or fittings may increase readings locally; bulkheads, frames, tanks, trapped water, leaking windows, wiring, plumbing, etc.
A high moisture content does not automatically mean she is in danger of structural failure, although again reading some reports, that is what they intimate. Many vessels which might be 15-20 years-old may well have a high moisture content but, there is no sign of blistering or de-lamination or any other indication of possible failure.
Some reports I have seen, also give the meter readings as the actual moisture content and with all machines this is not the case. In my opinion the actual moisture contents should be given; i.e. “readings were between 20-30 Tramex scale 1 (which equates to 0.09% by weight of moisture).” These readings can be considered dry.
It annoys me to read reports of meter readings on page 7 and an explanation of what that means on page 33 (a recent application).
Many years ago the Southampton Institute of Higher Education developed a table of comparisons between various meters and actual moisture content. This may have been updated but, I have not seen it. If you do not have a copy it can be made available in the Members’ area on the website; let
the office know.
When it comes to RECOMMENDATIONS, a judgement sometimes has to be made on rectification. In a 6-9m vessel the cost of full osmosis treatment could easily be more that the boat is worth on the open market; however, if she has not developed inter-laminar blistering then they should be allowed
to continue in their current state with maybe a recommendation they have their bottom shells examined every five years to determine if structural degradation has/has not set in.
There are many different types of lay-up which change with fashions and increases in technological application and these will affect the rate of moisture absorption. Gel-coat types have altered over the years, the use of surfacing tissues, hand lay-up, spray lay-up or a combination of the two, the use of prepregs and vacuum injection. There are some boat builders who offer epoxy coating from new to reduce the likelihood of osmosis damage in the future, but epoxy will succumb eventually.
The way the vessel has been treated by the owner will also affect the possible onset of blistering. Is she normally afloat 12 months a year or only eight months? Is she kept in fresh water or salt water?
In some of the reports I read, I do wonder how extensively the vessel has actually been examined. During report writing sessions I hear of 15m boats having a full survey in 4-5 hours; I would schedule 1.5 days for a vessel of that size to examine and record the information with the report writing additional.
So, going back to near the beginning… “Let’s be careful out there”