Ageing fibreglass hull?

[Refueler]

Laminar Flow mentions Baltic ... the problem there is that you are near tideless - the larger water level changes are from wind direction.
But majority cruising the Baltic are well aware of this and the areas to avoid.

 

[Refueler]

He's quite a character. I got him to do an insurance survey a couple of years ago and got just what I expected: a hand-typed report (using an actual typewriter) full of sensible suggestions.

He's still around ?

My first reports were on a Boots portable Typewriter - then I acquired an Amstrad ..... the printer was then a 9 pin and couldn't do fine detail and of course only black print.

 

[JumbleDuck]

On GRP ageing: after 1,000,000 (million) stress cycles, the strength of the laminate is reduced by half - a fair argument for increasing laminate thickness in boats intended for long distance voyaging.

That's meaningless unless you say how big the stress cycles are.

 

[Tranona]

On GRP ageing: after 1,000,000 (million) stress cycles, the strength of the laminate is reduced by half - a fair argument for increasing laminate thickness in boats intended for long distance voyaging.

A keel grid system should (and can) be designed to sustain a grounding at hull speed. The loads are entirely calculable. ABS offers specific scantling rules as a guide for appropriate design. Larsson & Eliasson dedicate an entire chapter to the problem; if someone is interested in the specifics.
In this respect grid or keel attachment failure is either attributable to poor engineering or bad construction or both.

On the frequency of groundings: I had a long chat with a local salvage operator who complained bitterly to me that GPS and plotters had ruined his business. In this respect it could be argued that either there are fewer boats out and about or that the number of accidental groundings has come down due to better navigational aids.
If, as Tranona claims, a lot of the yards are still kept busy making expensive repairs post grounding, then one might assume that Pantanius and the MRC are on to something and, consequently, there is something very wrong with the way keels are currently attached.
I think grounding is a fairly regular occurrence, particularly when cruising in areas with thin water. The Waddenzee and the Baltic come to mind, the latter also has a goodly amount of uncharted rocks to offer. Any serious boat should be designed and built to withstand such an impact.

Like JD I would like to know how you define stress cycles for GRP when used for boat hulls and your definition of strength. The weakness of GRP is flex in flat unsupported panels. Good example is the early Nic 32 which had large unsupported flat bow panels of thick GRP. The solution to the degradation was glassing in stringers rather than thicker layups. Today the layups are completely different and it is not just a question of adding more, but designing the laminate to resist flexing.

I suspect that in general keel grids do withstand grounding well, but the data is poor. One reference point is in the Cheeki Rafiki report. Around 900 of the class of boat were built and most intensively raced - that was what they were designed for. The research picked up around 15 boats that had structural damage from grounding, all successfully repaired. This is less than 2% of the population of a design that would be considered in the high risk category. I have made the point many times that by concentrating on the tiny proportion of boats that suffer from grounding and structural damage we ignore all the others that never have an issue.

The whole point of identifying and assessing these exceptions is to identify the root causes and take action to avoid them. As you suggest the number of groundings has probably declined because of better navigation, deeper keels which make owners more wary and fear of the consequences. You misrepresent what I was saying about repairs. The point was that it is not something new - keels and grounding have always been an issue - but the increase dates from the 1970s with the introduction of fin keels bolted to canoe bodies. The Solent example was used because it has all the factors that lead to grounding being common (in a relative way). Confined waters, littered with hard lumps just below the surface, intensively raced using waters where getting close to these hard bits is often advantageous. The ISAF survey a few years ago came to a similar conclusion.

While you are right that there are parts of the world where grounding is common because of the thin water and it makes sense to take this into account when buying a boat to use regularly in those areas. Look around the world and you will find many examples of similar situations - Pacific coral reefs, Bahamas and Florida sandbars as examples. Steel boats of relatively shallow drafts for the former and long keel centreboarders for the latter. More locally, my Eventide was perfect for exploring the backwaters of Poole Harbour - and I deliberately chose the longer shallow keel for my new boat so that I could use more of the channels in the harbour (as I did yesterday), and get over the bar of East Looe at low water.

On the other hand the vast majority of boats are used in areas that do not have these constraints and owners there would wonder what all the fuss in these types of threads was all about.

 

[Laminar Flow]

Right, on stress cycles.:
Subjected to repeated stress a panel with a breaking strength of 110N/sqmm, after 1 million cycles the strength is reduced from 110N/sqmm to 40N/sqmm or by 53%.
The first signs of degradation are microscopic fractures that occur at just 0.2% distortion, whereas failure occurred at 2%. The micro fissures start surprisingly early and virtually at the outset in the process. (Larsson/Eliasson, Principles of Yacht Design)

While in the past it was common that boats were developed to meet particular local conditions that is no longer the case. The only concessions, generally speaking, made today might be a choice of keel depth and /or rig. Whether short or deep, the keel is attached the same way. Of course a short keel root on a deep keel will exacerbate the stress of impact. Regardless, the GRP offerings that float in the Baltic, the Med, the Solent, even the Bahamas or the coral reefs of the South Pacific are the same.

I can only repeat: the loads of a grounding, whether on rock or sand, are calculable and as such the boat can and should be engineered to withstand impact up to at least hull speed. To give you an idea: deceleration, calculated at 0.25 of a second, as it should be, equates to hitting solid rock and impact should be assumed to be at the keel toe. The fact that grids are knocked loose or destroyed simply points to shoddy practice, either in engineering or execution, IMHO.

 

[Laminar Flow]

Laminar Flow mentions Baltic ... the problem there is that you are near tideless - the larger water level changes are from wind direction.
But majority cruising the Baltic are well aware of this and the areas to avoid.

Tideless or not, there are numerous uncharted rocks in the Baltic such as in the Stockholm archipelago for example or around the Danish South Sea. We even found one in the supposedly dredged, surveyed and marked fairway leading to Heiligenhafen that crushed the leading edge of our drop keel.
You don't need lunar tides to cause problems: in the North of Zeeland we had to abandon the harbour during a storm as the docks were two feet under water due to the surge and our mooring lines slipped off the top of the pilings.

 

[JumbleDuck]

Right, on stress cycles.:
Subjected to repeated stress a panel with a breaking strength of 110N/sqmm, after 1 million cycles the strength is reduced from 110N/sqmm to 40N/sqmm or by 53%.

Without - honestly - wanting to sound picky, that is still an absolutely meaningless statement without knowning what the amplitude of the repeated stress was. High-cycle fatigue is generally characterised with an s-N curve linking the stress range (s) to the number of cycles to failure (N), and while you are not talking about failure you are talking about the sort of accumulated microstructural damage which leads to failure. Metals fail by having cracks grow under cycling loading until one become big enough to grow unstably (fast fracture) which composites generally fail by progressive damage.

To take it to absurd extremes, with an undamaged UTS of 110 MPa you would not, I presume, expect 1 million cycles of 1 Pa to reduce strength by 53%, nor would you expect it to last 1 million cycles at 100 MPa.

So before knowing whether your figures matter, we really need to know the stress range involved and ideally the whole s-N curve. Many materials exhibit a fatigue limit; a value of stress range below which there are no fatigue effects at all. A quick look at the literature suggest that this is typically 25 - 30% of UTS. There is an interesting paper on the subject at Testing and modelling the fatigue behaviour of GFRP composites – Effect of stress level, stress concentration and frequency: "Testing and modelling the fatigue behaviour of GFRP composites – Effect of stress level, stress concentration and frequency" from which I have taken this

 

[Tranona]

Right, on stress cycles.:
Subjected to repeated stress a panel with a breaking strength of 110N/sqmm, after 1 million cycles the strength is reduced from 110N/sqmm to 40N/sqmm or by 53%.
The first signs of degradation are microscopic fractures that occur at just 0.2% distortion, whereas failure occurred at 2%. The micro fissures start surprisingly early and virtually at the outset in the process. (Larsson/Eliasson, Principles of Yacht Design)

While in the past it was common that boats were developed to meet particular local conditions that is no longer the case. The only concessions, generally speaking, made today might be a choice of keel depth and /or rig. Whether short or deep, the keel is attached the same way. Of course a short keel root on a deep keel will exacerbate the stress of impact. Regardless, the GRP offerings that float in the Baltic, the Med, the Solent, even the Bahamas or the coral reefs of the South Pacific are the same.

I can only repeat: the loads of a grounding, whether on rock or sand, are calculable and as such the boat can and should be engineered to withstand impact up to at least hull speed. To give you an idea: deceleration, calculated at 0.25 of a second, as it should be, equates to hitting solid rock and impact should be assumed to be at the keel toe. The fact that grids are knocked loose or destroyed simply points to shoddy practice, either in engineering or execution, IMHO.

That still does not answer my question. What does the 1 million cycles mean in terms of yacht usage? Is it related to years, miles, number of times you hit a rock.

You are wrong about boats related to local conditions. Although high volume production yachts are used widely round the world, there is still sufficient choice for people to tailor their choice to their sailing environment - for example Baltic built boats (until recently anyway) paid far more attention to keel type and construction than elsewhere, catamaran are popular in many shallow areas like the Bahamas and so on. Even then, most owners are sensible and if owning a boat with a deep high aspect ratio keel will tend to restrict their sailing to deeper waters. East Coast UK is a good example - used to be the preserve of shallow draft centreboarders. long keel and twin keels (championed by Maurice Griffiths), now populated by modern fin keelers whose owners are quite happy that their creek crawling is now limited.

Your last sentence will only have validity if you have quantifiable evidence that this is what is happening on a large scale rather than a small number o incidents repeatedly revisited - as you say IYHO. I accept the things like the MCA report which actually only concluded that more research is needed and the insurer's observations, but these again are not backed up by any hard evidence such as insurance claims history such that premiums are increased for such boats or insurance refused.

 

[Laminar Flow]

You are wrong about boats related to local conditions. Although high volume production yachts are used widely round the world, there is still sufficient choice for people to tailor their choice to their sailing environment - for example Baltic built boats (until recently anyway) paid far more attention to keel type and construction than elsewhere, catamaran are popular in many shallow areas like the Bahamas and so on. Even then, most owners are sensible and if owning a boat with a deep high aspect ratio keel will tend to restrict their sailing to deeper waters. East Coast UK is a good example - used to be the preserve of shallow draft centreboarders. long keel and twin keels (championed by Maurice Griffiths), now populated by modern fin keelers whose owners are quite happy that their creek crawling is now limited.

For a chap who likes to quote market popularity for what floats better, I find it a bit strange that you bring up the likes of Griffiths, in as much as, as far as I know, none of his designs are currently built in series. Just a couple of years ago I talked to the last commecial builder there ever was for the Golden Hind. I also used to own a Water Witch in my pimply youth. That doesn't mean I'd bring up 70-80 year old examples, of how currently boats with a specific use are designed & built for a mass market. Mass production by its very nature limits choice.

Your suggestion that Nordic builders pay more attention to how they attach their appendages, does not prove anything else other than it is entirely possible to make a better hull - keel connection with a bit more care. Also, the number of boats produced by the Nordic yards is a far cry from the comparative mass output of the French and the Germans. In your much touted popularity contest of the brands, the much more expensive Nordics hardly feature.

The MRC is mostly concerned with the safety at sea and potential loss of life. Pantaneus, however obviously worries about the extraordinarily high costs of having to put contemporary, deep keeled boats back together after a grounding. It stands to reason that this could very well up our insurance costs. The fact that they are compelled to publish a report proves that there is a problem and no doubt there will be financial consequences .

On the strength of GRP: Perhaps I was not clear. The test I quote addresses repeated flexing, i.e. bending of between 0.2 % and 2% , but under 2% as that causes failure. You are welcome to look it up together with all the pretty graphs, if you are interested (or worried) Eliasson&Larsson, pages 234, 235, 236. It specifically notes forces caused by: "waves and (pulsing) vibrations imparted by rig, keel and rudder".
Personally I am not worried about my own piece of 40 year old plastic in spite of the fact that she's been around the block, but because she is ridiculously overbuilt. This was not done to better withstand the rigors of the sea, but to accommodate the potential incompetence of the usual amateur fit-out.