Boat flex

[vyv_cox]

Presumably when one starts to thicken the core as shown above the issue of shear in the core becomes a problem. I would expect bridging points in the GRP to tie the 2 skins together laterally. However,I can envisage that may induce problems of a different sort.

I don't know about other makes but my Sadler 34 is a long way from being two separate skins with foam between. There are many points at which the two skins come together, particularly at the keel mounting grid but also locker tops, skin fittings, doorframes, ring frame, etc. The 34 has the reputation of being a very stiff boat, although the inner skin is very thin and can be cut with a sharp knife.

 

[Daydream believer]

The core should be stuck to both skins. There was a long-running case posted on here where a new boat was found to be severely defective because the core material wasn't properly stuck to the skins.

My friends Beneteau princess 36 motorboat was hauled to have copper tec antifoul applied. When they blasted the old paint off they realised that a large section of the hull below the water was sagging as it was not bonded to the core. The boat was fairly new with 350 hours on the clock. It had to have the skin removed & a new layer re bonded to the core, at the owners expense.

 

[geem]

My friends Beneteau princess 36 motorboat was hauled to have copper tec antifoul applied. When they blasted the old paint off they realised that a large section of the hull below the water was sagging as it was not bonded to the core. The boat was fairly new with 350 hours on the clock. It had to have the skin removed & a new layer re bonded to the core, at the owners expense.

My hull is solid glass from waterline downward. Above waterline it's 6mm inner skin, 25mm Airex core, 6mm outer skin. Strong and stiff. In the bow area the core stops and the last foot or so forward is 25mm thick glass. I guess good boat building is essential.

 

[doug748]

The core should be stuck to both skins. There was a long-running case posted on here where a new boat was found to be severely defective because the core material wasn't properly stuck to the skins.

Yes, I recall the boat, Joker, I think. Once the bond is breached the panel strength collapses be it Airex, Balsa or Honeycomb. Relatively low impact damage commonly starts the problem along with water, as already mentioned. Very rigid construction but as usual with boats there are downsides.

It's been said that the Sadler's 34 and 29 are a pretty standard hull construction with added foam and a light inner skin (as mentioned above), which are only nominally bonded for building convenience. The foam being present for buoyancy rather than any structural properties. Thus any moisture is not a concern and will gravitate towards the bilge.
No idea if this is true but it sound plausible.

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[Concerto]

The Polish built Huzar 30 had a problem with foam cored topsides not being correctly bonded. There used to be a web site by the British owner who had major problems from new. He picked the boat up in Poland and was sailing her to the Solent and started having major problems almost immediately. Once in the UK he contacted the UK agent who he bought it from. The agent denied responsibilty and it went to court, but he had no money. The builder did not want to know and the insurance company was unhlepful. Eventually the topsides had to be cut open and the foam core had dabs of adhesive, not bonded to the glassfibre. The topsides were then rebuilt at a cost of about £30,000 to the owner. The web site was very interesting but had been removed as at sometime in the future he would be selling his Huzar 30.

doug748 you are correct she was called Joker. I have just located the following threads on ybw by the owner. I have not had time to reread them, so some of the previous paragraph may be incorrect as it is all from memory.
A cautionary tale for Christmas
Advice needed ...
'Print Through' on dark hulls.
Thickness of GRP lay up
A Moral Dilemma

Hull delamination like the Huzar is also found with other boats. Just look at this link.
delaminated cored topsides - Google Search

 

[geem]

The Polish built Huzar 30 had a problem with foam cored topsides not being correctly bonded. There used to be a web site by the British owner who had major problems from new. He picked the boat up in Poland and was sailing her to the Solent and started having major problems almost immediately. Once in the UK he contacted the UK agent who he bought it from. The agent denied responsibilty and it went to court, but he had no money. The builder did not want to know and the insurance company was unhlepful. Eventually the topsides had to be cut open and the foam core had dabs of adhesive, not bonded to the glassfibre. The topsides were then rebuilt at a cost of about £30,000 to the owner. The web site was very interesting but had been removed as at sometime in the future he would be selling his Huzar 30.

doug748 you are correct she was called Joker. I have just located the thread on ybw by the owner. I have not had time to reread it, so some of the previous paragraph may be incoreect as it is all from memory.
A Moral Dilemma

Again poor building practise. It happens whether it's a new boat or old.
For all the hype about new boats being better built you only need to read about the Bavaria to see that a flexing hull can be a nightmare in bad weather especially with large windows. That's why there are blue water boats and production boats. They are not the same but maybe only the worst weather shows up the difference.

 

[Lucy52]

I am given to believe that all hulls flex, which is why you have to wait a day or two after launch for the hull to settle in the water before you can properly adjust the rigging tension.

Elasticity of structures would be a 101 of engineering design, the structure has to absorb the strain energy and then when recover when it is removed, or otherwise it would suffer permanent damage.

 

[Daydream believer]

I seem to recal that a Halberg Rassey 38 or 37 had bonding issues, which were found when the owner had an additional hull outlet fitted. That resulted in extended legal proceedings before HR accepted liability.
I further recall that Pete Goss had a boat that was meant to circumnavigate the globe. design issues were made worse by the core not being bonded to the hull correctly. That was a great shame with an expensive write off

 

[reeac]

R H Dana in Two Years Before the Mast recounts the mate telling him the ship, leaving what became San Francisco, crammed tight with buffalo hides rammed in using tackles, would 'ease her stays' after a day or two and sail a lot better.

I was about to quote this extract but you beat me to it. Fascinating book, incidentally.

 

[reeac]

That's a different issue. The problem was due to ignorance of ductile/brittle transition temperatures at the time. In freezing temperatures the steel in use became brittle and would crack.

Most famous was SS Schenactady, broke in two before completion in one of the Great Lakes. Solution is to increase manganese content to 4:1 against carbon.

I lived in California for a couple of years in the 1960s and my landlord there had worked in the Kaiser shipyard during WW2 when they were building Liberty ships. He said that sometimes when the workforce came in in the morning they would find that a big steel plate had cracked all the way across overnight. They would just cut it out and weld in another. Clearly lots of residual stress got built in during construction and then the ductile/ brittle temperature effect didn't help.

 

[fisherman]

Yes, I recall the boat, Joker, I think.

Hey! I was unable to call my boat Joker in 1983 (it was a partnership, JOnathan and KEith) as it was mortgaged so registered under part one and there was already a 'Joker' under part one. Had to call her Joker of Navax.