geem
Well-Known Member
Wow! That's not a great advert for Bavarias.
Boat flex
- Thread starter Jmc1764
- Start date
Concerto
Well-Known Member
At the Southampton Boat Show I was chatting to a Fairline Dealer from the South of France about the large hull windows in motorboats. They are experiencing flexing in hulls that are breaking the bonding with the windows. This is the biggest warranty problem for them. This will most likely become an even bigger problem as the boats age.
So hearing about a similar problem with a different brand and type of boat does not surprise me. Trusting the water integrety of a hull to an adhesive bond seems daft without a mechanical fixing as well (but that would cost more to build and not look as stylish). Adhesives are used in aircraft construction, but regularly checked. Car windscreens are bonded in, but can fail as per my car which has a small leak that I must get fixed, as it is part of the integrity of the passeger safety system.
Chris_Robb
Well-Known Member
Ha Ha - I was at Westerly when that was a problem! There were 4 sold to St Helier in Jersey, and the ware in frames so that they could ground on each time. After 6 months things were looking a bit spongy - they came back to Westerly and were substantially strengthened.
The one that was really embarrassing was the First Fulmar. Our cost accountant argued with the engineers on the layup round the Keel (fin) was not nearly strong enough (Ed Dubois first Westerly ). Anyway - It went to Dublin - and the keel fell off in from of the yacht club. The layup was substantially strengthened - and now they are considered one of the bomb proof Westerlies.
There may have been other incident's, but they are still sailing now!
johnalison
Well-Known Member
Everything flexes given half a chance, even buildings, but I'd sooner not be aware of it in the ordinary run of things. I believe that a feature of biplanes is that their rigidly braced wings makes the ride much less comfortable, but my memory of a ten-minute ride in a biplane sixty years ago is not clear enough to confirm this. Another feature of hulls is their tendency to twist. I believe that it wasn't until about the time of the clipper ships that this subject was even thought about.
fisherman
Well-Known Member
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.
Daydream believer
Well-Known Member
I know that this is not a cruiser but my sailboard, a Bic Bamba, has a certain amount of flex. When I use my sons FI race board it is very stiff & the difference is really marked. As the models changed one could feel how the stiffer versions just dance over the waves.
As an example of how stiffness affects sailing- I am not a Dragon sailor, but I understand that the Borenson dragon was the hull to have for a long time. However, Neal & petticrow found how to make the hull stiffer within the weight limit & dominated the class for a good few years.
When the Squib class changed builders they refused to let N & P make them because they felt that one of their hulls would make the older ones redundant even though made from the same mould.
Dinghies with stiffer hulls always perform better than their more flexible class built ones even though one design.
I do not know if something like a Pogo is very stiff, but I would expect the stiffer mini transats are the faster ones.
As an example of how stiffness affects sailing- I am not a Dragon sailor, but I understand that the Borenson dragon was the hull to have for a long time. However, Neal & petticrow found how to make the hull stiffer within the weight limit & dominated the class for a good few years.
When the Squib class changed builders they refused to let N & P make them because they felt that one of their hulls would make the older ones redundant even though made from the same mould.
Dinghies with stiffer hulls always perform better than their more flexible class built ones even though one design.
I do not know if something like a Pogo is very stiff, but I would expect the stiffer mini transats are the faster ones.
Last edited:
Concerto
Well-Known Member
Having a stiff hull ensures that the rig remains exactly tensioned in all conditions. On a lot of high spec race boats they now have tension guages on every piece of rigging and this is loaded into the onboard computer that also stores all the instrument readings. All of these are plotted to ensure they fit to what has been calculated and previously recorded. Gone are the days when lee shrouds flapped in the breeze.
William_H
Well-Known Member
OP doers not specify a size of sail boat that he is concerned about. Smaller boats say around 20 ft are inherently very stiff simply because you need a decent skin and reinforcement thickness to resist damage but this translates to a very stiff hull because of smaller dimensions. As you get to bigger boats the size with similar sort of hull thickness etc means less stiffness and strength until you get to supper tankers which can be susceptible to breaking in half.
Actual flexibility in say a beam is not bad providing flexibility is distributed along the beam. So bus chassis used to crack because of flexing. The simple answer was to fit fish plates either side of the cracked beam. The result was more cracking either side of the now very stiff area because the overall flexing remained the same now concentrated away from the repair.
Now a boat hull is much more complicated that a simple beam. You do however have load paths in a ring from mast base or support post at keel then keel itself to chain plates either side then across to hold chain plates together. Then you have the other load path forestay to mast base keel to back stay. So a good design would equalise the flexing through these load paths. With a total overall stiffness appropriate considering weight and cost. I think unlike metal, fatigue is not really a concern with GRP on a cyclic basis but an overload will delaminate and lead to failure.
I think all OP can do is gather as much service record of a particular design to determine if added stiffness would be desirable. Unfortunately the design and build industry of boats is haphazard. Often with small runs of production with lots of design change (in structure) so we don't have the consistency of design or the failure reporting mechanism like aircraft. Buy a design with large production and you get more experience of actual failures and fixes. Or to the OP don't worry! ol'will
Actual flexibility in say a beam is not bad providing flexibility is distributed along the beam. So bus chassis used to crack because of flexing. The simple answer was to fit fish plates either side of the cracked beam. The result was more cracking either side of the now very stiff area because the overall flexing remained the same now concentrated away from the repair.
Now a boat hull is much more complicated that a simple beam. You do however have load paths in a ring from mast base or support post at keel then keel itself to chain plates either side then across to hold chain plates together. Then you have the other load path forestay to mast base keel to back stay. So a good design would equalise the flexing through these load paths. With a total overall stiffness appropriate considering weight and cost. I think unlike metal, fatigue is not really a concern with GRP on a cyclic basis but an overload will delaminate and lead to failure.
I think all OP can do is gather as much service record of a particular design to determine if added stiffness would be desirable. Unfortunately the design and build industry of boats is haphazard. Often with small runs of production with lots of design change (in structure) so we don't have the consistency of design or the failure reporting mechanism like aircraft. Buy a design with large production and you get more experience of actual failures and fixes. Or to the OP don't worry! ol'will
vyv_cox
Well-Known Member
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.
geem
Well-Known Member
If you want stiffness in a hull you need to core it. If you take a 12mm thick GRP hull and compare it to a hull with 6mm GRP skins with 24mm of core in the middle, the cored hull will be 37 times stiffer.
AntarcticPilot
Well-Known Member
Weren't there also issues to do with their welded construction and stress concentration at hatch corners? The welded construction didn't stop cracks at the plate edges, as in rivetted construction, and the square hatch corners provided stress concentrations where cracks could start. The metallurgical issue compounded all these - without it they might have got away with it.
vyv_cox
Well-Known Member
Yes you are correct but I was not going to write an essay on my phone! These were some of the first welded shops, using semi skilled labour and poor practices.
johnalison
Well-Known Member
When we went to see our Sadler 29 being built in 1987 we saw that the hull was fitted out before the deck/coachroof section was put in place. We were told that it was only the great stiffness imparted by the cored construction that made this possible.
AntarcticPilot
Well-Known Member
I think that the majority of GRP yachts built since the 1980s have cored decks and cabin roofs, as it reduces the weight while imparting greater stiffness. Certainly, my Moody 31 does. But I have reservations about cored hulls - there have been some real horror stories on here about what can go wrong if water gets into a cored hull, and a friend had to have considerable work done to remedy a "wet" cored hull, which involved the boat being allowed to dry out for a very considerable period before remedial work could take place.
geem
Well-Known Member
There is core and there is core.
Balsa is a great core material until it gets wet. Our 42 year old Airex cored hull is dry. We had a moisture meter all over it before a respray last year. Biggest problems seem to come when skin fittings penetrate balsa cored hulls.
We had a leaking bolt on our ground plate. The penetration was badly done but no water had soaked the core even though it was exposed. It's now epoxied. If it had been a balsa core it would be a different story.
Porthandbuoy
Well-Known Member
My dad was based on this impressive looking warship in the early 1950's. Based in Malta she was LMS(E) 1023, aka "The Clank", a flat bottomed tank landing craft converted into an engineering support vessel for the 109th (IIRC) Minesweepers clearing up unexploded ordnance after the war.
Returning from delivering earthquake relief supplies to Greece she was reported as 'lost at sea' having failed to make Malta after a storm had swept through the eastern Med. Fortunately reports of her demise were premature and she limped into Valletta a week late. My dad told me the degree to which she flexed had the Maltese stewards crossing themselves and counting their rosary beads, convinced they were all going to die. So much water came through the sides of the bow loading ramp as the hull twisted the pumps couldn't cope, so they turned and ran before the storm. He went on to say he would not want to relive the bit where they turned the ship around.
Returning from delivering earthquake relief supplies to Greece she was reported as 'lost at sea' having failed to make Malta after a storm had swept through the eastern Med. Fortunately reports of her demise were premature and she limped into Valletta a week late. My dad told me the degree to which she flexed had the Maltese stewards crossing themselves and counting their rosary beads, convinced they were all going to die. So much water came through the sides of the bow loading ramp as the hull twisted the pumps couldn't cope, so they turned and ran before the storm. He went on to say he would not want to relive the bit where they turned the ship around.
vyv_cox
Well-Known Member
When I fitted my yacht legs I cut out two large rectangles of PU foam to be replaced by the backing pads for the leg sockets. I immersed one of them in a bucket of water, held down by a brick, for a week. I could not detect any moisture in the foam afterwards.
geem
Well-Known Member
It's pretty impressive stuff. Between two layers of glass it's even more impressive
Daydream believer
Well-Known Member
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. Certainly at bulkheads & large unsupported areas in the bow area for instance. However,I can envisage that may induce problems of a different sort.
AntarcticPilot
Well-Known Member
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.