How stiff is too stiff???

[gmakhs]

Hello ,
Back in 2003 we added 2 stringer on the aft beam sides of the boat (reason of the stringers was that the hull start show stress marks after changing the original location of the running backstays)

Lately I have been reading atom.voyages and how he added.bulkheads to support the hull .

So a question have arised in my mind , if I decide to make make.my hull stiffer adding stringers and bulkhead is there any limit ? Will eventually the hull be to stiff for the loads ?

[MartinR]

This needs either experience or engineering. The important thing is that you do not build up point stresses. Boats are designed to flex, the flex dissipates some of the energy transferred by the waves to the hull. Too much flex is of course a bad sign.

[coopec43]

Originally Posted by MartinR

This needs either experience or engineering. The important thing is that you do not build up point stresses. Boats are designed to flex, the flex dissipates some of the energy transferred by the waves to the hull. Too much flex is of course a bad sign.

I'm not sure I follow your post: I most certainly would not want a metal (aluminum/steel) boat to flex.

"Aluminum and steel boats are examples of vessels built to be completely rigid. By the nature of the material, these hulls will not tolerate flexing. Fiberglass boats, however, are another story. Fiberglass boats can be designed to be either flexible or rigid" (Pascoe).

Marine Surveying : Hull Design Defects - Hull Failure Part I - Boats and Yachts Surveys

(The whole chapter of David Pascoe's book is worth reading and I think would answer your question very well)


Clive

[MartinR]

Ever looked at aluminum aircraft wings?
They do tolerate flexing.

A lot.......

There is no way to build a boat completely rigid. Big steel vessels flex a lot. On a 300m ship I worked on we had a system to measure this. The alarm was set at 2m.

Old school fiberglass boats are almost rigid, though, like the ones built in the 60s. Overbuilt, because there was no experience with the material. The OPs boat is a First 26, relatively modern design, lightly built, will flex.

[coopec43]

Originally Posted by MartinR

Ever looked at aluminum aircraft wings?
They do tolerate flexing.

A lot.......

.

Wings of an aircraft are only supported at one end: they will flex Weight is extremely important in aeronautics so everything is pushed to a limit. Because of that maintenance costs are very, very high.
Airliners don't bounce across waves (we hope) so the stresses are different. A lot of supertankers sink every year:

March 30, 2014. “every year, on average, more than two dozen large ships sink, or otherwise go missing, taking their crews along with them.”Mar 30, 2014
Most of the cases of ship flooding have occurred because of hull damage, which had lead to creation of cracks in the ship's hull, allowing water to seep inside the ship.Feb 27, 2010

Because things flex on a 300M cargo ship doesn't mean we should design a 15M steel yacht that flexes!

Clive

[Benz]

A hull can't be too stiff overall...but if it has stiffer and flexier spots, then the flexi spots will flex more. If the transition from stiff to flexi is too abrupt, there will be great stresses there. Any stiffeners added need to gradually diminish into the hull, for example by overlapping each successive layer of glass 2 inches past the one below.
I'd be cautious about adding stiffeners willy-nilly without consulting an engineer, and I'd bear in mind that if a boat was badly designed or built in the first place, it might be impossible to make it good, no matter how many add-ons are added on.

[psk125]

Many good observations here. Making one part of the boat really stiff may have impacts on other parts. Like an umbrella, everything on a boat has to balance. If you made one rib of your umbrella stiffer than the others, the uneven stresses would destroy the whole thing in a gust of wind. You don't want something like that happening to your boat. Why did you move the location of the running backstays in the first place? Beneteau probably put them where they were for a reason.

[nortonscove]

Originally Posted by coopec43

I'm not sure I follow your post: I most certainly would not want a metal (aluminum/steel) boat to flex.

"Aluminum and steel boats are examples of vessels built to be completely rigid. By the nature of the material, these hulls will not tolerate flexing. Fiberglass boats, however, are another story. Fiberglass boats can be designed to be either flexible or rigid" (Pascoe).

Marine Surveying : Hull Design Defects - Hull Failure Part I - Boats and Yachts Surveys

(The whole chapter of David Pascoe's book is worth reading and I think would answer your question very well)


Clive

Yeah. This claim alone is why generalizations are often half wrong. The last boat we owned as a company had half to one inch steel and plenty of flex. Want to try something interesting? Put square tight fitting doors in a steel boat while empty or on the hard. Then put it in the water and fill the tanks and curse lots when you realize none of the cabins are accessible. I observed this while rebuilding a large motor yacht more than twenty years ago. Our yard cabinet fitter warned the guys doing the work to put in doors with rounded tops. They of course listened not and paid twice plus to fix their little mistake. Got our guy some work in the end.

[coopec43]

Originally Posted by nortonscove

Yeah. This claim alone is why generalizations are often half wrong. The last boat we owned as a company had half to one inch steel and plenty of flex. Want to try something interesting? Put square tight fitting doors in a steel boat while empty or on the hard. Then put it in the water and fill the tanks and curse lots when you realize none of the cabins are accessible. I observed this while rebuilding a large motor yacht more than twenty years ago. Our yard cabinet fitter warned the guys doing the work to put in doors with rounded tops. They of course listened not and paid twice plus to fix their little mistake. Got our guy some work in the end.

"half to one inch steel...."

Are we talking "boats" or "ships"!!!


Clive

[ontherocks83]

In the manual of one modern still in production Fiberglass boat, it states that after splashing and tuning the rigging you should check the prop flange alignment with a feeler gauge as rigging adjustments can affect prop shaft alignments. So with that I am assuming at least some flex is expected/desirable.

[Benz]

Originally Posted by ontherocks83

In the manual of one modern still in production Fiberglass boat, it states that after splashing and tuning the rigging you should check the prop flange alignment with a feeler gauge as rigging adjustments can affect prop shaft alignments. So with that I am assuming at least some flex is expected/desirable.

Expected, perhaps. Desirable, no. But it's too expensive to build a production boat perfectly stiff. Trouble with flexing is, as you work in the waves the rigging will slack and load, which isn't great for it long-term. If the boat is flexing around and you're motoring, who's to say the shaft isn't going into and out of alignment with every roll? The ideal is a boat that is perfectly stiff, and strong enough to absorb any loads without having to flex to do so. It's an ideal that's hard to engineer and expensive to accomplish.

[Dennis.G]

A boat that is "perfectly stiff" might be an ideal, but is also a dream. Every boat built out of any material you choose will flex under load. Just a matter of how much.

Proper marine engineering takes hull flex into consideration. As an example over sized mast rigging can cause issues as oversized stays/shrouds stretch less when a boat hogs and raises stresses on chainplates and fittings.

[Sailor 99]

Originally Posted by coopec43

I'm not sure I follow your post: I most certainly would not want a metal (aluminum/steel) boat to flex.

"Aluminum and steel boats are examples of vessels built to be completely rigid. By the nature of the material, these hulls will not tolerate flexing. Fiberglass boats, however, are another story. Fiberglass boats can be designed to be either flexible or rigid" (Pascoe).

Marine Surveying : Hull Design Defects - Hull Failure Part I - Boats and Yachts Surveys

(The whole chapter of David Pascoe's book is worth reading and I think would answer your question very well)


Clive

Rule 1 in life is that all things bend and/or flex or they break. It might not be visible to the naked eye but they do.


You ever looked out the window at airplane wings. The tips likely move up 3 feet when its flying. They also bend when the plane hits turbulence.



All sails flex. Kevlar sails stretch, they just go back to their original shape for x cycles before they don't any longer.

The Titanic sank because there was a fire on one side of the boat which made the steel brittle and it shattered when it hit the iceberg.

Some things are stiffer than others but they all move.

[coopec43]

Originally Posted by Dark Horse

Rule 1 in life is that all things bend and/or flex or they break. It might not be visible to the naked eye but they do.


You ever looked out the window at airplane wings. The tips likely move up 3 feet when its flying. They also bend when the plane hits turbulence.



All sails flex. Kevlar sails stretch, they just go back to their original shape for x cycles before they don't any longer.

The Titanic sank because there was a fire on one side of the boat which made the steel brittle and it shattered when it hit the iceberg.

Some things are stiffer than others but they all move.

Metal Fatigue


weakness in metal caused by repeated variations of stress.

1. "evidence suggested the plane broke up in mid air because of metal fatigue"

Fatigue



Boat hulls are exposed to two categories of stress: vibration and impact from the ocean. Impact tends to cause the greatest damage over time, especially from the constant and regular slapping of waves against the hull. Working boats, such as fishing units, also carry significant strain due the equipment they operate and the pulling involved with fishing net weight, etc. Fatigue cannot be avoided; it can only be addressed when weakness signs start to show strain on a boat hull.
Fiberglass hulls tend to also suffer aggregate stress over time from fatigue, studies have shown that fiberglass hulls will lose their rigidity against the outside ocean with the weakness growing gradually. This basically means that such hulls have a set utility life and then are no longer safe, period.(North Coast Boating)
https://www.northcoastboating.com.au...fatigue-signs/

[Sailorbob8599]

Originally Posted by Dark Horse
Quote: "The Titanic sank because there was a fire on one side of the boat which made the steel brittle and it shattered when it hit the iceberg."


Huh? I've always been under the impression the Titanic sank, strictly because an iceberg ripped her flank over too many compartments, allowing seawater to invade the hull until it slopped over the top of bulkheads that did not extend to the overhead decking, preventing the completion of a truly watertight compartment capable of being sealed off from other compartments, thus being classified as a 'Flawed design' or 'design error'. Call it what you may.

I thought steel quality (brittle) was another issue that exacerbated the damage caused by the collision. .

Never have I read anything about "fire on one side of the boat" contributing to the disaster . . . Does someone wanna explain???