Transverse frame calculation

Jean Marc has sailed enough miles in his 55 footer to have gone several times around the world . Except for one winter run to Xmas island ,and back to BC, most of it in Hecate strait , year round, for many years.
The disagreement is how much bigger we can go. 5 feet aint much bigger.

 

The dimples for and aft are in an extremely conic section of the hull where the curve is extreme.The stern is where webs transfer the loads on the skeg across the hull, adding greatly to the strength there.The one amidships is where a huge water tank top is the equivalent of a huge , fully welded bulkhead. In the case of a twin keeler, it is where the twin keels and their supports provide a huge amount of structural stiffness. Again, you are simplistically missing the complex connection between the number of bits and pieces that support one another.
The transverse bottom seams are covered by longitudinals, and the main water tank top, which add greatly to the strength. The chine seams, in the case of a single keeler, are covered by chine doubler plates. There is zero chance of the longitudinals breaking in the middle and even less chance of the tank top tearing in half to let the welds go.
Go ahead and make a model off the pattern shown, out of sheet metal. That will explain the principles involved far better than mere words and numbers can ever do.

 

Well youz guys might not be educating Brent but you are educating me.

I can clearly see both arguments now. Brent is looking at strength as a issue for impact resistance below the water line and a matter of twist and bending fore and aft but appears to completely ignore a top load down on the deck that would buckle the boat from top down. If I understand correctly this is exactly what the traverse frames would prevent.

Since I understand buckling and the effects of length on buckling and I understand that the pre stressing caused by pulling the boat together is in the wrong direction, I understand why it will not scale up well. Since I want a long skinny boat for my next boat, it will have adequate frames and mimimal pre stressing on the plating.

Thanks guys.

 

Tazzman

Not sure what 101 means?

You’re correct in saying that welding and its effects are applicable to any boat, origami or otherwise. However, perhaps I didn’t make my point clear enough.

1) The layout/”design” promotes poor detailing which in turn, leads to poor quality joints. The aforementioned reasons, from TWI et al, are the basics why. The poor detailing will lead to flaws in the weldment. These flaws have high localised stress concentration factors on the toes of the weld. When subjected to fatigue loads (as all boats are, from the sea to the rigging to machinery), these stress concentration significantly reduce the strength of the joint.

Therefore these details are not allowed with Classification approved structural joints/designs. Because of the inherent problems they create.

2) When a weld flaw is excited by cyclic loads and begins to propagate, we need to consider the mitigating effects of the structural design to ensure that it is what is called a “fatigue resistant structure”. What this means is simple. When a crack begins to propagate, the stress field at the crack tip becomes the driving force. So, if there is no stress, the crack cannot grow. This is the very basics of fracture mechanics.

So, as already noted previous, cracks can propagate in compressive cycles of preload structural members. But also when analysing the plating joints and their ‘supports’, these are all longitudinal; Brent has noted the chine and decks as the "supports". What this means is that the preload in the plate is transverse. Cracks propagate perpendicular to the applied stresses on the plate, ie longitudinally. So the preload is already encouraging the crack to propagate along the welded seam.

To make a structure “fatigue resistant” it requires structural redundancy. That is to say, structure which you could take away and it would not affect the overall static strength of the joint. This is because when, as mentioned, the crack tip grows because of the stress field at the crack tip, so, when the crack grows into a region of either a) thicker plate or b) stiffer structure, the stress field is lower, ergo the rate of crack propagate is retarded. If the structure is well designed, it can stop cracks growing all together. The down side is, this adds unnecessary weight.

So, finally, what redundant structure is there on an origami boats that prevents a crack from growing??...none! there is no stiff framing! Frames provide stiffness, which can also lower the stresses iwo of joints.

Thus, a framed vessel, apart from the elastic instability issues, when arranged properly and with an “eye” on fatigue, can prevent cracks from becoming anything more than “inconvenient”. Not to mention a frame vessel is Class approved whereas an origami boat is not, again, for the aforementioned reasons.

And I mean Class, NOT Flag. Flag authorities, such as your local Dept. of Transport etc, Flag only care about the people onboard and safety associated equipment to enhance their safety. Class, are concerned with the boat and its systems to remain ‘working’ in all conditions of operation for which it is Classed. That is the fundamental difference between the two, which I think many do nor appreciate or recognise.

Just like here, another claims class approved structures. Saying it, doesn't mean it is!

http://www.boatdesign.net/forums/metal-boat-building/metal-frame-detail-28371.html


As for a longitudinals covering a transverse seams. A seam is a weld in the longitudinal direction, a butt is a weld in the transverse direction. Please use correct terminology.

So, I have no idea how a longitudinal stiffener can “cover” a weld in the transverse direction, it covers it by a “ + “, ie one small spot where they cross.!

 

One swallow doesn't make a summer.

Nor do we have the final construction details from Mr Marc. Since you say you didn't design it where did the design come from and what are the structural details ?
Even if it were completely inadequate it might still survive the usage you describe in sheltered coastal summer cruising, and it's never cruised blue water. Claiming an equivalent number of miles to an ocean crossing is more rhetoric.

How often would a 60 footer be in heavy weather enough to dive into a wave as illustrated? (That's a 60 footer shown) what is the pressure on the topsides forward ? What would happen to your design scaled to 60 feet without bulkheads or transverse frames to shorten the longitudnial span?

I can tell you , so can any professional you'd care to consult.

Why do think class societies get it so wrong when they demand that structures be able to cope with large heads forward ? Even then they are absolute minimums and they get it wrong, ABS OSRY for example is deficiant in the fore-deck minimum allowed design pressures.

I think the reason you try and reject transverse framing so vehemently is that it's going to be so hard to put a bulkhead or transverse frame in after the boat is pulled together.
Then to build it properly will take longer than a plate on frame approach.

So your design can either produce a weak boat for the material used or a boat that will take longer to build than conventional methods and still have a limited shape.

Stick to the 35 1/2 footers and lets analyse the 40 footers in detail becasue they may be dangerously inadequate in heavy weather.

Your curvature arguments are not valid considering that the curve is not supported and that the shell thickness is inadequate for self support.

Can you provide your lines, offsets ? And I 'll ask you now if I can source these from someone who has your plans do you mind if they forward them to me? Lets put some real numbers to this.

 

Ad hoc
Ok I can see where your coming from there, even with best best welds in the world the stress is still there around them, slightest crack in the skin- plating would just keep cracking, to the point of!!
Welding 101 slang for just basic welding skills.
Thanks
Tom

 

I'm not sure what you mean. Are you saying you'd like to see the limit of scaling only in the longitudinal direction if 6mm plate were used? Or scaled 3D ?

 

Mike
I would be interested to see it scaled in length and width to see where 6mm without any internal bracing will survive the 3m head.

For Brent's benefit and others contemplating this method of construction I believe it would be useful to see what benefit various additions achieve. For example a longitudinal stiffener would overcome the particular failure mode you have shown. So after doing that where does it fail next.

The numerical analysis is about as close as you get to the real world. Of course you need realistic load cases and combinations. If you have access to numerical analysis then you can explore the modes of failure for different load cases.

Taking 6mm plate as the minimum that you accept for a cruising yacht you can get an idea of what siffening is required for various hull lengths.

For example:
1. What length can you get to with no stiffening.

2. What increase is there by a single longitudinal stiffener.

3. What increase is there with only bulkheads and where is their best location structurally.

4. What increase with both the longitudinal and bulkheads.

Rick

 

Some things are easily deduced without the need to re-run FE models.

For example your shape plated in 6mm collapses at 7kPa ( I checked) with the same buckling patterns as the 5mm model. If we want 30kPa then the scale factor for the 3d model will close enough at 1/sqrt(30/7) which is what... very close to half the size.


I really think deriving another scantling rule is unecessary. They go through these execises themselves. There's nothing magic about Brents origami boat structure, but it is the form we should be concentrating on.

I want the shape.............particulalry of the 40 footer, lines offsets anything to describe the full hull shape for a start. Then we can frame it to a safe standard and see how it performs structurally, where the weaknesses are and where the rhetoric breaks down into dangerous territory.

Come on Brent lets analyse your hull.

 

Tom
I cant help seeing this as drifting into the realmsvof ship rather than boat construction
Small boats fit snugly into wave patterns, in fact 40 feet is optimum
I have never seen or heard of even the most badly built , designed, small steel boat breaking or even cracking welds
It does not happen and I would love to see examples ALTHOUGH I do know of one, quite famous, ICEBIRD Dr D Lewis's small sailing yacht that broke coaming in antartic, the coamings broke at butt because there was no weld prep, Still I maintain that, a small steel sail boat or powrboat can survive, even when very poorly built, due to inherant strength steel
Whereas as we all know a ship moves, , the longer the more she moves having to cope with hog and hump
So perhaps we shoud maintain this thread within the bounds of BOATDESIGN

 

A longitudinal stiffiner will overcome the boat failing by buckling at the deck and folding upwards. It will not in any way prevent failure by the deck moving straight down towards the keel and buckling the sides of the boat outwards. Sort of pancaking at the bow. Only a traverse frame can control that. Since the sides are already bowed out and pre stressed it doesn't take much green water over the deck to do this.

The difference is that a smaller boat does not usually get into situations where there is as much green water over the deck as shown by Mike. The panels are also smaller on a smaller boat and the relationship to size is not linear.

 

Where did you get this from?

 

So the answer to my question is 0!

 

From the belly I suppose!

Like the "valid" and "proven" statements on other occasions:

http://www.boatdesign.net/forums/metal-boat-building/metal-frame-detail-28371.html

What did you expect?

We are hammering on poor old Brent since ages, telling him that not his boats fail, but his theory..............

....senseless in both cases.

PROVEN EXPERTS, you know?

"Look at my Gallery, I cannot fail!" .." Look at the miles I did, I cannot fail."

Eat more ****, 28 Billion flies do´nt fail neither!

Regards
Richard

 

many books (SEA SENSE) FOR ONE, PLUS A LIFETIMES SAILING smll boats dont have to span troughs, or hang over waves as in ships, In ships they have strain gauges on the bulheads to see how far the ship is flexing, too much and they either change cousre or slow down
take a looka t your PM,s Soren