Transverse frame calculation

But then the time would accrue again in adding the required framing since you'd be cutting transverses to a complex shape. But if it was all straight lines between chines on a transverse section this could be done easily. This is the basis of a lot of the barge type origami hulls buiult in Holland over the years.

 

Now if I buy what you are saying can you tell me what you feel is about the limit in size to gain this advantage? Aluminum rounded hull, canoe body type.

 

Mike
I am very interested to see actual results of the 3D FEA.

Your point on the sphere is as I understand it as well. Hence having a thicker shell increases the point load capability and confers considerably more strength on the shape. One of the challenges with monocoque structures is spreading the stress from point loads into the rest of the shell. Sometimes the force applied cannot be controlled such as running into something solid with a bit of speed up.

I have attached an IGES file of the simple monocoque structure. I am not too worried about the actual dimensions and plate thickness you choose. I am interested in the incremental loading as you proposed and how it fails. In addition I would be interested in point loading in the middle of one of the sides.

What I would like to know is how the monocoque stacks up against a framed structure of equivalent weight.

Rick

 

We can do that from a quick simple approach adequately .

Unframed it will have a primary principle failure on the long axis of the panel either one or 3 waves. With adequate framing it will yield before the shape will suffer elastic instability

Taking a head of pressure, say 3m and ....

By a rough calculation the single panel unsupported will need an equivalent section modulus of 1700cm3 [errata I actually used 335cm3 or thereabouts]. So 18mm plate would be close for the bulk of the plate

By dropping the plate thickness to 6mm and introducing 1 meter spaced frames in the centre portion that drops to 55cm3 for the longest frames say 6 frames

For moderate bar frames every meter the weight will be close to one third that required for an unstiffened plate.

An optimal structure with both transverse and longitudinal frames would drop the weight to around ¼.

 

Mike
Can you show me the results in an image showing stresses.

Rick

 

That's why I did the rough and ready so at least you have something.

I need the IGES or Rhino model as a full sold with all plate frames etc to save my time. But to set up and run and verify is not trivial and it would have to wait for a rainy day. (It took me two years to get around to modelling Brents frame).

 

I'd put the limit around 60 feet, altho larger boats can gain in time and expense saving , then have frames put in afterward.
If you are putting a few frames in , the logical place is around amidships where the sides of these frames are all straight, and thus quick and easy to put in. The conic sections on the ends not only make framing there more difficult, but far less structurally necessary, or relevant, due the greatly increased stiffness provided by the conic shapes.

 

Mike

I am not really interested in the framed model. I am mainly interested in the monocoque on say 6mm thick plate. See what pressure it collapses at.

Your rough and ready calculation seems a mile off. You are saying it would need to be 18mm thick restrained curved plate to support 30kPa over a span that tapers from nothing to a maximum of 2.8m across.

How did you calculate the section modulus of the 18mm plate? The value is very low for such a large curved plate.

 

Creasing would be a problem in a flat panel supported by the edges. When that panel has a longitudinal curve, supported by super strong edges , it's totally different picture. Throw some longitudinals in along that curve, and it becomes even more resistant to creasing, especially when the load is not a point load, but is more evenly spread across a considerable length of the panel.

 

Well I said it was rough
I see I made a blunder, replace that 1700cm3 with 335cm3 I think I calced it with a much lower Z but went back to give you the number and used the long distance rather than the short.

This is what happens doing it in my head and with a calculator. and jotting numbers on a bit of paper which I didn't keep.



Shell needs to resist the total panel force and from a buckling on a strain energy model its wise to limit the strip of shell to somewhere between 10 and 30 times the thickness. I expected 10mm to be close with a FOS of 1 and I used 25 times as the structural allowance with a possible mid panel load of rho.g.h.A . That's why shells are so poor structurally.

I'll try and run your model it willl be interesting. Could you give it to me in Rhino and thicken and 3d merge the shell to create a solid please.

 

Mike
I do not have a solid modeller set up here. It is a simple shape. Take three 6mm thick plates with a radius of 16m intersecting at 60 degrees to form a 3-panel football shape that is 10m long and 2.8m wide - or thereabouts.

 

Rick
You'll have to chop the ends off that funny shape since numerical methods and solid modellers don't like singularities.

What are you hoping to show, Or is it just to satisfy curiosity? Wouldn't it be better to actually model a boat hull shape now?

Hey Brent
Don't count too much on a 'cone shape' part of a cone is not a cone at all and in normal metal boatbuilding every undeveloped curved panel is 'conical' so there's no magic there.

Creasing of your topsides would be in the same direction as the longitudinals and I think with weak longitudinal frame the frame itself will induce and guide buckling end enhance the likelihood of a single wave buckle (or collapse).

We really need to see the hull lines, looking at pics they don't seem different enough in shape to warrant the extra shape strength claims considering the radius to plate thickness ratios.

 

Yes, the ends are sections of cones , supported by a fully welded deck and centreline. It is a fairly tight radius.
One also has to consider the weight of the boat in calculating how much pressure it will be exposed to. If hit the boat moves.I once sailed from BC to New Zealand with a piece of plywood covering the opening on an outboard well, in a 36 footer. It was held by two , two inch galvanized finishing nails, which I didn't really expect to hold. I hit some major bad weather off the Oregon coast and south of the Kermadecs. The plug never moved. Given that wood, , held by a fastening every 6 inches will hold around the horn, and has done in Robin Knox Johnston's Suhaili, I shouldn't have been surprised.

 

The shape is the simplest form I could come up with that has the curved surfaces similar to the bow of the origami boats. The symmetry should make the analysis easier. It certainly makes for a simple model.

What I was aiming to get an idea of is how it fails as I do not have access to experienced FEA modellers these days. You can see from Mike's quick calcs that it is easy to get it wrong.

If you take a view that 6mm plate is the minimum acceptable for puncture resistance on a long range cruiser so you have a good chance against a container, then I would like to see how the plate is stressed under distributed pressure on a 10m long hull.

You are welcome to have a go at the analysis. See if you get the same result as Mike. I would prefer not to wait 2 years for the result.

There has not been much support on this thread for monocoque structures, which seems odd when most high performance hulls are using monocoque construction these days. I wanted to see something concrete and credible in the way of analysis rather than simplistic exchanges.

 

Thats a bit surprising, when we take for serious what you provided on too many other threads.

>>>>simplistic exchanges<<<<

http://www.boatdesign.net/forums/boat-design/compromise-31901-7.html#post352558

Regards
Richard