aluminium thickness to prevent plate buckling

Hello,

I'm currently designing a 22m aluminium explorer yacht with a bare aluminium hull.

The topsides of the aft half of the hull are completely planar and I'm worried that the hull's skin will start buckling as fenders and dinghys will apply pressure. (see example in picture attached)


The idea is to add some one-directional curvature to the skin (retaining developable surfaces for easy of construction) to give the plate some additional form stability.

Is there any data available for how much radius I need for a given aluminium plate thickness to withstand a certain normal force?

Hope someone can help!

 

With the data you give us I do not think it is possible to give a valid answer. But, on the other hand, why not do what everyone else does: put rubber pads conveniently attached to the sides of the hull?

 

Well, if you are the designer, the solution is simple. Add sufficient stiffness in the contact area. That is how other designers do it. Your dilemma is not unique.

 

yes, there will be pads in the most worrysome areas, for instance aft at WL, where dinghies will bump. But a very large area will be prone to get pushed in due to fenders. I can't put padding on that big a surface. Would be too costly.
I was just wondering if I could solve most of the issue not by adding stiffeners (more expensive coz more welding time) nor by adding padding (more expensive too) but by being clever by using curvature to inherently stiffen the plating without adding material, weight or welding time. Just wondering if there is some data out there, maybe not even shipbuilding related..

 

Changing the curvature would change the shapes of the boat. I think the smart thing, the right thing to do, is to add reinforced interior (more frames or more local longitudinal stiffeners) and/or exterior (wood or rubber band).
As Ad Hoc very wisely says, you are the designer.

 

This approach is so wrong on so many levels which suggest you have not done this before.

Does a plate become curved all by itself?

And oh, wait a minute:

So, your solution is to pre-buckle the panels in one axis and direction only and you think your hull will remain planar and fair?!!

 

Tx for your input guys,

Ad Hoc, yes the idea is indeed to pre-buckle the plating, but in the opposite sense of how the outside forces will be applied. With a certain curvature (which is what I'm trying to figure out), the surface would not "cave in" on itself anymore for a given force.(up to a certain level of course). What I don't want is the bad type of buckling, where the buckling is in the direction of the force, and the hull skin is in a collapsed state and you can see the frames from the outside. (like on the image above)

Whether I need machinery to put a curvature in the plate, well, I suppose it depends on the size and thickness of the plate after all. A long plate will give me more leverage to bend it naturally by hand, which doesn't cost anything.

Also, I have no absolute requirement for the surface to remain planar. Will it remain fair? I guess that depends. At a certain curvature I think it would. (up to a certain level)

Compare it with a X mm. steel plate and a X mm.steel pipe to which you apply a normal force. The pipe can handle much higher normal forces because of its concave shape.

This is all theory of course. Maybe for the curvature to have any affect for opposing the normal loads the hull would need to sustain on a daily basis, it needs to be such a small radius that the design of the hull starts looking bizarre or blown up, which is prohibitive of course. I'm wondering rather if adding a bit of curvature would make already a big difference, hence my search for some data to work from.

 

Solutions adopted by other designers.

 

The fast catamarans used here for public transport look like they've been hit with sledge hammers a million times. Not just from fenders but also from hitting large waves at planing speed and contractions/expansions in a hot climate.

If you want good looks, do not use aluminium.

 

yeah, maybe using a navy patrol vessel to illustrate the problem wasn't the best in framing the purpose of the vessel. In the end it will be a yacht and will need to look fairly "clean". It won't get the abuse a ferry or workboat will get, but -being an explorer- will need to handle rougher usage than the average yacht. Maybe steel will need to be looked at. But from a maintenance standpoint, (bare) aluminium is more interesting as there is no paint to look after and I think chafing ropes and light scratches from mooring will be more of a concern in daily use than hard bumps against harbor walls. The yacht will spend most of the time at anchor, not in harbor. So for the concern of buckling, it really is limited to fenders. The lower area of the hull where dinghies will bump can be taken care off by adding more structure or a rubber bumper at the stern.

 

Thx CDK for your input. waves are less of a problem as this is a trawler with limited speed (11kn max). But climate might be something to consider, especially for an explorer yacht.

 

Ex Swedish Navy fast missile attack craft Vale, it has been on the River Wensum in Norwich UK since 1st September 2004, built 1978, retired from Navy use 1995. It replaced the Sea Scouts / Cadets previous Light vessel. I've been past it many times...

 

I suggest you start reading on some Class Rules as there is a wealth of information out there. Try Lloyd's, BV, or DNV. Calculations for a curved panel is pretty standard in plate design. There is also the intended use of craft from yachts to workboats wherein the scantlings (panel and substructures) are increased depending on the category. Increases can be from 1 to 1.5X.

Workboats are and other crafts that is constantly in contact with other boats, or boats designed for heavy seas are more robust in design plus the added fenders incorporated.

So a panel that works well for a fragile yacht will not work for a tug.

 

Using an ex-navy boat as the example for the results that you expect to achieve building with aluminum is a poor choice. The navy boat might have been built with thin plates to keep weight down and speed up, included many watertight compartments which required many vertical frames CONTINUOUSLY welded to the outside skin and many longitudinal stiffeners welded continuously as well.

Certainly curvature COULD help reduce buckling of the skin but perhaps at the sacrifice of having a normal looking boat.

Assuming that your boat will be a trawler and hence a pure displacement hull, there will be considerable curvature in the plates.

So the solutions are what hundreds or maybe thousands of aluminum boat builders around the world already use to get a FAIR and STRONG hull.

PLATE THICKNESS
The thickness of the plate will help reduce buckling and distortion caused by welding processes. At 22 meters, you will probably be around 1/4 inch (6.3mm) bottom plate and maybe as little as 3/16 inch (4.77mm)side plate. At these thicknesses, with proper welding processes and inside reinforcement, you can produce a fair and strong hull that will take most docking abuses. If your design calls for 1/4 inch 5mm side plates I would expect fair looking surfaces.

WELDING SEQUENCES AND PROCESSES
There are thousands of aluminum boats that have been built that are fair and strong enough to be tied to docks. ( and thousands of unfair boats built by first time boat builders). Your best bet is to purchase several aluminum boat building books that explain some of these processes.

PLATE STIFFENERS
You will require profiles to be welded to the sides of the hull to provide attachment points for internal bulkheads, sheathing, etc. Longitudinal stringers skip welded ( and with proper welding procedures) will limit or negate print through of the welds and weld induced distortion. These side stringers installed along the areas that you will likely see dock and tender impact, will do more to reduce distortion than merely increasing the plate thickness.
With thicker plate to reduce weld distortion and properly located longitudinal stringers
your boat can be as fair as a fibreglass hull and strong enough to resist dents.

So in your design, I would limit vertical frames attached to the outer side plates but you will need a few, ie engine room to living area. When you weld these, limit heat and bead width and observe the welding procedure that the books that you should buy suggest.

 

As I’ve noted above, you’re getting this wrong for so many reasons – clearly most not worth going into because you’re failing to grasp the basics of what you are saying and proposing.

A structure fails from several different mechanisms two of which are: deformation and buckling. One can argue the two are the same, but their fundamental causes can be totally different.

The force from any external source applied to a vessel must be withstood by the structure. The structure must satisfy the bending stress, shear stress and finally the deflection checks. The limits of failure of these are down to the limits imposed by the designer, that’s you by the way. Assuming you can estimate or calculate the applied loads to the side of the vessel, the rest is child’s play – to a designer. So either you can design the structure to accommodate said loads, or, you cannot. If there is excessive deformation – whose fault is that…the person in the dinghy bashing against the side, or the designer?

The other aspect is that of fabrication, already alluded to by Barry. Buckling, in the general sense, can be attributed to fabrication and poor sequencing. Vessels should never leave the shipyard with buckles present. This simply shows poor practice. What can exacerbate this is where the fabrication process has introduced such residual stresses it has taken the panels to their ultimate limit, prior to buckling, thus teetering on the edge of “popping”. So once any laod, often very minor ones, are applied to this “prone” structure – it pops, as the buckling strength has been exceeded. Note, the structural strength has not been exceeded, since the critical buckling is, in general, lower, for a given panel of plating v the max possible load prior to total structural failure. Walking across many decks, one can feel panels of plating popping, the dish in the deck popping from one side and then snapping back to the other. The deck has not failed, it has just exceeded its buckling limits.

However, buckling can be a serious case of instability and thus catastrophic structure failure. Thus you need to distinguish what aspects of buckling you are concerned with. Then place criterion or criteria to such, that you are satisfied with.

Everything comes at a cost. For example, why not make your plate thickness 25mm, will it buckle– most likely not. But will the vessel make speed – most likely not. And there is your dilemma, as the designer. What compromises are you willing to make and what mitigation are you willing to make to appease your desire to have a non-buckling side hull. Every designer does it differently – choose YOUR poison.