I think ‘broken’ needs some objectivity.
If it is a defective method then something would have broken by now..
The idea of scantling rules is to provide a minimum structural design without having to engineer it. In effect the scantling society designs the structure for you and you try to follow their guide.
The rules are based on a number of assumptions and even observations of other vessels that did indeed break. You don’t need your own design to break to prove the design is inadequate you can extrapolate this from some-one else’s prolonged heavy weather experience and learn from the damage.
Breaking doesn’t need to be catastrophic of course, it can just be that some of the structural welds fail or a fatigue crack slowly develops, maybe only with storm loads. This is unknown without inspection of the structure of a which is usually buried under a fit-out. When problems do become apparent in a steel boat they are seldom catastrophic and often simply and easily re-welded by some repair yard.
If the topsides are overly weak for docking and bend easily it’s an insurance claim and the dent is fixed. I think designers very seldom get sufficient feedback or are sufficiently objective to qualify this themselves.
The commercial field is a very good indicator of the strengths and weaknesses of design and building practices. Deficiencies in construction quickly become apparent through the mandatory scantling societies inspection process. The rules are altered if failures occur and of course every design has an unblemished record up until the first failure.
We have no way of assessing exact loads on any spot on a hull in rough sea conditions. ……….. Even finite element analysis has to make assumptions and generalisations …….
Pressure head is usually sufficient to assess local structure on boats of the size we are discussing, global strength is not really a problem with steel of this size.
The indeterminate loads tend to be for example the additional moments from the rig or the force on topsides from falling off a wave onto the vessels side.
Nowadays the techniques of prediction are being refined from the use of load cells accelerometers and strain gauges. The Wolfston unit at Southampton has put a lot of effort into studying smaller vessels, their research and other research institutes have been significant in driving many of the small vessel scantling rule revisions and exposing deficiencies in the same.
The preferred design process now amongst engineers is to design to the applied loads rather than follow a suggested minimum framing requirement. FEA analysis has radically simplified the design process for grillage based structural design. Scantling societies also accept this type of analysis when vetting a submitted design.
……. in the past steel boats were all over-built to allow for the inevitable erosion of material due to rusting……………
The acceptable corrosion allowance varies depending on usage and the society but it is not great for vessels under 24m, it is more a maintenance allowance of say 0.5mm or so. While for shipping there may be an allowance for 20% wastage before structural replacement is required.
I am appreciative of having the increased stress concentrations pointed out to me ..
Although this method is not as strong structurally as attached plating I can see the advantage to the builder, Increasing the area in the intersection would at the very least be easily accomplished with some fillets.
I use the ABS racing yacht rules as the basis of my structures. I believe strongly that this is a much maligned rule, which does not really deserve its bad reputation.
ABS OSRY minimum scantling for steel boats produces a light hull. Steel is more forgiving than any other material because steel vessel design is driven more by fatigue limits and yield than by ultimate strength. This provides a strong and able enough vessel providing the designer understands the material properties, and importantly the client accepts that the plans can not now be approved.
Metal construction gives the reserve strength to survive collisions that an equivalent composite craft would not, albeit dented but intact.
However for composite ABS has been cursed with some significant shortcomings . Some errors are current (such as foredeck design loads ) and the rules will not be updated, instead designers of smaller leisure vessels should migrate to another society as now recommended by ABS.
Around the world many regulatory bodies and insurance companies still accept ABS OSRY as a construction standard but just recently the issue has been raised that for a vessel to be legally compliant the designer must design to the scantling society rules at the time of construction. That creates a legal conundrum with designers adopting ABS OSRY. My opinion would be that with experience you use it as a steel design guide but preferably have an engineer check the design assumptions since the society will not.
Years ago …… I have a letter from him confirming that ABS would accept floating frames but they required that the stringers be continuous welded to the skin at each frame.
Any society will allow floating frames, but I think they should have recommended an increase in the section modulus of the frames. I will look into this if time allows.
The continuous welds I recommended earlier in this thread and was a concern since you have all the flange-web stress concentrated at those welds.
This may be contrary to what Mike's analysis suggests but does not get around the fact that we have not broken any, even in violent groundings.
In violent groundings the loads tend to be taken by the floors which look to be in full contact and very well welded in Wynand’s construction here.
The ABS calcs require the stringers to be slightly closer together or the skin thicker with floating than attached frames. This is due to the adjustment for panel aspect ratio.
People need to be aware that the panel aspect ratio relates only to impact (slamming loads) but not to pressure, grounding loads, docking loads etc.
I once ran a couple of my steel designs through ABS and Lloyds Rules for comparison before committing myself to the ABS route. The result surprised me. In some respects the ABS rules resulted in slightly lighter structure and in some respects slightly heavier. Overall, the ABS rule allowed a yacht structure to be more efficiently designed than Lloyds but careless design could well end up with a heavier boat.
……… 3mm deck skin. This is quite normal in yacht design. Dudley
Lloyds SSC, has been evolving and has probably changed several times since you initially assessed it. No longer is the old adage true ; that an elephant is a mouse designed to Lloyds register rules.
On decks,
3mm decks always feel excessively limber to me on anything over 45 feet, by the time you use thicker plate in all the high stress areas and allow for the closer framing the weight saving over 4mm is not as great as is initially apparent.
4mm is much less prone to damage, is easier to weld and repair without distortion. Commercial craft of mid-50ft often have 6mm decks. But all this comes back to the Righting moment and GM that the design of a powerful sailor and comfortable craft dictates within the choice of displacement.
