Welding the skin to the frames demystified

It was raining and cold in the weekend I ran some models to show just what is going on here. Added a few pretty pictures

Indicative only since the actual results will depend on the actual sizes but you will get a rough enough correlation.

cheers

 

Thanks Mike, this is great. I always appreciate your sharing of knowledge.

Looking at those model I would conclude, other than the well exposed distribution of the stresses, that ideally with an intermittent welding pattern the structure would benefit from the shell to frame beads being localized at those high stresses area?

cheers
Murielle

 

Mike, interesting images, just to clarify is the plating in your models continuously welded to all contacts(I think so from your phrasing) or just the longs to transverse with plating intermitent & also how do mouse/limber holes impact these intersections in the flush frame & stringer model & also what would be the ideal shape of these be too especially in regards to ease of manufacture. All the best from Jeff & keep the interesting stuff coming!

 

M&M
Thanks ....You are welcome.

If the transverse stands off the plate then the longs should definitely be continuously welded both sides a decent distance either side of the crossing.
When everything is flush on the plate it is not so important since there is a much greater contact area that the stresses flow through and the transverse is not shearing the longitudinal since it has its own attachment to the plating. This is similar to your query before : for strength you cannot over-weld the structure when the total strength relies on all the elements being 'fixed' together.

Wai...

Yes I did those as all fully connected/welded at every face contact , I wasn’t looking at the weld stresses, just illustrating the undesirable aspect of the technique.

Limber holes can be added safely close to the plating since they are close to the resulting neutral axis of the T section
Make them a half circle. Size depends on the other scantlings but you can make the radius one quarter of the bar height without making much difference, even up to a third in most cases. There is a lot of ‘redundant structure’ around the neutral axis of most structural elements which is where we cut lightening holes anyway if trying to save weight. Exactly what you can do depends on the overall design scantlings .


Cheers

 

Longs should never be continuously welded to the plates and never welded opposite a weld on the other side. That wraps the steel around the long, creating the effect of a chine there. A 2 inch stitch weld every 4 inches on opposite sides will never separate from the plate. The loads on these welds are simply nowhere near enough to break that weld.

 

Under normal conditions yes but not the scenario illustrated above when the standing off transverse also iduces an aditional moment in the longitudinal not noramlly present.

This is the qualifying statement about welding of course; the more weld you lay down the stronger it gets but ........................

 

Once you reach the poit where the weld is stronger than the cross section of the longitudinal , more weld becomes redundant.
Perhaps the best solution is the Fram solution, where more ice pressure simply lifts the boat. This means eliminating vertical sides, the one time when a multichine hull makes sense. The part along the water line , would then be the only part where extra reinforcing would be needed. as that would be the only part which would take ice pressure , and not being vertical , far less ice pressure.
Brent

 

Floating Frames

Thanks to LyndonJ for pointing out this thread to me. At the start, I apologise for the length but it covers a lot of territory.

I designed the boat that Wynand Nortje is currently building. If I remember correctly, that is the last steel design that I drew with attached frames. However, other designs of mine that he had built used floating frames and he asked me if he could do the same for this boat. I did some calculations then told him the basis on which he could make the change.

I have used the floating frame method on my steel designs for nearly 30 years without a single failure. If it is a defective method then something would have broken by now. Floating frames have been around from way before I started doing it. My first client commissioned the Pratique 35 in 1980 and requested floating frames based on its successful use by others, as covered in steel boatbuilding books that were available before that time.

All yacht design has developed empirically, from successes and failures over hundreds of years. We have no way of assessing exact loads on any spot on a hull in rough sea conditions. We have to rely on inexact theory to assess the loads. Even finite element analysis has to make assumptions and generalisations to come up with answers. The ocean provides the final proof. A boat is not a static building, with an easily defined structure and controlled load conditions.

A bunch of boats to my metal designs have sailed onto reefs, been blown ashore in hurricanes or fallen over on land due to inadequate propping in violent weather. One of them sprang a weld high up against the transom, the others broke nothing and suffered only dents and aeshetic damage to their hull skins.

I suspect that the discussion about increases in stress concentrations is of major importance to larger vessels like ships but it is of mostly passing interest to those of us who design and build small to moderate size yachts, with structures that are mostly over-designed due to the safety factors that have resulted from the empirical process by which our scantlings developed and the fact that in the past steel boats were all over-built to allow for the inevitable erosion of material due to rusting. We are still using those scantlings despite the improvements in coatings that have reduced rust problems to a much smaller maintenance issue. I am appreciative of having the increased stress concentrations pointed out to me so that I am aware of them for future designs and can watch out for situations where they may become a problem.

Yes, 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. It has some chinks that could have been fixed.

Years ago I had a meeting with the engineer in charge of the yacht rules in the Paramus New Jersey office and we discussed steel scantlings and application of the rules. Somewhere in my files 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.

The frames that I use are calculated along with a section of skin, with the I and Z values increasing slightly due to the 20mm separation. 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.

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. With attached frames each panel is bounded by frames and stringers. With floating frames the panel length is close to the length of the hull.

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.

Mike, you are surprised at a 3mm deck skin. This is quite normal in yacht design. I assume (hopefully correctly) that you work in larger commercial craft. These are not performance boats, they are heavy by modern yacht standards. We do whatever we can to reduce weight while still producing a safe boat. This may at times mean removing structure that we consider to be just deadweight. That is the primary reason why my smaller steel designs are frameless. They have also suffered no structural problems as a result of having no frames.

All of the above applies to my steel designs. I deal differently with my aluminium designs. For those I use attached frames.

Regards to all,

Dudley

 

Welding skin to frames

Your point iis well made. It's foolish to susgest that what has worked well over many years and many groundings"Won't Work" because the calculations say it won't

 

I think ‘broken’ needs some objectivity.

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.

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.

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.

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.

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.

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.

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.

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.

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.

 

as usual I have great respect for what Mike says, but really this is all a bit strange, small steel boats do not break, well unless they are so poorly built, like ICEBIRD, Lewis boat that split her coamings in the Antartic there are simply squillions of small(under 60ft) that are amatuer built and welded, that have survived for years and years
honest Injun pro boatbuilders rarely even think too much abt engineering equations, we just do it♠
Myself, I build transverse as you know, I fully weld floors forewards, lightly, acontinuous small weld is better than a triple pass staggered or chain
As I work aft and up the topsides I lighten up, same as the pl lightens up

steel requires more care as it pulls far more so leg and throat size do make a difference,
By the way I am ticketed
on fast powerboats this cont welding is a must forewards
groundings dont break welds fatigue does
it is interesting that the Euros have never taken to stringers, and I go along with them, , water traps,
A skilled builder wont get a hungry horse, and all first class metal boats need some filler
i maintain, and I am entitled to, given the VERY strong opinions here, that people build that way because they have neither the skill or patience to do otherwise.
Sorry if that offends but so be it
Mr Swain, I notice you take the Caveman hollow log principal, take some plate, weld it up, bung in some frames and what you see is what you get
But boatbuilding is not like that, its an exact art, in metal, requiring skill patience, feeling,
now prey why are there no examples of your work in the gallery?

 

Murielle
there are welds and welds, say a vertical DOWN, skid, in SMAW will have little penetration and small leg length and throat thinkness compared with a vertical UP weld, just say if I need explain more
Vertical downs are usufull with the right rod to seal tanks, they cause almost no distortion, because they are very fast to lay in, the longer you spend in a weld area the hoter the pl gets and the more distortion you will get
Vertical ups are used where throat and leg thickness is reqd on a single(or multiple pass) and great penetration is needed i would , if you are using SMAW, always use a vertical up in critcal areas, floors as they climb the bilge turn and forewards on the topsides
in Mig in alloy I am very experienced, in steel i know very little, so someone else shud come in there
if you go to my gallery you will see a blue sailing yacht, the owners originally wanted steel, perhaps you can think also abt alloy Alloy welding is many times as fast as steel

When you have non floating frames(and I never heard this term before now) and the frame is a close fit, you can use a piece of mig wire, jam it UNDER The frame and then when you stitch the weld will not pull the plate over the frame,
make up a weld stick, and mark off your weld space and then use that stick template everywhere Tidy welding is a plus, good luck

 

Murielle
there are welds and welds, say a vertical DOWN, skid, in SMAW will have little penetration and small leg length and throat thinkness compared with a vertical UP weld, just say if I need explain more
Vertical downs are usufull with the right rod to seal tanks, they cause almost no distortion, because they are very fast to lay in
Vertical ups are used where throat and leg thickness is reqd on a single(or multiple pass) and great penetration is needed
in Mig in alloy I am very experienced, in steel i no vewry little, so someone else shud come in there
if you go to my gallery you will see a blue sailing yacht, the owners originally wanted steel, perhaps you can think also abt alloy

When you have non floating frames(and I never heard this term before now) and the frame is a close fit, you can use a piece of mig wire, jam it UNDER The frame and then when you stitch the weld will not pull the plate over the frame,
make up a weld stick, and mark off your weld space and then use that stick template everywhere Tidy welding is a plus,
welding has nothing to do with hull stiffness, ( soory if others replied that)
adn if all gets too much then here is something for you'
* 4 tablespoons Dijon or spicy deli-style mustard
* 6 cloves garlic, crushed
* 6 tablespoons extra virgin olive oil
* 4 tablespoons fresh lemon juice
* 1 1 / 2 teaspoons ground black pepper
* 1 / 2 teaspoon salt
* cayenne pepper to taste
a great salad dressing
good luck

 

Stu

What I was trying to say in reply to Dudly's 'nothing has broken' was that not appearing to break doesn't necessarily mean that you used the materials to the best effect and that claiming that nothing has broken might be hard to verify without some detailed monitoring.

Although indeed steel boat construction tends not to catastrophically fail so as you would notice, but that fact doesn't mean the designer has adopted best practice or some welding or frame crossings have parted company.

As an example someone said in one of these threads that a few of one particular designers frameless boats have had problems with the plating fracturing at the hard spots caused by the engine beds. But if the designer is unaware then he will continue to claim their boats have never had a problem but in ignorance.

Either you design to a societies factor of safety or you don't, and whether their design loads are overdone or just adequate is another argument but the societies try to design for predicted load cycles that will never be experienced by most liesure boats. And yes steel is the most forgiving of all amatuer built materials. Even when framing and hull plate part company in large areas from lousy stick welding the plating remains intact and the vessel makes port with little problem but a worried skipper.

cheers

 

yes Mike Agree
Look Wnyns boat
only one in one 1000 would have so many butt welds?
a 15 m boat would have 2 per run,
I can not sanction this type of build, it all looks patchwork the stringers have no guts, what happens when you cut limbers in them?
How much time to do this idddy biddy plating? we do , 4 5m plates a day, and one man can stretch, form and another can fit the 4 in one day
in saying that, i am sure Wyn could build any way at all