Hull Water Loads

Mind you, quite a large part of the world uses the comma , for decimal numbers.
The period might be use to show thousands etc but not always.

 

It's a little sad....

It's a little sad though. When I started in this industry I had visions of doing serious investigations into who had done what research into hull loading, particularly on high speed craft. I wanted to make boats that were lighter, stronger and more fuel efficient that people were getting from the cookbooks. I found a number of studies at SNAME and RN USN and USCG archive stuff. I had run into too many so-called Naval Architects who design boats from tables and formulae. People who seem to be afraid and even unequipped to go into new directions. And some people who didn't even understand some of the basic hows and whys of structural engineering and were making a better living than me. Now I sit behind a desk, using tables and formulae, uninterested in going new directions. It's a little sad.

 

I agree, there is a component of sadness in getting old and wise. Immaturity and ignorance can be so joyful.

Think of all the houses built to building codes. Just about every one could have been better or cheaper if individually engineered to some optimum, but there would be many more failures, and it would require a much more stringent inspection each time a house was sold. Can you imagine a house inspector thumping each square foot of a house to detect possbile delamination?

Aside from the social value of building boats to "code", the target conditions for the "optimum" boat are too hard to pin down. The America's Cup boats are highly engineered to a very small envelope of allowed conditions. The results have not been encouraging. Perhaps 3% or 5% have suffered catastrophic failure of one kind or another (not counting rig failure).

Sad indeed.

 

You're only young once - but you can be immature forever.
One of my Rules for Life (tm)

However, when you look at the number of pages wasted on things like "An Orthoganal (typo intended) analysis of the plate stresses in mid-bow plates of bulbous-bowed VLCCs under statistically normal conditions in the English Channel", you begin to wonder where "first principles" gets you. How much different would those stresses be if it were a VLCC or if it were in a non-statistically-normal condition in the Bay of Biscay?
THe madness has to stop somewhere (except to keep grad students busy on something that has NEVER been tested before), and the Rules and Regs are the end result. We can go about designing boats without having to spend weeks analysing the likely conditions of use, plus the "worst-case" scenario (which will ALWAYS break the boat, like it or not, because that is, de facto, the worst case...)
Steve

 

Steve is right. The codes actually bring us to a point in design where we build what is safe, not necessarily the most efficient structurally. Structurally safe is the ultimate criteria, not structural efficiency. We cannot be structurally efficient because we cannot calculate the loads beyond a shadow of a doubt. There will always be doubt, and that is why the classification society rules are what they are--structural minimums based on statistical analyses of what is safe.

The America's Cup boats can afford to be as structurally efficient as they are because their performance envelope is extremely narrow, and the syndicates can through millions of dollars into research for that very narrow band of wind and water conditions. What works structurally in America's Cup does not necessarily apply to any other craft. Although the engineering models might be the ultimate benefit. If the engineering models ever distill down to something affordable and easy to use for us regular designers, then there is a benefit. Right now, they are still too expensive and require too much expensive talent to operate. My clients are not willing to pay those prices. I have a hard enough time getting them to pay for blueprint ink, never mind the design itself. The classic joke among naval architects and yacht designers is the client who comes in the door and says "I want to build a boat, but I don't have any money." This occurs almost every week.

If you want to be really structurally efficient, you have to be able to pay for the research, and that is expensive and clients are generally not willing to pay the fees necessary for the research. So we rely on codes to get the job done. It works, we design and build safe boats, and we get paid. There is nothing wrong with that. It pays for my sports car, my food, my house, and my kids' college education.

I take pride in the fact that I can understand and use structural codes, as well as first engineering principles, in a pretty thorough way. That comes from my education and experience, and it has all suited me well. I appreciate the fact that so many people before me have distilled all this engineering knowledge into workable rules that make my job easier and profitable. One of my pet peeves is articles in technical journals that give you absolutely no guidelines in how to engineer or design according to the topic at hand. The authors leave out critical data or equations or code. So what good is the article? Professional Boatbuilder magazine is very much better in that regard--a lot of the articles you can use directly in design.

After engineering, the key is the art--how do the styles of my designs stack up against my compatriot designers? I had a very nice compliment paid to me just today by a potential client who said my motoryacht designs were better looking than another designer's whose work I admire for its style. Thank you very much, I appreciate that.

So do not be discouraged by the hum-drum of structural codes. They are there to make your job easier, thanks to your predecessors who at one time did have the expertise and monetary support to advance the design and construction process. And maybe, just maybe, something that you do in design or construction will bring the rest of us forward to better boats. We have to go in small steps, and there is nothing wrong with that.

OK, I have pontificated long enough. I hope that helps.

 

Hi Karsten,

Here you find a pdf covering notation, symbols and coversion factors in the SI-system used in a few countries outside "the rest of the world"...

http://physics.nist.gov/Pubs/SP811/

might be of use

Good luck with your project
Olle

 

Eric, buddy , palllll....
Let me set up the scenario for you. This was a few years ago by the way.
I get a job working for a small boatyard making GAs for customers using their standard scantlings drawings, lines etc... I find that they recently increased their hull plating thickness because some earlier boats had shown signs of cracking. This was a knee-jerk response. They didn't bother to look at why the the cracking happened at that particular spot, or decreasing the unsupported panel size at that spot, they just made the whole boat heavier. Nobody was interested in what I considered to be an analytic engineering apprach. That put the first "hole" in my "boat". Later I got a job with a prominent production yacht builder, and my memos about code compliance problems were met with the old, "just do what you're told". I stick to the codes when its required, and rely on scanling rule like David Gerr's most of the time, but I'm often tempted to make a thing better by crunching some numbers, especially with yards that have been going with "what they've always done" structurally and hydrodynamically. I don't bother though. People have an affinity for the status-quo.

 

I find it is my responsibility as an independent naval architect to point out structural deficiencies as I go through a job. Even though the customer (builder, owner) may balk at my advice, I have nevertheless fulfilled my responsibility. If the decision is a really critical one, then I put the result down in writing, and if my advice is not followed, particularly if the structure is too light rather than too heavy, I disavow any responsibility for the design at hand. I have really done this on occasion. I also have the luxury of putting a client in his place if I find he is doing something really stupid, and I have done that on occasion. As I have said many times before, sailors (and builders) can be insanely conservative, literally to the point of madness and frustration. You can take your points only so far, sometimes, and you can't win them all. It is easier to give advice, I admit, as an independent consultant; it may be harder to give advice to a boss, particularly if your job is on the line. But then, if your advice is consistently not followed, then perhaps it is time to change jobs. If a builder is worth his salt, then he should respect the opinions of those whose advice he relies on.

Eric

 

I did find a new job. I work for a really intelligent guy and the crew here is really good. www.silverstreakboats.com
But I think I'll carry my scars around for a while.

 

This is all very interesting. I have been sailing all my life and when I was a teenager thought about becoming a boat designer. I was mostly interested in the structural stuff and aerodynamics. That's why I ended up studying Aeronautical Engineering. When the professor talked about aircraft I thought about how that's applicable to boats. After I finished studying it was much easier to earn a living working with aircraft than designing boats.

I think the biggest problem with boat design is that there is no money. Making a boat more efficient costs allot but you can't make money with it. If you safe a few kilos (or pounds) on an aircraft airlines can make more money selling more payload. So there is a good reason making the aircraft as efficient as possible. On the other hand, if you safe too much weight and the aircraft breaks you have hundreds of dead people.

Sometimes I'm quite surprised how wrong boat designers get it when they design something where cookbooks obviously don't work. Just think about big cats where the bow breaks away, around the world sailors who loose their keels, rudders and rigs and the famous Americas Cup "accidents". I think that's quite embarrassing and fortunately not too many people die because of these failures.

I don't know. Maybe the cookbooks are great to design a "normal" boat quickly and cheaply which is what 99% of all customers want and yacht architects are great at. I'm not quite sure who should work on cutting edge technology. In the very near future the designs will be done on 3D CAD systems, the structure will be analysed by FE and the parts will be cut by CNC machines. Not quite sure how well yacht architects with their tables are equipped for this.

 

There are a couple of reasons why boats break sometimes. First, a lot of designers are not engineers, and so they just don't know what they are doing, nor how to make good engineering decisions. Second, owners sometimes want to take the risk, and they'll advise their designers to go ahead and make it light anyway. Only in the America's Cup and in big budget grand prix racing yachts will you find engineering carried out to a great degree. As I and many others have said before, the perfect America's Cup yacht is the one that breaks completely apart two seconds after crossing the finish line to win the Cup. The extra one second is the safety factor. That is, the structure was designed perfectly for the object at hand, and once accomplished, it no longer needs to survive.

There is money in designing boats. I and many others make a decent living at it. It is frustrating sometimes because most owners to not appreciate what goes into a complete design, and they don't understand why we have to charge a fee (Hey, I thought you guys just designed boats for fun!). No, we have a living to earn, just like everyone else. But when the process works and you have a decent owner and a good builder, then you can accomplish some pretty remarkable designs. It is rare when everthing falls nicely into place, but I guess it is worth it, because I have been doing this for over 30 years, and I don't intend to stop.

Eric

 

What Eric said, with the following
It is fine to design boats to some "theoretical" approach when their operating conditions are fully known. But.... you put the same boat out in the Southern Ocean, with 100 knots of wind whipping up 80' (tall) waves, and some unkind things are going to happen. I would never advise using a cookbook formula for designing structures on an Open 60/50/40 that HAS to survive these conditions, but they can be helpful in showing what is a practical minimum. In some cases, the cookbook can be used as a starting point for the structure, but as I have said before, the "Worst Case Scenario" is always the one that breaks the boat. And that scenario is always out there.
Do you always deign for it? No - the boat wouldn't float
What I think I'm trying to say is that you can't design a boat to survive everything, just what it should reasonably be expected to survive.
Karsten, how many planes have you worked on that were designed to survive a cruising-speed crash?

Steve

Steve

 

I haven't designed any aircraft yet but the cabin is designed for 9g forward. At the moment I'm designing repairs for some old Boeing 707 ladies to safe them from being turned into cans for another year. Cans made form 7079 aluminium would probably crack in the shelves anyway.

You are correct. You can't design something for every possible load case. The art is to find the most likely load cases, calculate the corresponding loads and then make sure the structure survives it. I think the load cases should be known to the "user" and also the allowable damage should be defined to make sure that the structure is what the designer assumed in his calculations.

Therefore lots of engineered things come with users manuals and maintenance manuals. The user manual for example could tell the owner what the yacht is designed for. I once sailed on a 30ft yacht with water ballast upwind in 8-9bft in the North Sea. The boat was hard to slow down and we basically got airborne all the time and fell into the next valley. It was a bit of a bumpy ride and it would've been nice to know if the boat was actually designed for this sort of treatment. The boat was fine but we were a bit worn out after 12 hours or so.

A maintenance manual would also be useful. I mean after you ran on ground everybody looks into the bilge to see if there is any water coming. What about a manual that says if you hit the ground at speed X or more take a coin and do a tap test in area A to see if there are any delaminations? The owner would know that the keel is not going to fall off in the next storm because the structure was damaged previously. I guess if you inspected the rig for damage regularly you could probably safe 80% of all broken masts. To check for cracks in bolts you only have to apply a torque. If it shears it was damaged.

Do you guys provide information like this?

 

Here in the EU it is mandatory that all new boats with a length of more than 6 meter come with an owner's manual.

 

I have never seen an owner's manual that describes what conditions the boat was designed for, again, because they are so hard to describe. Basically, if a boat is designed to ABS or Lloyd's (which MAY be stated in an owner's manual) you can assume that it was designed for offshore sea conditions. But can you fine tune that to say it was designed for 20' waves or 30' waves or 40' waves?? No. That is because waves (waves are what kills you, not wind) come in an infinite variety of shapes, sizes, and directions.

I have been in 35' waves in the Bay of Biscay, and my little 27' Bianca rode out the storm faithfully under bare poles. It was not designed to any classification society rules that I know of (being built in 1968), but I had faith in the design and the structure. That is because I had read about storms and storm survival, and so knew what to do in those conditions, and I knew the boat pretty well. When the deck later collapsed under the mast in the Mediterranean, I did not quibble about what the structure was designed for, I found that the mast support was not built properly. Never mind, I knew how it should be fixed, and we sought out a builder who could do it.

I think most people who venture offshore are pretty prudent, and they seek out boats that suit their needs. They also understand sailing conditions, and know what their boats are going to be able to withstand. Yes, there are some dummies out there, but I don't think a statement that the boat was designed for such and such would save them.

In your case, with your 30' water-ballasted yacht in 8-9 ft seas, what would you have done if an owner's manual said this boat was designed for 7' seas? Would you have ventured out? When you fall off a wave, how do you calculate the loads? How fast are you falling? At what angle does the hull surface hit the water? How much boat mass are you assuming is facing the load that is about to impinge on the hull? How fast does the boat stop (half a second, quarter of a second, a millisecond) which you need to know in order to estimate the rate of change of momentum and, therefore, the force? You see, the loading conditions are so infinite in their variety that you cannot discretely determine what the real loads are. The conditions are not steady state as in an airplane, they are extremely dynamic, and that makes them hard to determine. Therefore, it is really pointless to say that a boat is designed for such and such conditions, because the desiger really can't tell what conditions it will or will not survive. You really have to take it on faith, buy your boat with an open mind and proper research, and add a big dash of prudence in your sailing.

In your case that you describe, if you were concerned about the survivability of the boat or yourselves, then you should have applied other tactics, changed course, and sailed the boat in a safer way.

It is interesting that in many cases, the boat can withstand the rigors of the sea better than humans. In a discussion I had once with C.A. Marchaj (who wrote Aero-hydrodynamics of Sailing) we joked about how the multihull transatlantic races would eventually evolve into crewless boats because the accelerations on them would be more than a human could withstand. Some people in Germany, I recall, a few years ago tried to sail a robot vessel around the world. I never heard how successful they were in completing the effort, but they did seem to have covered at least quite a few thousand miles.

Eric