understanding displacement

Thanks to all for the help! I thought long and hard on this last night and came to the conclusion I am wrong and I hope you folks (Gonzo) don't hold it against me. I reread my posts and they seemed sort of know-it-allish for that I am sorry. The whole thing is sort of embarising really considering I am so far off! I guess my only defense is my thoughts on design are the product of learning it the hard way, that is reading and studying the dozen or so books I have and looking at every boat I can. I have doodled with no real outside help from anybody (other than this board and you folks) so my views are warped a little by applying un-educated methods to my work. Next time I will make sure my ducks are in a row before blathering on so
Thanks for your time 8Knots

 

8knots - don't be embarrassed. I've seen people who learnt that stuff in college screw it up FAR worse than you did.
Steve

 

Probably because the payload is mandatorilly mentioned on the "builders plate"

 

Archimedes tells us that the weight of the displaced water must equals the floating object's (boat) weight. It follows that the mass of this water, the boat's displacement, must equal the mass of the boat.

Now, if my twisted mind is not mistaken, displacement is volume. The specific gravity of fresh water is 1000kg per meter cube, and saltwater about 1024kg per meter cube.

Hence, it is safe to state that the same boat will displace less volume of salt water than fresh water in a floating condition. Thus, although the weight of the displaced fresh or salt water will be the same as the boat, the displacement volume will differ.

Note of interest, if displacement is calculated to DWL / LWL on the lines plan, the boat will weight less in fresh water according to calculated displacement volume at DWL.

 

Wynand,

You're basically correct, but displacement is also a weight. You said it yourself

"....the mass of this water, the boat's displacement, must equal the mass of the boat"

And with that sentence you hit the nail on the head!!!!

On a given waterline the boat will displace a certain amount of water equal to the weight of the boat for that same waterline!
The DWL is used as an estimate, starting to design a boat you need a reference line, a desired draft, a desired weight. That's where you start. Eventually the boat will have it's ready-to-sail or fully loaded waterline somewhere near the DWL.

The confusion is there when displacement and weight is used for the same boat, but the corresponding waterline is not mentioned!!!!!!!!!!!

 

Displacement

Peter, Gonzo, 8knots and others,
Thank you guys for providing such a good, educational info worth reading. I'm new to the boat building world but would like to know the following:

If my boat weighs 900 lbs for instance, how much more weigh can it hold for a safe ride in salt water. The 900 lbs i'm talking about here is including an outboard, gas tanks but no passengers. Could anyone of you guys out there shed some lights?.

Mandy

 

That's a difficult question. If your boat is fairly new and built to the EU standards it will have a plate stating the number of persons (or kgs) it can hold. If it's old and built without a plate from DNV or simmilar you'll have to use common sense. Of course there are rules of thumb, if your boat weighs 400kg (900lbs), 20% of that is 80kg or 180lbs. That would be a rule of thumb for larger crafts, for a small craft it's obviously inadequate.... For small dinghies, the payload is often the same as or twice the weight of the empty boat....

 

Mandy,

Try to determine the area of the waterline for it's current draft (at 900lbs).
Multiply that area with an inch, you'll get a volume x s.g. (fresh or salt water) = weight per inch draft.
This is a roughl figure, and just for indication purposes.

Also, it only tells you how much more weight you can put in the boat, and not if it's safe to do so! You're mentioning ''safe ride on salt water'', I'm saying: " that all depends on the sea state and the weather!!". So, good seamenship is in order!

Good luck

 

DWL is mainly of concern to commercial or military vessels where there is a readily determined minimum freeboard, (or other criteria for military craft) set by statute. This then sets a fixed maximum displacement and bulkhead spacing via floodable length and so on through the traditional cargo vessel design process, input to classification structural rules, etc. etc. Then by determining the lightship weight, we find deadweight, subtracting fuel and other consumables, we find weight for cargo and so on.

Yachts (other than small craft subject to 33 CFR flotation requirements) generally don't have a hard maximum displacement, since they generally don't comply to any statutes, so it is difficult to set these specific cases up.

ISO sets hard displacement limits based on compliance to 12217, so again we can presumably find maximum dispalcements, and then work up specifc load cases to a standard.

 

Displacement

Gonzo,

You are absolutely correct. Displacement is the weight of the water displaced by the boat. The confusion arises when we ask, Salt water or Fresh water? Salt water is denser than fresh water, hence it is heavier. The boat simply floats higher on the water because its weight is the same. Engineers insist that weight and volume (displacement) are two different measurements. Try floating the boat on molten lava.

Somewhere on this thread, there is a mention of DWL. Designed Waterline is the volume expected to be displaced by the boat fully loaded as envisioned by the owner or the designer himself. The designer then calculates if he/she will use it on fresh water or salt water and adjust the shape of the hull to correspond the estimated volume that will be displaced.

When the boat is "dry" or when the envisioned cargo,passenger, fuel, oil, ect. is not on board, it will be light and float high on the water but it will not go anywhere.

Wait until the architects talk about Gross Registered Tonnage and Net Registered Tonnage. It will be utterly confusing.

 

Before an architect designs any boat he makes a calculation of weights. Without, he cannot make his calculations and establish what will be the wetted surface and what will be the approximate draft.
We use therefore the CWL = construction water line as a basic reference. CWL = equal to DWL

Often - sometimes during the building process, but it can have 1001 reasons, weightshifts are made or are just happening. Other specs, you name it.

Sometimes, even well known shipyards, try to bend the rule of physics, the law of Archimedes.

A very well known yard in Holland launched a big motoryacht.
Fortunately before the official launching. It heeled 10 degrees to port and remained there. After correcting the heeling problem, the yacht was 5cm deeper in he water, visibly under the waterline.

Our yard received a commission from a German client. He choose a specific design that went back to the respective NA who designed it. There had to be done because the superstructure was too high to pass a bridge. I had been asked to do it but my principle is not to meddle in another designer's work, so
I refused. When the designs came back I did not notice any changes.
I said so to the CEO, who shrugged his shoulders and said: no more delay, the customer brings tons of own stuff on board and we cannot delay the building any longer.
The client came for the official Flag protocol and refused to take the boat.
"The boat is too high," he claimed. His lawyer came, a very reasonable guy, and some official measureres. The boat was indeed too high and nothing we could do about. The difference was just to much.
Based on the hull parameters another superstructure was designed.
After 10 months his new boat was ready. We were just waiting for the hauler's to bring the clients posessions.
None came, only the client and his family to take delivery. They owned one of Germany's well known Bone China factories and the delivery was the worst I ever experienced. Paul Allen couldn 't be more difficult.
The lawyer came with the measurers - boat too high, however, this time we could correct it. And I saw our escape.
I went to the lawyer. I said "Look, where are the personal belongings of the client as described in the building specifications. I had sensed the trap. By just not bringing her personal stuff, the boat could be still too high and therefore we could forfeit the last payment term, always the most difficult one to get.
Nevertheless we had to bring the clients party to a first class hotel, on our charges and we started to confer with the lawyer. The result was that a large truck arrived the next day and everthing that belonged to the clients inventory was brought on board.
The measurers came afterwards and - the boat was exactly as was required.

So what is displacement?

Ask a submarine commander. He has to calculate with many displacements, after firing a torpedo, his boat is abt two tons lighter; In the Med his boat needs more ballast than in the Atlantic, less when going upriver.

Therfore the distinction Deadweight and Full Displacement. In the Med a yacht's displacement is different from that in the Northsea.

Tell me.

 

Theoretically, a boat floating just right on the waterline in fresh water will float higher in denser sea water assuming the water temperature is the same.As the water gets colder, whether it is fresh or saltwater, it gets denser. the boat will float higher.

 

Displacement

D,Artois,

On the basis of your article, you are a designer or an NA. My respect for you. I know you know the answer but for the benefit of the others, here it is.

An average boat owner would not care much if his/her boat sinks a fraction of an inch deeper in salt,fresh, or brackish water. They would probably be concerned that the boat is not pulling up to speed.

But when a ship, not aboat, travels on an international waters, the regulating body takes over. They require that a permanet mark be placed on both sides of the ship at or near the midship. This is a disc with a horizontal line in the center of the disc. This is the "summer load line" but more on that later. Forward of the disc is a grid composed of of several lines indicating loads for different loads.

This is called the PLIMSOL Mark.

The Oceans of the world are divided into Various zones and seasonal areas according to probable weather severity of the weather. During winter, where more severe weather may be expected, The vessel is not permitted to load as deeply as in the summer. Where there are seasonal zones, the winter mark is applicable. Temperate zones are still divided into two. The summer load line remains the base. In tropical zones where the weather is less severe, the vessel is allowed to load deeper.

The summer load line is the base by which the vessel is allowed to carry more or less load depending on the zone where it will go.

The plimsol mark tells at a glance the number crunching done by the naval architect(s).

This explains why "different zone" have "different displacements". It is simply how safe the boat is permitted to load.

The calculations for arriving at the weight V.S. displacement remains the same.

regards

 

Plimsoll Mark

The plimsoll mark saw the daylight in 1836 and was meant as an optical control on the loading condition of commercial trading vessels.

It is an indication of the loading conditions of ship in the different waters around the globe in different seasons;

Hence this thread: displacement

My comments were in relation to yachts - and yachts are exempt from the obligation to carry a plimsoll mark.

If you are close to deliver a fast-going yacht of several million dollars value and you see that the yacht sinks already unloaded till above the designed full load waterline, you would be very concerned!

In some cases you are right and is such phenomenon insignificant; that I have experienced too.

In my corner it could lead to a court case and refusal of the Flag ceremony.
But that has nothing to do with yachts also.

To comment on your assumption of what I am I can say that I do designwork - if I am up to - but our type of clients tend to come with already designed ships - what I do is to make sure that no harm comes to the yard and/or to the client. This in all respects.
Furthermore, we do a lot of material related investigations, soemtimes even ship's cases.

 

design water line design load line . the boat should float !