Discussion in 'Boat Design' started by Sailcy, Feb 11, 2016.

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### SailcyJunior Member

Hi to all!
I'm still a bit confused about the correlation between displacement and loaded weight.
Could anyone highlight this for me please

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### gonzoSenior Member

The weight and the displacement are the same. A floating object displaces a volume of water equal to its weight. The loaded weight or displacement is the maximum weight/displacement a vessel is design to.

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### TANSLSenior Member

You have to explain things a little better. A volume is not a weight (force). They are totally different things. Both are measured in different units. It is like saying that an area is equal to a length.
The weight of a ship afloat, is loaded or not, is always equal to the weight of water it displaces (displaced water volume multiplied by its density).

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### DCockeySenior Member

Displacement of a vessel is almost always expressed as the weight/mass of the water displaced, not the volume of the water displaced. The volume displaced depends on the density of the water, which changes with salinity (fresh water, salt water or in-between) and temperature.

Frequently specifications for a boat list a "displacement" without specifying the load condition of the boat. The listed displacement may be the assumed displacement for design purposes.

Sometimes advertisements and similar list "weight" and "displacement" as two different figures, with "weight" representing the unladden or shipping weight of the vessel, and "displacement" representing a typical operating displacement or maximum intended displacement.

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### philSweetSenior Member

It is understandable that this would be confusing to non-native speakers, but it is usual among native speakers because English lacks an appropriate case identifier and has no compact alternative. Its is referring to, and applying the characteristic of weight to, both the subject and the object at the same time. This is an ungrammatical, but fairly common, usage.

A floating object displaces a quantity of water. The weight of the floating object and that of the displaced water are equal. Ugh, but grammatical.

A one-ton floating object displaces one ton of water. This should get the point across. But it isn't quite the same statement.

If all else fails, there are the Incompressible Navier Stokes Equations.

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Notwithstanding all the 'interesting' replies above, i suspect, correct me if I am wrong, that you mean something else. This:-

A vessels displacement can be broken down into 2 simple parts.

The Deadweight = all the things that are 'consumed' and/or not part of the vessel. Such as fuel, water, people, etc.

The Lightship = everything else!

In a nut shell.
However, I am assuming this is what you are attempting to clarify?

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### TANSLSenior Member

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### SailcyJunior Member

))))
Thank you guys comprehensively trying clarify this matter for me.
Actually, this is a school program if I remember correctly what Archimedes says - mass of the displaced water equal to the mass of the object.
By correlation I meant is there any rule how the boat design should be described?
As different designers have different approach. Usually its two types - displacement till waterline or loaded weight

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### SailcyJunior Member

...What empty displacement (weight empty) should include by default apart from bare hull?

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### fredrosseUSACE Steam

".........if I remember correctly what Archimedes says - mass of the displaced water equal to the mass of the object."

I think not. In this case he was displacing water with trinkets that were made of gold and silver, neither of which would float on water. He realized that a given weight of pure gold would displace a water volume different than a counterfit trinket having the same weight (and thus different bouyant force, which is equal to the weight of the water displaced), but not made entirely of gold. Eureka!

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### SailcyJunior Member

Yes, it was originally described as a force pushing object out of a water/liquid,, hence knowing initially the weight of the metals he was able to discover was the crown made of pure gold or not ))))

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### SailcyJunior Member

Unfortunately, I could not find any legal definitions for such thing as "empty displacement' which many of small boat designers refer to. Apart from ship displacement where many categories exist, for example - loaded displacement, full loaded, deep loaded, standard.. Etc, our European regulation has a " light displacement" which often used for small craft (up to 24m) description. But light does not mean "bare hull".
According the ISO 12217and ISO 866 norms (boring to read!), the light displacement refers to:
– all structural elements
– all the comfort equipment included in the standard version of the boat (as mattresses, cushions, cockpit table),
– the heaviest engines available,
– standard batteries,
– electronics,
– standard deck equipment (ladder, winches, anchors chain and anchor, mast, boom, standard sails, sheets, halyards…).

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### SailcyJunior Member

According to norm ISO 14946, the max loaded displacement refers to the light displacement with in addition:
– the maximum number of persons that the boat can accept (75 kg each),
– all the extra equipment,
– the basic personal equipment,
– provisioning,
– all tanks full (fuel, water and grey tanks),
– Liferaft.
Now, when you read the specifications of a boat on a brochure you should ask yourself:
– what type of displacement is the boatbuilder talking about?
– is this displacement definition in compliance with ISO norms?
Indeed some builders do not specify if the displacement complies with ISO norms. The answer matters if you want to compare 2 boats because if you have the right info so you can evaluate with precision the payload of the catamaran and the equipment that you can take with you.

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### SailcyJunior Member

So, coming back to our business regarding weight. When you do not know a lot about boats you can feel a bit dizzy when someone talks about boat displacement. In fact there are 2 radically different definitions: light displacement and max loaded displacement. These terms are defined by ISO norms which are used by the European Economic Community regulations. A builder like Lagoon has to respect these norms and others like boat design EEC classification.

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### gonzoSenior Member

Most sales brochures make claims that are not strictly true. They use hype to attract customers. You could request the data necessary as a condition of buying the boat.

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