Weight and Displacement - cause for confusion

By definition displacement equals weight unless the body sinks in which case it equals volume.

So why so much confusion? The way in which a yacht designer or naval architect uses the term is strictly speaking the same as the definition, but with some variations that are at the root of the confusion.

One of the first jobs in the design process is to determine how much hull (or displacement) to put under the DWL (Design Waterline or Datum Waterline) on the model or the drawings.

So this is what the designer calls "displacement DWL".

A small production trimaran might weigh 950kg, but might be certified to carry between 400kg and 500kg in crew, supplies, and personal gear, possibly more than half the boat’s weight. 

Same for a larger cruising cat. In these boats there is much more flexibility and variation in the build process, the internal fitout, and the gear and equipment that is put on board. Again the add ons can be half the boat weight or more.

In both of the above cases the designer has to be conservative and allow for the highest expected load case so that the boat trims at or above its designed waterline, so the stern is not dragging in light air (or the bridge clearance too low in the case of a cat) and the owner can be confident to drive the boat hard without fear of structural problems.

 

The designer has to allow for this in taking account of stability, trim and structural calculations. The number for disp. DWL is the foundation of the information the designer gives to the rig designer, the composites engineer, and the naval architect for stability data.

 

The number that most potential boat buyers are looking for is not the displacement but the weight. Unfortunately there is no clear standard for defining weight for sailing boats and different designers and builders use terms like built weight, empty weight and bare weight without saying specifically what that includes, and so it is open to interpretation.

 

To get an idea of what a particular type of sailing multihull might typically weigh the best reference source is the OMR ratings spreadsheet on the QMYC web site. This is an extensive and generally reliable data base of boats measured to race under OMR with the inclusions clearly defined in the weight measurement. You can see it here;

http://ec2-54-206-9-177.ap-southeast-2.compute.amazonaws.com/joomla/index.php/2014-03-23-06-43-01/2014-03-23-12-36-47/omr-ratings