Wetted Surface Area ( WSA )

Hello all,

I'm trying to calculate WSA for small ship for which I have AutoCAD lines and body plan.
To do I choose to :

Case A:
Integrating spline (waterlines) length from bottom to given level (draft,main deck).
Case B:
Intergrating spline ( frames) length from aft to fore.

I believe that result in the end of both calculation should be the app. the same , but difference is 1,5 times..
In order to test algorythm I take rectangular shaped vessel with same main particulars ,put rectangular ship data in Excel, but then WSA is twice less than by hand calculation ( 2 x ( area of sides ) + 2 x ( area of front and aft ) + area of bottom of rectangle ).

I was thinking to create 3D model of ship ( surface area would be WSA ) just for rough compare, but difference between my result and rectangle in previous step made me thinking that my way of thinking ( formulas ) that I use to find WSA are wrong.

Any advices would be hinghly appreciated

PS : I'm sure that my integration is correct, because when I plot those curves in AutoCAD, I get same value under curve. So error should be in formulas?

Best regards
Todor

 

The calculation of the wet surface is, although it seems to be a lie, one of the most complicated when it comes to obtaining the properties of a hull. In fact, one way to check whether a naval architecture program is serious or not is to see how it calculates the wet surface.
If you calculate the surface of a sphere by integrating cross sections or water lines, you will see that that you get values that are different from each other and different from the correct value.
The only way to get a fairly correct value, as you have thought, is by creating surfaces using the command "SWEEP" between various lines and, depending on the type of hull, the calculation with water lines will be more accurate than with frames. In general, the first one is more accurate.
If you create a surface in AutoCAD, the "MEASUREGEOM" command will give you the results you are looking for. AutoCAD calculates with amazing accuracy.
I hope I have helped

 

Hi and thanks for reply Ignacio,

I was trying to avoid generation of 3D computer based model. I was thinking that with Excel and with help of 2D based program, I would be able to do such calculations.. but results disapointed me.
I just wonder how they calculate WSA before computer era.. or maybe that factor isn't important in past.

Any way thanks for reply
Regards
Todor

 

Hi Todor,
It was calculated using traditional methods of integration, trapezoids, Simpson, parables of 3rd order, etc., using the water lines. In the old boats, with rounded shapes, rounded sterns, without a bow bulb, the results were closer to reality.
Scale models were also made, in wood, they were covered with thin veneer of wood or cardboard and their area was measured, for example, by weighing it.
BR,
Ignacio