|
|
|
General HydroStatics
Ship Stability Software |
|
Command of the Week
(New or interesting aspects of GHS that you may not know about) |
|
COMP /WET vs. COMP /SKIN
|
|
The area of the enclosing surface of any component of the geometry model is made available by either of these two
commands, though they have different purposes as will be explained.
COMPONENT /WET finds the wetted surface area: the area of the portion of the component that lies below the waterplane (or if it is a tank, below the interior liquid level). On the other hand, COMPONENT /SKIN calculates surface areas without regard to liquid levels. That immediately brings up the question about differences. For example, is the wetted surface of a hull that is completely submerged identical to the surface area of the same component as found by these two methods? To answer that we submerge our trusty Supply Vessel and issue the commands for both reports: |
project surf
read sv.gf
report /box:color
depth 99
comp hull\hull.c /wet
page
comp hull\hull.c /skin /profile
print preview /spawn
report off
|
|
The wetted surface number appears along with various other measures and ratios, some of which are of little use for a ship in this unfortunate condition: |
|
| On the next page we find the /SKIN report. What you see below is actually the second page of the skin report, showing the last few sections followed by the totals. |
|
|
Note that the surface areas in the two reports are somewhat different: 2306 and 2316.8, for a ratio of 0.9953.
One would think that closer agreement could be had by reducing the distances between sections in the model, which can be done easily by means of the FIXUP command. For example, FIXUP (HULL\HULL.C) MAXGAP%:1.0This yields 2310 and 2318.1, for a ratio of 0.9965, which is only slightly better even though the number of stations in HULL\HULL.C was doubled. The reason for the divergence is that two different methods were used to calculate the surface areas. The next page pictures the girth curves that were integrated by the /SKIN method. |
|
|
Note the discontinuity. This is caused by the raised foredeck that is incorporated in the
hull model and the abrupt end of the inboard portion of it at a bulkhead located at 41.82
meters aft. The surface integration has to include the area of the face of that bulkhead, which
is about 26 sq. meters.
The next graphic shows the discontinuity plus one section on either side of it. What appears to be three stations is actually four. The two stations at the discontinuity are closely spaced. |
|
|
The complex shape at the bulkhead and aft of it (represented by the hump in the curve) is the cause of the
difference. The two methods give closer results for smoother surfaces.
The differences are minor in any case.
COMPONENT /SKIN is good for estimating paint or construction material. The center of gravity it provides may be useful in that application. With subparameters ALL, DECK, NODECK, and BOTTOM, it detects the points at which each station transitions from bottom to side to deck. Of course /SKIN:BOTTOM works better when a pronounced chine is present, and /SKIN:DECK and /SKIN:NODECK prefer a definite deck edge. COMPONENT /WET is intended to provide wetted surface for use in resistance estimates performed outside of GHS. Wetted surface for a range of drafts is available too. Below is the graph that comes out after the curves-of-form table. The command was, comp hull\hull.c /wet /depth:2, 2.1 ... 4.5 |
|
|
If it is hard to read the legend in this graphic, wetted surface is represented by the
blue line on the right, which starts at about 2.8 x 200 = 560 sq. meters at the initial 2-meter draft.
The precise value is 564.7, taken from the table (not shown).
|
|
Questions, comments, or requests?
Contact Creative Systems, Inc. USA phone: 360-385-6212 Fax: 360-385-6213 Office hours: 7:00 am - 4:00 pm Pacific Time, Monday - Friday Mailing address: PO Box 1910 Port Townsend, WA 98368 USA Click here for an index to this and previous COWs |