General HydroStatics Ship Stability Software

Command of the Week (New or interesting aspects of GHS that you may not know about)

DEFLECT

Hull bending, or deflection as it is called, can have a significant effect on drafts compared to ideal drafts with no bending. If the deflection is known, the DEFLECT command is a convenient way to correct for this. After applying deflection, the displacement derived from drafts will be more accurate. It's no secret that GHS accounts for deflection by bending the waterplane, not the hull model. To illustrate this, here is a ridiculously large hogging deflection:

DEFLECT 4.0

Imagine what this would do to the coordinate system (which is tied to the ship) if the geometry were somehow bent! Since bending is usually small compared to the length of the vessel, an acceptable approximation of the deflection effect is obtained by a second-order or parabola-shaped curve applied to the waterplane. Here is a more reasonable amount of deflection:

DEFLECT 0.25

To correct displacement and LCG respecting the deflection:

SOLVE WEIGHT, LCG STATUS

In practice, the DEFLECT command takes a back seat to the DRAFT command in which a series of drafts can be given from which deflection is calculated.

DRAFT 7.0 @ 45F, 6.75 @ 0. 7.0 @ 45A SOLVE WEIGHT, LCG STATUS

Here is the screen output comparing the result of the DEFLECT command with the result of the 3-point DRAFT command.

Deflection can also be calculated using the Longitudinal Strength feature in GHS. The system variable DEFLECT can be used to capture whatever deflection has been calculated into another variable so it may be restored later by the DEFLECT command. It is important to know the length over which deflection is referred. The full syntax of the DEFLECT command allows for specifying a length. To keep this COW simple we will leave deflection lengths to the GHS manual and the GHS training class.

Questions, comments, or requests? Contact Creative Systems, Inc. support@ghsport.com 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 www.ghsport.com Click here for an index to this and previous COWs