General HydroStatics
Ship Stability Software
Command of the Week
(New or interesting aspects of GHS that you may not know about)

DRAFTS


The captain of the chemical tanker TANKER.GF wanted to find out how much the hull was sagging with the current load, so he sent for drafts at the perpendicular locations, and what he got back he plugged into the GHS DRAFT command:

 LBP 0, 95f
 variable L
 set L={FP} plus {AP} div 2
 LBP {FP}, {L}, {AP}
 draft 4.04 @ FP,  4.95 @ MP, 4.34 @ AP

And the program responded with this output:


The captain knew this couldn't be right. That much deflection was impossible. So he called for drafts every ten meters.

While he was waiting, he had the program spit out the drafts he would expect to see with zero deflection, assuming the FP and AP values were correct (he had more confidence in them since the crew were used to getting draft readings at the forward and aft perpendiculars where they had tables giving the conversion of freeboards to drafts). Just for fun he displayed the drafts in a little dialog box:



By the way, here is what he did to make the template for the dialog:

 draft 4.04 @ FP, 4.34 @ AP
 macro drloc
  set DRLOC8=%1
  set L=%1
  "{L:3fa}" | "{DRVALU:3}"
 /
 template drafts "Drafts, straight WL"
  expand drloc (10,10) {FP}
  expand drloc {AP}
  expand drloc {MP}
 /
 .drafts

Finally he got the list of drafts at ten-meter intervals, and he plugged them into the DRAFT command:

 draft 4.04 @ FP,
 4.06 @ 85f,
 4.15 @ 75f,
 4.14 @ 65f,
 4.66 @ 55f,
 4.20 @ MP,
 4.21 @ 35f,
 4.23 @ 25f,
 4.28 @ 15f,
 4.34 @ AP

with the following result:


Right away Capt. Chemical noticed that draft at 55m fwd was out of line. So he deleted it and reissued the DRAFT command without it, and he got the following surprise:


This was entirely unexpected—the first time in his entire career that he found the result of a deflection calculation to come out so small. Obviously it was accidental since it was impossible to resolve drafts within even a centimeter. Added to that, he knew the ship had a permanent sag. No one would ever believe these results. What was he to do? We felt sorry for him, but we couldn't help him.

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

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