3669. Light Ship or Load? Uncertainty in Shipboard Weight Surveys

Paper

Abstract

Adequate stability for a ship is critical to the vessel’s safety and operation, the safety and well being of its crew and passengers, and its ability to accomplish its mission. To validate that a ship has acceptable stability, a detailed inventory of the ship is conducted to determine what weights must be added or deducted in order to arrive at an accurate light ship center and weight. This determination is important in arriving at the ship’s true light ship weight and center, a basis for calculating the ship’s draft, trim, and stability characteristics. Small vessels may only require a couple of people and a few hours to complete such a survey while a larger vessel, on the other hand, can require several thousand manhours and more than a week to perform. The accuracy of this important survey depends on the available manpower, the experience of the surveyors, the size and complexity of the ship, and, most importantly, the accuracy and consistency practiced by the people conducting the survey.
Currently, there are two guidance documents: the Navy’s Naval Ships Technical Manual (NSTM) Chapter 096, Weight and Stability, and the Society of Allied Weight Engineer’s (SAWE) Recommended Practice M-9, Shipboard Weight Surveys. However, it has been observed that there are still many areas of inconsistency from one surveyor to the next, one type of ship to the next, one shipyard to the next, and even from one Navy observer to the next. There are also new trends observed on naval vessels that need to be factored into a ship survey such as the growing trend of increased shipboard items introduced by the ship’s force to improve quality of life, increased stores, and increased equipment that may or not be a permanent fixture on the vessel. Currently, these items default to a judgment call by the individual surveyor or the organization conducting the survey. This inconsistency causes inaccuracy and reduces the value of conducting these surveys. It also reduces the value of the data to determine trends and to compare one ship to another. How much inaccuracy is inherent in this process is not known and cannot be easily measured.
It is the intent of this paper to identify areas of actual or potential inaccuracy and inconsistency and offer suggestions on how to resolve them Rationale for making a decision as to whether a newly encountered item falls into the light ship or variable load category will also be offered to better enable surveyors to make a reasonable decision during a survey. Guidance will also be provided to determine into which load category new or questionable items should be placed. The main objective is to increase the accuracy of shipboard weight surveys, reduce the inaccuracy of shipboard weight data, and improve the consistency of weight surveys across multiple ship classes and across the industry.