Friday, March 25, 2011

My Work Hat.....

Small Passenger Vessel Assumed Passenger Weight Change

On December 1, 2011, all US passenger vessels will be required to operate under limitations imposed using an assumed average passenger weight of 185 lbs. Operators are responsible for determining before that time how they will incorporate this change into their vessels. Intent must be submitted to the Coast Guard in writing. This discussion is specific to subchapter “T” vessels with stability verification via 46 CFR, part 178, also refered to as the Simplified Stability Test (SST.)

Background fromWikipedia:
The Ethan Allen was a 40-foot, glass-enclosed tour boat operated by Shoreline Cruises on Lake George in upstate New York. On October 2, 2005, at 2:55 p.m., with 47 passengers–all from Michigan and Ohio and mostly seniors–aboard, the Ethan Allen capsized and sank just south of Cramer Point in the Town of Lake George. Twenty passengers died and 27 survived. The accident caused government regulators to consider new laws on passenger boat capacity.

The final rule making stemming from this accident is now implemented. In addition to several procedural changes, the rule stipulates that the assumed weight per passenger carried will be increased to 185 lbs. Vessels previously underwent stability analyses for assumed average weights of 140 lbs each, 160 lbs each or 165 lbs each depending on factors such as vessel size and service. Without re-evaluation, a vessel will be assumed capable of carrying the same total passenger weight, but divided by a larger weight per person. Most small (under 65’) launch vessels operating in protected water would have been evaluated using a simplified stability test and an assumed 140 lbs per person. These vessels, without new analysis, will be required to reduce passenger count by approximately 25%.

The Simplified Stability Test (SST) is a procedure intended to alleviate the need for 3 dimensional hull shape information. As such, it is favored for smaller vessels whose value or construction documentation level do not support a more detailed naval architecture approach. The limitation of the SST is that it yields a simple pass/fail indication and not adequate numerical data upon which to base later modifications. After modification, a new test must be conducted. This was the primary underlying cause in the Ethan Allen disaster, and is common to a lesser extent in modified production vessels granted sister status from an earlier test. Further limitation is that the test is conducted for a previously determined passenger capacity, often based on seating or standing room. In other words, the test is not designed to optimize capacity based on stability. A trial and error approach is necessary to correct for failure to meet the applicable standards at reduced passenger count.

Viewed from another perspective, the new passenger weight regulation provides vessel owners with an incentive of up to 25% passenger capacity to update or at least become familiar with their vessel’s stability profile. Most operators and many builders do not understand or retain the stability test report that determined their passenger capacity. This is recognized from the Ethan Allen accident as a great danger since it often masks the effects of cumulative changes over the life of the vessel.

Moving Forward:
To comply with the new required passenger weight, vessels not otherwise in question regarding stability will be given the option to take a passenger capacity reduction. This will be in direct proportion to the original assumed weight divided by 185 lbs. Many owners will chose this option based on cost, convenience, and normal operation patterns. Since I have never encountered a commercial vessel operator who wished for less allowed capacity, I believe this approach is in error. Even if the allowed capacity is carried in only a small percentage of operations, it is lost on each and every trip. Given the short run of this vessel type, the loss during peak demand will rapidly exceed the cost of a new evaluation. While true that convenience will be the impact most often felt, peak demand such as special events will potentially result in fewer passengers able to be served overall.

The first step for any passenger vessel owner is to obtain a copy of the stability test report (form CG-4006) that authorized their current passenger capacity. This should not be confused with a stability letter or inspection certificate. In some cases these SST reports are on file with the Coast Guard office that provided construction or modification inspection oversight. This document is as vital to the safe operation of the vessel as the certificates of inspection, documentation, and insurance. It is also the one piece of information that can allow a naval architect to discuss the cost benefit advantages of a new SST. In all cases, the last conducted test is of great value in planning modification or a new test.

If this document cannot be obtained, the vessel is being operated on faith that it exists as it was tested. Whether this is true or not is subject to many considerations, but it is irrefutable that no operator ever has complete accounting of cumulative modifications to a vessel while it is floating right side up. Since the foundation of a vessel is fluid, there is no insignificant change. The immediate penalty for this head-in-the sand approach is an automatic capacity reduction, whether safety standards deem it necessary or not. The ultimate penalty was paid in the Ethan Allen disaster.

Finally, for those vessels lacking direct documentation, and for those whose limitations are shown in the original SST to have been imposed by factors other than total passenger weight, the vessel should be re-tested. A new SST is not an expensive undertaking. One working day is often adequate for the procedure. Materials can be as simple as empty 55 gallon drums, a good pump and wood blocking.  In some cases the SST is conducted entirely by Coast Guard inspectors. Where optimization of owner concerns is preferred, and for increased veracity of the results, the involvement of a naval architect is advisable. If modifications are contemplated or the starting point is otherwise uncertain, a preliminary “dry run” is useful.

It cannot always be guaranteed that the outcome from this work will match that desired. Particularly for vessels lacking all prior documentation, owners are cautioned that the result obtained will be safe, if not all that was requested. There are cases where original tests are discovered to have been in error. There are others where modifications have augmented or reduced safe capacity. More than half of the new tests I have conducted have resulted in full passenger capacity being retained at the higher assumed weight. There are, however, no cases where a passenger count either higher or lower than imposed by safety standards is in the best long term interests of the vessel owner.


  1. Great post Chris, definitely something everyone should read and take note of.

  2. Very very interesting! A lot of things to consider for boat owners everywhere. You did a thorough explanation too. Great post!