Effect on design characteristics of Great Lakes bulk carriers by physical operation limitations and diversion of water from Lake Michigan 1

Abstract

The familiar type and proportions of Great Lakes cargo vessels differ from ships operating on ocean waters due to dimensional limitations peculiar to inland water transportation. Lock sizes; available dry dock capacity; river, channel and port depths; maneuvering ability in harbors and rivers; seasonal operation averaging about 245 days per year; are all restrictive factors forcing the designer to adopt dimensions somewhat abnormal for optimum construction costs.

Length overall of 730 ft. and beam of 75 ft. are the present maximum dimensions specified by the St. Lawrence Seaway operation. Draft, which is the least costly dimension, is currently limited by project channel depths of 27 ft. The effect on construction costs by the above restrictions is reflected in the ratio of hull block cubic to hull surface, i.e., cargo capacity to hull structural cost. For example: $\begin{array}{ccc}\text{Ocean ore carrier}\dots \hfill & \text{Relative bulk/surface}\hfill & =14.94\hfill \\ \text{Great Lakes ore carrier}\hfill & \text{„}\text{„}\text{„}\hfill & =12.50\hfill \end{array}$

A further direct handicap affecting the operating costs on the Great Lakes waterways is the continuing diversion of water from Lake Michigan by the City of Chicago for sewage disponal. Surveys indicate Lake Huron is ultimately lowered 1 inch per 1000 cu.ft./sec. diversion to other continental drainage regeons. Lakes Erie and Ontario suffer about $\phantom{\mathrm{(}}5\mathrm{/}\phantom{\mathrm{(}}8$ inch level drop for the same unit diversion.

The above artificial loss of water results in a corresponding decrease in loading drafts for the larger Lake carriers. The tons/inch of lighter draft, and hence cargo, runs from 50 to 113 long tons per ship. The potential loss in operation income equals: tons/inch of a given ship × inches of lowered lake level × number of trips/season × freight rate/ton, and represents a potent factor unfavorable to the economy of lake transportation.