SURVEYOR'S NOTEBOOK

The Really Big Picture

By Rik van Hemmen

The April issue of (mt), the SNAME house magazine, will feature an article by Dr. Wayne Neu, professor of Aerospace and Ocean Engineering at Virginia Tech. Dr. Neu updated the classic 1950’s Gabrielli – von Karman plot using more recent vehicle data collected by the students in one of his classes.

The Gabrielli – von Karman plot is one of those devices that allows one to think in terms of the real big picture. In essence, it plots vehicle drag over lift against vehicle speed. It is a cornucopia of transportation devices and allows one to compare one mode of transportation against another as far as basic efficiencies at various speeds is concerned.

In the simplest terms (and there are much more complicated refinements of this plot) vehicles that are lower on the plot tend to be more efficient at their corresponding speed.

When Gabrielli and von Karman looked at the plot, they noticed that there were no transportation devices that extended past a straight line on the plot and they named it the Garbrielli – von Karman line. They also noticed that there was an empty area behind the line in the mid speed range of 20 m/s (about 40 knots) to 100 m/s (about 200 knots), except maybe rail, but that stuff only works on land.

Newer designs have broken that line and that was most evident when Naval Architects began designing very large ships.

When Dr. Neu sent me the updated graph I noticed that besides the GvK line there is another line and that is the line that contains all the waterborne transportation devices. This version of the graph contains that line and I have modestly named it the Neu – van Hemmen line (or the NvH line).

This line is anchored at the bottom by very large bulk carriers and container ships and at the top by high speed recreational vessels (whose true speed I tend to doubt) and the SES 100B, which is still a boat but only barely so. One can debate the angle of the NvH line, but its presence is remarkably clear and provides guidance as to where ships can and cannot compete.

On this plot and right in the GvK mid speed zone is a vehicle that provides a fascinating alternative; the WIG (Wing In Ground effect¬†vehicle¬†). This is essentially a slow aircraft that can fly no higher than ground effect, which extends roughly the length of the vehicle’s wingspan. Large WIGs take off and land on water like flying boats and are closely tied to marine design even in their air mode. The few WIGs that exist are classed by Lloyds and regulated by IMO.

On (or rather, over) land such a vehicle is a pretty dicey proposition, but over water a large WIG can be a grand vehicle of the type that we envisioned airships to be. In the big picture, maybe it is time to take a closer look at WIGs.

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