— March 2003
By Richard Thiel
|“Just throwing a speed limit on the harbor might be the worst thing you could do.”|
It’s not in our nature to look backwards. Whether we’re walking or driving, our focus is naturally directed forward. This does have its disadvantages, however, not the least of which is a blissful unawareness of any problems we may have left in our path.
That characteristic takes on special import when we’re driving our boats, because unlike Reeboks and Volvos, boats create a palpable disturbance that if of sufficient size can cause substantial damage. The formal name for this is, of course, wake, and today a lot of boaters and landlubbers are concerned with it. In more and more places, governments are passing laws designed to essentially ban wakes through the imposition of speed limits. This movement will have a direct affect on how you will pursue your favorite pastime in the future.
Technically, a wake is any disturbance caused by an object that displaces water. In its smallest and most innocuous form, a wake is the ripple formed by tossing a pebble into a pond. That is obviously not the kind that’;s causing speed limit signs to proliferate along waterways at about the same rate as Zebra mussels.
Wake is a function of an object’s mass, shape, and velocity. Without getting wrapped up in hydrodynamic theory and arcane formulae, a vessel’s waterline length—from where the bow enters the water to where the stern leaves the water—dictates its displacement speed, which is how fast it can travel through the water without causing a big wave. To go faster than displacement speed, a hull must plane—climb on top of the water and glide over it, instead of plow through it.
All this is relevant because of the findings of a study conducted by the Stevens Institute of Technology of Hoboken, New Jersey, on wave patterns in New York Harbor and the damage they cause both shoreside and waterborne property. After placing sensors on the harbor bottom, Stevens scientists determined that the harbor's wave patterns were generated not by tides and currents but by vessel traffic, mainly ferries, which have proliferated in recent years. They set about finding ways to minimize the size and number of the waves.
What's surprising is that after all their research, the Stevens scientists did not recommend the imposition of speed limits. In fact, Michael S. Bruno, director of Stevens' Davidson Laboratory, told The New York Times, "Just throwing a speed limit on the harbor might be the worst thing you could do." He pointed out that ferries generate most of their wake not at low or high speeds, but in the transition zone between the two--when the vessels are between displacement and planing modes. The same is true of pleasureboats.
This is something most boaters have known all along. You need only look aft while on a high-speed planing boat to see that the wake is small--not as small as when the boat is at displacement speeds, but much smaller than when she's trying to get over the hump. This simple conclusion might be something those government agencies should take into account the next time they decide to attack a wake problem by simply posting a 9-mph speed limit. It might lessen the wake for some vessels, but for others it just might be the point at which they generate their largest wake--and the most damage.
This article originally appeared in the February 2003 issue of Power & Motoryacht magazine.