See How They Run, Part II

Spectator - November 2001

Spectator — November 2001

By Tom Fexas

See How They Run, Part II
Further observations on the elusive "nice running boat."
 More of this Feature
• Part 1: Running
• Part 2: Running
 Related Resources
• Spectator Index

 Elsewhere on the Web

Last month we analyzed entry angle and trim, two running characteristics of hulls that can be determined by simply studying photos of the vessels underway. This month we examine three more. The numbers superimposed on the photo correspond with those in the text below.

Generally, the cleaner a hull parts the water (no. 1), the softer the ride. But forebody entry angle is only one determinant of softness, which also depends a great deal on the shape of the bottom forward. Blunt sections here present flatter areas to impact the sea. Of course, everybody will tell you that their hull rides better than everybody else's, and while a measurement of how soft the hull rides can be attained by fitting decelerometers to a hull, the verdict is usually via SOTP (seat of the pants).

Concave sections produce a dry ride with good spray suppression but "focus" the impact of waves, producing a jarring ride. Convex sections, on the other hand, are soft-riding but monsoon-wet. What's a poor yacht designer to do? The answer is: compromise like hell. Add a little here, take a little off there, put a bulge here and a hollow there. After many long years of shape-shifting, a designer will arrive at a hull form that works best (usually about the time he is ready to retire). 

If you are receiving mouthfuls of water when running at high speed into seas off the bow, you have a wet boat. The trick to avoiding this is bottom shape manipulation and sprayrail placement (usually determined by divine inspiration). At best, the bow wave should be caught close to its source and deflected as a sheet downward (no. 2). Deflecting the bow wave in droplets or a disturbed pattern will result in more water in your puss. The spray deflection in the photo at left of the Midnight Lace 52 was taken around 1985 (before we had perfected the black art of sprayrail placement), and while good, it is not quite as clean as that of some recent designs. Today we would place the sprayrail a bit lower and configure it differently.

Generally, vessels that create the least fuss aft (no. 3) run more efficiently than those that make you look over your shoulder, wondering who the hell is following you. Wake generation has to do with bottom shape and, more important, overall weight. A porky hull--even one with an optimum hull shape--will produce much more wake than that same hull form lightly loaded.

Next page > Running, Part 2 > Page 1, 2

This article originally appeared in the June 2003 issue of Power & Motoryacht magazine.

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