See How They Run, Part II Page 2
Spectator — November 2001
By Tom Fexas
See How They Run, Part II
|Further observations on the elusive "nice running boat."|
Beam also plays an important part here. A wider boat is less efficient than a skinny one at semiplaning speeds and more efficient at planing speeds. The original Midnight Lace 52, for example, was very light (23,000 pounds) and had a beam of only 13 feet. With two tiny 220-hp engines, she ran about 23 knots. Mostly due to creeping comfortism, the new Midnight Lace 52 is somewhat heavier (39,000 pounds), has a beam of more than 16 feet, and will require twin 320-hp engines to run at 23 knots. While the wider, heavier boat will never be as efficient as the skinnier, lighter boat, it is possible to refine the hull shape such that efficiency is not severely compromised by increased beam.
Deadrise aft also has an effect on efficiency. No matter what you may hear, the simple fact is that flatter sections aft develop more lift (and are more efficient) than high-deadrise sections. This is common sense and has been proven in tank tests and on full-scale hulls. That's why you will never see high-deadrise surfboards or water-skis. What high deadrise aft does well is soften the landing impact when a hull becomes airborne (such as the famous Wellcraft in the opening credits of Miami Vice), since a boat that becomes airborne will land on its tail first. If the aft sections are flat, the impact will be harsh; if it is highly V'd, reentry will be softer. As far as cruising boats are concerned, the question has to be: "How often do I take Mamma's Mink airborne?" Most cruising boats over 35 feet will never go airborne, and that's why high transom deadrise is not usually found on cruising hulls.
In general, a hull should be shaped for the cruising speed the boat is locked into by the power installed. If a hull is powered such that it will never reach planing speeds, it should not have a planing (hard-chine) bottom shape. (Ever see a duck with chines?) Water does not like hard corners unless true planing occurs; that is, when the water cleanly separates from the transom and chines at speed. Otherwise, a semiplaning (or penetrating) hull form will be more efficient. There are far too many boats designed with planing bottoms that never have a prayer of planing--except, maybe on the face of a 50-foot wave.
In the end, it all comes down to guile and cunning on the part of the yacht designer. How do you make the transition from a very deep, fine entry (for softness) to a flatter bottom aft (for efficiency) without twisting the bottom such that the stern will suck itself down? How do you shape the forefoot to be soft yet dry? How do you form the hull for a steady ride in following seas? Well, at these prices, none of these secrets will be revealed here (you'll have to wait for the book). All I can say is that after 35 years of pushing volumes around like Jell-O in a balloon, I have come up with a killer hull form: our "MCR round V" configuration. ("MCR" stands for "Magic Carpet Ride." Since everybody else has foolish names for their bottom configurations, I thought I'd join the party. Sorry about that.)
But, like I said, everyone will tell you their hull is the softest, driest, most efficient ride in town. Boat salesmen may outright lie to you. Yacht designers can be evasive and change the subject. The only way to tell for sure is to plant your butt in the helm chair and mash the throttles forward in a 20- or 30-knot breeze. Then carefully analyze posterior input. The butt knows.
Tom Fexas is a marine engineer and designer of powerboats. His Web site is www.tomfexas.com.
This article originally appeared in the June 2003 issue of Power & Motoryacht magazine.