A buyer's guide to intelligent design
Posted on 31 March 2011
Written by Eric Sorensen
All boats involve trade-offs - knowing what they are will help you make an informed decision
During more than 20 years spent testing and evaluating boats, a number of observations consistently emerge as I conduct dockside inspections and sea trials. Let's have a look at what I've observed in three key areas: ride vs. efficiency, seaworthiness vs. space and visibility at the helm.
Ride vs. efficiency
The fundamental truth about any product is that it has capabilities and limitations. Of course, that includes boats. No boat can do everything well, yet some boatbuilders say they offer better fuel economy and a smoother, more comfortable ride in rough water. The bottom line is that these assertions frequently are at cross-purposes. For example, getting top fuel economy in a planing monohull requires several key design elements. Lighter boats with flatter bottoms, and to some extent longer and narrower hulls, are more easily propelled. The only design element that contributes to a smoother ride is the longer, narrower hull.
The lighter a hull is for its shape and size, the more reactive it will be to wave action. It's a simple matter of kinetic energy - how much energy is stored in the vessel as a result of its mass and how the hull's mass counters the mass of each wave that it impacts successively. The lighter the boat, the more relative impact (literally and figuratively) the wave will have on the hull. Less reactivity to wave impact and lower accelerations, especially the vertical kind, is what we feel as a smoother ride. A heavier hull of the same shape simply runs better - within limits, of course. A boat can grow so heavy that it becomes grossly inefficient and wet, and it will handle sluggishly.
A planing hull must have flatter bottom sections for optimal efficiency. When you think about it, a perfectly flat bottom is the most efficient water wing, or lifting surface, and that's what you get with a ski boat. A ski boat gets on plane at very low speeds - about 10 knots if it's not too heavily loaded - and it leaves a flat wake astern, a function of its flat and efficient bottom shape. Also, the wake itself is an excellent indication of form drag, the resistance created by the shape and cross section of the underwater portions of the hull.
The deeper the hull is immersed in the water, the heavier it is, and the more deadrise it has, the bigger the wake. If you look at the cross section of a planing ski boat hull in relation to the waterline, the bottom of the hull is just a few inches below the surface of the water, so its form drag and its wake size are very low.
The flat bottoms seen on the planing boats that builders say are more efficient often require less power to reach a given speed. However, heaven help you once you take the boat out of a river or small lake and into a bay or ocean. The ride is going to be rough. Although the deep-deadrise hull is a less efficient lifting surface and takes a little more power to push, the difference in efficiency between a well-designed deep-vee and a low-deadrise hull is often just 5 to 10 percent. Because nothing in life is free, the price you pay for the improved efficiency is a boat you can't take offshore when the wind is blowing harder than a light breeze. I'd give up 5 or 10 percent efficiency for a boat I can use (almost) whenever I want.
Why buy a boat knowing you'll only be able to use it 18 or 20 days a month, when a good-running boat will let you go out comfortably and safely 25 or 28 days a month? This becomes an even more compelling argument if you have a 90- or 100-day boating season. If you've blocked off a three- or four-day period for a short vacation, but can't use the boat on one or two of those days because the wind is blowing hard, then buying that full-bodied, queen-berth, his-and-hers-closet-forward boat seems a little foolish.
The reason the vee hull rides better is because the bottom more gradually meets and absorbs the energy of the waves. Both buoyancy and dynamic lift develop more sequentially in a vee hull, as first the keel and then gradually (in microseconds) the rest of the bottom, from keel to chine, impacts the wave.
The wave meets the flat-bottom boat essentially all at once, creating a higher impact level and a jarring ride. We feel the more gradual absorption and dissipation of kinetic energy as a smoother ride.
There is a way to get a good ride with improved efficiency: Just take a lot of weight out of the boat. You can either build the boat light and cheap, which means it will likely end up falling apart - there are more boatbuilders than you might think that take this approach - or you can pay for high-tech construction to deliver a strong, light boat.
You can cut hull and deck structure weight by 25 percent or more over an open-molded fiberglass boat with prepreg, post-cured epoxy construction. But you'll pay big bucks for the stronger hull and lighter displacement, and it is doubtful that you will ever make up the increased costs of construction through reduced fuel costs over the years unless you use your boat for thousands of hours. And even with a hull and deck structure that weighs 20 percent less, by the time you add engines, systems, fuel, water and passengers, the difference in gross weight, compared with the conventionally built boat, will be closer to 5 or 10 percent than 25 percent.
The bottom line is that you're better off with a slightly less efficient and smoother-riding boat that you can use on a given day offshore.
Seaworthiness vs. space
The next observation involves accommodation volume vs. seaworthiness. If you choose your boat at a boat show without inspecting the bottom shape or taking the boat for a rough-water test ride, you pretty much deserve what you get. When you walk around at a boat show, your first clue that something may be amiss is a curtain wrapping the hull from the waterline down. Some builders have a lot to hide, with shapeless full sections forward that guarantee a hard and wet ride. But regardless of the hull design, what sells at a boat show for many cruising yachts is the volume of the interior, and this includes the hull and superstructure.
At the last few shows I attended, I took a close look at a couple of boats in the mid-40-foot range that had very generous accommodations. Unfortunately, the trade-off usually is reduced seaworthiness in rough water.
With one, I was taken aback by the sheer size of the superstructure for a boat with a beam of 14 feet and a length of just 47 feet, including the swim platform. This boat has a full-beam master stateroom below the saloon, which is what drives its overall height. The company had done a great job carving out usable space in what is essentially a three-level layout - master stateroom, saloon and flybridge.
If boats such as this are classified for Recreational Craft Directive Design Category D, for use in sheltered waters with winds of as much as 17 mph and wave heights of as much as 1.6 feet, I would be satisfied that such vessels are being responsibly represented - as long as the prospective owners are made aware of the boat's classification and inherent limits of seaworthiness in rough water. The builders should let customers know that buying boats with a high vertical center of gravity and substantial top hamper, or windage, means trading a considerable measure of seaworthiness for the boat's equally considerable accommodations. In my opinion, they should be making a clear delineation between the ride and seakeeping abilities of a boat like this and, say, an express sportfisherman of the same overall length.
My concern with designs such as these is that there is so much fiberglass structure up so high, with the bridge deck extending out over the cockpit and a huge radar-arch-supported hardtop high off the water. If I were in the market for such a boat, I'd ask about the range of positive stability, which is a measure of the vessel's static stability and a function of its hull form and fixed center of gravity. Also, and just as important, I'd ask about dynamic stability calculations, which take into account the forces of wind and wave and the momentum of a rolling, pitching, yawing, surging, swaying and heaving hull.
As I discussed in previous columns, it's not just the center of gravity that matters. It's also the distribution of weight that counts for so much in ascertaining a boat's seaworthiness. Add a lot of weight in the ends, say hundreds of feet of anchor chain in the forepeak, or a hardtop weighing hundreds of pounds 17 feet or so above the waterline, and the gyradius effect (flywheel effect) will accentuate pitching and rolling motion and will contribute to excessive yawing (and, therefore, rolling) when cruising down sea. Add to that a flybridge that looks as if it could comfortably seat seven or eight 200-pound people, and the center of gravity and the gyradius effect go up.
The old Chris-Crafts and similar cruisers built in the 1960s and '70s may have been similarly proportioned, but they also tended to have enclosures made of canvas, not fiberglass, and they often were powered with heavy diesels. Having canvas tops removes weight from up high, and heavy diesels add weight down low. The center of gravity moves toward a weight increase and away from a weight reduction.
This is not to say that these boats aren't worth considering. Indeed, as Phin Sprague pointed out in an earlier column, these boats are just the ticket for dockside living and inshore cruising, and they are perfectly suited for people who don't intend to head offshore when the seas start to build. However, it is safe to say that as boats continue to get higher and more voluminous, their seaworthiness, seakindliness and ride quality diminish correspondingly.
I remember taking one such 52-footer out in Miami in the Intracoastal Waterway some 10 years ago, and it pounded in a 6-inch chop. Once I got out of the Government Cut and offshore in a brisk breeze, I found that the boat was almost uncontrollable when running down sea above 8 knots in 3- to 5-footers. There was just too much mass up high too far forward. However, if I wanted a big boat for its overall length that I could comfortably spend a week or a month aboard when cruising sheltered waters, it would have been fine. So in terms of capabilities and limitations, these boats draw a clear line in the sand for the buyer.
My final observation deals with visibility at the helm. A helm station that has large segments of horizon blocked off by interfering structure reduces situational awareness. If you can't see a 10-degree chunk of horizon because of a 30-inch-wide radar arch a few feet abaft the helm, whatever is hidden behind that sector could reach out and touch you with little advance warning if it closes on the same bearing.
An approaching boat that maintains a constant bearing, accompanied with decreasing range, will result in a collision every time if someone doesn't change course or speed soon enough. Of course, you have to know there's something out there before you'll have a reason to change course or speed. This is especially a problem at night because all you can see are the other vessel's running lights, which are likely to be more tightly clustered than the hull itself.
The same thing goes for windshield frames, or mullions, between sections of glass. If the mullion is 6 or 8 inches wide and just 2 or 3 feet away, the sector of horizon blocked from your vision may be as great as it is behind that 30-inch radar arch. A recent trend has been to put enclosed helm stations on express and center console boats. This can be great when it comes to looks and doing away with canvas. But if you end up with a 7-inch top support right next to where you stand at the wheel, your situational awareness decreases, making the boat inherently less safe for the operator and passengers. And you could miss fish breaking or birds working a few hundred yards off your beam. I'd rather have more visual interferences, such as narrow T-top support pipes, that I can see past with binoculars without having to move my head.
The color of the helm station is another element that affects visibility and situational awareness, especially if you have an aft-sloping windshield, as you'll find in the majority of powerboats. From a visibility perspective, bright white is the worst possible color for the dash at the helm. The whiter and brighter a dash is, the more it will reflect sunlight and artificial light from gauges and electronics onto the windshield. Just put a piece of white paper on the dash of your car to see what I'm talking about. The more glare, the less you can see. Although this is true in bright daylight, it's especially true when helming at night.
Another problem is the hardtop overhead and the boat structure forward of the helm, whether it's the deck of a center console or the bridge of a convertible. If these areas are also bright white, you will never have good night vision, which is what allows you to see dimly lit objects at night. The flashing red 4-second light three or four miles away is not so much of a problem. In my experience, the problem is the small, dark-hulled and unlit kayak or canoe a few hundred feet away. Any glare on the windshield makes spotting these nearby craft very difficult.
Of course, that also underscores the folly of building or buying a dark-colored canoe or kayak; they blend in so well with the water. Sometimes (mostly after nearly hitting one) I think there should be a law requiring that these boats be white, bright pink or maybe orange and white with diagonal stripes, as they are so hard to see, even in bright daylight.
On the Navy and Coast Guard ships I used to drive, the overhead was flat black and the bulkheads flat pea green, which did not cause windshield reflections or glare. And the forward-sloping windshield glass was less susceptible to catching glare.
Lighting also was very important. A sure way to get thrown off the bridge was to momentarily turn on a white light, even a dim one. Only red or blue lights were allowed because they enabled us to see much better in the dark. I could stand 15 feet away from a window at night and see a side light at four nautical miles. This made the ship much safer to operate, both for us and for the other guy.
Flat black paint is best at the helm station, but an off-white or cream color around the dash is a great start in the right direction without making your bowrider look like a missile boat. I also like Cobalt's upholstered dash treatment a lot, not because it's so pretty but because it practically eliminates glare.
So you can't have it all - the most efficient and best-riding hull, the roomiest and most seaworthy boat, or the brightest and best helm station. Intelligent boat design demands that you make choices, assign priorities and fully understand the capabilities and limitations. Understand the basics. Don't kid yourself into thinking you can have it all, and don't let anyone tell you otherwise. You'll be a happier boat owner as a result.
In a future column, I'll discuss more basic truths of boat design, and I'll also talk about Europe's Recreational Craft Directive, which specifies minimum stability, buoyancy and freeboard requirements for vessels, depending on the conditions in which they are designed to operate. There are four categories, designated A through D: Ocean, Offshore, Inshore and Sheltered Waters, respectively. What I like about these standards is that they label a boat's capabilities in terms of seaworthiness better than anything we have on this side of the Pond.
Eric Sorensen is a consultant to boat- and shipbuilders and to the government. He was founding director of the J.D. Power and Associates marine practice and is the author of "Sorensen's Guide to Powerboats: How to Evaluate Design, Construction and Performance." A longtime licensed captain, he can be reached at
This article originally appeared in the April 2011 issue.