What the Hull?
We tapped Yacht Designer Bill Prince to break down the basics of Hull design. then he started talking about bread and buttocks.
I received a question out of the blue from a friend in the auto industry the other day. He asked, “When you are designing a vessel and approximating the final weight and hull surface area, do you know where the waterline is going to be? I suppose the answer is yes, but how close? And do you know how the vessel will sit in the water before it actually gets wet?”
Since he’s affiliated with Ford’s design studio, I have to wonder what the next Mustang is going to look like after a question like this.
But, as the adage goes, whenever one person asks a question, it’s safe to assume other people want to know the same thing. Most of the experienced boat owners I know have at least a vague idea of how Archimedes’ principle relates to the way their boat floats, but there’s so much more to understanding hull design than just the way a boat sits in the water.
A boat’s weight—and its distribution fore and aft—is central to hull design of any kind. We design the underwater volume of a hull to be distributed in a way that matches the optimal fore and aft center of gravity of the assembled boat plus her occupants and gear. This “center of buoyancy” is the volumetric center of the shape of the underwater part of the hull.
Once this mathematical exercise is complete, the boat must be built so her actual center of gravity matches the designed center of buoyancy of the hull. Archimedes’ principle being what it is, the submerged volume and center of buoyancy will match the weight and center of gravity of the floating boat, so the designer and the builder should work in concert to come as close to the planned center of buoyancy as possible. This is the only way to ensure the boat will float “on her lines,” not down by the bow or stern, or generally too low.
Displacement-speed boats, which do not get up on plane, have their mass distributed farther forward than a faster boat will, owing primarily to the optimal underwater shape for the two boats’ vastly differing speeds. A tugboat, for example, will have its mass centered over a deep bilge amidships, whereas a triple-outboard center console will have its mass centered farther aft. The reasons for this have a lot to do with a funny-sounding term in naval architecture called the “buttock angle,” which we’ll get to in a minute.
The beauty of a well-designed hull form is one result of the constantly evolving surface curvature from bow to stern. As in nature, beauty is a by-product of function, so naval architects need ways to quantify hull shapes in order to optimize them for performance, seaworthiness and fairness for buildability. How do we develop and quantify these compound curves? By creating a 3D hull shape from four sets of two-dimensional curves: stations, waterlines, buttocks and diagonals.
Think of a loaf of bread from which you might make toast. One end is the bow, the other end the stern of your “boat.” Better yet, imagine four loaves of bread on your kitchen counter. Cutting one loaf of bread like a normal person does results in stations through the loaf of bread (your hull). Stations define the shape of a hull section at any point from bow to stern. They are viewed from head-on, as though the boat were coming right at you. Since the loaf of bread has the same section shape from “bow” to “stern,” all the cuts through the bread will be the same shape. Such is not the case with a hull, unless it’s a barge. Usually we divide the hull into 10 stations from bow to stern, beginning where the tip of the bow intersects the waterline and dividing the hull into equal stations from there to the transom.
If you cut the second loaf of bread parallel to the kitchen counter, you’d be cutting waterlines. Waterlines are contours cut—surprise!—at the design waterline and at even intervals above and below it, all parallel to one another. They are viewed in the plan view, looking down on the hull from above. Waterline contours below the design waterline can generally be thought of as the path water takes as it interacts with the hull bottom, creating lift and drag at speed. Straighter waterlines in the aft section of the hull’s running bottom mean a shorter, more efficient path for water to take along the bottom, resulting in less drag.
Cutting the third loaf of bread vertically, like you would a cake, provides buttocks. (Or as Forrest Gump would say, BUTT-ocksss.) These lines are viewed in the hull’s profile, indicating the rise or fall of the hull bottom from bow to stern. Buttocks are a critical indicator of a hull’s speed potential. A high buttock angle in the aft half of the hull (usually coupled with a lot of curvature) limits a hull to not much more than displacement speeds. Sailboats, or tugboats with deep bilges amidships and very shallow draft at the transom, are examples of boats with high buttock angles aft. These hulls are incapable of high speeds unless they are dropped off a cliff.
Conversely, a typical flats boat or sportfisherman will have a low buttock angle, resulting in a nearly flat run from amidships to the transom. This provides low resistance to water flow and plenty of surface area for the hydrodynamic lift required to get on plane. The speed potential of a hull with low buttock angles is limited less by its underwater shape than by the boat’s power-to-weight ratio. You can use this information to identify a given hull’s speed potential (relative to the boat’s length) by sight in a boatyard.
And if you’re a complete lunatic, you might cut the fourth loaf of bread into four or five pieces at about a 45 degree angle the long way from an imaginary centerline. This would give you diagonals. Diagonals are another way to approximate the path that water takes along the length of the hull, but this is more useful on boats without a hard chine like sailboats.
Finally, much is made of a hull’s deadrise angle at the transom. Deadrise angle is simply the degree to which a boat’s bottom is angled up in a “V,” measured athwartships from the keel to the chine. A truly flat-bottomed boat has zero degrees of deadrise. Most powerboat hulls have some deadrise, giving the hull bottom its “V” shape when viewed from the bow or stern. The deep-V hull was developed in the late 1950s and proved to be optimal for high-speed offshore vessels, with transom deadrise of 18 to 24 degrees. Boats with less than 18 degrees of transom deadrise are generally considered to have modified-V hulls, offering greater stability than a deep-V at rest and at moderate speeds, but being more prone to pounding.
But transom deadrise is only part of the story, for a good planing hull’s deadrise angle varies from bow to stern. One of the most important factors in gauging a boat’s comfort in rough seas is how the hull shape minimizes vertical accelerations to the human body, the motion to which we are most sensitive. For this reason the most important measure of deadrise is taken forward of amidships, typically 30 to 40 percent of the distance from the bow to the stern, where the hull first engages oncoming seas.
This primer will help you understand the hull shapes you see in the convention center at the next boat show. But before you go, buy a few loaves of day-old bread and have fun identifying your stations and your waterlines, diagonals and BUTT-ocksss.