Smooth the Ride - Soundings Online

How a boat handles in rough water depends in large part on its hull design. Here’s a look at the capabilities and limitations of the three basic forms for monohulls: displacement, semi-planing (or semi-displacement) and planing, which includes stepped bottoms.

Displacement Hulls

Displacement hulls are shaped to go through the water, unlike their planing and semiplaning cousins, which are shaped to fly on the water’s surface. The biggest limitation for a displacement hull is speed, which can’t be faster than an open-ocean wave of the same length as the hull at the waterline. That slow speed also reduces a skipper’s options when trying to avoid bad weather and reduces the boat’s agility, making it harder to zig and zag around breaking waves. Deep-draft displacement hulls are also restricted in their ability to take shortcuts through shallows.

For pure ocean-crossing capability, however, there’s no substitute for a displacement hull. These boats are often ballasted to increase their range of stability to 90 degrees or more and can use very little fuel. A 100,000-pound, 60-foot trawler with a 54-foot waterline, drawing more than 6 feet and powered with a pair of 160-hp diesels, might get 2.4 nmpg running at 7.34 knots. That’s four times the economy of a typical 44-foot planing hull at 30 knots.

The Nordhavn 63 has a proven displacement hull. 

The Nordhavn 63 has a proven displacement hull. 

A displacement hull’s round bilges, upswept buttocks and emerged transom create very little form, or wave-making, drag at low hull speeds. The ballasted hull has a low center of gravity, and with much of the hull below the waterline, the boat is much less susceptible to the wind. The most seaworthy vessel has a big hull and a proportionately small superstructure. A large deckhouse reduces static and dynamic stability, and makes the boat harder to control in high winds and heavy seas. Situating the deckhouse and helm aft also makes for more comfortable cruising, with less pitching than when it’s located forward.

On a displacement vessel, not blowing around like a kite makes handling easier and more predictable. In heavy weather, especially breaking seas, the odds are with the well-found displacement hull.

Because of a displacement hull’s speed limitation, the stern tends to get tossed around when running downsea in waves that are longer and faster than the hull can go. On a displacement boat, the skipper must start to counter the overtaking wave with the rudder well before it hits.

The keel is usually thought of as providing directional stability, but it has the opposite effect when running downsea if an overtaking wave catches the keel and tosses it to the side. The resulting yaw, combined with a roll, can lead to a broach if these two rotations are powerful enough. The key is for the skipper to be alert and stay ahead of events.

The keel also works against you in a hard turn when the boat heels away from the direction of the turn. This can be disconcerting for guests and potentially present a safety issue. Heeling away from the turn makes the rudders less effective, with much of the prop wash deflecting energy upward and sideways. A boat heeled over outboard in a turn also has less righting energy available if it were to be hit by a breaking sea from the upwind side.

Although it can cause problems when the boat is running slower than overtaking waves, the displacement hull keel is essential to course keeping, provides grounding protection, anchors the boat against the wind, and holds ballast down low for greater stability. It’s essential that aft sightlines be unrestricted, since you’ll need to see those waves and overtaking vessels approaching from astern.

According to Nordhavn Vice President Jim Leishman, early Nordhavn models pitched more than the builder liked, so they added a stern bustle for extra buoyancy. “It gave us better results on our smaller boats than adding a bulbous bow, and in any event we don’t put bulbs on hulls smaller than 70 feet because they pound. All that weight in our boats, including thousands of pounds of lead in the keel, make them much more comfortable, as do the slack bilges, which make for easy rolling motions.”

The Trident 620’s semi-planing hull has enough power to climb over its own bow wave.

The Trident 620’s semi-planing hull has enough power to climb over its own bow wave.

Semi-Planing Hulls

The Trident 620’s semi-planing hull allows it to get on plane, giving it some of the seaworthiness characteristics of a displacement hull while at the same time offering more speed. This hull type may be the least understood, because it can operate in a speed/length region partially supported by buoyancy, like a displacement hull, and partly by dynamic forces, like a planing hull.

A semi-planing, or semi-displacement hull, has nearly flat buttocks aft. With enough power and speed, the stern can create lift so the boat can climb on plane. The shape of the bow should be able to lift as well as displace. A boat can never drive through its bow wave; it has to climb up over it. For boats with operating speeds of 12 to 16 knots, the bow and the stern must be able to generate lift within this range.

Boats that spend most of their time below hull speed benefit from round bilges, which create less wake-making resistance. However, boats running faster than 16 knots benefit greatly from having hard chines, because the hard chine’s sharp edge creates flow separation. That allows a sheet of water to break away from the hull, which reduces frictional drag. The hard chine hull also has more surface area for lifting, which reduces the dynamic bottom loading.

The Maine lobster boat, with its full keel and round bilges, is one of the best-known semiplaning hull forms, though these boats often are overpowered so they can make 25 or 30 knots. At these speeds, the keel and round bilges are not what you want if you value efficiency and rough-water handling, though the keel keeps the boat rooted when working lobster traps. These boats are typically heavier than full planing boats, but with round bilges and fine bow sections, they have a comfortable if wet ride and easy motions.

In terms of agility and speed, they are firmly situated between displacement and planing hulls. They can make excellent rough-water boats if properly designed. And they are most appropriate for owners who are happy with a 12- to 16-knot cruise, and a dash capability in the low 20s.

Left: Grady-White’s Canyon 456 is a classic planing hull.

Left: Grady-White’s Canyon 456 is a classic planing hull.

Planing Hulls

In a planing hull, the most important factors are ride quality and dryness going into the waves, and tracking when going with the seas. Planing boats need to make good speed reliably in bad conditions. Most cruising boats with planing hulls are poorly suited to venturing offshore because they’re too wide for their length. The result is that they pound and run wet, and are hard to control downsea.

In addition, some planing cruisers have ever-larger superstructures, which makes them top-heavy, less stable and a bear to handle. These undesirable design elements also severely limit a skipper’s ability to outrun a storm.

That said, if I were running a well-found, deep-V planing hull with a moderately proportioned superstructure, a responsive propulsion and steering system, plenty of freeboard forward, and fast deck drainage aft, I would be confident offshore in almost any weather.

Compared with a displacement vessel, the planing hull has shallow draft, hard chines (so it can plane efficiently), and flat buttocks aft, to allow the boat to accelerate over and past its bow wave. The planing hull feels deceptively stable, a result of the distribution of buoyancy, but has a narrower range of positive stability than a displacement hull. The typical planing hull cannot survive extremely rough conditions. So the planing boat must, in the words of boxer Muhammad Ali, float like a butterfly, dancing around the biggest breaking waves rather than confronting them.

In all but the most severe sea conditions, a well-designed deep-V is an excellent choice when the objective is to transit from point A to point B. You’ll get there a lot faster, and you’ll be back in your slip sipping a strawberry daquiri while the slower displacement trawler is still headed home in the building seas. And when you have to cross a bar to get home, the planing hull is fast enough to ride the back of a wave.

A good planing boat has to be able to run well in all directions to the sea. A moderately sharp entry and generous deadrise allows the boat to keep running at high speed in rough water without pounding passengers into submission. There should be adequate deadrise aft so the boat will run in a straight line, rather than constantly yawing off course. This will also allow it to heel into a turn for better control, increased reserve stability and passenger safety.

The forefoot shouldn’t be too fine or the bow will plunge into a wave. Excessive bow immersion, especially when running down sea, turns the bow into a rudder. This makes the boat difficult to control and in very rough water can result in a broach or even capsizing. So, the bow should not be too flat and full to cause pounding up sea, but still have enough buoyancy and dynamic lift to minimize immersion down sea.

The Edgewater 37o’s stepped hull gives it  a higher top speed.

The Edgewater 37o’s stepped hull gives it a higher top speed.

Stepped Hulls

A subset of planing hulls, stepped bottoms have one or more transverse step pockets in the bottom. The hull surface immediately abaft each pocket is slightly higher than just forward, and at speed an enlarged cavity at the chines ensures uninterrupted airflow. At high speeds, the water flow below the steps creates a low-pressure area that draws in ambient air from the chines, pulls it in toward the keel, and blankets the hull just aft with a layer of air bubbles. This aerated water has less frictional drag than solid water, allowing the boat to go faster.

While steps add turbulence and drag at lower speeds, they can add as much as 6 or 8 knots at higher speeds.

Stepped hulls aren’t for everyone. The high pressure points at each step tend to resist efforts to trim up the bow.

It’s important not to immerse the steps at the chine because the air supply will then be blocked. This can happen if the boat is overloaded, or if the boat heels too much in a hard turn. Because a high center of gravity increases heel in a turn, adding a tower must be carefully considered. In a turn, if airflow is suddenly interrupted it could create a vacuum and stop one side of the boat while the other keeps going. That can result in hooking—where the stern violently spins around—and has killed people in a number of cases.

I hesitate to recommend stepped hulls as family boats, but if properly designed—with sufficient bow-up attitude, large enough step pockets to ensure continuous airflow, moderate deadrise to limit heel in a turn, ample deadrise and vertical hull surfaces aft to create resistance to hooking—they can make sense for skilled, attentive operators.

Johnny Sebastian, a naval architect at Vectorworks in Titusville, Fla., designed the Edgewater 370 with a stepped hull. “You live and die by the weight study,” he says. “The boat must be precisely balanced longitudinally on the steps and transom so it runs at the correct trim. Each planing surface is calculated for lift and drag, as is the height and location of each step, so we know where the water will reconnect with the hull bottom.”

The Recap

In summary, a displacement hull is slow, but also tends to have the greatest range. Its lack of speed makes it harder to avoid storms and handle breaking waves, but in heavy weather, it is by far the most survivable hull type.

A planing hull lacks a displacement vessel’s stability and range, but it can outrun bad weather, has more agility in breaking seas and provides more options crossing a bar.

Stepped hulls take less energy to go fast, but dynamic stability and handling can be problematic compared to a straight planing bottom.

Semi-planing boats can cruise at low hull speeds and sprint when needed, although heavier, larger, more expensive machinery is needed to provide the propulsion.

Which hull type is right for your type of cruising plans? It really depends on your plans. Using a planing hull when you’re in displacement cruising mode is like trying to fit a square peg into a round hole. Learn your shapes, and you’ll have a much more pleasurable experience out on the water. 

This article originally appeared in the May 2020 issue.

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