How different hull types react in rough water - Soundings Online

How different hull types react in rough water

Author:
Publish date:

This is the first in a series of stories on rough-water boat handling. Although you might not intend to go out in seas taller than your VHF antenna, you may well find yourself in such conditions if you venture offshore often and far enough. In this first article, I’ll consider the capabilities and limitations of the three basic hull forms: displacement, semidisplacement and planing. I’m assuming in this discussion that each of the examples under consideration is the best of its kind in terms of hull form, seaworthiness, helm sightlines and so on. Each hull form has its pluses and minuses. This overview should help you think more deeply about the issue of seaworthiness.

A good turn of speed is one of the advantages of planing hulls. (A Beneteau Barracuda 9 is shown.)

Displacement

The biggest limitation these vessels have is their speed, which is constrained to that of an open-ocean wave of the same length as the hull at the waterline. The square root of the waterline length (LWL) multiplied by 1.34 tells you precisely how fast a displacement hull can go in knots — 7.9 knots for a 35-foot LWL, for instance. Such low-speed capability greatly reduces a skipper’s options when trying to avoid bad weather at a macro level. It also reduces agility locally, so these boats are less able to zig and zag around breaking waves. Displacement hulls tend to draw more water than their planing counterparts, which, of course, limits their ability to take shortcuts through shallows.

For pure ocean-crossing capability, however, there is no substitute for a displacement hull. These boats, which are often ballasted to increase their range of stability to 90 degrees or more, use very little fuel when run well below their full-displacement speed. A 96,000-pound Krogen 58 (52 feet LWL), drawing more than 6 feet and powered with twin 158-hp diesels, gets 2.4 nmpg running at s/l of 1 (7.2 knots). Its hull speed of s/l 1.34 (9.7 knots) reduces efficiency — and range — to less than half that: 1.1 nmpg.

A displacement hull’s round bilges, upswept buttocks and emerged transom create very little form, or wave-making, drag at these low speeds. All those molecules of water being displaced by the hull separate and then regather gently and gradually, so wave-making resistance is very low.

The ballasted displacement hull’s deep draft creates a very low center of gravity and results in a lot of hull below the waterline, which makes the boat much less susceptible to the wind. A well-designed displacement hull is a lot easier to keep pointed into the wind at low speed than a planing hull, which is good because low speed is all that this genre is capable of. The trouble is that some recent displacement yachts are getting larger and larger superstructures in relation to their underwater hulls, and this makes them much less stable dynamically and harder to control in high winds and heavy seas. A good scenario would be to find yourself offshore in a boat with a big hull and little superstructure. To the degree that the ratio of mass between the underwater hull and above-water deckhouse favors the hull, your boat will be far more controllable in a seaway and, therefore, more seaworthy.

With all that draft and ballast, and comparatively little power, the displacement vessel accelerates more slowly and to a lower top speed. But the generous mass and righting arm give you some degree of immunity to the elements not granted to shoal-draft planing hulls. Being less susceptible to the wind, which you cannot see and can only feel when it hits you, makes boat handling more predictable. In very heavy weather, especially breaking seas, the odds are with the well-found displacement hull that cannot dodge these seas as well as a planing craft but can survive them more reliably when they are encountered.

The EdgeWater 268CC rides a deep-vee hull designed for offshore conditions.

Because of the displacement hull’s speed limitations, the stern tends to get tossed around running down-sea in waves longer — and therefore faster — than the hull itself. This is in spite of their easy sections aft that pick up buoyancy gradually, unlike a flat-sterned planing hull. This must be anticipated and reacted to proactively by the skipper, who should start to counter the wave, well before it hits, with the rudder. The keel, usually thought of as providing directional stability, has the opposite effect, working against you in down-sea conditions as the overtaking wave catches the keel and tosses it to the side. The resulting yaw, combined as it invariably will be with a roll, can lead to a broach if these two rotations are powerful enough.

The key is for the skipper to anticipate and stay ahead of events. The keel also works against you in a hard turn, with the boat heeling outboard away from the direction of the turn. This is not only disconcerting and potentially hazardous for passengers, but heeling away from the turn also makes the rudders less effective in turning the boat, with much of their prop wash deflection energy directed upward as well as sideways. A boat heeled over also has less righting energy available if it were to be hit by a breaking sea from the up-heel side. Although it can cause problems when the boat is running slower than waves overtaking on the quarter, the keel on the displacement hull is essential to coursekeeping, provides grounding protection, anchors the boat against the wind and holds ballast down low for greater stability.

So what we have at sea is a vessel that plods along, something like a bulldozer, while using comparatively little fuel. The hull’s motions are gentler in every direction, so the crew is long to tire and able to stay alert for longer periods on watch — a crucial boat handling element when offshore. A large rudder directly behind a large, slow-turning prop creates a good amount of lift to both sides, giving the skipper who knows how to anticipate the sea’s effect good directional control. The full keel can be a help or a hindrance, depending on the situation. On a slow boat, having a lot of draft — along with ample directional control — makes the boat better able to hold its own in heavy weather. If seas are breaking, the displacement vessel is usually the one you want to be aboard because it is better able to survive an encounter with big water on deck, thanks to its greater displacement, often stouter construction and larger area of positive stability.

Planing

It’s important to qualify any discussion on planing hulls because their ability to take on and not only survive but also thrive in heavy seas is greatly dependent on hull form and, specifically, their ability to reliably make good speed in these conditions. In my experience, the majority of the planing boats sold today are poorly suited to venturing offshore because their hulls are too wide for their length, too flat and full forward, and often too flat aft.

In addition, like the contemporary displacement trawler with its ever-ballooning deckhouse, there is even more of a gap between the area of the hull under water and the boat above the waterline, creating top-heavy boats that are hard to control in a stiff breeze. In addition to making a boat very difficult to handle in rough water, these design elements also severely limit a skipper’s ability to outrun a storm. These boats also ride so harshly and roll so heavily that the people on board are soon too exhausted to react properly when they need to most.

That said, if I were running a well-designed deep-vee planing hull with a moderately proportioned superstructure, a responsive steering system, plenty of freeboard forward, deck drainage aft, a reliable and powerful propulsion system and predictable handling, I would be very confident offshore in almost any weather. I also like the speed and agility of planing boats, so I would default to one in most circumstances.

Compared with a displacement vessel, the planing hull is characterized by relatively shallow draft, hard chines instead of round bilges and flat rather than upswept buttocks aft, which provide lift and allow it to accelerate over and ahead of its bow wave. The planing hull feels very stable, but this is a result of the distribution of buoyancy — called form stability — not weight, and has a narrower range of positive stability. The typical planing hull cannot physically survive extremely rough conditions, as it is built to lighter scantlings than many displacement trawlers. So the planing boat must, in the words of prize fighter Muhammad Ali, “float like a butterfly” on its own terms, dancing around rather than confronting the biggest waves.

In all but the most severe sea conditions, a well-designed deep-vee is an excellent choice when transiting from Point A to B is the objective. You’ll get there a lot faster, and you’ll be back in your slip sipping a drink hours before the trawler makes it home. And if you have to cross a bar coming home, the planing hull’s speed advantage keeps you in better control of the situation because you choose the wave to ride in on.

A good planing boat has to be able to run well in all directions to the sea. A moderately sharp entry and generous deadrise with high chine elevation in the forward half of the hull 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 tends to run in a straight line, rather than constantly yawing off course; deadrise in a deep-vee acts like a keel while allowing it to heel into a turn for better control and passenger safety. Running down-sea, the forefoot shouldn’t be so fine or deep that the bow plunges deeply into the back of each wave. Excessive bow immersion, especially running down-sea, turns the bow into a rudder and makes the boat very difficult to control, easily leading to a broach, which in very rough water can result in capsizing. So a well-designed deep-vee planing boat’s bow is not so flat and full that it pounds up-sea, and it carries enough buoyancy and dynamic lift so immersion is minimized down-sea.

Nordhavn's full-displacement trawlers have circled the world.

Stepped hulls are a subset of planing hulls. These boats have transverse step pockets in the bottom, with the hull section immediately abaft the pocket slightly higher in elevation than just forward. At high speeds, the water flow past the recessed pocket creates a low-pressure area that draws in ambient air and blankets the hull just aft. In simple terms, reducing hull surface in direct contact with the water reduces drag, and the boat goes faster. Hull steps work extremely well at high speeds — in the 60-plus-knot realm — adding 5 to 10 knots to some boats, according to designers.

Stepped hulls aren’t for everyone. They have limitations in trimmability — a big limitation when running down-sea — and adhere tenaciously to the wave gradient, rather than letting you “fly” the boat with more autonomy using tabs and engine trim. They are also more sensitive to weight additions, particularly extra weight up high, with the risk of excess chine immersion blocking air flow to the steps and causing a sudden low-pressure spike and loss of directional control. They demand a higher level of skill and attention by the operator. That said, they make a boat faster and more efficient at high speeds.

Semidisplacement

This hull type is the least understood, as it operates in a speed/length region partially supported by buoyancy, like a displacement hull, and partly by dynamic forces, like a planing hull. A semidisplacement hull has a nearly flat buttocks aft, with the transom immersed below the waterline, which with enough power applied allows the stern to create lift so the boat can climb on plane. The shape of the bow should also be such that it lifts as well as displaces. A boat can never drive through its bow wave; it has to climb up over the wave, with the operative word being climb. For boats with operating speeds of 12 to 16 knots, it doesn’t matter much in terms of planing efficiency whether the boat has hard chines or round bilges, but it matters a lot that the bow and the stern can generate lift within this range. Boats that spend most of their time at or below hull speed benefit from round bilges, which create less wake-making resistance. Boats running faster than 16 knots benefit greatly from having hard chines, however, because the sharp corner at the intersection of bottom and sides creates flow separation, breaking a sheet of water away from the hull, which reduces frictional drag. The hard chine hull also has more surface area for lifting when on plane, which reduces the dynamic bottom loading and helps the boat to plane with less energy and at a lower speed.

We know that a pure displacement hull is limited by its waterline length to an s/l of 1.34 (square root of the LWL times 1.34), but where is the demarcation between planing and semiplaning, the point at which most of the boat’s weight is supported by dynamic forces? Mathematically (and inexactly, because hull shapes and bottom loading vary) this occurs at an s/l of 2.5. So a boat with a LWL of 35 feet will plane at 14.8 knots, and a 45-foot LWL at 16.8 knots.

A semidisplacement hull tends to have a center of gravity that’s farther forward than planing boats, so they tend to plane more easily and at lower speeds. The Maine lobster boat, with its full keel, round bilges and lift-generating hull shape, is one of the best-known semidisplacement hull forms, although even this class of boat has a lot of variety in lines and proportions. Semidisplacement boats are often heavier than full planing boats, and they often have deeper and finer hull sections forward, producing a comfortable 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, with much of the solidity and comfort of the displacement hull but also a good run of speed, often into the high 20- or low 30-knot range. At these speeds, a high percentage of propulsion power goes into pushing the keel through the water and overcoming the added resistance of the hull shape. For this reason, the hull designed for semidisplacement speeds is most appropriate for owners who are happy with 12- to 16-knot speeds, with a dash of capability in the low 20s.

A summary comparison

Of the three types, the displacement hull has the longest range, gets the best mileage, has the greatest range of positive stability and has the highest level of survivability in extremely rough conditions. It does best in a head sea, tends to roll heavily in a trough if unstabilized or unballasted and is often squirrelly down-sea; I’ve often wished I had a rudder in the bow, as well as in the stern, when running down-sea in a displacement vessel. They are ultimately very survivable, but they also need to be, given their inability to dodge bad weather.

The well-found deep-vee planing hull offers surprisingly good handling in rough water, with a smooth ride up-sea, good coursekeeping down-sea and stability in a trough. The deep-vee has a very broad speed range, which gives it the ability to get out of harm’s way, assuming that can be accomplished within the boat’s fuel range. It is also quick and agile, so it’s the best platform of all for avoiding patches of breaking waves. In my experience it is by far the best for crossing bars, with its twin-screw stationkeeping ability, speed, agility and natural coursekeeping ability. It’s a little like the personal watercraft that dart around 40- to 50-foot breakers in Hawaii, towing surfers out to the surf line. You’d never survive those conditions in a 10-knot boat.

Hull steps incerease speed by drawing in air beneath the hull, which reduces drag. (A Contender center console is shown.)

The planing hull also tends to be drier in rough seas. The chine flats and spray strakes forward deflect spray down and out, the higher speed capability allows these boats to drive past the spray, and spray is generated farther aft along the hull in the first place, so less of it lands on the windshield.

The semidisplacement boat has some of the advantages of the other types. It’s solid and comfortable and rooted by its keel and draft like the displacement boat, but it’s capable of going twice as fast to avoid bad weather or get home before seas really start building. It’s probably the most comfortable of all up-sea, but it can have issues down-sea, with its full keel at times interfering with the rudder doing its job. Furthermore, putting the rudder over when running down-sea, and the subsequent keel-induced heel outboard, diminishes steering control that much more, just when you need it most. Most semidisplacement hulls are flat aft, and although this helps with efficiency and load-carrying ability, it also detracts from down-sea directional stability.

I’ll discuss specific offshore boat-handling tactics, tips and strategies in my next installment.

September 2014 issue