Last month I discussed the capabilities and limitations of planing, semiplaning and displacement hulls in rough conditions. Now let’s take a look at offshore boat-handling tactics, tips and strategies.
Meeting steep waves
The more head-on you meet a wave, the faster the relative motion between boat and wave will be and, therefore, the more violent the boat’s reaction and motions. If your destination is exactly 000 degrees (due north) of your present position, a course of 000 will be the direct route. However, if the seas are high and steep enough, it may not be the most prudent course to take. In these conditions it might make more sense to zig-zag 20 or 30 degrees to either side of your intended track to decrease the relative speed between hull and wave and to allow the boat to move more gradually and gently as it crests the wave peak.
I’ve often run in rough conditions by changing course abruptly to one side and perhaps slowing just before cresting a particularly big wave, launching more gently over the crest, and then resuming course and speed. This also lets the energy of the hull/wave peak encounter be dissipated in both pitch and roll, rather than just pitch.
Although dissipating energy in head seas is desirable, so is avoiding capsizing. In the case of a breaking wave, you have to be careful about meeting the crest too broadly on the bow because wave impact from this direction will induce roll and pitch rotations at the same time, increasing the danger of a capsize. In this situation, with waves that are higher than the deckhouse and breaking, and therefore very steep, the best solution is often to meet the wave head on, slowing to reduce impact but not slowing so much as to lose the steerageway you will need to stay in control of your heading.
As you slow to meet a cresting wave, you likely will need a strong burst of power and a good deal of rudder just as it hits to maintain heading control and keep the bow from getting tossed to the side. This is a big advantage of the azimuthing propulsion system — pod, outboard, sterndrive. When meeting a large breaking wave, you can slow the boat to a crawl in terms of headway, reducing the force of wave impact, and still have powerful, nearly instantaneous steering control.
It may be helpful to think in terms of wave gradients and how to avoid following them too tenaciously when they are very steep. A boat running directly into or with the wave is the most stable in relation to the gradient, and a boat with the beam to the waves is in the next-most-conforming position. Therefore, putting the bow 25 to 45 degrees to the wave face is often the best strategy to maintain control in both trim and heel.
It is also possible with a keel-less planing hull to use sudden course changes to maintain heel in a way that helps you stay in control. That’s why I am not a fan of 20-plus-knot boats with keels in very rough water; you will heel outboard in a turn, and this can make the situation worse when trying, for example, to turn back up-sea when running with the seas abaft the beam. A keel-less planing hull, on the other hand, is far safer not only for its occupants, but also it is far more stable and controllable dynamically in these conditions precisely because it heels into, not away from, a turn.
Speed and the ability to use it
I have run many 12-knot boats in very rough conditions, and I don’t remember ever thinking, Boy, I’m really glad I can’t go any faster than this. There is just no substitute for speed capability in rough water, assuming the hull form delivers in terms of controllability (particularly down-sea), dynamic stability, dryness and ride quality (particularly up-sea).
A planing hull with sufficient deadrise in the forward half of the hull to produce a smooth ride and sufficient deadrise aft to keep it from yawing all over the place, while still allowing it to plane at 12 or 13 knots, is ideal. Very few planing hulls meet this standard, unfortunately. Most are too flat and full forward to allow you to maintain speed in rough conditions, are too wide for their length for comfort and efficiency, and are either too high or low in deadrise aft.
Although the Hunt hull form is not the only planing hull that works well, it’s by far the best as an all-around sea boat in very rough conditions, in my experience, and this is why most of the pilot boats in the United States are Hunt designs. The ability of this capable hull to maintain speed in a seaway — tracking well in quartering seas, stable in a beam sea and able to keep running at high speeds up-sea — can mean the difference not only between comfort and discomfort and injury, but can also allow one boat to survive while another is overcome by nature. Such a hull can maintain a higher rate of speed in rough conditions and beat the worst of the weather home, and it is also far more agile when crossing a bar or avoiding local breaking waves.
One of the reasons I am a fan of a well-designed planing hull over a slower vessel in very rough conditions is that you can overtake waves at your discretion when running down-sea. If you are in a wave that is 200 feet long from trough to trough, you have to be able to run at 19 knots just to stay up with it. Being able to run at 25 or 30 knots allows you to position the boat anywhere you like on the back of a wave and stay right in the middle, where it’s safest and calmest. A 10-knot boat does not have this capability, nor does a poorly designed planing hull that is too wide or flat or has a keel that interferes with its agility and course-keeping abilities.
A displacement hull has far greater range, but it is more at the mercy of storm systems and local breaking waves because it lacks the speed and agility to avoid them. For this reason, the displacement yacht counts on rugged construction and a wide range of positive stability to survive.
Although running into a wave subjects you to the highest impact and acceleration, running down-sea is much more dangerous if the boat’s heading and speed are difficult to precisely control. This is a function of hull form, rudder effectiveness, dynamic stability and speed capability. Although the stern of a planing hull is broad to give it lift aft and allow it to achieve high speeds, running such a hull below wave speed down-sea can get you into trouble quickly because of all that buoyancy aft.
A planing boat cannot be successfully run like a displacement boat down-sea. The displacement hull is greatly limited in speed, but the very shape that limits its speed also helps keep it out of trouble in severe conditions. The stern sections of the displacement hull are fine, so they pick up buoyancy gradually. This means the stern lifts less dramatically upon encountering a sea from astern, and that results in the bow plunging less. A boat gets into trouble down-sea because the stern is lifted and the bow is immersed as the boat rotates longitudinally about its center of flotation. When the bow digs in, it turns into a rudder that is much more influential when determining heading than the rudder back aft. Such a boat not only needs a large rudder with a lot of prop wash moving over it for control, but the rudder also needs to turn a full 35 degrees for maximum lift, and the bow must be buoyant enough so it immerses only so far.
In many boats the best strategy is to run with the waves as best you can, trying to match their speed and staying in position on the back of the wave. This is not always possible unless the wave has the same destination you do. Otherwise you have to run the boat with the seas on the quarter, between the beam and the stern. If the waves are going faster or slower than your boat in this condition, you are at risk of heavy rolling as the crest passes or you pass the crest, combined with a sudden yaw or course change. If the boat rolls and yaws at the same time deeply enough, it can broach, which can lead to a capsize. Unless you have a very good hull under you with plenty of power and are experienced operating in these conditions, you may be better off minimizing the number of times you overtake a wave.
Running an inlet
There are many texts on running an inlet, and for good reason. As the bottom shoals up near shore, the waves get closer together and steeper, and if they get steep enough they break and collapse on themselves. The breaking face of a wave inclines to 90 degrees and then beyond as it breaks. If you are caught on this inverted gradient in any other heading than bow-on, you will almost certainly capsize.
It’s tough to stay bow-on to a breaking wave when returning home through an inlet, absent backing through at 15 knots, so the next best strategy is often to ride the back of a wave in. Waves in an inlet, like those offshore, have a certain pattern that you can detect after watching maybe 40 or 50 waves successively. This will take awhile to study closely, but this is not the time to rush things.
In addition to figuring out where the shoals are, make sure you know where the deepest part of the channel is, as this is where the waves are least apt to break. In other words, pay attention to the surf line, which is where you’ll be most vulnerable, note where the waves are smallest and head for that part of the channel. Wait until the occasional freak wave has gone by, and then make your way inshore and through the inlet.
There are plenty of videos on YouTube showing boats doing it the wrong way, and they can always edify and entertain us. Be sure to ready the boat for crossing the bar. Close exterior doors and hatches. Make sure the scuppers are clear. Have everyone put on life jackets and sit down low (to lower the center of gravity) and near an exit so they can get out of the boat if they have to. Don’t let anyone down in the cabin, as they will be staying with the boat for sure if it capsizes.
Tabs, steering response, rudder design
Trim tabs allow the hull trim — the bow attitude — to be adjusted. You can lower the tabs for quicker planing or for a smoother ride in a head sea, with the sharper deadrise forward meeting the waves. Smart designers also choose a slightly aft-oriented center of gravity so the bow naturally runs a bit high, which is what you want in rough conditions down-sea, with the tabs used to drop the bow when desired. This balancing act is the way to go because trim tabs can only raise the stern and not lower it. Tabs used independently also allow the operator to correct for heel from a beam wind (the boat heels into the wind because of the rudder angle effort required to keep the boat running straight) or for list from an off-center weight, such as your uncle Mort over on the starboard settee.
With inboards, rudder design is important. The rudder’s job is to create lateral lift, and enough of it, quickly enough to provide good control at high or low speeds. Creating enough lift is a function of the rudder’s surface area (more is better in terms of control) and deflection. It should turn a full 35 degrees to each side, for a total arc of 70 degrees — any more and it will stall, any less and it will not generate lift to its potential. Small rudders provide poor handling around the dock, but they can add a couple of knots at high speed with their lower drag. Builders get to market this speed capability, but you get to find out how poorly the boat handles on your own time.
Steering reliability comes from the rudder being positioned far enough from the transom to prevent ventilation when the rudder is turned. The shape of the rudder also is important. An axe-head profile is best for planing boats because it produces proportionately and predictably more lift as it turns increasingly to one side, and it resists stalling. A foil shape is good for displacement hulls, as it creates less drag at lower speeds. However, it stalls and loses lift at planing speeds.
Next month, I’ll look more specifically at handling considerations for waterjets and open-bow boats, as well as situational awareness and how propulsion types factor into all of this.
October 2014 issue