Rough Water Seamanship
by David Pascoe
Small Craft Warnings
Small craft is a relative term and means ships less than ships. Because small craft entails so many sizes of boats, it's up to you to know what sea states are dangerous to your boat. The authorities are just giving a general warning to let operators know that conditions may be dangerous to some boats. If you have a small outboard, then obviously all small craft warnings apply to you. No official agency could possibly gather all the possible differing factors and put them into something more specific.
The criteria to use is wind speed and sea state. Twenty knot winds make for nasty, if not big, seas. Your obligation is to be sufficiently educated in order to understand when conditions become a threat. No one can tell you that because they do not know you, your boat, or your skills.
Big Seas, Little Seas
In Part I we discussed why it is difficult to have a general discussion about seamanship, and why experience is so important. Water and waves behave differently under different circumstances. Smaller waves, say in the 4-6 foot range are often more dangerous than larger waves when in open waters. I've made crossings of the Gulf Stream from Florida to the Bahamas in 8-12 foot seas that were far less threatening and uncomfortable than 4 footers.
Since different boats behave differently under differing circumstances, it is up to the operator to learn how to become a good seaman, learning about his boat's strengths and weaknesses. In maritime law there is an axiom that no boat is seaworthy without a skilled captain.
No one is going to learn much without making the effort to learn. You can try to learn from others, but nothing is going to take the place of actually being out there under the kinds of conditions you want to learn about. That means that you have to challenge yourself and your boat a bit and test the waters.
The sensible way to start out is with moderate conditions and circumstances that are less dangerous. As a young racing sail boat sailor, I learned rough water seamanship simply by taking the opportunity to go out when conditions were bad. I got a big thrill of going out and challenging the ocean. People thought I was nuts, but what they didn't realize was that I didn't start out challenging 12 foot seas. Like every other good seaman, I started at the small end of the scale, but didn't stop once I'd learned the easy stuff.
Then, when the big blows came along, I'd round up my crew and out we'd go into conditions that would make most boaters hair stand on end. Granted, there is not much point in doing something like that in a floating motel room, but if you're a cruising boat owner who is going to sooner or later get caught in bad conditions, the best way to learn to deal with them is simply a matter of practice.
Not only will you learn about seamanship, but you'll also learn a lot about boat design, equipment and safety. Who knows, you might even have fun doing it.
In a power boat, you have an advantage that sailors don't have. That is the ability to control your boat speed relative to wave speed, and along with the direction of travel, are the two most important factors. Controlling speed controls the effect a wave will have on a boat. Most people don't want to slow down when conditions get rough, but that is an inescapable necessity.
Following seas are those conditions where the direction of the waves is anywhere aft of a line drawn through the beam. When heading straight downwind, with waves heading exactly in the same direction as your boat, is known as a following sea. When that direction is a number of degrees off a line drawn down the center of your boat, these are called quartering seas. Seas at an angle off the bow are said to be on the forward quarter. Seas parallel to the centerline are called beam seas. Seas directly on the boat are head seas.
Regardless of direction, it is necessary to control your boat speed and choose the one speed at which the boat becomes most responsive and controllable. Going too fast in a following sea means that you'll fly off the top of one wave and bury the bow into the back side of the next. That's not good, so we need to find the right speed which yields the most comfortable ride while still keeping good control of the boat.
When following seas start to get really big, we have only two choices: either we slow down to the appropriate speed, or we have to change direction. If we put the seas on the aft quarter we can maintain a higher speed without stuffing the bow into the backside of a wave. On the other hand, we may not end up going in the direction we wish. (And here you may have thought only sailboats engaged in tacking) It then becomes a matter of whether our higher speed makes up for the extra distance we have to travel. Often times it does, making it advantageous to alter course 20-30 degrees.
I briefly touched on the subject of broaching in Part I. Controlling a broach is a function of controlling boat speed. It is best to avoid broaches by never running into the backside of a wave at such a high speed that you lose control. Once it happens, the only thing you can do is chop the throttle immediately.
As with a skid in a car, don't try to steer out of it. It is best to hold the wheel where it is and let the slowing speed get you out of it. The danger is in allowing the boat to suddenly go broadside to the waves and possibly capsize. Once the bow is no longer buried in the wave ahead, you can use the rudder and throttle to quickly right your course.
Big head seas bring just about any kind of pleasure boat to a grinding halt. When waves get to around 5-6 feet and we want to head upwind, we're pretty much stuck with idle speeds. Unless, you want to try quartering the head seas and see if that won't get you where you're going at a higher speed without taking blue water over the bow. At a 45 degree angle you can increase speed only slightly. At 60 degrees a bit more, but makes it tough to get where you want to go.
Tacking back and forth in this manner certainly is not a prescription for going anywhere fast, but it can get you there eventually. That's why the experienced pilot will try to use lee shores to advantage. If you can use a course alteration to get in the lee of a shoreline, you may be able to get to your destination in a round about fashion without beating your brains out. What I'm referring to, of course, is finding ways to use irregular shorelines or islands to your advantage.
Many novice boaters get themselves in a pickle by heading out the inlet on a fairly rough day and go charging off in beams seas only to find that they can't get back so fast because to return they now have to take seas on the bow. The moral of this story is don't head out without considering how you're going to get back.
When waves get big, the steepness is normally (tides and currents excepted) a function of whether winds are steady, increasing or decreasing. As all fishermen know, just because waves are big doesn't automatically mean that these are impossible conditions.
Building winds are most dangerous and those likely to get us into trouble, for conditions are likely to worsen. Decreasing or dying winds means that the energy that causes or creates waves is decreasing and so wave steepness will decrease and spacing from crest to crest will increase. Though waves may still be quite large, they will permit the boat to go at higher speeds. The seas left over from storms can be very large, but not necessarily uncomfortable.
I learned this lesson back in the 1960's when, docked at Bahia Mar we were waiting for the Stream to calm down before crossing to Bimini. A cold front had been through and the wind had blown for nearly a week. Looking out over the ocean we could see nothing but big lumps on the horizon. There was big surf along the shoreline. Well, we waited and waited, and being on vacation, were seeing it go down the tubes.
Finally, I said to hell with it, let's just go anyway. It was pretty nasty along the shoreline, but much to may amazement, once well out in the Stream we had 12 foot swells rolling down from the north, but they were so big that it was like driving a car over rolling hills. It was a completely comfortable crossing. That was one of my first lessons that taught me that all waves are not created equal. The question becomes, what kind of waves are they? and from what direction? Arriving on the other side was a different story, where we couldn't enter the harbor for the huge surf breaking on the bar, so we went somewhere else instead.
Several day later in Nassau, for reasons I couldn't fathom, winds freshened out of the northeast again and kicked up a nasty chop in the 12-15 waters of the Bahamas Banks. Now where can we go, I schemed? We can't head into it toward Abaco, where I wanted to go, but we could head south to the Exumas, or go visit the mosquitos and no-see-ums in Andros. Just one problem: If we got down there, and the wind continued to blow northeast, as it is prone to do in winter, we wouldn't be able to get back.
Or could we? By studying the charts, I could see that it might be possible to plot a course down the less (western side) of the Exumas and stay in the lee of the islands. This was risky because there are nasty rocky shoals jutting miles out from each of the islands in the chain so that one cannot follow the lee shores closely. If you get too far out of the lee, the chop will get real nasty with the tidal currents between islands.
I decided to go for it and hope for the best. This proved to be a mistake. The shoals were worse than they appeared on the charts. I couldn't get anywhere near the lee shores of the Examas (since you can't read depths in real choppy water) and found myself surfing southbound in a thousand feet of water in the tongue of the ocean. We were going to end up trapped by sea conditions and topography and I had no choice but to turn around and head back. Sometimes you can't win.
These are among the worst conditions, and don't necessarily have to involve large waves to be dangerous or make life at sea miserable. Confused sea conditions are best avoided since there is no getting around them and nothing one can do to make things better. Confused sea conditions occur as a result of major shifts in wind direction that occur quickly. This causes waves coming from differing directions, resulting in waves that are irregular and unpredictable.
They are mostly an oceanic phenomenon but do occur on large lakes or very large bays during or after thunderstorms, but will die down quickly on smaller bodies of water. But on oceans, confused seas can last for days after major fronts or hurricanes. Even large thunderstorms can have a significant affect on the ocean. Like throwing a rock in a pond, a storm or front can send out waves in different directions from the winds that caused them previously. The waves come together and make the surface very bumpy.
So called rogue waves are caused by two waves from differing directions coming together at oblique (very wide) angles. Like two boat wakes coming together, the net effect is to create a yet higher wave, up to two or more times the height of the originals. These can be downright dangerous due to their unpredictability. The best way to deal with them is to stay tied to the dock.
When caught out in confused seas, one needs to be particularly alert for those big ones that suddenly pop up out of nowhere. With a bit of experience one can come to anticipate them soon enough in advance to take evasive action.
We often hear reports of skippers describing boats "falling off a wave." They don't mean slamming in the ordinary sense, which is avoidable. A situation occurs that is the opposite of a rogue wave; instead of two waves coming together to make a taller wave. it happens that undulations from confused seas can create exceptionally large troughs. The boat hull can be on a hump and suddenly that hump just disappears out from under the boat. What happens is that the undulation moves away, the boat is left with nothing supporting it and it simply drops. Weird but true.
One can observe this very clearly on those days when there are confused swells as opposed to waves. It's very common in the wake of tropical storms.
Waves on Top of Waves
This is another dangerous sea condition. It occurs when waves get very large, and at a time when winds and seas are still building. No pleasure craft should be out in these conditions.
David Pascoe is a second generation marine surveyor in his family who began his surveying career at age 16 as an apprentice in 1965 as the era of wooden boats was drawing to a close.
Certified by the National Association of Marine Surveyors in 1972, he has conducted over 5,000 pre purchase surveys in addition to having conducted hundreds of boating accident investigations, including fires, sinkings, hull failures and machinery failure analysis.
Over forty years of knowledge and experience are brought to bear in following books. David Pascoe is the author of:
In addition to readers in the United States, boaters and boat industry professionals worldwide from nearly 80 countries have purchased David Pascoe's books, since introduction of his first book in 2001.
In 2012, David Pascoe has retired from marine surveying business at age 65.
On November 23rd, 2018, David Pascoe has passed away at age 71.
Biography - Long version