With waves, it is not always just the height that matters. Direction and frequency are equally important factors when considering comfort and safety. Pretty much regardless of direction, I would rather be in 6-foot seas with a period of 12 seconds than be in 3-foot seas spaced four seconds apart. The 3-foot seas at four seconds are going to have rather steep faces that will strike the boat with significant force. The 6-foot swells will lift the boat gently and lower it again every 12 seconds, producing a much softer, more comfortable ride.
Different hull styles will handle waves quite differently. At 54 feet length overall and 90,000 pounds, our boat is a fairly heavy displacement hull with a bulbous bow, active fin stabilization and a shallow amount of submerged transom. At our 7- to 8-knot cruise speed, waves with a frequency of four to five seconds on our bow are comfortable up to about 2 or 3 feet. Above that height, the boat begins to slam into the face of the wave, and starts to pitch up and down or hobbyhorse somewhat uncomfortably.
Compare this with a cruising friend’s boat of similar size and weight, but with a semi-displacement hull and much higher horsepower doing 10 to 12 knots. Their boat handles these waves a little more comfortably, because at their higher speed, their bow is lifted slightly so the boat is coming down into the wave, not meeting the vertical face.
With either boat, if you spread the same-height wave out to a period of eight seconds or greater, but still on the bow, both would handle them more comfortably.
Waves or swells on the beam of a boat can be the least comfortable, inducing a side-to-side roll that tends to perpetuate itself. Some form of stabilization will be necessary for a comfortable ride in any kind of beam sea. A long passage with extreme roll can also wear on the crew, not to mention being dangerous for the boat if something is loose. Taking any significant sea state on the beam should be avoided, even if it means alternate tacking to increase safety and comfort.
A displacement-hull trawler with a non-submerged transom handles a following sea quite well by allowing waves traveling faster than the boat to pass under the hull without pushing it faster, or pushing the stern sideways, risking a broach. In most cases, boaters would prefer to run with the waves instead of into them. In a head or beam sea, the boat has a more abrupt encounter with the waves. The downside to this is the uncomfortable ride. However, the upside is an awareness if the seas are building in height.
In a following sea, the waves usually pass beneath the boat with a gentler motion. While this is a more comfortable ride, vigilance must be maintained that seas are not building to a hazardous state. A gentle following sea can lull an inattentive captain into believing everything is fine, right up until it’s not.
And, waves on the quarter of many trawlers tend to push the boat around in a corkscrew pattern that can be uncomfortable. Semi-displacement or planing-hull boats may experience this less because they have the ability to better match their speed to the speed or frequency of the waves, providing a more comfortable ride.
Whenever possible, take advantage of land as a natural breakwater. Running in the lee of an island or a shoreline will always be more comfortable, because the wind coming off the land will not have enough fetch to build large waves. You can estimate the height of waves by knowing your eye height above the water. If, when standing at the helm, your eyes are 10 feet above the surface of the water, then waves at eye level on the horizon will also be 10 feet in height. Using this as a gauge, I’ve turned around in an inlet more than once when I estimated the seas to be more than I wanted to handle.
When running in large seas, it is crucial to pay attention. It’s difficult to predict what the ocean will throw at you, as waves can be unpredictable. A large wave may pass easily under the boat, when a small one might break at the right angle to slam violently into the boat. Always stay alert and ready to act. Don’t focus on a single area in front of the boat; instead, scan from side to side, as rogue waves don’t always come from the same direction as the primary set.
In addition, just because the wind stops blowing doesn’t mean the seas stop rolling. In the last issue, we described swells as being seas that continue after the wind has subsided. If the wind has been steady for a long period of time, then the waves generated by them could be substantial. When planning to get underway for an offshore run, don’t just look at the current forecast. Research what the winds were doing for the week before as well. You may like the current forecast of light and variable winds out of the west, but if the winds were blowing 15 to 20 knots out of the east for the previous week, significant wave height could be double what it should be, based on current local conditions.
New computer technology and wave modeling are helping boaters forecast more accurately for longer offshore runs. Weather-routing websites have introduced predicted-wave routing, where you input your boat’s characteristics along with your tolerances, and the programs offer specific routes that will include swell, wind waves and direction. This type of modeling offers numerous benefits. Even if the route is predicting seas higher than you would like, it does give you time to prepare for them, or it lets you see that a different route would be better.
Waves are a fact of boating. Understanding wave characteristics and how your boat handles them will help you make better decisions and, hopefully, keep you from regretting that you went out on a day when you should have stayed in.
This article was originally published in the May/June 2023 issue.
