Last Updated on 05 August 2016 by Eric Bretscher
This article is a follow-up from Heavy Weather Dynamics: Yachts in Following Seas published some months ago. It has since become one the most read items on this site.
While the original article applied to most sea-going monohulls, the following discusses more specifically fast modern hulls, or yachts in very fast following seas, such as can be found in the Southern Ocean. A lot of it is also applicable to fast power boats in exposed heavy weather areas.
Earlier in the year, I was up for a change of scenery after more than a decade in southern New Zealand and so I loaded up the sloop Nordkyn for a 1000-mile trip northwards along the near entire length of the country. Sailing up and down the New Zealand coast can require a bit of effort due to the extremely changing nature of the weather and it is a windy country.
As often when relocating, we tend to carry extra items that we don’t quite feel like abandoning behind, and this was no exception. On top of the standard ocean cruising gear, food and stores, a lot of tools, paints, materials, books and other items made it on board. This extra gear was stowed into lockers forward of the mast, some more found its way into some boxes left on the floor, and some more items went aft, eventually achieving a perfectly level trim. All up, the sloop may have gone down some 50mm, so say a good 1500kg above full cruising displacement.
After a few pleasant days in the little port of Oamaru, a southwest gale on the forecast appeared as a perfectly valid opportunity for making some good distance to the north. Counting on an average speed above 10 knots, 35-40 knots of true wind would be reduced to a more relaxing 25-30 knots apparent, one of the big perks of fast ocean cruising boats. Not only distances covered in a day can be quite phenomenal, but on the run the wind is not as strong for manoeuvring on board and one can sometimes literally sail away from unpleasant conditions in a matter of hours. Leaving port late in the morning, the first milestone was Banks Peninsula some 120 miles away and we were going to reach it the same evening.
With a single reef in the main and a #2 jib forward, the big sloop was fully powered up and light at the helm. As we gained more and more of an offing from the shore, the sea built up to its normal proportions for the wind blowing parallel to the coastline; around 4 metres at times and somewhat confused with the occasional snarly wave and patch of white foam.
I decided to take the helm over from the steering vane and get a first-hand feel for the conditions. The first difference I noticed was that the bow often didn’t alight as per usual, not that we seemed to sail any slower. Instead, we were sailing in a different, unusual mode. In normal sailing trim, the nose lifts and the hull parts the water almost at the mast at times; it transitions into planing mode. Instead, here I observed the sloop clipping along at over 14 knots for long periods trimmed dead flat, still cutting the water at the bow. The apparent wind dropped down another notch, and sailing so fast with so little wind was rather puzzling at times. Peak speeds touched 18 knots. We were faster than the surrounding sea, catching up, climbing and crossing over the waves, something I frown upon in bad weather and consider as asking for trouble.
Offshore I would have reefed the main further, brought the speed down to just keep up with the sea and gone down-below to read a book or have a rest. However, this was just a moderate coastal gale, the sea wasn’t very high and I was happy outside hand-steering for a while.
Watching the flat wake behind and picking a pathway in the sea at the tiller was exhilarating. It was spectacular sailing. Sensing and balancing the huge forces at play, the feeling was one of awe and huge respect. It was like holding a tiger by the tail. Alone on board, as I was watching the silvery grey waves form, develop and disappear with no land in sight, I was transported thousands of miles offshore again.
Fast and Too Fast
Too fast doesn’t mean much in terms of absolute speed. Some old hulls were often becoming completely unmanageable near their hull speed, often a mere 7 or 8 knots, and people used to say this was “going too fast”, even stopping completely at times. The issue wasn’t speed, but hull hydrodynamics.
Too fast, for us here, is as fast as a fast sea or faster than the sea on boats that are manageable in these conditions.
The risk with going too fast on the run is planting the boat into the sea
Typically, there are two pathways for achieving this.
If the sea is high and breaking, one can ride up the back of a wave, catch up with the crest in front and cross it as it is about to collapse: at this point the hull falls off the edge as there is no water to support it behind the crest, because the upper part of the front face of a wave about to break is near-vertical. The consequences of “jumping off the cliff” are almost always disastrous. Fast power boats that can exceed some 20 knots also face this risk.
I only heard directly of one such instance once from a sailmaker who was racing downwind in a gale in Cook Strait, New Zealand, very tidal and exposed. They planted the boat, a Farr 1020, into the sea after crossing a crest that had “nothing behind it”. What immediately followed was so violent that they were instantly dismasted.
Average cruising yachts don’t have a chance of getting there no matter what, but high-performance hulls driven quite hard do.
The other way is far more common and arises from “hitting a bump”. The sea in a gale is often anything but well-formed and regular; there are secondary waves in the main troughs, often coming from a different angle. A fast yacht coming tearing down the hill can engage into such a secondary lump while the hull is pitched bow-down. If the lump is higher than the freeboard forward in particular, there can be potential for unpleasant developments. The bow may bury and trigger a nose-dive or a high-speed broach. The outcome ranges from a fright to damage or injuries; if unlucky enough, one can also get subsequently rolled by the following crest as it sweeps past.
When I designed the Nordkyn hull, I deliberately built more volume into the upper forward sections than what is usually found on modern yachts today to account for the downwind speed potential and inertia. On that day off the South Island, the stage was set. The Yarra had never really been fast enough to enter this territory, being held back by its pinched stern sections, but with its slippery hull and superb course stability, Nordkyn just wanted speed and the sea wasn’t high enough to get into serious trouble with a boat that size.
In fact we had already hit a couple of modest lumps without much in the way of result. Again, the sloop accelerates smoothly above 14 knots, catching up with the crests and riding down into the troughs. I overtake a few waves, looking for a decent bump down in a trough, find a good one and steer into it.
The bow hits it squarely in a spectacular fashion before we have quite reached the bottom of the through; a shiny wall of solid green water parts away on each side. The thought that there is no way back now crosses my mind as the scene appears to unfold in slow-motion. From the cockpit, I am peering forward and I can see the water almost reaching the cathead, but the deck remains clear. At the helm, it feels as if the boat just plowed into something very soft, but massive. I can sense the deceleration, the stern lifting and the helm becomes very light and less responsive… I correct with a lot of rudder to counter a broach as the bow pops up, the sails are still filled solidly and the hull powers through and over the mass of water. I didn’t like the feeling at all and the experiment is not repeated. The volume in the hull forward provided considerable margin and the deep rudder blade was an asset, but there is clearly a limit in this direction.
Going too fast only matters for boats that are capable of getting there in the first place. Ocean-cruising sleds like Nordkyn are different from average cruising boats: new options and possibilities are suddenly open, but with those come new limitations and risks sometimes. Those need to be understood. Having designed the hull, I wanted to witness what was going to happen at the relatively small scale of a common gale if I simply forced the issue. Had I thought for an instant that it might go very wrong, I wouldn’t have done it. Had it gone wrong anyway, the boat is built like a submarine, the rig is phenomenally strong, there wouldn’t have been anything to break or carry away and no one else to lose overboard.
One normally learns about the capabilities and limits of a design over time and when circumstances challenge those limits. This often takes years even sailing offshore and in the high latitudes. It is usually just as well this way.
I would have never pictured the Yarra sailing up into a huge breaking sea, disappearing under the collapsing wave crests and repeatedly coming out unscathed on the other side, but once it did. This is how I discovered it could be done. Afterwards I tried to avoid finding myself there again, but having seen it was a huge confidence factor for crossing bad stretches of ocean with a fairly small boat to visit very remote places.
Sail Configuration and Hull Trim
The above wouldn’t have been easily achievable under normal loading conditions, because the bow rides up at speed. It may also have ridden up on that day if I hadn’t been carrying as much canvas: carrying a lot of mainsail downwind in particular tends to push the bow down.
Running under a single headsail doesn’t lift the bow as often claimed because the driving force is not acting on a line below the centre of gravity, but it is less detrimental, mainly because the part of the sail actually working tends to be lower.
Running on a headsail alone also offers more stable tracking, but dropping the mainsail is a double-edged sword: it can be quite challenging to get it back up downwind later if needed and if the wind is still strong. In any case, the boat should be set up to allow it, even if it means winching it up.
Trim is an important aspect when running before the sea at high speed in the sense that it influences freeboard forward and can reduce the volume of water displaced by the bow sections when entering a wave. A bow-up trim contributes to staying on top basically. As sails contribute to a bow-down moment, modern offshore racing yachts usually include a stern water ballast tank to offset it and allow carrying more canvas for longer on the run.
On a cruising yacht, it is irrelevant. Nordkyn will sail steadily and effortlessly at over 10 knots and it would be foolish to design for more speed than this in bad weather when one can already cover 250 nautical miles or more in a day – if the wind holds throughout.
Dynamics of a Yacht Hitting a Secondary Wave at Speed
The Concept of Inertia
The other important aspect relates to inertial forces. When a yacht on the run engages the bow into a wave, a buoyancy force is produced forward. This force contributes to lifting the bow and – given enough time – it will always succeed. Under some circumstances, this time may not be available and this is where the concept of rotational inertia comes into play.
A heavy vessel or a vessel heavily loaded in the ends has a slower pitching motion than one that is light and/or has its mass more concentrated near its centre of gravity
A force acting on a heavy object is slower at producing results than if the object is light: try applying the same force pushing a car or a shopping trolley.
The same principle applies to rotational motions, but there the distribution of the mass also matters. This can be readily illustrated by attaching two weights at the extremities of a bar and trying to rotate it, and then moving the weights together to the middle and rotating it again. Rotation is a lot easier when the mass is concentrated.
Pitching in the sea is also a rotational motion, around an axis transverse to the boat. Pitching inertia is defined as the sum of the products of the masses on board by the square of their distance to a horizontal transverse axis passing through the centre of gravity:
Iyy = Σ mi . di2
A vessel with a 12-metre waterline will have its centre of gravity just slightly aft of the half-waterline, so about 6 metres back from the bow. Now let’s say that the ground tackle forward weighs 250kg. Its contribution to pitching inertia is Iyy = 240 x 62 = 8640 kg m2. The separation distance makes a huge difference: 300 litres of water near midships, at half-a-metre away from the centre of gravity will contribute Iyy = 300 x 0.52 = 75 kg m2 only! For the same reason, the rig is also a large contributor to pitching inertia and bulb keels are worse than straight fins. It is a matter of balancing benefits and drawbacks. Bulb keels work best with hull shapes that strongly damp pitching motions.
The Effect of Inertia and Speed
A vessel with high pitching inertia reacts much more slowly to trimming forces, like a sudden buoyancy force acting forward when engaging the bow into a sea. Boat speed aggravates the situation because it requires quicker changes in trim to keep the bow up. Unfortunately, one doesn’t always choose how fast a boat sails in following seas and high pitching inertia adds an uncontrollable risk into the equation: planting the boat into the sea.
Before even leaving the South Atlantic during his Golden Globe 1968 southern circumnavigation, Bernard Moitessier  insists on alighting his boat and concentrating weights amidships to make it safer for running in heavy weather. He goes as far as throwing provisions and valuable gear overboard to achieve this, without providing much in the way of explanation. Moitessier was an instinctive sailor and acute observer and it is clear that he could sense the difference reduced hull resistance and inertia made to sea-keeping in heavy weather, even with his long-keeled steel boat and solid wooden masts.
In the technical appendices of his previous book  recounting his passage from Tahiti to Alicante via Cape Horn, he also briefly discusses engaging into secondary waves in the troughs. Since his ketch Joshua was only keeping up with the sea, his answer was not slowing down, but rather taking them at an angle.
Fast modern yachts can and often should readily go down to a very small rig in high winds and heavy following seas. They still easily keep ahead of breaking crests. In extreme conditions, even a bare pole can provide enough drive downwind, but personally I prefer keeping some sail up if I can for manoeuvrability. A yacht under bare pole immediately loses all drive if it ever strays from its downwind course. Recovery can be slow and during this time, one is at the mercy of a beam hit.
When running under very small canvas, especially a single small headsail, I watch like a hawk for any sudden drop in wind speed. Following the passing of a front, there can be a considerable sea running and I want to maintain sufficient drive to keep ahead even when the wind starts abating. This is often when the mainsail needs to go back up and rounding up to hoist is not an option.
Too slow, especially with hulls that offer more resistance, can lead to planting the boat into the sea by suddenly getting launched forward by a steep crest; the most famous example was probably Tzu-Hang , because it was well documented. On the other hand, accelerating to keep up with the sea often means reaching unusually high speeds and negotiating “obstacles” can become a challenge, especially if the bow won’t lift. Sailing faster than the sea with a high-performance design and crossing the crests downwind quickly becomes a gamble if the sea is high, because there is no indication of what the hidden wave face may look like; occasionally, it will be a vertical drop.
 “The Long Way”, Bernard Moitessier
 “The Logical Route”, Bernard Moitessier
 “Once Is Enough”, Miles Smeeton