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» Evaluating Stability and Capsize Risks For Yachts (original) (raw)

Over the last couple of years we have had a number of discussions about the mechanics of stability and capsize risks. Recently these issues have come up again, and we thought this might be a good time to revisit this fundamental safety issue. In the ensuing blog we are going to show you some rather unpleasant photos and video. These are worth watching as they will give you a feel for the real world risks involved, albeit small, in this watery world for which we all share a love. We’ll discuss stability, its impact on comfort, and what enhances or degrades it. We will also go into some detail on the process used to evaluate stability in the design phase.

“There is excellent upwind and downwind control in adverse conditions, watertight integrity during a knockdown, and ability to recover from a wave-induced capsize (better inverted stability curve than even their sailing designs)…”
–Northwest Yachting Magazine

Our goal is to convince you that heavy weather risks, while remote, should be taken seriously. And that sailors and powerboaters should be realistic in assessing the capabilities of crew and vessel.

Let’s start with a short video of a commercial trawler in a strong gale.

There are several things to note about this event. The two vessles are running with the seas, but the breaking crest which capsizes the lead trawler is from the beam. This is often referred to as a “sneaker” wave. However, waves coming from a different direction than the norm are not at all unusual, and if there has been a frontal passage you will get a 90-degree wind shift with its accompanying new sea direction. We don’t know the stability figures for this vessel in its present trim. However, we can assume that it met the minimum stability levels required by authorities, which are typically much more rigorous than for pleasure craft.

Have we got your attention?

This photo is from the 1979 Fastnet Race in which 23 yachts were abandoned and numerous crew lost their lives. It is of an RNLI lifeboat out on a rescue mission after the seas had substantially flattened.You can download the post race inquiry here. Aside from a tragedy, this race was a seminal event in yacht design, and gave rise to a variety of research on capsize mechanisms. Author (and good friend) John Rousmaniere was in the race and wrote an interesting piece on the 20 year anniversary which you can read here.

Forward now to the 1998 Sidney Hobart Race.

This is a tough part of the world to cruise, worse yet if you are scheduled to race with no precedent for postponing due to weather which is almost always bad at some point in the race, with 98 being especially awful. Like the 79 Fastnet the 98 Sydney Hobart was a yacht design and storm tactics crucible.

Our own direct experience? Limited. A brief 65/80 knot blow off Cedros Island in Mexico, three days of strong gale to storm force winds off the coast of South Africa, and eighteen hours in a 65 knot (gusting higher) northeaster near Cape Hatteras (the photo above was taken after things had calmed down). Of course there have been other gales and the occasional storm, like the 55 knots we saw one long evening on our way to the Austral Islands, but these all took place in deep water, unaffected by wave steepening counter current.

Not much exposure in the context of 250,000+ miles at sea. Yet we know for sure that in spite of all our efforts to the contrary, one of these days we will get caught again. Hence, the emphasis on understanding the mechanics of what creates problems in heavy weather and how to mitigate them.

OK, time for some basic principles of naval architecture. Stability, capsize resistance, and recovery, is a complex subject. Yet the basic principles are quite simple. Stability is created by several aspects of design:

• Waterplane stability derives from the distribution of area at the waterline. The beamier the boat, the higher the initial stability. High initial stability can be good for sailboats as it allows more sail area, but can be a negative for motor vessels because of quick, uncomfortable motion associated therewith. High initial stability almost always implies low ultimate stability (capsize resistance).
• Vertical center of gravity is next. The lower the VCG the higher the stability. For sailing designs there is almost never too low a VCG. More stability is always faster, hence the pressure on increasing keel draft, and keeping weight low in the boat. For power vessels, pleasure and commercial, the combination of high initial stability and a low VCG is uncomfortable, and can be dangerous, to the point where freighters will shed deck load if stability is too high with the accompanying snap motion.
• The next factor is the buoyancy and added stability provided by the hull and any structure mounted on the deck as the boat heels.
• Every vessel has a stability curve, the shape of which determines how it reacts to waves, how far you can heel before losing the ability to come back, and the restoring force required to get the boat upright again if capsized.
• VCG plays an increasingly important part in recovery as heel increases.
• It is quite possible to have a design which will float on its side, not capsize, and not recover from a heel past the point of positive stability. Refer to the lead photo of the Japanese car carrier in the Gulf of Alaska for a good example of this.
• In order to have heeled buoyancy work for you the windows, doors, and vents need to stay water tight. Once there is a major inrush of water, perhaps just an off center vent, the situation quickly becomes grave.
• Free surface effect of liquids in fuel and water tanks can be a major contributor to capsize, if the tanks are not well baffled, or filled.
• Water trapped on deck by bulworks is another major risk factor.

How does a designer or builder know what the heeled stability curve really looks like? In the olden days this was a long, tedious, and costly process. Today it is quick and cost effective. There are numerous computer programs that will roll a 3D model through specified heel angles, calculating the new waterplane and stability at each point. The other part of this is the VCG and that is a spreadsheet exercise, albeit in some detail, to determine where the weights which make up the boat are located.

For sailboat designers, builders, and buyers, analyzing stability is at the top of the list. Almost all sailboat design and sea trial articles will publish a stability curve. There seems to be less concern from the power boat end of the spectrum, which may be acceptable in protected waters, with shelter close at hand. Offshore, with wind and wave risks factored in, one could argue for a more proactive approach to stability analysis.

Let’s start with a modern sailboat stability curve, this one for the Halberg Rassey 40 (German Frers design). There are several interesting items to note:

• Initial stability is moderate and rises gradually with heel.
• Stability peaks at 60/65 degrees, and then gently falls.
• Ultimate stability is lost at 130 degrees.
• There is some inverted stability and wave energy will be required to right the capsized vessel (but not a lot).

The graphic above is from a research project done to investigate commercial trawler safety as a result of numerous capsize incidents and the associated loss of life. The type one design(blue line) is a traditional moderate freeboard trawler while the type two (magenta line) is a high freeboard enclosed deck design. Note that both reach maximum stablity at around 25 degrees and then righting force drops precipitously. The traditional design loses positive stability at 50 degrees while the design with more superstructure will go to 85 degrees. However, neither design will right itself from a capsize.

Next, we will investigate the FPB 64 stability curve. The curve which follows is from our preliminary studies. The final shape shall remain quietly secured – there is a limit to how much information we want to give away. But this is in the ballpark.

There are several unique things about this stability curve:

• Righting moment climbs in a straight line and peaks at 60 degrees.
• The righting force then remains almost the same until 90 degrees. This means the boat has maximum recovery force working at a point where normal yachts – power and sail – have long since lost most or all of their righting moment.
• Although the FPB 64 specifications call for 130 degrees this shows ultimate stability remains positive until 150 degrees.
• There is virtually no stability below the line indicating little wave energy is required to right the boat in a full capsize.

Why don’t other ocean going motor yachts have this type of curve? There is a complex relationship between form stability from hull shape, VCG, the way weight is distributed (polar moments), where you live and work relative to the roll and pitch centers, and how this impacts comfort and cargo security. As we mentioned earlier, as you design in more beam – which equates to form stability – the roll period shortens. The shorter the roll period, the faster and less comfortable the motion. Passenger liners and many freighters have ballast tanks up high to reduce stability and lengthen the roll period. Of course their ultimate stability is compromised (we refer you to the lead photo in this regard), but for the ships, their odds of meeting a sea large enough to capsize them are reduced by their scale. Smaller yachts obviously do not have the scale effects working for them like the big ships and are more at risk..

When designing a beamy yacht, relative to a narrower configuration, for a given roll period (or comfort level), the VCG has to go up. Too low a VCG combined with extra beam means a short, uncomfortable roll period. Raising the VCG at small angles of heel has little impact, as hull form stability is dominant. But as the boat begins to heel, the VCG assumes greater importance, to the point where it is a major determinant of ultimate stability.

Here are some things you can do to improve stability:

• Keeping tanks pressed (filled). For example, if you have four fuel tanks and carrying half your fuel capacity, it is better to put the fuel in two filled tanks than four half full tanks.
• Keep deck gear, dinghies, anchor rodes low.
• Store heavy supplies low.
• Avoid adding weight high (likes solid fly bridge tops, extra roller furling sails, and oversized rigging.

We started using 3D CAD software back in the 1980s. The programs we ran then would take all night for one set of calculations. Today the calcs are much more sophisticated and happen in seconds. As a result, we can play “what if” games ad nauseum, working towards the optimal combination of characteristics. One of the areas we study are the shapes of the hull at various heel angles, and how we think the boat will react in a breaking sea. Lets look at some graphics showing floatation in different states for the FPB 64.

The images which follow are derived from the FPB 64 hull shape, basic house volumes, and a variety of heel angles. We are looking for:

• Heeled stability properties.
• How this configuration floats at various heel angles
• Hatches and vents that are at risk from flooding.
• What the heeled shape looks like relative to how it will skid, or trip, when impacted by a breaking wave crest on the beam (skidding is a highly desirable reaction).

Let’s start at 30 degrees of heel. The deck edge is clear of the water, and there are no air intakes, hatches, doors, or other items about which to worry.

Still heeled 30 degrees (above), now looking underwater at the hull and fins. The bow is barely immersed relative to the stern’s floatation plane. There is little in this combination of hull and fins to resist skidding sideways with wave impact and with most of the resistance to skidding aft (skeg, rudder, and hull), the tendency will be for the boat to rotate on its axis bow down the wave. In our opinion the closer the bow is to heading up or down wave, and the further you are from having the waves abeam, the safer you will be.

At 60 degrees the deck edge is immersed, but all openings are still well clear.

The fish eye view shows the skeg and rudder still immersed, but rapidly losing their grip on the water. The windward stabilizer is clear of the water

90 degrees and we are just now immersing the Dorade vents, which is why they have closure plates adjustable from inside the boat. The centerline air intake for the engine room is well clear.

Here we are completely inverted. Note that the house and just a touch of deck are required for floatation, with the deck barely immersed This means the hydrostatic pressure on the entry door and deck hatches is low. The hatches on top of the house are less than five feet/1.5m below the waterline. Fully inverted like this the hull is highly unstable, meaning very little force is required to get the boat rolling right side up.

And now an often overlooked issue, wave impact and how the energy is absorbed. If you have raced planing dinghies or catamarans you know that when the wind is puffy, and you are occasionally overpowered on a reach, pulling up the centerboard (leeward fin on a cat) allows the boat to skid sideways with the puffs, dissipating the wind energy. Modern, relatively light keelboats with high freeboard, will skid on their topsides when knocked down due to being overpowered, typically with a spinnaker flying.

When a wave crest wallops your topsides it imparts energy to start the heeling process. If the boat is heavy, with a deep hull or keel, immersed surfaces and inertia of the mass, tends to hold the boat in place, allowing more energy from the wave to be transferred to the topsides. If the boat skids off to leeward with the wave impact, then the wave crest cannot impart the same force to the hull. The boat slips away from the wave, absorbing less energy over a longer period. Think of this like a boxer rolling away from a punch, taking the sting out of it as it were. The ability to skid is a primary safety factor in dangerous seas.

To recap, the key skidding elements are:

• Heeled floatation planes which present surfaces that do not lock the hull into position.
• Light displacement relative to the topside area.
• Reduced vertical surface area below the waterline.

Do the weather and sea state risks warrant our concern with stability and capsize, especially with modern weather forecasting? The answer depends on your cruising grounds and personal tolerance for risk. We would urge the following be considered in evaluating risks:

• Cruising grounds, season, normal and abnormal weather patterns.
• Conditions affecting sea state such as bottom topography, surface currents, and lee shore wave bounce.
• Possibility of secondary and tertiary wave trains interacting with primary wind driven seas.
• Length of passages (in time rather than distance).
• Secondary destination options if the situation deteriorates, or the primary goal becomes to risky.
• Primary and secondary destination approach considerations (lee shore, difficult navigation in adverse conditions, breaking bar).
• Possibility that it may be necessary to stand off in deep water until seas calm down and allow shelter.
• Vessel steering control up and down wind (can you run with the seas if required?).
• Range of heavy weather tactics available.
• Level of crew experience and skill.
• You and/or your crew’s need for emotional security versus living with nagging doubts.

We are expert enough in weather forecasting to feel confident in our ability to avoid most severe weather. We cruise at 11 knots, so exposure is minimized. We can run with a storm and not worry about steering control and broaching. We understand heavy weather tactics. Yet we know that there is always a risk, especially on passages of more than two days, that the weather models, the professional forecasters, or we, will be in error. Maybe its a major malfunction that slows or stops our progress. Bottom line is neither of us likes worrying about what might happen if the voyage does not go as anticipated. We want to be emotionally secure, knowing if (or when) we are caught in truly dangerous conditions, our vessel is going to give us the best chance of success.

How do you determine if your yacht, or one you are considering, has sufficient stability? First, decide what you want in this regard. Is a conventional stability curve for the class of yacht you are considering OK? Are you willing to go to sea on a vessel that will not recover from a capsize? If not, what is the minimum range of positive stability that is acceptable?

If you are buying a yacht, ask to see the stability curves (something common with commercial vessels and yachts built to code, and most sailing designs). If these are not available, there are naval architects who will check the righting moment, and then using this plus a set of lines, calculate a stability curve.

OK, enough of this. It is a lovely winter day in Tucson, Arizona, we are starting to think about some testing we need to do with Wind Horse, including a gale or two in the Gulf Stream.

PS: If you want to learn more about storm tactics take a look at our Surviving the Storm. You can read excerpts and learn more about the book here.

[Click here ](/FAO Small Vsl Stability.pdf)to download a basic pdf file on stability.

For more information on the FPB Series, e-mail Sue Grant: Sue.Grant@Berthon.Co.UK.

Posted by Steve Dashew (January 8, 2012)