SAIL.SailMagazine

Photo by Drew Harper/Spinnaker Sailing

A statement from Artemis Racing

Artemis Racing today held a private ceremony commemorating the memory of our friend and teammate Andrew “Bart” Simpson. After eight bells, a wreath was cast upon the water by representatives of the four teams of the 34th America’s Cup. Then the morning’s rain parted and sunshine spread across San Francisco Bay. The Artemis Racing team thanks everyone for their support. Bart, may you rest in peace.

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Written by Ben Ellison on May 17, 2013 for Panbo, The Marine Electronics Hub

One problem with cruising north early during a late spring was that Gizmo’s open fly bridge was usually too cold to work with electronics like the Furuno TZT14 that I’d installed there just before finishing the trip south (as seen in this photo). I look forward to much more testing here in Maine but this I already know: For me, there is no other MFD or PC navigation program that does underway chart plotting so well. TimeZero software from MaxSea and Nobeltec are obviously excepted — and note that TZ for iPad is on its way — because Furuno essentially built the NavNet3D and TZT series as very specialized marine computers that run TZ with tight integration to their radars, sounders, etc. as well as to NMEA 2000, Ethernet and analog cameras, and more. Let’s look at some screens that illustrate what TZT does so well…

First, let me encourage you to click on some of these screens to see them in their full 1280 x 800 pixel glory. And whereas your display is probably denser, remember that the TZT 14 spreads these pixels across 14.1 inches with bright LED backlighting. There is also a certain “aliveness” to a TimeZero display that’s hard to describe. Videos at Furuno’s NavNet site and on YouTube (check out FurunoDan) nicely illustrate TZT’s fast and smooth chart handling and multi-touch controls, but I’ve yet to find a video that really shows TZT well underway. It’s almost as though the imagery responds not only to the boat’s COG but slightly to its pitch and roll. Garmin does something similar and I think it helps make the chart plotting relate better to the real world around you…

What the screenshots do illustrate well how the super high-resolution satellite photo maps with “PhotoFusion” (that are unique to all the TimeZero products) work toward the same goal. I almost invariably use this overlay, at least in one window, as it gives me much more detail about what’s going on ashore and in shallower waters without badly obscuring the essential chart data. Note how the screens above show Gizmo about to get underway in Norfolk’s Lafayette River and then having just passed beneath the low bridge that required a special air draft sensor. Incidentally, the NOAA raster chart seen on the TZT without overlays provided the easiest and surest reference regarding the bridge’s height on my way in, one of many reasons rasters are often my preference. Note too how well the little Furuno DRS2 radome — last discussed on Panbo here — is doing in close quarters. The Navico solid-state 3G and 4G radars may shine in these conditions, but the high-end HD domes from Furuno (and Raymarine) do pretty darn good.

Above is another example of PhotoFusion at work, here arguably indicating what you’ll actually find in lovely Solomons Island better than any other photo/chart mashup I’m familiar with. In fact, it helped me anchor right near the Calvert Marine Museum I much enjoyed visiting.

Properly mixing photos with charts is not easy, however, and in the tight confines of the ICW I saw a fair bit messyness like that in the left window above. It’s definitely made worse by the 3D mode you can instantly invoke and control with a two-finger gesture (or RotoKey command) but often I like TZT 3D as it too helps unite the plotter imagery with what I was actually seeing, and maybe concerned about, as Gizmo moved through unfamiliar waters.

The screen above was taken just south of Solomons where I noticed that some electronic charts did not do a great job of notifying you of that restricted target area just behind me. The NOAA raster and the Jepessen C-Map both did well, while the NOAA ENC seemed surprisingly the worst. Also seen is how easy it was to start a route (previously imported as a GPX file) at a particular waypoint I tapped, as well as the TZT’s slick onscreen tidal level and current graphics.

Tapping on tide and current icons is also one way to get to prediction screens with a certain elegance and utility that you’ll find a lot in TZT, though it is true that some other MFDs can also calculate the local rise and set of the Moon.

Getting back to charts, I’m impressed with the catalog facility that’s built into TZT, which is much like what you’ll find at MapMedia, the MaxSea subsidiary that processes all the different chart formats, photo maps, and bathy info that can display in the TimeZero family. The loaner TZT14 came with two loaded 64 gig SD cards and it’s easy to see what’s on them. I have not yet tried to update any of the data but I understand that Web downloads have gotten easier and that there is a MM3D hard disk available. I still suspect that the extra MapMedia processing puts a lag between, say, actual NOAA updates and MM updates but many chart systems are hard to keep truly current (with some notable exceptions like Coastal Explorers automated NOAA updates and Navionics Freshest Data).

I’m also not sure about MapMedia high res photo map coverage beyond the U.S., but the screen above also showing a Jeppesen Explorer raster chart for the Northern Bahamas looks great.

While this entry is mostly focused on TZT chart plotting, these last screens also show how data can be shown in dedicated windows like the ones above and in the easily popped-out and customized data overlays like the one seen in last image. Note above that once I’d used a CZone module to translate Gizmo’s tank senders to N2K, at least the two fuel levels were available in TZ and I could name them Port and Starboard (instead of the “0″ and “1″ instance names native to NMEA 2000).

Let’s also note how incredibly rugged the TZT hardware is. Its depth could be problem for some flush installs but have you ever seen a bracket anywhere near as heavy duty? The two-wire power cable is 3/8-inch in diameter due to three layers of insulation with a steel mesh protective jacket sandwiched in. That’s the Furuno way, and it almost seems weird to see it combined with charting plotting that’s functionally beautiful nearly to the level of artfulness (I think). I hear people moan about the TZT 14′s $7,695 retail price, but it seems that the bigger Garmin Glass Helms are in the same ball park and so may be the Raymarine gS Series. With both those systems about to get real, the premium glass bridge MFD niche is going to get interesting…and who will be surprised if Simrad soon refreshes its NSO line?
Deciding among the Big Four will likely become tough, though without a significant down side as all of them seem to be working on the cutting edges at this point. (Promising details that come to mind are the multiple NMEA 2000 ports on the Garmin Black Boxes and the multiple Power Over Ethenet ports on the Ray gS.)  But here’s hoping that lots of developers some time underway with a TZT, and also that TZ can be made to shine on an iPad so that many boaters can experience it.

Written by Ben Ellison on May 17, 2013 for Panbo, The Marine Electronics Hub

Ofta ser jag bilder på facebook av vänner som sitter och smuttar på ett glas rosé efter jobbet och firar en bra jobbdag, after work… När jag ser dessa bilder blir jag smått avundsjuk och känner att åh vad kul det vore att ha kollegor att jobba med. Min kollega är Andy, vilket jag tycker är alldeles underbart, men ibland vore det kul att ha fler jobbkompisar…

Vi har nu kommit tillbaka från Bermuda och jag blir verkligen påmind varför jag gillar mitt jobb så mycket. Under event jobbar vi hårt och mycket och som jag skrev tidigare måste man helt enkelt lägga allt åt sidan och fokusera helt på eventet. Vi är upp tidigt och är ofta på kontoret innan klockan åtta, medan vi äter frukost tänker vi ofta och diskuterar dagen, vi jobbar hela dagarna och ofta är det event på kvällarna och kommer inte hem förrän sent på kvällarna, många kväller är det middag och sängen på en gång för att orka med nästa dag. Vi är verkligen allt i allo under eventen och gör alla möjliga jobb och omöjliga jobb ;)

En bra dag på jobbet! Båtarna seglar nu ut från bermuda och mot Azorerna!

Det är dock otroligt roligt. Vi jobbar tillsammans med vänner och äntligen har vi jobbkompisar. Kompisar vi känt under flera år nu tack vare eventen. I Bermuda var det Lyall och Kieran. Vi besöker underbara platser som BVI och Bermuda och lär känna folk på plats eftersom eventen kommer tillbaka år efter år. De som seglar är trevliga och intressanta människor som är duktiga att ge os komplimanger för hur hårt vi jobbar och hur mycket de uppskattar vårt jobb. vi är runt seglebåtar och blir ständigt inspirerade att segla vår egen båt till spännande platser…

I onsdags seglade båtarna ut från Bermuda och efter att vi packat ihop kontoret fixade vi vår egen after work. Det blev några drinkar i baren och sedan tog Brian med oss ut på en båttur…  Till kvällen tog vi våra vespor och åkte till Swizzle Inn och mumsade god mat! Jag måste säga att vi nog förtjänade detta efter en lång och hårt jobbande vecka! :)

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Following the first meeting of the America’s Cup Review Committee on Thursday in San Francisco, teams have been asked to suspend all sailing in AC72 and AC45 catamarans until the middle of next week.

The Review Committee is scheduled to meet with the teams for the first time on Friday morning.

Following the first meeting of the America’s Cup Review Committee on Thursday in San Francisco, teams have been asked to suspend all sailing in AC72 and AC45 catamarans until the middle of next week.

The Review Committee is scheduled to meet with the teams for the first time on Friday morning.

There is no good beach here in Puerto Escondido so today we dinghied out about a mile south to see what we could find. What we found was a shallow, rocky, seaweed covered expanse. So thick with seaweed that we had to shut down the motor and row to shore. The kids didn’t mind though. There was just enough sandy bottom for Lowe to get on his “surfboard” and rock around, and there were tons of hermit crabs for Ouest to play with. Or torture, depending on your view.

We dug the crabs a swimming pool because apparently the ocean isn’t enough. Then when they started to escape from there we began to turn it into a prison, fortifying it with rocks and shells. And when those tenacious buggers still got out we just threw them back in and built bigger walls.

In the afternoon we hit the pool where Lowe continued to grow more daring. He’s now willing to dive without quite holding my hands, instead just diving to my hands. And he doesn’t mind a bit that I don’t catch him enough to keep him from dunking under every time. But when I take him underwater for the sole purpose of taking him under he isn’t too thrilled with me. He’s doing great though, probably right on track with where Ouest was at his age. I’m telling you, we’ve got two Mexican Olympians on our hands here.

There is no good beach here in Puerto Escondido so today we dinghied out about a mile south to see what we could find. What we found was a shallow, rocky, seaweed covered expanse. So thick with seaweed that we had to shut down the motor and row to shore. The kids didn’t mind though. There was just enough sandy bottom for Lowe to get on his “surfboard” and rock around, and there were tons of hermit crabs for Ouest to play with. Or torture, depending on your view.

We dug the crabs a swimming pool because apparently the ocean isn’t enough. Then when they started to escape from there we began to turn it into a prison, fortifying it with rocks and shells. And when those tenacious buggers still got out we just threw them back in and built bigger walls.

I just looked through my notes and was astounded to find that we are only ten days into this trip! It seems like so much longer. In that ten days we’ve already covered about 450 (nautical) miles, developing a rhythm of passages and rests which seems to suit us well. Our first leg, the shakedown so to speak, took us about two hundred miles from New Orleans to Choctawhatchee Bay, near Destin FL and was about as close as you come to a perfect sail. We had two days of reaching so comfortably at six knots  that we were preparing full meals at least twice a day. After a weekend with friends our next leg of 250 miles to Tarpon Springs was more of a shakedown. Again we were doing mostly six knots but this was in a blustery front, skipping along in unpredictable twenty and thirty knot winds and ponderous swells. Nothing too serious, but after a couple of slightly hairy late-night sail changes I have plenty of notes on ways to rework the foresails! We like to take a good wind and run with it but even with four of us aboard we have all been getting a bit tired of handling what is often a rather heavy helm. To this end I’ve begun experimenting with sheet to tiller steering.

Ok, ok, I know the modern world all steer with a wheel, but at least on a small boat such as mine I’m still a die-hard tiller advocate. A tiller is simpler, far easier to maintain, more intuitive, more responsive, and easier to jury-rig in the event of a steering failure. Sure, a wheel is less fatiguing but that seems like a lot to trade off just for an easier time at the helm, especially when a tiller allows you to rig up self-steering without an autopilot or windvane. It’s this that I started messing with on our last passage with help from a book given to me by an old sailing friend. The book is Self-Steering for Sailing Craft by John S. Letcher, Jr. and it’s a real gem.
If you have a tiller and you come across this out-of-print book, don’t pass it up!

 The windvane section, which I haven’t gotten into yet, makes up the bulk of the book but there is also a sizeable chapter on sheet-to-tiller steering. The concept is fairly simple: to the tiller (for you must have a tiller) are lashed two lines which are led to opposite sides of the cockpit. The first is (or incorporates) an elastic and is led to leeward while the second is led through a small block to windward and then tied to either the jib or mainsheet. You then adjust the tension on each until the boat steers itself.
The basic concept. This shows the jib sheet led directly to the tiller while most iterations involve a separate line hitched to the jib or main sheets.

This is how it looked in the only truly successful iteration I have tried so far:

The block on the right is looped around the windward winch and through it is a led a line which is tied to one piece of the mainsheet with a rolling hitch. When a gust hits the mainsheet tightens up, pulling at this line and keeping the boat from rounding up. The other line is a length of shock cord hitched to the tiller and tied through the leeward caprail. If the wind starts to fall off or a wave pushes the boat off course this shock cord steers to windward until the sails once again fill enough for the mainsheet to begin pulling in the opposite direction.

That is the theory, at least for most points of sail from close hauled to a broad reach. In practice I have a lot of kinks to work out. My experiments on most points of sail were failures but I did have great success when we were close-hauled. The rig pictured above actually steered the boat for hours with a reefed main in gusty winds upwards of twenty knots.
The boat steers itself while I take some photographs

 On other points of sail I seem to be mainly having issues with tension, either not enough or too little. One of the issues I have been having is that this shock cord does not have quite the right stretch so I’ve spent the last day trying to track down some of the surgical tubing which Letcher recommends. We’ll see how it goes. If nothing else, fooling with this stuff is a great way to break up the monotony of a shift at the helm!

We have previously discussed both form stability and ballast stability as concepts, and these certainly are useful when thinking about sailboat design in the abstract. They are less useful, however, when you are trying to evaluate individual boats that you might be interested in actually buying. Certainly you can look at any given boat, ponder its shape, beam, draft, and ballast, and make an intuitive guess as to how stable it is, but what’s really wanted is a simple reductive factor–similar to the displacement/length ratio, sail-area/displacement ratio, or Brewer comfort ratio–that allows you to effectively compare one boat to another.

Unfortunately, it is impossible to thoroughly analyze the stability of any particular sailboat using commonly published specifications. Indeed, stability is so complex and is influenced by so many factors that even professional yacht designers find it hard to quantify. Until the advent of computers, the calculations involved were so overwhelming that certain aspects of stability were only estimated rather than precisely determined. Even today, with computers doing all the heavy number crunching, stability calculations remain the most tedious part of a naval architect’s job.

There are, however, some tools available that you can use to make a sophisticated appraisal of a boat’s stability characteristics. If you dig and scratch a bit–on the Internet, or by pestering a builder or designer–you should be able to unearth one or more of them.

Stability Curves and Ratios

The most common tool used to assess a boat’s form and ballast stability is a stability curve. This is a graphic representation of a boat’s self-righting ability as it is rotated from right side up to upside down. Stability curves are sometimes published or otherwise made available by designers and builders, but to interpret them correctly, you first need to understand the physics of a heeling sailboat.

When perfectly upright, a boat’s center of gravity (CG)–which is a function of its total weight distribution (i.e., its ballast stability)–and its center of buoyancy (CB)–which is a function of its hull shape (i.e., its form stability)–are vertically aligned on the boat’s centerline. CG presses downward on the boat’s hull while CB presses upward with equal force. The two are in perfect equilibrium, and the boat is motionless. If some force heels the boat, however, CB shifts outboard of CG and the equilibrium is disturbed. The horizontal distance created between CG and CB as the boat heels is called the righting arm (GZ). This is a lever arm, with CG pushing down on one end and CB pushing up on the other, and their combined force, known as the righting moment (RM), works to rotate the hull back to an upright position. The point around which the hull rotates is known as the metacenter (M) and is always directly above CB.

The longer the righting arm (i.e., the larger the value for GZ), the greater the righting moment and the harder the hull tries to swing upright again. Up to a point, as a hull heels more, its righting arm just gets longer. The righting moment, consequently, gets larger and larger. This is initial stability. A wider hull has greater initial stability simply because its greater beam allows CB to move farther away from CG as it heels. Shifting ballast to windward also moves CG farther away from CB, and this too lengthens the righting arm and increases initial stability. The angle of maximum stability (AMS) is the angle at which the righting arm for any given hull is as long as it can be. This is where a hull is trying its hardest to turn upright again and is most resistant to further heeling.

Once a hull is pushed past its AMS, its righting arm gets progressively shorter and its ability to resist further heeling decreases. Now we are moving into the realm of ultimate, or reserve, stability. Eventually, if the hull is pushed over far enough, the righting arm disappears and CG and CB are again vertically aligned. Now, however, the metacenter and CG are in the same place, and the hull is metastable, meanings it is in a state of anti-equilibrium. Its fate hangs in the balance, and the least disturbance will cause it to turn one way or the other. This point of no return is the angle of vanishing stability (AVS). If the hull fails to right itself at this point, it must capsize. Any greater angle of heel will cause CG and CB to separate again, except now the horizontal distance between them will be a capsizing arm, not a righting arm. Gravity and buoyancy will be working together to invert the hull.

Stability at work. The righting arm (GZ) gets longer as the center of gravity (CG) and the center of buoyancy (CB) get farther apart, and the boat works harder to right itself. Past the angle of vanishing stability, however, the righting arm is negative and CG and CB are working to capsize the boat

A stability curve is simply a plot of GZ–including both the positive righting arm and the negative capsizing arm–as it relates to angle of heel from 0 to 180 degrees. Alternatively, RM (that is, both the positive righting moment and the negative capsizing moment) can be the basis of the plot, as it derives directly from GZ. (To find RM in foot-pounds, simply multiply GZ in feet by the boat’s displacement in pounds.) In either case, an S-curve plot is typical, with one hump in positive territory and another hopefully smaller hump (assuming the boat in question is a monohull) in negative territory.

The AMS is the highest point on the positive side of the curve; the AVS is the point at which the curve moves from positive to negative territory. The area under the positive hump represents all the energy that must be expended by wind and waves to capsize the boat; the area under the negative hump is the energy (usually only waves come into play here) required to right the boat again. To put it another way: the larger the positive hump, the more likely a boat is to remain right side up; the smaller the negative hump, the less likely it is to remain upside down.

Righting arm (GZ) stability curve for a typical 35-foot cruising boat. The angle of maximum stability (AMS) in this case is 55 degrees with a maximum GZ of 2.6 feet; the angle of vanishing stability (AVS) is 120 degrees; the minimum GZ is -0.8 feet

The relationship between the sizes of the two humps is known as the stability ratio. If you have a stability curve to work from, there are some simple calculations developed by designer Dave Gerr that allow you to estimate the area under each portion of the curve. To calculate the positive energy area (PEA), simply multiply the AVS by the maximum righting arm and then by 0.63: PEA = AVS x max. GZ x 0.63. To calculate the negative energy area (NEA), first subtract the AVS from 180, then multiply the result by the maximum capsizing arm (i.e., the minimum GZ) and then by 0.66: NEA = (180 – AVS) x min. GZ x 0.66. To find the stability ratio divide the positive area by the negative area.

Working from the curve shown in the graph above for a typical 35-foot cruising boat, we get the following values to plug into our equations: AVS = 120 degrees; max. GZ = 2.6 feet; min. GZ = -0.8 feet. The boat’s PEA therefore is 196.56 degree-feet: 120 x 2.6 x 0.63 = 196.56. Its NE is 31.68 degree-feet: (180 – 120) x -0.8 x 0.66 = 31.68. Its stability ratio is thus 6.2: 196.56 ÷ 31.68 = 6.2. As a general rule, a stability ratio of at least 3 is considered adequate for coastal cruising boats; 4 or greater is considered adequate for a bluewater boat. The boat in our example has a very healthy ratio, though some boats exhibit ratios as high as 10 or greater.

You can run these same equations regardless of whether you are working from a curve keyed to the righting arm or the righting moment. The curve in our example is a GZ curve, but if it were an RM curve, we only have to substitute the values for maximum and minimum RM for maximum and minimum GZ. Otherwise the equations run exactly the same way. The results for positive and negative area, assuming RM is expressed in foot-pounds, will be in degree-foot-pounds rather than degree-feet, but the final ratio will be unaffected.

GZ and RM curves are not, however, interchangeable in all respects. When evaluating just one boat it makes little difference which you use, but when comparing different boats you should always use an RM curve. Because righting moment is a function of both a boat’s displacement and the length of its righting arm, RM is the appropriate standard for comparing boats of different displacements. It is possible for different boats to have the same righting arm at any angle of heel, but they are unlikely to have the same stability characteristics. It always takes more energy to capsize a larger, heavier boat, which is why bigger boats are inherently more stable than smaller ones.

Righting moment (RM) stability curves for a 19,200-pound boat and a 28,900-pound boat with identical GZ values. Because heavier boats are inherently more stable, RM is the standard to use when comparing different boats (Data courtesy of Dave Gerr)

Another thing to bear in mind when comparing boats is that not all stability curves are created equal. There are various methods for constructing the curves, each based on different assumptions. The two most commonly used methodologies are based on standards promulgated by the International Measurement System (IMS), a once popular rating rule used in international yacht racing, and by the International Organization for Standardization (ISO). Many yacht designers have developed their own methods. When comparing different boats, you must therefore be sure their curves were constructed according to the same method.

Perfect Curves and Vanishing Angles

To get a better idea of how form and ballast relate to each another, it is useful to compare curves for hypothetical ideal vessels that depend exclusively on one type of stability or the other. A vessel with perfect form stability, for example, would be shaped very much like a wide flat board, and its stability curve would be perfectly symmetrical. Its AVS would be 90 degrees, and it would be just as stable upside down as right side up. A vessel with perfect ballast stability, on the other hand, would be much like a ballasted buoy–that is, a round, nearly weightless flotation ball with a long stick on one side to which a heavy weight is attached, like a pick-up buoy for a mooring or a man-overboard pole. The curve for this vessel would have no AVS at all; there would be just one perfectly symmetric hump with an angle of maximum stability of 90 degrees. The vessel will not become metastable until it reaches the ultimate heeling angle of 180 degrees, and no matter which way it turns at this point, it must right itself.

Ideal righting arm (GZ) stability curves: vessel A, a flat board, is as stable upside down as it is right side up; vessel B, a ballasted buoy, must right itself if turned upside down (Data courtesy of Danny Greene)

Beyond the fact that one curve has no AVS at all and the other has a very poor one, the most obvious difference between the two is that the board (vessel A) reaches its point of no return at precisely the point that the buoy (vessel B) achieves maximum stability. A subtler but critical difference is seen in the shape of the two curves between 0 and 30 degrees of heel, which is the range within which sailboats routinely operate. Vessel A achieves its maximum stability precisely at 30 degrees, and the climb of its curve to that point is extremely steep, indicating high initial stability. Vessel B, on the other hand, exhibits poor initial stability, as the trajectory of its curve to 30 degrees is gentle. Indeed, heeling A to just 30 degrees requires as much energy as is needed to knock B down flat to 90 degrees.

Righting arm (GZ) stability curves for a typical catamaran and a typical narrow, deep-draft, heavily ballasted monohull. Note similarities to the ideal curves in the last figure

To translate this into real-world terms, we need only compare the curves for two real-life vessels at opposite extremes of the stability spectrum. The curve for a typical catamaran, for example, looks similar to that of our board since its two humps are symmetrical. If anything, however, it is even more exaggerated. The initial portion of the curve is extremely steep, and maximum stability is achieved at just 10 degrees of heel. The AVS is actually less than 90 degrees, meaning that the cat, due to the weight of its superstructure and rig, will reach its point of no return even before it is knocked down to a horizontal position. The curve for a narrow, deep-draft, heavily ballasted monohull, by comparison, is similar to that of the ballasted buoy. The only significant difference is that the monohull has an AVS, though it is quite high (about 150 degrees), and its range of instability (that is, the angles at which it is trying to capsize rather than right itself) is very small, especially when compared to that of the catamaran.

The catamaran, due to its light displacement and great initial stability, will likely perform well in moderate conditions and will heel very little, but it has essentially no reserve stability to rely on when conditions get extreme. The monohull because of its heavy displacement (much of it ballast) and great reserve stability, will perform less well in moderate conditions but will be nearly impossible to overturn in severe weather.

What Is An Adequate AVS?

In the real world you will rarely come across stability curves for catamarans. If you do find one, you should probably be most interested in the AMS and the steepness of the curve leading up to it. Monohull sailors, on the other hand, should be most interested in the AVS, and as a general rule the bigger this is the better.

Coastal cruisers sailing in protected waters should theoretically be perfectly safe in a boat with an AVS of just 90 degrees. Assuming you never encounter huge waves, the worst that could happen is you will be knocked flat by the wind, and so as long as you can recover from a 90-degree knockdown, you should be fine. It’s nice to have a safety margin, however, so most experts advise that average-size coastal cruising boats should have an AVS of at least 110 degrees. Some believe the minimum should 115 degrees.

For offshore sailing you want a larger margin of safety. Recovering from a knockdown in high winds is one thing, but in a survival storm, with both high winds and large breaking waves, there will be large amounts of extra energy available to help roll your boat past horizontal. There is near-universal consensus that bluewater boats less than 75 feet long should have an AVS of at least 120 degrees. Because larger boats are inherently more stable, the standard for boats longer than 75 feet is 110 degrees.

The reason 120 degrees is considered the minimum AVS standard for most bluewater boats is quite simple. Naval architects figure that any sea state rough enough to roll a boat past 120 degrees and totally invert it will also be rough enough to right it again in no more than 2 minutes. This, it is assumed, is the longest time most people can hold their breaths while waiting for their boats to right themselves. If you don’t ever want to hold your breath that long, you want to sail offshore in a boat with a higher AVS.

Estimated times of inversion for different AVS values (Data courtesy of Dave Gerr)

As this graph illustrates, an AVS of 150 degrees is pretty much the Holy Grail. A boat with this much reserve stability can expect to meet a wave large enough to turn it right side up again almost the instant it’s turned over.

Other Factors To Consider

Stability curves may look dynamic and sophisticated, but in fact they are based on relatively simple formulas that can’t account for everything that might make a particular boat more or less stable in the real world. For one thing, as with regular performance ratios, the displacement values used in calculating stability curves are normally light-ship figures and do not include the weight that is inevitably added when a boat is equipped and loaded for cruising. Even worse, much of this extra weight–in the form of roller-furling units, mast-mounted radomes, and other heavy gear–will be well above the waterline and thus will erode a boat’s inherent stability. The effect can be quite large. For example, installing an in-mast furling system may reduce your boat’s AVS by as much as 20 degrees. In most cases, you should assume that a loaded cruising boat will have an AVS at least 10 degrees lower than that indicated on a stability curve calculated with a light-ship displacement number.

Another important factor to consider is downflooding. Stability curves normally assume that a boat will take on no water when knocked down past 90 degrees, but this is unlikely in the real world. The companionway hatch will probably be at least partway open, and if the knockdown is unexpected, other hatches may be open as well. Water entering a boat that is heeled to an extreme angle will further destabilize the boat by shifting weight to its low side. If the water sloshes about, as is likely, this free-surface effect will make it even harder for the boat to come upright again.

This may seem irrelevant if you are a coastal cruiser, but if you are a bluewater cruiser you should be aware of the location of your companionway. A centerline companionway will rarely start downflooding until a boat is heeled to 110 degrees or more. An offset companionway, however, if it is on the low side of the boat as it heels, may yield downflood angles of 100 degrees or lower. A super AVS of 150 degrees won’t do much good if your boat starts flooding well before that. To my knowledge, no commonly published stability curve accounts for this factor.

Another issue is the cockpit. An open-transom cockpit, or a relatively small one with large effective drains, will drain quickly if flooded in a knockdown. A large cockpit that drains poorly, however, may retain water for several minutes, and this, too, can destabilize a boat that is struggling to right itself.

This boat has features that can both degrade and improve its stability. The severely offset companionway makes downflooding a big risk during a port tack knockdown or capsize, but the high rounded cabintop and small cockpit footwell will help the boat to right itself

Fortunately, not all unaccounted for stability factors are negative. IMS-based stability curves, for example, assume that all boats have flush decks and ignore the potentially positive effect of a cabin house. This is important, as a raised house, particularly one with a rounded top, provides a lot of extra buoyancy as it is submerged and can significantly increase a boat’s stability at severe heel angles. Lifeboats and other self-righting vessels have high round cabintops for precisely this reason.

ISO-based stability curves do account for a raised cabin house, but not all designers believe this is a good thing. A cabin house only increases reserve stability if it is impervious to flooding when submerged. If it has open hatches or has large windows and apertures that may break under pressure, it will only help a boat capsize and sink that much faster. The ISO formulas fail to take this into account and instead may award high stability ratings to motorsailers and deck-saloon boats with large houses and windows that may be vulnerable in extreme conditions.

Simplified Measures of Stability

In addition to developing stability curves, which obviously are fairly complex, designers and rating and regulatory authorities have also worked to quantify a boat’s stability with a single number. The simplest of these, the capsize screening value (CSV), was developed in the aftermath of the 1979 Fastnet Race. Over a third of the more than 300 boats entered in that race, most of them beamy, lightweight IOR designs, were capsized (rolled to 180 degrees) by large breaking waves, and this prompted a great deal of research on yacht stability. The capsize screening value, which relies only on published specifications and was intended to be accessible to laypeople, indicates whether a given boat might be too wide and light to readily right itself after being overturned in extreme conditions.

To figure out a boat’s CSV divide the cube root of its displacement in cubic feet into its maximum beam in feet: CSV = beam ÷ ³√DCF. You’ll recall that a boat’s weight and the volume of water it displaces are directly related, and that displacement in cubic feet is simply displacement in pounds divided by 64 (which is the weight in pounds of a cubic foot of salt water). To run an example of the equation, let’s assume we have a hypothetical 35-foot boat that displaces 12,000 pounds and has 11 feet of beam. To find its CSV, first calculate DCF–12,000 ÷ 64 = 187.5–then find the cube root of that result: ³√187.5 = 5.72; note that if your calculator cannot do cube roots, you can instead take 187.5 to the 1/3 power and get the same result. Divide that result into 11, and you get a CSV of 1.92: 11 ÷ 5.72 = 1.92.

Interpreting the number is also simple. Any result of 2 or less indicates a boat that is sufficiently self-righting to go offshore. The further below 2 you go, the more self-righting the boat is; extremely stable boats have values on the order of 1.7. Results above 2 indicate a boat may be prone to remain inverted when capsized and that a more detailed analysis is needed to determine its suitability for offshore sailing.

As handy as it is, the CSV has limited utility. It accounts for only two factors–displacement and beam–and fails to consider how weight is distributed aboard a boat. For example, if we load our hypothetical 12,000-pound boat with an extra 2,250 pounds for light coastal cruising, its CSV declines to 1.8. Load it with an extra 3,750 pounds for heavy coastal or moderate bluewater use, and the CSV declines still further, to 1.71. This suggests that the boat is becoming more stable, when in fact it may become less stable if much of the extra weight is distributed high in the boat.

Note too that a boat with unusually high ballast–including, most obviously, a boat with ballast in its bilges rather than its keel–will also earn a deceptively low screening value. Two empty boats of identical displacement and beam will have identical screening values even though the boat with deeper ballast will necessarily be more resistant to capsize.

Another single-value stability rating still frequently encountered is the IMS stability index number. This was developed under the IMS rating system to compare stability characteristics of race boats of various sizes. The formula essentially restates a boat’s AVS so as to account for its overall size, awarding higher values to longer boats, which are inherently more stable. IMS index numbers normally range from a little below 100 to over 140. For what are termed Category 0 races, which are transoceanic events, 120 is usually the required minimum. In Category 1 events, which are long-distances races sailed “well offshore,” 115 is the common minimum standard, and for Category 2 events, races of extended duration not far from shore, 110 is normally the minimum standard. Conservative designers and pundits often posit 120 as the acceptable minimum for an offshore cruising boat.

Since many popular cruising boats were never measured or rated under the IMS rule, you shouldn’t be surprised if you cannot find an IMS-based stability curve or stability index number for a cruising boat you are interested in. You may find one if the boat in question is a cruiser-racer, as IMS was once a prevalent rating system. Bear in mind, though, that the IMS index number does not take into account cabin structures (or cockpits, for that matter), and assumes a flush deck from gunwale to gunwale. Neither does it account for downflooding.

Another single-value stability rating that casts itself as an “index” is promulgated by the ISO. This is known as STIX, which is simply a trendy acronym for stability index. Because STIX values must be calculated for any new boat sold inside the European Union (EU), and because STIX is, in fact, the only government-imposed stability standard in use anywhere in the world, it is likely to become the predominant standard in years to come.

A STIX number is the result of many complex calculations accounting for a boat’s length, displacement, beam, ability to shed water after a knockdown, angle of vanishing stability, downflooding, cabin superstructure, and freeboard in breaking seas, among others. STIX values range from the low single digits to about 50. A minimum of 38 is required by the European Union for Category A boats, which are certified for use on extended passages more than 500 miles offshore where waves with a maximum height of 46 feet may be encountered. A value of at least 23 is required for Category B boats, which are certified for coastal use within 500 miles of shore where maximum wave heights of 26 feet may be encountered, and the minimum values for categories C and D (inshore and sheltered waters, respectively) are 14 and 5. These standards do not restrict an owner’s use of his boat, but merely dictate how boats may be marketed to the public.

The STIX standard has many critics, including many yacht designers who do not enjoy having to make the many calculations involved, but the STIX number is the most comprehensive single measure of stability now available. As such, it can hardly be ignored. Many critics assert that the standards are too low and that a number of 40 or greater is more appropriate for Category A boats and 30 or more is best for Category B boats. Others believe that in trying to account for and quantify so many factors in a single value, the STIX number oversimplifies a complex subject. To properly evaluate stability, they suggest, it is necessary to evaluate the various factors independently and make an informed judgment leavened by a good dose of common sense.

As useful as they may or may not be, STIX numbers are generally unavailable for boats that predate the EU’s adoption of the STIX standard in 1998. Even if you can find a number for a boat you are interested in, bear in mind that STIX numbers do not account for large, potentially vulnerable windows and ports in cabin superstructures, nor do they take into account a boat’s negative stability. In other words, boats that are nearly as stable upside down as right side up may still receive high STIX numbers.

The bottom line when evaluating stability is that no single factor or rating should be considered to the exclusion of all others. It is probably best, as the STIX critics suggest, to gather as much information from as many sources as you can, and to bear in mind all we have discussed here when pondering it.

Here is a response submitted to SailingScuttlebutt.com in response to the pickup of The Prototype blog. I would note that it was not “submitted.”

From Dan Meyers – Newport, RI:
As I get older I figure that I have seen all of the foolishness in the world, but this week the nonsense submitted to Scuttlebutt is appalling.

Ouest and I have been playing Mailman lately. I’m the mailman and I make deliveries to her. Before I give her the mail I say, “Special delivery for Ouest. Oh, hello, who are you?”

Reaching out for her stack of cards she blurts out, “O-U-E-S-T. Ouest Lill Schulte. Forty-three pounds. I live on Bumfuzzle. I’m from Mexico.”

And really, that’s all you need to know about her. With that information she should be able to find her way home from anywhere else in the world. At the very least her mail will always find her.

By my reckoning, the Dad-Kid Humour Index peaks when the kids are about ages 3-6.  Dad specializes in Kindergarten funny.  Puns, bodily functions, and even the odd dubious word are used to hilarious effect.  When I opened this photo of Erik yesterday, Indy laughed until she almost cried.  Dad with a blue head?  Comedy genius.

There's no better way to waste a few minutes of your work day.

(via)

Last night we heard the noise that cruisers without watermakers dread—the long-cycling water pump—signaling we’d reached the bottom of the tank. A day or two earlier than expected, but not a big deal. We returned to Puerto Escondido, filled the tanks, grabbed a mooring ball, went to shore for ice cream, and then jumped in the pool. Needless to say the kids were not disappointed in how this day went down.

This was a fast passage with very little motoring. My mate Mr. Lassen and I covered the 830 some miles between Fajardo and St. Georges in less than six days and burned only about five gallons of fuel in the process. Not my fastest passage ever between the Onion Patch and the W'Indies, but I think it's the fastest northbound trip I've ever made at this time of year.

41 months Ouest, 21 months Lowe

Another month zips past. Ouest’s Spanish is improving greatly. She knows dozens of words and is using them pretty regularly. And of course her swimming continues to improve daily. She is to the point now that we let her go out swimming to the deep stuff by herself. We watch her and call her in when she goes too far. And now when she gets tired while swimming she just flips right over and back-floats her way in.

Sander van der Borch