Sunday, May 17, 2015

Transpac Weather and Tactics by Stan Honey

Transpac Weather and Tactics
Stan Honey

Stan Honey has navigated in nineteen transpacific races and has won eight times.  As navigator, Stan has held the single-handed, double-handed, Pacific Cup, and Transpac passage records for monohulls to Hawaii.

Overall race structure and necessary decisions

The primary feature that dominates the Transpac is the Pacific High.  Typically there is no wind in the center of the high, and increasing wind as you get farther south, up to a limit.  The central question of the Transpac is how close to sail to the high, or how many extra miles to sail to get farther from the high.  In years when the Pacific High is weak (or weakening) and positioned well south, there can be strikingly more wind to the south.  There have been Transpacs where yachts that are 10 miles to the south of competitors can experience 1 knot more wind.  A sled, in 1 knot more wind will sail 1/2 knot faster, and therefore would gain 12 miles per day on the northern competitor.  This condition can persist for the entire middle third of the race, eliminating any chance of recovery for the yachts that are positioned too far north.  Note that all yachts in this middle third of the race are nearly fetching the finish on starboard pole, so the boats caught too far north cannot gybe out of their predicament without sailing a dramatically unfavored angle,  passing far astern of the competitors to the south.  This condition, dominates the results of most Transpacs.

Occasionally, however, the Pacific High will be strong (or strengthening), and located far to the north.  In these conditions, it IS possible to be too far south. The boats that sail closer to the high will not only get more wind, but will sail the shorter distance.  Typically in these sorts of years, the wind stays "reachy" throughout the middle third of the race, so the boats that paid extra distance to get south cannot even "cash in" the southing and reach up in front of the northern boats, because everyone is reaching fast.

The beat to the West End of Catalina

Generally, tack up the Palos Verdes coastline until the Westerly has filled in, and you can at least lay the Isthmus.  When you tack onto starboard to cross the channel, continue all the way across.  Do not tack on the shifts in mid-channel. There is substantial adverse current and lighter wind in mid-channel.  It is better to get right across into the accelerated wind and reduced current at Catalina.

Think of the Transpac in three sections:

1.    The windy reach to the ridge;
2.    "Slotcars" through the middle third; and
3.    The run for the last third.

The Pacific High nearly always has a ridge extending off of its southeast corner. On the weather map this is visible as "U" shape of the isobars on the southeast corner of the high.  After rounding the West End, you will have a windy reach for a couple of days, depending on your yacht's speed, but when you get to the ridge, the wind will lighten and veer very quickly.  For this reason you will find that after reaching in lots of wind for two days, when you finally get the spinnaker up, and are struggling to carry it, within 6 hours or so, the pole is back and you're running on your downwind vmg angles in much lighter air; you just crossed the ridge.

The most critical decision of the Transpac is where to cross the ridge.  The reason this is critical is, once you get to the ridge and the wind veers, you can not get farther south.  You are already sailing as low as you can on your polars, and you can not gybe without huge penalty.  That is why the middle third of the race is called "slotcars."  You have to stay in your slot, on starboard pole, until the wind eventually veers enough so that you can gybe out on port, if you choose.

The middle third of the race begins as soon as you cross the ridge, and the pole comes aft.  Throughout this part of the race, every yacht sails as low as it can (e.g. sails its downwind polars).   At the west end of Catalina you made your decision where you wanted to cross the ridge, you sailed there, and now you have to live with it for three days or so.  If you are too far to the north, you will be slowly destroyed by the yachts to the south of you, and there is nothing that you can do about it; you cannot gybe, you cannot sail lower.  As the wind gets lighter, your polars force you to sail higher and higher, until you "spin out" up into the high.  When you eventually gybe to avoid starvation, your angle on port pole has you heading due south, far behind your competitor's transoms.  The "slotcars" leg ends when the wind eventually veers far enough so that both gybes are symmetrical around the course to the finish, allowing you to sail either gybe.

The final third of the race is "the run."  This is why we sail Transpacs, the wind picks up as you approach the Islands, and you are surfing in tradewind swells. Generally the right hand side of the course is favored in the final third of the race, because the wind slowly veers as you sail further west.  Therefore the best course is to favor starboard pole until the last gybe to the vicinity of the Islands,
and come in on port pole to approach Molokai at Kalaupapa.  Be sure to account for the fact that the wind will continue to veer, and do not overstand Kalaupapa. In the final third of the race the wind speed is generally even across the course. Oddly, those boats that get too far north in the middle of the race and stew about it for 3-4 days, gybe onto port as soon as they can and sail to the south after
there is no longer a windspeed advantage to the South.  These boats then miss the right shift in the last third of the race and lose even more.

Approaching the Finish

Pick your approach to come into Molokai at Kalaupapa on port pole.  Gybe close to Kalaupapa and sail along Molokai in the accelerated wind.  When you get to the west end of Molokai, if you have been lifted away from shore, gybe back on port to get close to Ilio Point, where there is accelerated wind.  Gybe onto starboard off Ilio Point and cross the channel.  Never approach Oahu much above Koko Head, take another hitch on port in mid-channel if you have to.  It is fine to sail close to Koko Head, and from Koko Head sail a straight line to the finish. As you approach the finish, plot your track on the chart, and take GPS fixes as well as periodic bearings with your hand bearing compass.  The finish line is deceptive, and many yachts get too close to shore when they can not see the red buoy.  The best technique is to plot your position and navigate to the buoy, rather than expect to see it.  With spectator boats around, the buoy often cannot be seen until it is within 100 yards.


In contrast to popular perception, squalls do not always work the way "catspaws" do.  Catspaws have diverging wind in front of them.  Surprisingly, some tradewind squalls can have converging winds at their leading edge.  The wind converges because there is an updraft in front of the squall.  In addition,
the average wind in the squall is veered about 15 degrees or so to the right of the prevailing surface wind, and the squall itself moves about 15 degrees to the right of the path of the surface wind.  Behind squalls the wind is light, particularly near dawn.

Heavy boats:   As the squall approaches, gybe to port pole, stay on port pole right through the squall, and then gybe back when the squall has past completely over you and your wind speed and angle has returned to the prevailing conditions.  If you gybe back to starboard pole too early, you run the risk of crossing behind the squall and getting into the light air in the wake of the squall.

Light (fast) boats:  Gybe to get in front of any squall within reach.  Gybe back and forth in front of the squall for as long as you can.  Each gybe "back" towards the squall will be at a horrible angle, because of the way the wind "toes-in" in front of the squall, but do it anyway; the velocity makes up for the horrible
angle.  When the squall finally passes you, exit on port pole and get away from the squall to avoid getting becalmed behind it.  Port pole is more effective to avoid the calm behind a squall because the squall itself is moving to the right of the path of the surface wind, so port pole allows you to diverge rapidly from the light air area behind the squall.  It is perilous to exit a squall on starboard pole because of the risk of getting becalmed behind the squall, particularly near dawn.

Weather Information

The best source of information about the future position and strength of the high comes from the 500 mb progs, but they take some practice to interpret.  Zonal flow, or a straight E-W path of the jetstream is characteristic of sourtherly and weaker surface highs.  A jetstream with a large “omega block” is characteristic of a strong high.  The next best source of data is the surface data, either from weatherfax or grib files.   Satellite imagery via NOAA APT satellites is fun, but not really of much use in a race in the tropics other than to monitor the position of tropical depressions

Author's Disclaimers

All of the above comments are relevant to typical Transpacs.  There are unusual races in which you have to break the above rules to win.

Pay attention to your boat's polars.  If you are racing a sled, it is worth sailing extra miles to get extra wind, because no matter how hard it blows, a sled will sail still faster if you get more wind.  On the other hand, if you are racing a moderate displacement boat, do not sail any extra miles in order to get more wind than necessary to reach hull speed.  If you sail farther to get more wind,
you will have more fun, but your average speed won't increase enough to pay for the extra distance.

Watch for tropical depressions.  The inverted troughs that extend north of a tropical depression can cause the tradewind direction to shift from normal.  This can make a huge difference as you are picking your approach to the Islands.

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