Polars is a short-hand term identifying a set of data that predict sailboat performance for different wind conditions. The data are usually derived from computer-based velocity prediction program (VPP) algorithms that factor in hull and rig characteristics. Yacht designers use this in designing hull shape, weight, and rig configuration for a sailboat. Rating organizations like Offshore Racing Congress (ORC) use this to assign a boat's ratings for different sailing conditions, using her specific sails inventory, that tend to be fairer than a single number.
- VPP best performance diagram - Traditional graphic form for all points of sail in true wind speed ranges.
- VPP best performance table - Tabular format for all points of sail in true wind speed ranges from 6 to 24 knots, assuming mainsail, overlapping genoa, and running spinnaker.
- VPP polars chart - Excerpt of the tabular data for best VMG upwind and downwind, along with beam reach. We keep this in the cockpit for reference. Here is a copy:
measured in degrees; think of this as tacking angle or gybing angle
Some tips on interpreting and using the data for competitive benefit on the race course:
Boatspeed - Polars are often over-simplified to just target boatspeed for different wind conditions. Granted that target BS should be our goal for every point of sail. Observing that we are sailing slower or faster than target BS should clue us to work harder for sail trim or steer a better course. They are a partnership. Many other factors affect boatspeed, of course: sea state, puffs and lulls, windshifts, wind sheer, wind affects from nearby boats, current, rig tune, condition of the hull's bottom, motivated zebra mussels, condition of the sails, condition of the running rigging, condition of line stoppers and other line handling equipment. Oiyyy - that's a lot!
Tacking - Look at TWA and Turn for upwind. Expect to tack through a greater angle in light air vs. a moderate breeze. For example, Volant tacks through 85.2 degr in 6 kts TWS compared to 76.0 degr in 12 kts. Use this when deciding where the layline is and when to tack. Don't forget to account for current in addition to tacking angle.
Heeling - Notice that when sailing upwind in typical moderate wind conditions, the maximum heel angle is between 10 and 22 degr. Even in very strong wind, maximum heel is 23.7 degr. The goal is to work hard to not exceed target heel angle for the wind condition and keep heel angle steady. Use shroud tension, backstay, boom vang, traveler, sail choice, sail trim, reefing, weight distribution, feathering steering. Otherwise, the keel loses lift and the boat slips sideways through the water with unacceptable leeway. We may be pointing at the mark, but the course is different from the heading. Sailing downwind, the boat should be very flat to maximize exposure of the kite. Only when winds are very strong does heel angle necessarily increase.
Beam reach - What's up with the beam reach data? Of course, VMG is 0 because course is paralleling a mark to weather or downwind. So, these data are just one example of a point of sail other than a beat or run. The graphs and tables supply data for other points of sail. But the beam reach is useful because it gives an indication of what to expect for AWS, AWA, and Heel when sailing off the wind.
Run TWA - Consider rounding the upwind mark. In light to moderate wind conditions, selecting a TWA of about 145 degr is a good and safe default for the initial course after hoisting the spinnaker. Helmsperson and spinnaker trimmer work together to adjust the course thereafter based on pressure in the kite. However, different spinnakers are designed to perform in different wind angles. The A2 will sail lower, and possibly the A1. The A3 and A5 will hold shape better sailing higher angles, closer to a broad reach.
Gybing - Look at TWA and Turn for downwind. Expect to gybe through a greater angle in light air compared to a moderate breeze. For example, Volant gybes through 73 degr in 6 kts TWS compared to 53 degr in 12 kts. This is a big difference. When gybing angles are this narrow, it is easy to sail past the layline and then be forced to reach to the leeward mark, covering extra distance. This reduces the spinnaker douse options to one.
Rolling - Look at TWA in a run downwind on a windy day. In moderate-to-strong breeze of 14-20 kts TWS, course downwind of about 160 degr TWA is only about 20 degr from a dead downwind run. Be careful of sailing by the lee, death roll, broach, accidental gybe - all bad outcomes. If a roll starts, steer up slightly to stabilize the heel angle, ease the boom vang for more mainsail twist, sheet in the spinnaker, then soak back down under control and ease the kite to a curl.
Rudder - Look at the interesting dissonance in optimal TWA from 20 to 24 kts TWS when running - from 160.9 to 141.4 degr. That is an indication that the keel and rudder may start losing the ability to keep the hull stable at some point over 20 kts TWS. This is exacerbated if there are following seas that can lift the rudder out of the water if they are overtaking the boat. Steering up maintains rudder control, builds a lot of extra speed, and does not compromise VMG. In fact, look at how fast VMG increases.
Sails - Notice how fast the AWS increases as TWS builds when upwind. Sails are designed and built to sail within a wind range that is safe for the fabric strength, and the sail feels AWS, not TWS. The G1 is not designed for 17 kts of wind, and that would be 12 kts TWS close-hauled. Need to change down to a stronger headsail before then. Sailing downwind, because of boatspeed, the AWS is somewhat light, and a light spinnaker can withstand that. The trouble occurs if the boat heads up intentionally or is forced up due to traffic. The AWS builds dramatically. This is when spinnakers rip apart.
Reefing - Volant has only one reef point in the mainsail. The truism goes "when you start to think about reefing, it is too late"! According to the chart, we should start thinking about reefing at 16 kts TWS and commit to it by 20 kts TWS when close-hauled. Downwind, with expectedly high boatspeed and likelihood of keeping the boat quite flat, the AWS is so light that we really don't want a reef in.
AWA vs. TWA - Not obvious to derive from the chart, but the AWA is the better guide to steering proper course upwind than TWA. AWA is what the sails feel and dictates how they are trimmed. The AWA on the instruments matches the behavior of the telltales on the headsail. Many skippers also use AWA as the primary steering guide downwind, but I find that TWA is more stable and reliable. AWA is too variable downwind because it changes quickly in puffs and lulls, and you tend to chase it ineffectively up and down.
There you have it - some tradecraft technique for applying VPP polar performance characteristics of the Beneteau 10R and her sails effectively! We can use this on the race course. If you have other observations from the VPP data to add, let me (Chris) know!
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