Sunday, June 13, 2010

Speed Polars

rev. 9-16-2013

A common question posed by non-sailors is "how fast will she go?" As a question, it feels like "how long is a piece of string?" It depends on many things. The full answer, for another sailor, is a speed polar plot, expressing the velocity of the boat in a variety of winds and on a variety of headings. Conventionally, these are produced by prediction programs and are used in sales literature and as goals for racers, who will remove every scrap of weight from the boat, scrub the bottom weekly or even dry sail the boat, and buy new magic-cloth sails annually. The rest of us, and we are legion, have only the stretched-out sails that came with the boat, mounds of stuff in lockers we can't even remember, scrub the bottom only before painting, and sail with either no crew or lounging crew.

Predictions are typically made for flat water (like that's going to happen with a 20-knot breeze), everything unloaded that can be, empty tanks, zero water absorption, a questionable factory weight figure, and a polished (not bottom paint) bottom. Capsize is calculated based upon wind speed, when in reality waves are a very significant factor. These projections are not rooted in the real world, unless the boat is actively campaigned as a racer.

I've been curious how my real-world boat would do, compared to these projections. Certainly not the screaming speed on my Stiletto 27, often sailing above wind speed. As a starting point, I found a velocity projection for the PDQ 36:
PDQ 36 Speed Polar Plot
I also looked at a Gemini 105Mc plot: a bit faster to the weather, and a bit slower on a reach. A bit more heeling when hard pressed. I did not use those numbers.

I took a bunch of measurements from my own boat, under non-scientific conditions:
  • Open Chesapeake Bay waters, with typical long-fetch sea state
  • Clean bottom, but nothing special
  • Full fuel but variable water tanks
  • Tender with some stuff in it
  • 1-5 people
  • Old sails
  • Typical cruising load. Lots more "stuff"
  • Uncalibrated instruments, though I did check the GPS against the knot meter, and calculated apparent values against measured true readings. The compass was recently swung.
  • 130% (or what ever percent) genoa. IT would be slower and heel less with the stock self tacking jib.
I will also add that I did not fly the chute in 20 knots - that lies beyond my comfort zone with my typical crew. There is little question that with the chute up in 20 knots you are flirting with capsize, should the boat round-up. The numbers were projected from the PDQ 32 actuals using the PDQ 36 methodology. They should be good... if scary. The 20-knot main and genoa numbers are also fudged hard-on-the-wind. We are pressing too hard there for my old sails, so I reef at about 15- to 18-knots sustained.

I have also observed that just because I can carry more sail, it is not always much faster on the PDQ 32. Because the keels are about 2 feet further forward than good sailing design would place them (they are so located for drying-out on tidal flats, so it is not a design error), the boat develops a very heavy helm when pressed hard on the weather, and reefing the main and rolling up a few turns of jib lightens the helm with no perceptible loss in speed; the rudder becomes straight, rather than being cocked to one side and behaving as a brake.

So, it is not so simple. But here I present a PDQ 32 speed polar for cruisers. About 40% of the values have been confirmed in actual practice, spread across the full range. The gaps have been filled using both the PDQ 36 study methodology and some other forgivable fudging. Achievable goals for real world sailors, I think.

(Click for larger image)

PDQ Altair 32 Classic, 45-foot mast height

There are some jumps in speed that beg explanation; I saw the same affects on the Stiletto.
  • When the speed passes ~ 7.5 knots the water begins to leave the transom more cleanly, and the waterline, in a sense, becomes longer. Thus, a 0.5-1.0 knot jump occurs rapidly, up to ~ 8.0-8.5 knots.
  • When the wind speed reaches somewhere around 15-20 knots it is easy for the rudders to become overpowered on certain courses, so it becomes critical to either steer well and keep the speed up, drop the traveler a lot, or reef the main. Reefing the jib only makes this condition worse. My Stiletto had larger more efficient foils and did not have this characteristic - it just kept going faster, until the helmsmen lost nerve or flipped.
  • When the speed passes ~ 10 knots the hulls begin to plane a bit. I suspect this is very load dependent. I have noticed a number of times, reaching with the genoa, that the transition from 10 to 12 knots occurred rather suddenly. A little unnerving a first. The Stiletto did the same thing.
  • The PDQ 32 is probably just as fast as the PDQ 36, in general. It's a little slower under power, because of the shorter waterline; the jump in speed happens at a different point, so it will be a little slower in 8-12 knots winds as well. In 0- to 8-knot winds, it is a bit faster due to lighter weight and less wetted surface. In 15-20 knots it will depend on the waves; the PDQ 36 will win in chop, due to mass and length.
A challenge for you; try to get boat speed to equal apparent wind speed. It is very possible on a reach with the chute up.

The apparent wind plot is simply calculated from the speed plot. There are no predictions there, just high school trig.

Running wing-and-wing is not evaluated, because the wind speed instrument fails to operate correctly when the wind is less than 30 degrees from the stern. However, when sailing deep, particularly in a strong breeze when a chute might be scary or simply too much work, it makes more sense. I admit, I resisted this for a year, a product of sailing very fast cats. A mono-hull sailor convinced me to try it, and he was right. As soon as the genoa begins to get well blanketed, consider it" you will maintain the same speed you had at about 140 true. The proper set-up is a must; see this post:

Rigging for Wing-and-Wing

However, wing and wing is only practical straight down wind. It is still slower than a chute, though much easier in a blow.

The data, for anyone who cares....

PDQ 32 Speed Polar Data File

For comparison, here is a Gemini 105 mC speed polar from a velocity prediction program. Notice that the speeds are very similar on most courses, except hard on the wind where the boards make the Gemini more weatherly. Not the other hand, the heeling data suggests that the Gemini is significantly less stable, becoming very light in 15 knots true and going over in 17-knot gust; of course, that is with everything in tight, hard on the wind. This is about 5 knots less tolerant than the PDQ 32, though either act would require pretty poor seamanship; the strain would be quite obvious and the sailor would certainly have reefed down and/or eased sheets.

Gemini 105 mC Speed Polar

The PDQ 36 has similar speed characteristics, only it can handle a lot more wind. instead of getting light around 20 knots, something more like 27 knots is required. The large difference is due, in part to the large genoa used on the PDQ 32 in place of the stock self tacking jib. Beam and mass account for the rest.

PDQ 36 Speed Polar

Need a polar for your boat? Search the web; lots are out there. Try this collection.

No comments:

Post a Comment