speed polar plot on PDQ36.com

[thinwater]

http://www.pdq36.com/PDQ36%20polar%20diagram.pdf

Is this based on sailing trial or theory?

[thinwater]

I found the data in a 1991 Multihulls magazine review of the PDQ 36.

FYI

I assume it would be asking too much to wonder if one exists for a PDQ 32?

[Loki]

That sure looks like the output of a Velocity Prediction Program (VPP); I'd be willing to bet on on it. It may be that the magazine verified that the results are reasonable, but I can't believe they actually collected all of the data points reflected in the report. Also, the equivalent report for the 42 was done before it it was launched.

[thinwater]

That sure looks like the output of a Velocity Prediction Program (VPP); I'd be willing to bet on on it. It may be that the magazine verified that the results are reasonable, but I can't believe they actually collected all of the data points reflected in the report. Also, the equivalent report for the 42 was done before it it was launched.

But clearly they did not take all of the data. They either took a few points and connected the dots with a program, which would be good practice, or made up lots of it, which is not.

It did strike me that they were suggesting carrying the chute closer to the wind in stronger winds than I would think is good sense - the ragged edge of capsize and certainly sail stretch territory. It is also going to be tough to take good data at the edge!

[Loki]

But clearly they did not take all of the data. They either took a few points and connected the dots with a program, which would be good practice, or made up lots of it, which is not.

Perhaps I should clarify: there is virtually zero doubt in my mind that the polar is completely computer generated. However, VPP software is seeded with real life data, traditionally the result of running many tests on slightly different models in towing tanks. Every time the America's Cup competition generates a new fleet of well tested designs, the programs get updated. Catamarans, however, don't have such a large sample to draw on, so the results are not of as high quality. As Steve Killing (the NA whose VPP was used for the PDQ Polars) said, "While the computer VPP is an incredible tool, it should be relied on only when the parameters it is assessing are within the range of a 'normal' boat." He goes on (in his book "Yact Design Explained) to describe infamous examples of failures of VPP programs. The book also includes a review of the PDQ 36, so he had some experience with the boat.

I would take the VPP numbers with a grain of salt, since the one of primary determining factors, the displacement, was listed as "8604.4 lbs" which is probably 15-20% under what a lot of PDQ's cruise with. In fact, I can say that we came close to the VPP numbers a month or so after delivery when we were reaching at 13+ knots in a 20 knot breeze, but after fitting out for cruising we've hardly even been over 10 knots, and more generally are happy with 8-9 knots.

It did strike me that they were suggesting carrying the chute closer to the wind in stronger winds than I would think is good sense - the ragged edge of capsize and certainly sail stretch territory. It is also going to be tough to take good data at the edge!

The computer doesn't get very nervous about anything, let alone carrying the chute in 20 knots, but it does note that the heel angle will go over 9 degrees, which is within the stability range. And the computer certainly cares little about stretching the sail! And remember, the computer doesn't "suggest" anything; it simply predicts a speed for a situation.

[wake up]

Steel Sails can carry the spinnaker ( about 760 sq ft ) to about 20 knots, though the sail may not take it. Stripped down for racing we made 13 knots on the gps with wind in the 18-20 knots range. Blew the spinnaker about an hour later in a gust.


Paul Haggar

Steel Sails
36067