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Page history last edited by Kevin 15 years, 5 months ago



I've had a few chances to get some hard data on the boat's performace.  I took a handheld, old, non waterproof GPS out a few times in 2007, but didn't get any tracks out of it, I was just happy to see the boat go over ten knots.  Laurent started to bring his more modern GPS, which he could download into an Excel spreadsheet.  I found that one good way to look at the data is to chart it as boatspeed on the x axis and course on the y axis.  The first such chart I made is from April 2008:



Windspeed was about 15 mph or so, perhaps a bit more at times, out of about 160 degrees.  Max speed was 14 mph, we spent a lot of time at 11 and 12 mph.


The next time we used the GPS much was during the Texas 200.  Day 1:



Max speed was 15 mph, windspeed was mid 20s, course was a broad reach.




Day 2:


Not quite as fast, but still moving pretty well.  Also a broad reach day, wind about 20 mph.




Day 3:



Day 3 had a good bit of close reaching and beating.  We still had pretty good numbers, I was happy about that.




Day 4:



Day 4 also had some windward work, which can be seen more clearly in this graph.  We were also in some open bays, which limited our speed some.





Day 5:


A few miles of a reach, then a run to the beach.






I had a few suggestions as to presenting the data in a better format than the above, as neato as they may seem at first glance.  The big problem with the above format is that it fails to take the wind direction into account, so you know what the course was, but not what the true wind bearing was.  Not very useful.  I tried to fill that in below the graph, but it's hard to go back and figure out what the course was just by looking at the thing, and I discovered upon looking at the wind data available online that my memory had played me a little false anyway. 


So, first, the wind data; the best I can find is the record of the Corpus Christi station, which is here.  Sadly, it only records the directions of the max gusts, not the average wind directions throughout the day.  Still, the wind wasn't very variable, so I'm going to use this.  So, our Day 1 was June 9, so our wind data was, I think:



               Direction        Average strength

Day 1      140                  13.9

Day 2      160                  11.2

Day 3      140                  12.2

Day 4      130                  11.4

Day 5      140                  10.4


We certainly got more wind than that at times, but the averages are probably also about right for where we were except for Day 1, we had sustained winds in the 20s in the Land Cut.


So I took all the data, corrected for wind direction for each day, and messed about with it.  I was unsatisfied.  I was trying to clump the data together in 'legs' of about the same course, to come up with an average speeed on that course, but the resulting polar was very lumpy and odd looking.  When I finally just gave up and plotted *all* the data on a polar the reason for that became clear:



So, clearly we were on the starboard tack almost the whole time, and clearly we did more reaching than anything else.  But what was messing me up was the big arm of points that stick out down and left.  Those are our speeds on the first day, they were what was messing up my attempt to make it all fit on one polar curve.  We had quite a bit more wind that day, compared to the rest of the trip, and very flat water in the land cut.  On other days we would often hit bigger winds in bays, when we also got bigger waves, which slowed us down.  It's very dramatic to see what you get when you have a lot of wind and no waves to worry about, we were moving. 


This makes me think that Pete Brigaitis' idea of everyone stopping at the mouth of the land cut and then us all taking off at the same time and having a drag race to Camp 1 really has some merit.  Wouldn't that be a blast?


So now I think I could draw a pretty good polar for the boat in 11 or 12 miles per hour of wind, I would just ignore that arm of data.  But this graph of all the data points seems to me to tell at least as much as the polar would, so I may just stop here.





It occurs to me that I was for the most part focused on the off the wind performance, since that's where the trouble was with making the data fit.  But this graph sure makes the boat's windward performance look good, doesn't it?  Really good.  Too good.


The scales on the x and y axes are not the same, so a visual 90 degrees is not really 90 degrees.  To help with this I tried to locate the (5,5) and the (-5,5) points, and then draw lines from the (0,0) point through them.  The resulting angle is a true 90 degrees:




Huh.  I have no illusions my boat shunts through 30 degrees.  I must thus conclude that the data that's inside the yellow angle is not in the wind I thought it was; on both tacks we must have been lifted quite a bit, to get those data points.


The problem is, I don't have any better wind data now than I did when I made the graph.  Each day is corrected for the wind direction shown on the Corpus Christi NOAA site.  I could go back and find those data points and make up a wind direction that makes them look better, but I don't know what that direction would be.  I rather think that we were about 50 degrees off the wind in the ship channel, for example, but I could be wrong.


So I'm tempted to leave it like this, with the note that the stuff off the wind is likely right, and the stuff inside the yellow angle should likely be ignored; it's possible that everything shown above a beam reach should be thought of as having been sailed in a fifiteen degree or so lift. 


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