Anonymous wrote:

Arne wrote:

Robert,

that was very impressive numbers. I have never seen polar diagrams where the boats sail 30 deg. from the true wind direction before. 

Could this be apparent wind angle? In that case, the  VMG to windward will not make sense.

Arne

I've thought of something that may have a bearing on this:

We rig the anemometer on a short mast way out on the quarter, as this is where we can get it furthest way from the rig. Yet it's not a perfect position. If you race, you know that if a competitor gets onto your lee bow, he deflects the wind that you feel, heading you so that you fall away into the windshadow behind his rig if you don't tack away. Our anemometer is reading a wind that is a few degrees more "on the nose" than the ambient wind, I think.

An anemometer on a short mast at the bow is in a worse position, in the upwash as the wind approaches the rig.

An anemometer at the masthead is also not in the true wind, and is probably going to be somewhat affected by the tip vortex at the top of the rig (and, dynamically, much affected by pitching and rolling). How much affected, I don't know, but that's the source of all the data that's been collected to date on sailboats' performance, and is the source of our feeling that 30˚ apparent, 45˚ true is what we should be aiming for as an achievable target.

I think David may well be right that our anemometer position has meant that the upwind angles are being affected by the effect of the rig on the airflow. So what can we do about it?

We could apply an arbitrary correction to the data we have, so that, for example, we increase the wind angle by 10 degrees at 35 degrees true and taper it off towards zero degrees and towards 70 degrees using a sine curve. That would bring the polars we have back into "sensible" territory.

I suggest something along the lines of

If TWA<70 then

TWA corrected = (180/PI()) x (TWA +(10 x Sin((PI()/180) x (90/35) x TWA))

applied during the true wind calculation would achieve the desired result. NB; The PI()/180 term converts from degrees to radians and the 180/PI() converts back from radians to degrees.

Longer term, we can, and I think we should, retest some or all of the boats tested so far, with the anemometer mounted on a long pole attached to the bow to get it as far forward as we can, and at the same height as in the aft position. The cable on the anemometer is long enough to do this. We can than compare the upwind results we get with the results we already have to see what the effect really is of mounting the anemometer aft. Then we can adjust the correction factor above if needed based on observed differences.

Obviously we should ignore any results with the wind aft when the anemometer is forward as they will be seriously affected by the sail!

Then, for testing any further boats, we should put the anemometer forward for upwind sailing, and aft for downwind. It would not be hard to do that.

Looking through the polars Robert has produced I can't help thinking the results are being skewed by taking the median value for each group of points. Taking Poppy at 12 kts wind speed for example, the values at 40, 50 and 60 degrees are pulled in by the number of low speed data points, while at 30 degrees the median appears near the top of the range of values.

The display of the data points makes me think that taking the highest point at each angle would produce a better curve. Possibly taking, for example, the mean of the top 5 values would produce an even better curve, smoother and with a better defined optimum VMG.

Just a thought!

Anonymous wrote:

Summry PDF's of polar plots for all 4 boats at various true wind speeds and angles.

Ancillary graphs, jpg's of all the polars and a cleaned-up version of the original data are here: Calisto, Miranda, Poppy, Weaverbird.

Bravo Robert for a fascinating set of results. I particularly like the analysis of the raw data which shows how different the conditions were for the different boats. (See attached pics) Unfortunately I had to add the boat name to all the files to make these comparisons as when taken out of their own folder they all have the same file name. Maybe that's something to change for the future?

It would be interesting to see the results for each tack on each boat, as the indications I have are that the results are significantly different. Again I like the histograms that show the data variation between tacks.

Anthony Cook and I have been working on filtering the data to try and remove the suspect or dodgy data points which can be generated by rounding up, as discussed, or by the wind dropping suddenly, as the boat speed decays quite slowly compared to the drop in wind speed. I hope we will be able to share the results soon. At the moment we are eliminating any data points where the wind speed drops by more than 0.7 knots per second, and any where the wind direction changes by more than 5 degrees per second. We also eliminate all subsequent data points until the boat speed changes. These filter values eliminate about 10% of the data points.

It is early days, and we are definitely still on the learning curve!

i really like the polar plots for vmg (even if some of them miss the zero point at 90 deg…)

…but something tells me that the interpretation of the data misses an essential point. i’ll try to put my finger on it:

as a helmsman i never steer a really straight course. while meandering some 5 to 10 degrees to either side of the main course, the hull speed will drop a little bit, but the data collected on the windward side might show far better results than you could sail in this direction.

a solution could be to sail a strait course for a couple of minutes and then interpret 30 second or one minute blocks of data to interpolate the main direction, main boat speed and main wind speed.

imho this would give more accurate (but lower) performance results.

ueli

I think it's worth saying that these were the very first trials, and we're learning how to gather data at the same time as learning how to record and analyse it. The numbers we recorded give Robert something to work with, but let's not read too much into the results just yet.

I agree with Arne and Ketil, a cruiser is doing pretty well if it points at 45˚ true and 30˚ apparent. Weaverbird, Calisto, Miranda and Poppy ought to manage it, in a reasonable seastate. Not closer, though, at best VMG.