So here I am at Preston dock. I set up the kit yesterday afternoon, but was too tired to do a run, after having to fix the outboard in the morning. However, everything connected up properly.

This morning it was flat calm and still water, and we did the calibration run. That went OK, except that at speed and horizontal, quite a lot of water was going over the top of the float. It seems to be best to tow it at 3˚ bows-up.

Then I did a run with miniplex3 config recording a txt data file. Of course the actual figures are meaningless, but it will give you something to look at and process.

It took a couple hours to set up the first time, but familiarity will improve on that. It took an hour to break down and stow the kit again.

I don’t fancy my chances of doing any real testing until I reach Falmouth, unless I stop in Milford Haven and do a bit there. The wind looks turning fair on Wednesday and Thursday for the passage down to there.

Jerry wrote:

David,

I was reading about the polar performance experiments and was somewhat surprised about the upwind performance of your rig and the aerojunk versus the bermudian and split junk. Paul McKay mentioned he would narrow the slot and flatten the main on his rig to improve it. It seems to me that the things you are doing on the newest version of your rig will help as well. One thing I was wondering about was if the difference might be due to the lower aspect ratio of both rigs compared to the others? Or maybe we just need more data? 

Thank you for keeping up at the "quest"! It's very exciting to me to follow along with your progress.

Thanks,

Jerry Bart

Jerry,

I think it's not advisable to read too much into the testing we did last year, particularly comparing different boats. As Alan said in JRA magazine 79, there were other factors affecting the results. I was fully familiar with Weaverbird and her rig by that time, so could sail her reasonably well, but it was literally the first time that the test equipment was deployed, and Alan and I were learning how to operate it to gather data and process it. Then there was a question about whether Calisto was being sailed in optimal fashion. Then Miranda's rig was not in a fully shaken-down and optimised state. With Calisto, Miranda and Poppy, Alan was having to go aboard boats that were new and unfamiliar to him, and try and get them sailed at their best while gathering data. Not an easy task. And then the various venues and sea states will have posed more questions. Calisto and Poppy are somewhat larger than the other two - were they handling any given sea state better (or worse) because of their size and/or hull type?

So I think it will always be a bit risky to read too much into boat-against-boat tests. We will be on much firmer ground when we can do some "before and after" testing on one boat in similar wind and water conditions. Weaverbird with her current JR rig will bear comparison with any other cambered JR, and I wouldn't expect the instruments to show that one is markedly better than another - but when the rig is changed to the new wing sail, will it be possible or not to record a noticeable difference in performance? If so, will we be able quantify the performance improvement that I get, versus the extra work to make the rig? Could we do the same on a boat that changes from flat to cambered JR?

It's early days, though. Let's see if we can add to our store of knowledge this summer.

I have today added a page under the Technical Information heading which shows the performance of the 4 boats tested so far at each wind speed (6, 8, 10, 12, 14 kts). They can be seen here.

These were created in Excel from data tables produced by Anthony Cook. he has spent time over the last couple of months developing software that creates these tables from the raw csv files. We have tried many different filtering options to try and cut out any misleading data points caused by rapid rounding up or bearing away, or by rapidly dropping wind speeds. We have found that a maximum wind speed drop of 0.7kts per second is a reasonable limit. We have tried several different limits for the rate of change of the wind angle, and have found that +/- 10 degs per second gives the best results.

I will be interested to see what everybody thinks of these results!

Anonymous wrote:

In prep are pdfs showing polars for 1) all boats, all even wind speeds, 2) all boats , all odd wind speeds, 3) scaled stw's (Alan's factors) all boats, max upwind vmg's, max downwind vmg's and beam reach (90 deg) stw's.

The rescaling, i.e. multiply STW's by Alan's factors, I think attempts to isolate the effects of the rig type by correcting for the boat's characteristics ...length, displacement, sail area, etc.

rself

Here are the results of comparing boat speeds, after scaling by Alan's factors (see his spreadsheet below). My approach is to compare the frequency distributions of boat speed in three categorys: max upwind vmg's, beam reach and downwind. The histograms are unambiguous if sufficient sample size is available. These data were collected at 1 second intervals. Around 1000 points or seconds or 17 minutes of data appears to be a minimum to get a close approximation to a bell-shaped curve. To gain sample size I've had to combine boat speed results from more than one true wind angle and more than one true wind speed.

Upwind vmg: combined 40 & 50 deg twa and 6-12 knt tws.

Beam reach: combined 80, 90, 100 deg twa's and 6-12 knt tws's.

Downwind: combined 170 & 180 deg twa and 6-12 knt tws.

The more points the nicer the shape and less ambiguous the comparison across boat designs/rig types/before and after conversions to junk rig (in the future). I'd advocate more data (keep or increase the current data rate) not less.

In Alan's scaling factors Poppy is deemed fastest (longest waterline, fairly high SA/D) and calculates a factor of 0.94. When applyed (multiply every boat speed by 0.94) reduces Poppy's boat speeds by 6%. Miranda, deem slowest of the four, had a factor of 1.29...all boat speeds were increased 29%. Weaverbird's factor was 1.05 and Calisto's 1.14.

The Poppy and Weaverbird histograms tend to overlap on all courses. Conclusion, the scaling correction explains boat speed differences in the original data.

Miranda is the slowest boat on heeling courses (upwind and beam reach) even after the 29% factor boost. Downwind, although the data count is sparse at only 40 points, it tentatively looks like Miranda would keep up with the other boats. My best guess at an explanation is that Miranda had the lowest ballast-to-displacement (B/D) ratio and (B/D) is not one of Alan's criteria. A higher ratio would mean less heel, carry more sail, reef later probably resulting in higher speeds on heeling courses.

Calisto (bermuda rig) was the fastest boat upwind but on the other courses there was overlap with Poppy and Weaverbird. Combining Poppy and Weaverbird mean upwind vmg's by eye I get around 2.6 knts. Calisto's vmg is about 1 knot or 38% faster. Alan's 1.14 factor is reasonable meaning that a 14% boost to boat speed for this comparison does not explain the difference. The Varne 27 design is moderate in most respects: SA/D, B/D, LWL. The design does have the highest D/L which would tend to make it slower not faster. Given everything that is known the 38% higher upwind speed is anomalous.

I look forward to analyzing next summer's data. For me the hard part of writing the code is done. Today it'd take me hours not days to process 40,000 lines of data ( 4 boats 10,000 lines each) from raw csv's to polars to summary histograms.

rself

Anonymous wrote:

Alan,

another idea:
When I see the clouds of data on one of the diagrams in Reply# 6929495, I wonder; could it be that it is the sensors, or rather the sampling rate of them which is not optimal. I only own a simple Garmin 72 GPS, myself. However, I mean I remember that I could set up the damping of the speed. What was presented was an average over 2, 5 or 10seconds, something like that. With the shortest setting, the speed would fly up and down with over a knot, all the time. With the 10s damping, the speed only jumps with 0.2 – 0.4kts, at most, at each updating (once a second?). If the sampling of your sensors could be dampened in a similar way, there may be less of a job for later algorithms to struggle with.

Just a thought.

On the first test results we had a problem with the multiplexer being overloaded with incoming sentences, which meant that the boat speed was not changed every second, but at random intervals, sometimes as long as 20 seconds. This meant that the response to increasing and more importantly decreasing wind speed was delayed. That is why with Anthony's software we are eliminating not only data points where the wind drops by more than 0.7 knots per second, but also all subsequent data until the boat speed changes.

In Polauto we have the option to average over 1, 5, 10, and 15 seconds and what that showed is that we ended up with too few data points and very spiky results.

Personally I think it would be best to record wind speed, direction, and boat speed every second so that we can see how the boat speed changes as the wind speed changes, and get a sense of how the boat accelerates and slows down. We can always apply averaging to this data if we want to and see what effect it has, but if the original data is averaged, we are stuck with it.