Instrument calibration: Part two

Last issue we considered the basic calibration needed to get good, valid numbers out of our instrument systems. Now the basics are correct in this article I consider possible measurement refinements to produce the most meaningful data for the tactician.

There are a number of refinements we can make to the basic calibration to ensure the most accurate data. Just like with basic calibration, we must do this in the proper order so we do not have to repeat steps.

Boat Speed

Your paddle wheel is likely on the centreline, just forward of the mast. Great location when upright, motoring along, but what are the effects of the yacht heeling over and planing? What happens as we go faster? Is the paddle wheel linear across the speed range we expect the boat to sail?

The answer to this question is that without more calibration the paddle wheel will not record speeds consistently when the yacht is heeling and planing. We need to negate the effects of heel; these generally cause the paddle wheel to end up in a zone of water that flows differently around the hull. We also need to negate the effects of planning; these tend to make a paddle wheel over read at higher speeds.

The paddle wheel is calibrated using a boat speed calibration table. This table allows us to change the boat speed on a non-linear basis. In turn, this means various correction values can be applied at various boat speed and heel values.

However, it can be hard to know how much correction to apply. To determine this involves a lot of sailing and careful analysis. Monitoring the relationship between speed through the water and speed over the ground is key. But remember current and its effects too. Another trick is to use the change of the current vectors from tack to tack to determine if your speed is fast or slow when sailing upwind.

True Wind Direction

True wind direction is a critical tool for tactical decisions as a shift of just 4° can add up to a one minute gain or loss on a typical windward beat.

Of course you can derive this in your head based on compass headings on each tack, but if the instrument data is accurate then the tactician will use this instead. This will make their job much easier.

At first glance you might think that the computed data will be accurate immediately. After all, it is just a calculation of vectors with boat speed, heading, apparent wind speed and angle, known as the wind triangle.

However, sails also play a major part in bending the wind. This affects the angle that the wind gear “sees.” Upwind this bending, known as upwash, tends to increase the measured wind angle from reality. If we just left this alone with no corrections, when we tack the true wind direction will move giving the impression that we were always tacking on to a lift. We don’t want to know this “bent” wind direction we want the true wind without the sails influence.

How do we sort this out? A good instrument system will have a true wind angle correction table. It will allow for a correct value at a given wind speed and angle, interpolating for wind speeds and angles between. Higher end systems allow for more data points thus giving better control over the interpolation.

Sail upwind and note the true wind direction on each tack. After a while you will notice a trend. If the TWD moves in the opposite direction to the way you tacked, this is an indication that the TWA is too wide. As with the AWA correction what we do allies equally to both tacks, so we need to subtract half the difference of the TWD on each tack.

Conversely if the TWD tacks in the same direction as the boat tacking then the TWA is too narrow and we need to add some to the TWA.

As we are sailing up wind noting these corrections we need to apply them to the upwind part of the table in the wind speed we are sailing in. As you can see this requires us to gather this information for every wind speed in the table. This can take some time to achieve so is treated as an ongoing process.

The same issue applies downwind. With your normal sails set for that wind. Run downwind at your normal VMG angles. Note the TWD and gybe, again the TWD will gybe with the boat and a correction will have to be made.

Once you can get your instrumentation calibrated correctly and have confidence in the numbers you are seeing, you can start using the data to make smarter tactical decisions. The trimmers will be able to make better sail selections and the navigator can give more accurate racing information. 

– Ross VickersFor more stories like this subscribe to Australian Sailing magazine at Great Magazines or you can download it to your iPad with iTunes.

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