Thanks for the answers.

Regarding the first point, it's a matter of definition and consistency. TWS and TWD, although named "True", are usually the wind components experienced by a boat that would be static on water, i.e., moving with any current that prevails. So TW includes the wind resulting from the movement of the boat due to the current.
Ground Wind is the data provided by weather forecast: this is the wind that would be felt at 10-m height by instruments that would be static on Earth, so without any effect of current.
Since we can calculate Ground Wind on a boat equipped with a GPS, I was wondering why you chose TW instead of GW.
This is really of interest in high current and/or low wind situations.

As for the rest, I really like the concept but find it difficult to work without the ability to shift in time and space. I sail in the Med, where forecasts notoriously challenging. They are becoming better but still tend to suffer a time lag and a spatial shift of some extent. Without accounting for this, it would be dicy to bet on a forecast because it seems to do a better job "here and now" over a given time frame on the boat's course. It will be interesting to see how you handle these shifts.

Looking forward to these developments.

A fresh start on Magnus' questions and the first answers that he received, and some additional thoughts.

1. I am not sure that Instrument systems actually apply a Leeway correction to the Tide Set and Rate calculations. Instead, they tend to bundle the two... which creates strange Current readings in certain situations (low current - or no current as in Magnus' case). Please let me know what the facts are on this for various systems.
2. When it comes to Wind calculations, my understanding is that the Tide component is ignored, except for angles. On the H5000 system, applying Leeway amounts to taking Course as angle reference instead of Heading. But there is no correction to the magnitude of True Wind or Ground Wind to reflect the Leeway component.

I am pretty sure of these answers for the B&G H5000 system (but always ready for surprises - I haven't seen the code!!). Why would that be? Why not "do things right" and include Leeway properly in calculations?

One issue in using Leeway to correct for Current or Wind quantitatively is that the way we generally get to Leeway is rather imprecise for the accuracy we would need in these corrections. Using an empirical formula (usually k x Heel / BS^2) with a fixed factor is fine for navigation to correct courses under certain limitations, but it does not provide the level of detail required to perform the instantaneous corrections on Current and Wind, even with damped data.

One way to check this is to get a Leeway table from the naval architects of your boat, together with the optimum Heel and of course the polar curves. This allows you to back-calculated k factors for each pair of TWA and TWS...
In the only example I was able to run (only complete data set I was able to get), k factors vary between 5.2 and 17.7 - and I am only limiting the calculation to TWA less than 75 deg to make the point. Now, assume you had applied a reasonable value of, say 10 for the k factor and see where this gets you. The differences between "T

Hi,
Interesting development I have been trying to get from other developers... without success! Which makes me very curious about your software I just discovered via the Expedition forum. Do you know whether you are (still!) the only ones to do this so far?

A few technical question:
1. Why use TWS and TWD and not Ground Wind data, which is what the forecasts provide?
2. What corrections do you apply to TWD and TWS before comparing to forecasts?
3. Is your correlation "static", simply cross-correlating (adjusted) boat measurements and forecasts along the boat course, at the time she was there? Or do you allow for time and space shifts as forecast may be accurate wind-wise but just offset in time by a few hours or in space by a few (tens of ) NM?
4. Do you have the ability to work with Ensemble data sets?
JL

Hello Magnus,
In response / to complement your question and discussion on Leeway-corrected Current.

It seems that we are both experimenting with the same subject, albeit with different systems (B&G H5000 and Adrena for me). I ran sensitivities in Excel and as you rightly point out, the perpendicular component of Leeway has no practical bearing whatsoever on Tide Rate (the name in H5000, aka Current Speed) calculations.
Most systems therefore will calculate Tide as the straight difference between the SOG vector and the Boat Speed vector. Where it gets tricky is that 1/ TIDE Set is affected by Leeway and 2/ Boat Speed actually contains the longitudinal component of Leeway. The latter component is never discussed because we can't measure it directly. It is generally embedded in correction tables. But there are mathematical tricks to try and calculate it.
I am still playing with this... Will report back if I get anything useful out of it.