Expedition Expedition Navigation Software

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Questions about Expedition features » Current Set/Drift and Leeway » 7/01/2019 12:56 pm

JL
Replies: 28

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Thanks for the info ZeeZee. Not surprising from NKE... they already offer an electromagnetic speed sensor - although I don't believe it provides a direct Leeway measurement yet. It won't be long...
Meanwhile there's a DX900+ from Airmar that is advertised to provide a direct Leeway measurement. Would Expedition accept the measured Leeway as the legitimate Leeway in its Current and Wind calculations?

Questions about Expedition features » Current Set/Drift and Leeway » 6/30/2019 11:38 pm

JL
Replies: 28

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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

Questions about Expedition features » Current Set/Drift and Leeway » 6/30/2019 1:49 pm

JL
Replies: 28

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ZeeZee, Magnus,

Reading Magnus' original post again, I see that I got his point wrong: 1/ he knows for sure that there is NO current, and 2/ I suppose now that he started from a situation with a Leeway coefficient of 0 rather than say, 10. That would then indeed allow for a 0.5 knot Leeway effect in order of magnitude.

Thanks for catching me out and sorry for the confusion.

JL




 

Model Accuracy » How does Model Accuracy actually work? » 6/30/2019 12:59 am

JL
Replies: 3

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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

Questions about Expedition features » Current Set/Drift and Leeway » 6/30/2019 12:10 am

JL
Replies: 28

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Hi again Magnus,

Magnus86 wrote:

Using Stripchart and my observations duting no sea current have been that the current barbs points 90 degrees to leeward of the boat independent of tack and I can get around 0.5 knots in current at 5 knots boat speed. I guess this must be due to missing Leeway compensation. Hdg (+-1 degree) and Bsp (+-0.1 knots) shouldn't give that big affect.

We tend to assume no error in SOG, which is reasonable in many cases.
I agree with you, Leeway will not give that much of an effect, unless an error in the Leeway coefficient (way too high) creates the anomaly, or any other spurious effect.
And then there is always the possibility that there is actually a funny current... but more likely that it is a Boast Speed issue.

Magnus86 wrote:

Am I right if no sea current and I get current barbs:
1. Pointing forward, boatspeed is calibrated too low.
2. Pointig backwards, boatspeed is calibrated too high.
3. Pointing to leeward independent of tack, leeway underestimated.
4. Pointing to windward independent of tack, leeway overestimated.
5. Pointing to leeway/windward dependent of tack, bad heading calibration.

In principle, yes for 1 and 2. But the actual current needs to be of sufficient magnitude and orientation to significantly affect the sensors (paddlewheel, Heading, SOG and COG) within their sensitive range.
3 and 4 in theory only. In practice, the ratio of perpendicular Leeway to Boast Speed is way too small to be reliable. Maybe a paddlewheel alignment issue?
5, possibly.

Magnus86 wrote:

Its a paddlewheel located in the middle of the boat. Does it still get affected by heeling?
My observations have been that it get 0.1-0.2 wrong when beating. I thought i was the leeway, but after some calculations I guess the leeway shall only increase SOG with around 0.03 knots.
Am I right than that I should calibrate BSP = SOG-0.03 knots (which is insignifcant) while beating.

0.1 to 0.2 error on Boat Speed is remarkable, but how do yo

Questions about Expedition features » Current Set/Drift and Leeway » 6/29/2019 11:30 pm

JL
Replies: 28

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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.

Technical Q&A » AWA reading to quickly » 6/29/2019 11:10 pm

JL
Replies: 13

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Reading AWA and AWS from the MHU by-passes the H5000's calculations of these data. B&G have a particular way of producing Apparent Wind data, which consists in back-calculating them after going through a first iteration based on what they call "Measured Wind " (MWA and MWS are accessible for display) to calculate True Wind. Once this is done, including all corrections to TWA and TWS, they back-calculated AWA and AWS.
In summary, you really should use the CPU's Apparent Wind data to rate full advantage of the H5000 system. This does require that you populate the correction tables properly, however.

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