Expedition Expedition Navigation Software

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6/26/2019 1:23 am  #1


Current Set/Drift and Leeway

Hi,

Question 1:
Is Set and Drift calculations in Expedition compensated for Leeway?
If not how can I get a good estimation of the Current set and drift?

Facts:
We are using a Nexus system, that also calculates Set and Drift, but I choosen to activate the Expedition calculations.
I also let Expedition calculate the Leeway using the Leeway equation.
The Log, it is mounten in front of the keel in the boats centerline. Calibrated while running with engine.

Problem:
My problems is that when boatspeed increases during beating the system is indicating Drift to Leeway, that looks to be linearly increasing with speed.
Compas is calibrated and the problem is independent of direction.


Question 2:
Has Expedition any variables for current compensated TWD/TWS?

Kind regards:
Magnus Olofsson

 

6/26/2019 1:38 am  #2


Re: Current Set/Drift and Leeway

1. Yes. Normally the instrument calculation ought to be better as they have the raw data, but sometimes not.

What sort of cal value are you using for the leeway coefficient in Exp?

Normally current errors are most likely due to bsp or heading errors. Are you calibrating the bsp at about the same speed as you are sailing?

2. Gwd & gws

 

6/26/2019 9:54 pm  #3


Re: Current Set/Drift and Leeway

Agree with Nick that having the instruments calculate Leeway, drift and set is preferred.   Then display those values in Expedition.

To have set/drift calculated correctly your leeway calculation needs to be be correct. Likely the heel sensor in your boat is very accurate and you mentioned HDG is also accurate.  Then BSP and the K factor are the uncertainties. K normally is somewhere in the 8-12 range.   
BSP calibration can be non-lineair for differen boat speeds, especially with a paddle wheel (ultrasonic tends to be much better here). Make sure your BSP is calibrated for the typical upwind BSP you sail.
Can your Nexus system calibrate BSP for different heel angles?  This can make quit a difference when not properly calibrated.

When you sail on a lake without current it can be very helpful to have Expedition display the current barbs along the track. Make sure the 'size' of the barbs is quit long (I uses a value of 0.2 in Settings-Weather-Current-Shade/size max).
When your calibration is correct,  these barbs should almost be zero in length.  Depending on which way the barbs point you can quickly see if your BSP or leeway is not properly calibrated.
Do mind thare may alway be a smal surface current as result of the wind.  This depends how sheltered your sailing area on the lake is.
 

 

6/26/2019 10:21 pm  #4


Re: Current Set/Drift and Leeway

Also, you want to bias the leeway cal value for upwind. You don't mind if it is wrong downwind as an error in almost nothing is still small.

Agree about the current arrows on the track. If they change when you tack, either your heading or bsp is causing it.

 

6/26/2019 10:43 pm  #5


Re: Current Set/Drift and Leeway

And of course Stripchart is always your best friend as well when calibrating!

 

6/27/2019 12:24 pm  #6


Re: Current Set/Drift and Leeway

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.

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.

Bsp is calibrated at sailing speeds of around 5-6 knots.
I only have settings for starboard and port, so can't compensate at diffrent boatspeeds and heeling. Can I do it in Expedition?
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.

Regarding the Leeway index, I haven't had time to experiment. Have previously used the instrument Set and Drift and just recently started trying to get correct values using Leeway in Expedition. Just did a fast test to see if the Current barbs was affected by the leeway by setting it very high, but I didn't get and changes in the barbs, thats why I asked i Drift and Set was compensated by Leeway.
I guess I use the current Barbs also to find the leeway index, by adjusting it until the barbs don't point to leeward and windward.

I will investigate more!

Very thankful for your support, still learning alot and its a field and on a level where it is very hard to find information. But as Engineer in control systemes it is very intresting! As usual its hard to get time by the team to do controlled tests.

     Thread Starter
 

6/27/2019 9:52 pm  #7


Re: Current Set/Drift and Leeway

Your assumptions on how the current barbs are pointing (outlined in your 5 bullet points above) is correct.  If leeway is underestimated:  increase the K factor, if it is overestimated decrease K  (assuming BSP and heel are correctly calibrated).

Yes, even if the paddle wheel is on centerline your BSP calibration is impacted by heel.  When your BSP is calibrated perfectly when motoring (no heel),  your BSP will normally indicate too high when heeled. At my boat BSP is about 3% too high at max heel when I wouldn't compensate for heel. 

If your instruments cannot compensate BSP for heel, then Expedition can do. Expedition can also calibrate leeway in case your instruments can't (can you enter a K factor in your instruments?).    But as Nick has indicated, the data rate within the instrument system is likely much higher than the data that Expedition receives,  so it's always better to do the calibration in the instruments when possible.

If you instruments cannot calibrate for leeway  I recommend you have expediiton doing it.  But in that case you should also have Expedition calculate set and drift because if you would still use set/drif from the instruments the leeway (as calculate dby Exp) would not be part of the equation.


 

Last edited by ZeeZee (6/27/2019 9:54 pm)

 

6/29/2019 11:30 pm  #8


Re: Current Set/Drift and Leeway

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.

 

6/30/2019 12:10 am  #9


Re: Current Set/Drift and Leeway

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 you know, I mean outside the calibration assumptions under engine?
Yes, Heeling will affect the centrally-positioned paddlewheel too, and that varies with Boat Speed and Trim. But if the sails are properly trimmed, corrections are generally done based on Heel because that reflects Boat Speed to an extent. The H5000 system provides a table for Heel correction of Boast Speed.

Magnus86 wrote:

Regarding the Leeway index, I haven't had time to experiment. Have previously used the instrument Set and Drift and just recently started trying to get correct values using Leeway in Expedition. Just did a fast test to see if the Current barbs was affected by the leeway by setting it very high, but I didn't get and changes in the barbs, thats why I asked i Drift and Set was compensated by Leeway.
I guess I use the current Barbs also to find the leeway index, by adjusting it until the barbs don't point to leeward and windward.

Even if Leeway is accounted for, the Current barbs will not vary appreciably in length, only a few degrees in direction. That is if everything is properly set elsewhere (Leeway coefficient, Boast Speed...). My guess in this case is an asymmetric response of boat speed depending on tack.

Magnus86 wrote:

I will investigate more!

Me too!

Magnus86 wrote:

Very thankful for your support, still learning alot and its a field and on a level where it is very hard to find information. But as Engineer in control systemes it is very intresting! As usual its hard to get time by the team to do controlled tests.

Very much the same here. Glad two have found people to exchange thoughts and experience with!

Last edited by JL (6/30/2019 1:05 pm)

 

6/30/2019 12:20 pm  #10


Re: Current Set/Drift and Leeway

JL,
Not sure if I undestand exactly what you say,  but from above I get the understanding that you say that leeway has hard any impact on the current speed.

I don't thing that is true.   With a boat speed of 7 knots, a leeway of 4 degrees equals a current of 0,5 knots perpendicular to the course.   In other words: if you won't calibrate for leeway it would look like there is a 0,5 current.

But maybe I get you wrong?.

 

 

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