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eclipsemullet

Expedition problem

19 posts in this topic

I'm no Exp expert but it looks to me that our Pol Bsp% is being exceeded by too much.

 

Anyone got an idea of what inputs I should check? I've done my best to centre the speed transducer and realigned the wind vane.

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I'm no Exp expert but it looks to me that our Pol Bsp% is being exceeded by too much.

 

Anyone got an idea of what inputs I should check? I've done my best to centre the speed transducer and realigned the wind vane.

First: Calibrate everything.

Second: Check that the polars are correct - where did you get them - are they designer polars or based on experience? If the latter they may have ongoing log errors built in. Are they flat or rough water polars? Are they up to date?

 

If the same problem exists upwind, downwind and reaching its possibly a calibration problem or the polars are universally wrong. If the problem only exists at some angles the polars may be wrong in certain areas.

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They are polars I got direct from Farr a couple of years ago.

 

I think the problem was manifesting on port tack before, so I turned the speed transducer a few degrees in its housing. Surely the speedo can't be that sensitive to alignment?!!

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Eclipsemullet,

 

Boat speed is a building block vector, it is fundamental to the problem your are trying to solve.

 

Comparing your performance to your polar is at the higher order of complexity end of the scale.

 

Start simply by eliminating your sailing ability and the vagaries of wind and waves.

 

Find a calm location at slack tides. Then prove that your boat speed is in the right ball park with

running measured runs, up and back, under power. Keep going until your calibration factor converges.

 

Keep track of the range of corrections need for each set of runs. Errors at this level will haunt you later. This information will help you understand the size of your blind spot and limit your dithering later.

 

Depending on your instrument system and sensors this technique can be refined.

 

Pm me if you would like more specifics.

 

H

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Thanks for help.

 

Using Expedition's Stripchart, I think our AWA is +5 degrees too large on port tack.

 

I shall try and dial this into our Raymarine system and see what difference it makes to the calcs.

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eclipsemullet - as well as the angle correction factor - also check that your masthead unit is not moving - ie it's bolted on securely.

I've seen issues with the wind angle changing on each tack (and only sometimes) because the masthead unit or the slot the vane was located into was worn - which lead to assumptions down the wrong path.

As per above - go back to the basics.

Once that's sorted look at the exped polars again and make sure the boats weight, waves etc is the same when comparing numbers

At times we struggle to make our polars and other times we break them bigtime - including twa and awa - which is sometimes due to trimmers, weight distribution, etc, so you need to understand the variables and why it's occurring as well.

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As previously mentioned, the inputs for Pol% are:

 

-Boatspeed

-TWS

-TWA

-Polar

 

If any of these are wrong then your pol% will show it.

 

The most likely culprit is Boatspeed. Monitor d Bsp - Sog - if using a raw number then dampen it by maybe 20sec, else use stripchart and look at averages.

 

TWA and TWS are rarely well calibrated - especially at angles where pol% is used - i.e. non-VMG angles.

 

I've never met a polar or VPP from a designer that isn't optimistic (consider it a marketing polar) as they are typically run in perfect conditions - but your case seems to show the opposite!

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Thanks.

 

I drilled into the menu options of our Raymarine ST70s yesterday. For some reason there was a +5 degree manual offset in favour of port tack. I reduced this to zero.

 

I will see if this gets rid of the problem.

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I hate to break it to you but using Rodneymarine as instruments and expecting something close to reality is a wet dream at best. You cannot calibrate the instruments well so what you get is garbage in- garbage out. If you get withing 10% consider yourself lucky

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I hear what your saying but we might as well try and get the system we've got working as good as possible. A few top boats around here sail with Raymarine, so I'd question whether its a bad as you suggest.

 

I'd love B&G but aint got the £££££ at the moment.

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Absolutely - BUT - be careful putting too much faith in the numbers. You could go cheaper than B&G and still get very good data - Nexus is an option

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it can be really hard to get good BSP out of even the top level B&G

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^^^ This. At least on my (now a bit dated) B&G's the correction is linear and the error is not. So, if your boat has a big range of sailing speeds, perhaps, calibrate at the speed that is most critical to you or at a speed which minimizes the error over the range of speeds you typically sail at.

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Eclispemullet, is it possible to publish plots of your tacks? A strip of Boat speed, True wind angle, magnetic wind direction, Heading, True wind speed and target speed and angle. Something about 6 minutes long centered around head to wind in a tack would be ideal. Group the parameters to keep the scales reasonable. I like to use absolute value for the wind angles. Try to find data from a day with a gradient breeze on shore, as long as the water in not too warm.

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Christian is correct, ray marine instruments don't have the capacity to do the calibrations you need to get reliable racing data. However since you are using Expedition you can do all the calibrations you need there.

 

Expedition has full calibration tables which let you do high end calibrations across all wind angles and all wind speeds. In fact it was sailing on several boats that had ray marine instruments that lead to the development of Ventus Navigation.

 

Using Expedition to do the calibrations and race data calculations, and then displaying the data with Ventus, gives you a very accurate instrument package at a fraction of the price an instrument system with a real race processor costs.

 

I use that on my own boat that has an older set of Nexus instruments and get very accurate data displayed on my phone. We only use the instrument displays to view boat speed and apparent wind data since those three things can be calibrated in the instrument system. All the true data and all the tactical data is handled by Expedition.

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Christian is correct, ray marine instruments don't have the capacity to do the calibrations you need to get reliable racing data. However since you are using Expedition you can do all the calibrations you need there.

 

Expedition has full calibration tables which let you do high end calibrations across all wind angles and all wind speeds. In fact it was sailing on several boats that had ray marine instruments that lead to the development of Ventus Navigation.

 

Using Expedition to do the calibrations and race data calculations, and then displaying the data with Ventus, gives you a very accurate instrument package at a fraction of the price an instrument system with a real race processor costs.

 

I use that on my own boat that has an older set of Nexus instruments and get very accurate data displayed on my phone. We only use the instrument displays to view boat speed and apparent wind data since those three things can be calibrated in the instrument system. All the true data and all the tactical data is handled by Expedition.

 

I can vouch for this...even on high end B&G systems I use the instruments mainly as a raw data supply, and calculate then calibrate speed, leeway, true wind etc in Expedition - unfortunately B&G (well at least the GFD) struggles with too many remote channels written out from Exp, so if I can't write out the Exp calculated/calibrated data on a B&G remote channel then I use Ventus on a couple of 10" and 7" tablets.

 

An alternative is Tinley Electronics and Expedition wrote some code for me a couple of years ago in order to get performance and other Exp data on deck on Stowe Dataline displays - very nice solution with cost effective displays (http://www.stowemarine.com/dataline.htm). PM me if you would like more information.

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