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tobi_in_gemany

B and G upwash calibration

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we have on our boat, an IMS 41' (ex Reckless, Commodores Cup Winner 96) a Hercules System for the TWD. The vertical mastheadunit is about 1 meter long.

 

Altough I understand that calibration of AWA shall be done when you have no windshear or gradient, I find it very difficult to get consistent readouts for TWD. AWA is the same on both tacks when beating.

 

We are unsure which values we have to enter into the TWD cal Table for beat, reach and downwind. Does anyone have any recomendations or experience what typical values are, in order to give us a starting point? What values do you use?

 

The boat is 7/8 rigged and has no masthead kites.

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I see in the preview that this does not format very well on this forum software so please send me an email if you wish and we can discuss this in more detail but here is a rough matrix to use as a starting point.

 

5 10 15 20 25 30

 

Up -5 -2.5 1 0 0 0

 

Re -2.5 -1.5 -1 0 0 0

 

Dn -2.5 -1.5 -1 0 0 0

 

TWS -.5 -1.0 -1.5 -2 -2.5 -3

 

 

I generally leave the heavy air stuff empty to start with because you will find that all things being equal you will probably be correcting for mast twist in this range rather than upwash. The beauty of this matrix is that it give you the control over this variable that can not be achieved with any other system because all others apply, if at all, a linier function to this area and B&G lets you make specific corrections through all wind ranges and upwind reaching and downwind. It may look slightly more complicated than others but it really works.

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Bob,

 

Are the values you list for TWS correction the same for a given windspeed on all points of sail?

 

Is it typically easier to use the Tactician Software or just build the matrices through a FFD?

 

Thanks for a helpful thread.

Andrew

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I'd use either Dfw or a RemoteVision - but then I loathe having to do much with the FFD's - comes from being too lazy to learn the interface!

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Would this be related too?

The instruments on our boat jumping up to 50knots boat speed and then slowly coming back down to the speed we were doing which was around 8.5 what is the reason for this?

 

 

The yacht is a cookson 50, instruments B&G Hercules 2000

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Would this be related too?

The instruments on our boat jumping up to 50knots boat speed and then slowly coming back down to the speed we were doing which was around 8.5 what is the reason for this?

 

 

The yacht is a cookson 50, instruments B&G Hercules 2000

Your bowman is down in the bilge blowing on the impeller.

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I don't think that the boat speed jumping to 50 knots is related to the upwind calibration table. We have the same problem on our instrument system too - the boat speed suddely jumpes up to 20 knots or so and the slowly coming back to the original speed. I assume this to be some sort of connection problem between the sensor and the interface box and have never worried about it much. The interface box gets a wrong signal from the sensor, shows that value and then slowly comes down again (due to the damping).

 

Upwind calibration however allows for the twist in apparent wind that the Hasthead unit sees due to the wind deflected by the sails.

 

There are three values put in the upwash calibration Table for a given TWS (5, 10, 15, 20, 25, 30 knots of TWS) - one for beating, reaching and running. This table is the interpolated for the actual TWS and TWA.

 

From what I understand from the B&G manual, Upwash cal should start with the boat beating. Make sure that you pick a day with no or little wind gratient and windshear. Calibrate the Mastheadunit Angle (unter AWA). That should be the same on boath tacks.

 

Now - continue beating and check if TWD changes as you tack. the change in TWD / 2 should be the correction for Upwash.

 

As the values in the upwash calibration table are interpolated, make sure that you do not have any big jumps or gaps in the table. i.e. if you set the upwash cal to 10 when beating in 5 knots and -10 at 10 knots, results will be screwy when beating in 7 knots TWS.

 

This all sounds pretty easy in theorie - but is rather hard in practice...

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Would this be related too?

The instruments on our boat jumping up to 50knots boat speed and then slowly coming back down to the speed we were doing which was around 8.5 what is the reason for this?

 

 

The yacht is a cookson 50, instruments B&G Hercules 2000

I'm no expert - but do you have performance processor + Hydra - if so, could it be that the dynamic damping is coming down to too short a period through the manoeuvres? IIRC the dynamic damping is a function that only comes whn Hydra is added in - we dont have it so I dont recall that section of the manuall too well!

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Another possibility could be garbage in the speed linearity table, either at very low speeds, or low heel angles.

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Actually I just got a reply from B&G technical service which is quite helpful as well. The numbers quoted there are:

 

"

TWA

TWS 5kts 10kts 15kts 20kts 25kts 30kts

Upwind -7 -3 -2.5 4.5 6.5 8.0

Reaching -2 -1 0 1 1 1.5

Downwind 4 3 1 -1 -1 -2

 

TWS

TWS 5kts 10kts 15kts 20kts 25kts 30kts

Downwind -1 -1.5 -2 -2.2 -2.5 -3

"

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While we at this I was thinking about another thing I have no answer to.

 

You can either calculate TWA taking the leeway into account or leave the leeway out. As far as I know, B&G TWA does not take Leeway into account, whereas some software like Expedition does.

 

As we started building our polar tables from the IMS certificate, does anyone know if the optimum TWA for a beat and a run stated in the IMS certificate take leeway info account or not?

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I'd use either Dfw or a RemoteVision - but then I loathe having to do much with the FFD's - comes from being too lazy to learn the interface!

Roger that. Lotsa buttons on the FFD's. A tip: you can get it all done simply with the arrow keys.

 

"Up arrow" selects the upper readout, "down arrow" the lower. You can do it all with the two arrow keys and the enter key.

 

Select one half of the display (upper or lower), then scroll thru the categories: Performance, Wind, Nav, etc.

 

Select one of the categories, then continue with the arrow button to select which readout you want (TWS, TWD, TWA, Pi R Squared, etc).

 

I put myself thru a crash course with those things, and it's not all that bad.

 

I wish the FFD's had the same color scheme as the jumbos. The red on dark is real easy to pick up. The dark on red takes the eyes a moment at nite. But they are cool gages.

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Sorry not to chip in sooner guys - just a couple of pointers on the B&G stuff.

 

1. Boatspeed overeading:- I suspect that this is being caused by the paddle wheel coming out of the water in bigger waves and then being blown by the (much faster) breeze. I have not seen it for a while but it is a known issue for boats where the paddle is quite a long way forward.

 

2. True Wind Angle Cals:- These vary of course from boat to boat and are accounting for a number of factors that can effect the wind angle measured by the masthead unit. The two principle effects are the tip vortex of the rig (sometimes called upwash) and rig twist induced by rig loads. These two effects actually work against each other with aerodynamics dominating in light airs and twist winning in heavy airs. At some point they cancel each other out (normally around 12-15kts). The magnitude of the corrections varies from boat to boat (masthead vs fractional, runners vs swpet back spreaders etc). You should be looking for a smooth curve of corrections though so if you edit one also tweak the ones either side in the table.

 

3. True Wind Speed:- When sailing downwind the Measured Windspeed will over-read as the wind acclerates to get over the mass of sail you have put in its path. You can see this quite clearly when you bear away at the top mark. Generally corrections in True Wind Speed are in the -10% to -15% range. The Hercules system and WTP allow you to apply this correction at your downwind sailing angle. Hydra applies it at 165 TWA it is then reduced to zero at 90 TWA

 

Attached are typical cals for a Swan 45 Hercules system with a vertical masthead unit

 

4. Leeway:- All systems that have leeway functionality calculate it as it is not easy to measure, normally this is done as a function of heel and boatspeed. VPP outputs from designers and I suspect IMS normally include leeway in TWA as does our WTP system. At the moment Hercules and Hydra do not, but I suspect we may revise this as it is strictly not correct. It will not affect the majority of our users who tend not to calibrate their leeway, those that do will hopefully be pleased if we make this change. So if you are using Herc or Hydra and Polars from your designer it is not uncommon to be able to sail higher than the TWA predicted by 3-4 degrees as your instruments are not including the leeway, so adjust your polars to account for this.

 

5. Point of Fact:- America's Cup - Three teams use the Spanish Bravo system with B&G sensors and displays (of these only one is a "top" team), one is testing it vs the B&G WTP system, all the rest use the B&G WTP system. No other suppliers are used.

 

Hope this helps.

 

Nat Ives

B&G Race Specialist

racing@bandg.com

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Nat - thanks for your excellent relpy!

 

I just asked Fietje Judel if IMS polars inculde leeway or not. IMS polars do not include leeway, so we should theoretically see the same TWA for VMG sailing in the IMS certicitate and on the B&G system (Given that the IMS Polars have some connection to the "real world", which may or may not be the case....).

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Nat - thanks for your excellent relpy!

 

I just asked Fietje Judel if IMS polars inculde leeway or not. IMS polars do not include leeway, so we should theoretically see the same TWA for VMG sailing in the IMS certicitate and on the B&G system (Given that the IMS Polars have some connection to the "real world", which may or may not be the case....).

 

 

The instruments will agree if the VPP includes leeway. For the very few VPP which do not include leeway in their wind triangles the instruments will be out. If you sail a boat, such as an AC yacht, that sails at about zero leeway then this is not a problem.

 

In any case, you should take the VPP AWA as your targets and not the TWA. Then any difference in leeway is then trivial.

 

In any case, if there is not leeway in the VPP then TWA will be out (opposite to what you are saying)

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To clarify....

 

This is all about the axis of reference for Wind Angles - boat axis (centreline) or course axis (centreline plus leeway angle). If we use the course axis then the TWA will be bigger than if we use boat axis (by an amount equal to leeway angle). Old B&G Hercules, Hydra and h1000 systems use boat axis, the WTP system uses course axis. With the new Hercules system I think we are going to give you a choice.

 

Similarly if the VPP works to course axis the TWA will be bigger than if they work to boat axis. I am surprised to hear that the IMS VPP uses boat axis but I guess I should have expected it! In this case VPP and B&G instruments are on the same reference axis and should in theory agree.

 

You may feel that this is all a bit pedantic but if you find your B&G's saying 33degrees true when your VPP says 37 you can now think about the possibility that you are not in fact an upwind genius but that leeway is 4 degrees and the VPP has taken account for it but the B&G's have not.

 

AC boats, Volvo boats and even CBTF boats always have leeway, in fact on the Volvo boats it seems quite large. It is not efficient (even on AC boats with tabs) to sail at very low leeway angles.

 

Nat

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Not that it's likely to be critical - but are we talking a software upgrade to the performance processor - or new processor? One is achievable, the other less likely, particularly as I want some new sails too ...

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