Bramblemet under reading?

[bdh198]

A few times over the last couple of months I've noticed that the wind speed recorded by Bramblemet in the Solent appears to be quite a bit less than we have been reading when we've been mid-Solent at the same time. At first I thought this might be because our own instruments were slightly over-reading, but I've now noticed a similar under-reading at Bramblemet on three different boats.

Most recently was yesterday when we were taking part in the Hamble Winter Series. During the half hour leading up to the first warning signal at 10:25 Bramblemet never showed a mean wind speed of more than 21.1 kts. However, during that half hour our own instruments never read less than 24kts, with a mean wind speed around 25-27kts.

I'm assuming Bramblemet takes its measurements as a mean over a five minute period as that is the period shown when you look in the Bramblemet archives. I am also assuming Bramblemet is actually located on Bramble bank and hasn't been moved to somewhere where there is a wind shadow from certain directions (closer to Calshot?).

I appreciate that it is possible all three boats I've been on might have been over-reading the wind speed by a similar amount. However, my suspicions about the reliability of Bramblemet has been compounded when I compare the data from yesterday at Bramblemet to the data over the same period at Chimet. The wind was NW, and so if anything you'd expect Chimet to have a lower reading as it's in the shadow of Hayling Island. However, Chimet's readings from yesterday are much closer to the readings we were actually experiencing north of Bramble Bank.

Is there something I have misunderstood about using the data from Bramblemet, and if not, has anyone else noticed an apparent under-reading at Bramblemet?

 

[lw395]

How tall is your mast?
Do we assume your boat's wind speed is computed true wind., taking into account boat speed? Is that log or GPS?

 

[Giblets]

Have you tried the one on Calshot Spit light float?

http://rock7.com/wind/

 

[BabySharkDooDooDooDooDoo]

Comparing bramblemet with the other 'mets' it often seems to be understating the wind even when making some allowance for the different locations

 

[bdh198]

How tall is your mast?
Do we assume your boat's wind speed is computed true wind., taking into account boat speed? Is that log or GPS?

Mast height is about 18 meters. It would be useful to know what height Bramblemet is taking measurements at.

The readings I've taken were true wind calculated from the log.

 

[bdh198]

Have you tried the one on Calshot Spit light float?

http://rock7.com/wind/

Thanks! It will be interest to see how that compares.

 

[ShinyShoe]

Mast height is about 18 meters. It would be useful to know what height Bramblemet is taking measurements at.

The meterological standard is to use a 10m mast. Not sure if Bramblemet is doing that. That is quite tall for a buoy.

I'm assuming Bramblemet takes its measurements as a mean over a five minute period as that is the period shown when you look in the Bramblemet archives.

Industry standard is to average over 10 minutes, but that doesn't mean you can't upload over lapping averages. However, according to this: http://www.bramblemet.co.uk/(S(o2orzi55kbhugy45qgto0l45))/tech.aspx they average over 5 mins

So they take a reading every second, get 300 readings and average that out. I don't know how their anenometer works. it could be a simple rev counter. In which case there will be a degree of inaccuracy based on what fraction of a revolution it can measure per second. Rev counters work better over longer periods.

I am also assuming Bramblemet is actually located on Bramble bank and hasn't been moved to somewhere where there is a wind shadow from certain directions (closer to Calshot?).

See: http://www.bramblemet.co.uk/(S(1o2gmy2w2hjrop55tz4amx25))/about.aspx

 

[Giblets]

That is quite tall for a buoy.

It's not on a buoy but a bloody great red post stuck in the sea bed! Granted it's not 10m high though.

 

[DJE]

It's not on a buoy but a bloody great red post stuck in the sea bed! Granted it's not 10m high though.

It might be a low tide.

 

[DJE]

Time for another plug for the Hill Head Sailing Club weather station I think.

http://www.hillheadsc.org.uk/weather-station/

(We seem to be agreeing pretty well with Bramblemet at the moment.)

 

[Sandy]

When was the instrument last serviced, perhaps it is lacking in some 3-in1 oil?

 

[Nico]

Maybe the discrepancy was because you were in a slightly different position to the Bramble Bank post - not a long way away but maybe enough in a gusty offshore wind. I noticed the same thing the week before when we were experiencing over twenty knots but Bramblemet was showing 15-20 knots. Both weekends were sailed in the same place (more or less) and in both cases the wind was NW so possibly accelerated off the land and reducing by the time it reached the post. Just an idea.

As mentioned elsewhere 10m is the standard height for measuring wind speed but the post would vary with tidal height.

 

[Judders]

If you take the final beat on Sunday, we had thre or four more knots on the left than the 2nd & 3rd placed boats did on the right. That's the Solent for you. It varies all over the place.

Plus virtually every yacht overreads.

 

[Seajet]

The www.Chimet.co.uk/ Bramblenet/Cambernet/Southamptonmet sensors usually tell one when they are being inaccurate, but I think it best not to rely on them 100% in case some poor shipwrecked yottie is clinging onto the anemometer cups, distorting the readings

 

[BabySharkDooDooDooDooDoo]

Maybe the discrepancy was because you were in a slightly different position to the Bramble Bank post - not a long way away but maybe enough in a gusty offshore wind. I noticed the same thing the week before when we were experiencing over twenty knots but Bramblemet was showing 15-20 knots. Both weekends were sailed in the same place (more or less) and in both cases the wind was NW so possibly accelerated off the land and reducing by the time it reached the post. Just an idea.

As mentioned elsewhere 10m is the standard height for measuring wind speed but the post would vary with tidal height.

Passing around high water today it didn't look all that much higher than deck height!

 

[Keen_Ed]

It's not on a buoy but a bloody great red post stuck in the sea bed! Granted it's not 10m high though.

About 20’ ish above the high water mark, IIRC?

[Pedant hat]And yellow, not red. [/Ph]

 

[bdh198]

About 20’ ish above the high water mark, IIRC?

[Pedant hat]And yellow, not red. [/Ph]

I was racing in the Hamble Winter Series on Sunday and someone mentioned that the anemometer and other Bramblemet equipment had been moved from Bramble Bank to Calshot. I'm not sure if that is to a buoy like Calshot Spit, or to the actual spit itself. I can't find any information confirming if this is true, and if it is where it has moved to. Has anyone heard anything about this?

If it's been moved closer to Calshot it could explain why there appears to be an under-reading in northerlies.

 

[BabySharkDooDooDooDooDoo]

If it has been moved then they haven't updated the website http://www.bramblemet.co.uk/(S(curbj445eoejnm2cxemvowqk))/tech.aspx

 

[alahol2]

About 20’ ish above the high water mark, IIRC?

[Pedant hat]And yellow, not red. [/Ph]

Maybe it's a perspective thing but it looks like the wind generator head is at the same height as the anemometer. If that is the case then in certain wind directions it may well affect the readings. I can't believe they would make a mistake like that though.

 

[burgundyben]

has anyone else noticed an apparent under-reading at Bramblemet?

Yes, I think so too. I live in Cowes. i'm on the shore at Gurnard in the park almost everyday, often check from the office or while there, always look at B'met and Sotonmet by way of comparison.

On the whole, I agree, it does seem that B'met under reads.