tonybarebones
Well-Known Member
Mike Johnson
Marine Officer/Harbourmaster Gloucester Harbour Trustees
Tide Tables. I am quite sure that many of your members will by now have cottoned on to the fact that the Arrowsmiths Tide Tables for this year are related to Chart Datum throughout. Regrettably, this includes the predictions for Sharpness. We are all used to seeing the Sharpness predictions relative to dock sill height and some members may use the predictions as a guide to when they may be able to get on or off moorings, and into/out of the Pill. If members have been using the data in the tables for this purpose (or for estimating on which tides it will be possible to safely cross the sands or race in certain areas) they must now remember that the heights given for Sharpness in this years tables are related to Chart Datum Sharpness (1.83m below OD) rather than Sharpness Sill (1.33m below OD). Thus for this year we must remember to subtract 0.5m from the predicted heights for Sharpness if you wish to relate those predictions to sill level. I have suggested to Arrowsmiths that the tables for next year should revert to the traditional relationship to Sharpness sill.
I have given Barry copies of the Sharpness tide tables relative to sill height, which I hope he will display in the club house.
Narlwood Reservoir, Counts Rock. Clearly the comments about the tidal predictions are of significance if people are not aware that the reference datum has changed, as they may be under the impression that there is more water available. There are occasional times when cruisers have been caught out and have spent a period aground in the vicinity of the Narlwood Reservoir and Counts Rocks on neap tides. Whilst it may not particularly bother us, there are certain other organisations who get quite excited about it.
I have written on this matter before, but thought it worth doing so again. Here is an example - not a complaint (although the Reservoir is a prohibited area!) and not a safety issue, but just an observation in light of the changed datum in the tide table.
On Wednesday 21st January, yacht "Celeste" departed the Pill just before 1400 and went to Cone Pill via Counts buoy, arriving at 1456. HW Sharpness was 1601, height 5.5m (on CD Sharpness). Vessel departed Cone Pill at 1548 heading for Narlwood Beacons, crossing about 200m ahead of an outbound ship at 1607 before crossing the NW corner of the reservoir wall at 1610 (tide ebbing at this stage). The wall at this point is 1.4m above OD. Maximum depth over the wall at this point and time would have been no more than 2.1m ( 7' ) which is obviously adequate for a bilge keel craft. So what's my point? Had anyone planned this passage for a vessel with a deeper draft on the assumption that there was 5.5m on the sill (as opposed to 5.5m on CD), they might well have only just scraped over the wall.
Narrow Channels. Now that sailing is beginning again, please would you remind members that Rule 9 applies in respect of ships in the channel. There are also byelaws relating to the required clear distance ahead, astern and to the side of ships.
That's enough of my woffling!
Best wishes,
Mike
--
Mike Johnson
Marine Officer/Harbourmaster
Gloucester Harbour Trustees
www.gloucesterharbourtrustees.org.uk
01453 811913
07774 725270
Marine Officer/Harbourmaster Gloucester Harbour Trustees
Tide Tables. I am quite sure that many of your members will by now have cottoned on to the fact that the Arrowsmiths Tide Tables for this year are related to Chart Datum throughout. Regrettably, this includes the predictions for Sharpness. We are all used to seeing the Sharpness predictions relative to dock sill height and some members may use the predictions as a guide to when they may be able to get on or off moorings, and into/out of the Pill. If members have been using the data in the tables for this purpose (or for estimating on which tides it will be possible to safely cross the sands or race in certain areas) they must now remember that the heights given for Sharpness in this years tables are related to Chart Datum Sharpness (1.83m below OD) rather than Sharpness Sill (1.33m below OD). Thus for this year we must remember to subtract 0.5m from the predicted heights for Sharpness if you wish to relate those predictions to sill level. I have suggested to Arrowsmiths that the tables for next year should revert to the traditional relationship to Sharpness sill.
I have given Barry copies of the Sharpness tide tables relative to sill height, which I hope he will display in the club house.
Narlwood Reservoir, Counts Rock. Clearly the comments about the tidal predictions are of significance if people are not aware that the reference datum has changed, as they may be under the impression that there is more water available. There are occasional times when cruisers have been caught out and have spent a period aground in the vicinity of the Narlwood Reservoir and Counts Rocks on neap tides. Whilst it may not particularly bother us, there are certain other organisations who get quite excited about it.
I have written on this matter before, but thought it worth doing so again. Here is an example - not a complaint (although the Reservoir is a prohibited area!) and not a safety issue, but just an observation in light of the changed datum in the tide table.
On Wednesday 21st January, yacht "Celeste" departed the Pill just before 1400 and went to Cone Pill via Counts buoy, arriving at 1456. HW Sharpness was 1601, height 5.5m (on CD Sharpness). Vessel departed Cone Pill at 1548 heading for Narlwood Beacons, crossing about 200m ahead of an outbound ship at 1607 before crossing the NW corner of the reservoir wall at 1610 (tide ebbing at this stage). The wall at this point is 1.4m above OD. Maximum depth over the wall at this point and time would have been no more than 2.1m ( 7' ) which is obviously adequate for a bilge keel craft. So what's my point? Had anyone planned this passage for a vessel with a deeper draft on the assumption that there was 5.5m on the sill (as opposed to 5.5m on CD), they might well have only just scraped over the wall.
Narrow Channels. Now that sailing is beginning again, please would you remind members that Rule 9 applies in respect of ships in the channel. There are also byelaws relating to the required clear distance ahead, astern and to the side of ships.
That's enough of my woffling!
Best wishes,
Mike
--
Mike Johnson
Marine Officer/Harbourmaster
Gloucester Harbour Trustees
www.gloucesterharbourtrustees.org.uk
01453 811913
07774 725270