Civil Service Sailing Association – Channel Sailing Division
SKIPPERS NEWSLETTER – Yacht Sea Essay of Hamble
Newsletter No 01/12
This is an occasional document issued by the Yacht Secretary, which will be placed on the CSD website and/or sent to all CSSA approved Skippers in CSD when there is new and/or important operational information to promulgate. There is no intention to produce newsletters on a regular basis. Each one is numbered in sequence every year so recipients can identify if any are missing.
Items for this issue of the Skippers Newsletter are as follows:
River Hamble Tidal Streams
There have been a number of incidents when berthing alongside our berth in the Mercury Yacht Haven, several of which have been attributable to the direction and rate (speed) of the tidal steams in the HambleRiver and around our berth.
Following theoretical and practical measurement (see Annex below) it has been established that the rate of the ebb will be significantly greater than the preceding flood tide and can be increased further with the amount of heavy rain (land) water flowing in the river. The direction of flow also varies with location.
It is recommended that departure and arrivals are planned to coincide with low tidal flow rates. It is appreciated that this will not always be possible and preparation for leaving or berthing should take account of tidal predictions and prevailing conditions on the day. The need for a clearly formulated plan, which has been fully briefed to the crew, cannot be over emphasised.
The passage across the tide between C and B pontoons may have to counteract a tidal stream the rate and direction of which can vary considerably. As a result the course steered may have to be as much as 45 degrees off the rhumb-line and a suitably higher boat speed and crabbing angle may be needed during peak tidal flow to avoid loss of control.
It is recommended that the boat is positioned in the centre between the two lines of pontoons and a crew member available with roving fender ready for action if required. In spite of the predictions obtained there is no substitute for personal observation of the tidal stream before leaving and during your passage up the river. Situational awareness is the key and use of transits, back bearings etc. can be used but the position can change rapidly which prevents the use of anything too complicated.
Particular care and use of these techniques is particularly relevant between spring and half-spring tides at about half tide. The same manoeuvre at Neaps and at or near High or Low water or during a Stand can be more straightforward though the effect of wind must also be born in mind.
Berthing bow or stern in
The decision to berth bow or stern in is one that only the skipper can make, bearing in mind that directional control is generally better going bow in. It is also true that boarding and leaving the boat over the stern with the bathing platform down is easier for the less mobile. The use of the berthing spring rigged with both ends on the boat’s amidships cleat and the bight over the outer pontoon cleat provides a degree of additional control.
Skippers, when needing to oversee and advise the helm berthing the yacht, should ensure that they have sufficient crew capability to undertake the berthing safely.
CSD Yacht Secretary
Sources of Information
The amount of detailed information of the tidal steams is surprisingly limited. The most comprehensive information comes from the Raymarine Plotter by placing the cursor over the “T” at Warsash, pressing “OK” (centre of rotary control) then using the soft keys for data etc. Other sources include easytide.ukho.gov.uk where predictions for 7 days are free. The notes in Reeds under “River Hamble” are also of interest.
The YH Team has, as an experiment made one physical measurement (using a floating object passing under the walkway and a stopwatch) of flow towards the outer end of ‘C’ pontoon at mid ebb which was approx 0.8 knots. It is the Team’s intention to continue such measurements into the rate of the tide at various times and dates on pontoon C as and when the opportunity presents itself. This physical measurement is significantly less than the figure given by the chartplotter software presumably because the water depth is significantly different.
Interpretation of Data
From these sources the Warsash tidal curve can be obtained and the times of high and low water. From the curve the prediction for a 1st and 2nd HW at or near Spring tides and any “Stand” which can be as long as 2 hours can be seen. The steepness of the line can be an indication of the expected rate. It should be noted that with a 1st and 2nd high water and a stand of up to 2 hours the ebb tide has to complete its ebb cycle in 3-4 hours instead of the usual 5+ hours. This means that the rate of the ebb will be significantly greater than the preceding flood tide. This rate will also increase with the amount of heavy rain (land) water flowing in the river.