Marineflex Mooring Trial

The Club’s mooring spaces are allocated by the HKSAR MarDep and we have not been able to be allocated any more for a very long time. Members’ needs have increased over the years and we now have a long waiting list for moorings and also for moorings for longer boats.

Our moorings are basically a concrete sinker on the seabed with a chain leading up to the surface buoy to which the members mooring lines are secured to. During calm weather the chain lies on the seabed but in strong winds the weight of the boat pulls the chain tight, and it rises off the seabed and stretches out moving the boat further away from the mooring sinker.

The circle that the boat swings around, with the mooring sinker in the centre, is named the swinging circle. The length of the chain has to be a minimum of the depth of the mooring area at low tide, plus an allowance of 2.7m for a spring tide and then another allowance for storm surge.

To try to decrease the swing circle, stop the chain from dragging on the seabed and just possibly convince the MarDep to allow us to place slightly longer boats on our moorings we have examined various methods of alternative moorings. The Marineflex system is similar to the SeaFlex system that holds our pontoons in position at Kellett Island. It consists of a series of elasticated rubber bands located between two yokes. One yoke is fitted to the sinker and the other to the mooring buoy. The strength of the Marineflex rubber keeps the complete mooring tight when the water is low. However when the tide rises the Marineflex stretches out to the depth of the water and During typhoon Mangkhut the storm surge was 2.3m.Therefore, we have to have a length of chain that is at least 6m longer that the water depth to leave an allowance for another massive storm surge. As some of our moorings are in 4m of water this means that at extreme low tide there is 4 to 5m of chain dragging along the seabed as the boat moves with the tide and wind. If the NE monsoon is blowing hard the chain will stretch out giving a large swinging circle, especially at low water. It is this swinging circle that dictates the distance allowed between moorings by the MarDep. Regretfully the actual formula that MarDep use to calculate the distance between boats is a state secret that we are not allowed to know.

Long chains dragging on the seabed are bad for the environment as any underwater life will be destroyed by the chain when it scrapes over the seabed as the boat swings. This area of chain also wears away when in contact with the sandy bottom reducing the diameter of the chain.

thereby keeps the buoy close to the position of the sinker. If the wind blows stronger then the Marineflex will expand to allow the boat to move with the wind and the tide together. This results in the boat swinging circle being reduced.

To monitor the position of the boat, which will be Kellett VII in our test at Shelter Cove, we will be fitting an instrumentation system which will record 24/7 the wind speed and direction, boats heading, speed of the boat moving in the water, strain on the mooring ropes, depth and GPS position of the boat. The information will be recorded in a cloud through a SailMon processor, which will collate the instrumentation data, and transmit it to shore through a router. On completion of the trial the instrumentation, which will be “borrowed” from the fourth Seaward committee boat, will be returned and the SailMon unit will be transferred to the Kellett VIII so as to be able to keep track of the conditions on board our committee boat when racing.

With the data we hope to be able to convince the MarDep to allow us to put longer boats on our existing moorings.