Tidal mishaps and risky bottoms

潮汐災害與危險的海底

by Stu Pryke

The late Maurice Griffiths in Magic of the Swatchways once wrote, "Well, what a great old tide,” as I slapped the mate on the back and waited for the flood to send us off the bank.

"It is never the right time to hit any shoal in waters like this Bob,” with the mate slinging the lead again to see how the increasing inches were beginning to work their magic.

In Hong Kong, we are blessed with little tide compared to waters such as the Bristol Channel, The St Lawrence Seaway or even The Mersey, though, in periods of spring tides, we can experience ones of over 1.5 metres. In the whole pool of tidal science, Hong Kong remains at the shallow end but that does not mean we should neglect the impact of its local extremes.

It was only recently that a spring high tide caused the HHYC dinghy dock to separate and the boatyard outlet’s sumps to flood. These are expensive events however, the contingency measures for a typhoon surge and an accompanying low-pressure system could cause as much as a five-metre swell, introducing another set of parameters—ones that see major breakages of both dock-ware and boats alike.

But, what of these low tides—those that Maurice Griffiths mentioned above?

Just off the South Dock by barely 15 metres lies a rising seabed. Thankfully, it is an area of mud, light silt and shale. Yet, on a spring and neap low tide, it can present a keel strike problem for yachts with a draught of over two metres. Boats have been stranded there and sat incongruously for a rising tide to save the day. Certainly, a Sunday spoiler if the crew were on their way to Yau Ley or Cascade!

Soft groundings on the ‘Hebe Hump’—red faces aside, have generally been lowkey events and light on damage. Below are a few tips that may just avoid a low tide grounding for those boats in and around the eastern HHYC marina entrance and those setting off from the south dock for the red and green channel marker cans.

First, take time to research tidal conditions. Reference can be made via the E Sea App, Government Tide tables, The HK Observatory or simply the watermarks around the Club’s slipways and steps. For example, water over the sampan steps at the end of the garden bar indicates high water and a good water altitude over most seabed threats around the Hebe area.

已故 Maurice Griffiths 在《斯沃琪威的魔法》(Magic of the Swatchways) 中曾寫道：「『多麼偉大的浪潮』 說著我拍了拍大副的背，等待洪水把我們推離岸邊。

『在這樣的水中撞上淺灘真不是好時機，Bob 』大 副再次回頭看看越發高漲的潮汐如何開始施展魔 法。」

與布里斯托爾海峽、聖羅倫斯海道甚至默西河等水 域相比，香港幾乎沒有什麼潮汐，縱然在春天潮汐 期間會有超過1.5米的海浪。在整個潮汐科學範疇 中，香港仍然涉足不深，但這並不代表我們可以忽 視其對地區產生的極端影響。

直到最近，春天潮汐才導致白沙灣遊艇會的船塢分 離，令船塢出口的水槽受淹。然而這些都是代價昂 貴的事件。颱風突然來襲的應變措施以及隨之而來 的低壓區可造成如5米海浪的損毀，並增加了另一 組參數——碼頭設備和船隻的重大損毁。

但 Maurice Griffiths 所說的那些退潮又如何呢？ 就在離南邊碼頭僅15米遠的地方，有一個漸升的海 牀。幸好它是一個混集泥沙、輕淤泥和頁岩的區域。 然而，在春天潮汐和退潮時，可為吃水深度超過兩 米的遊艇帶來龍骨受撞的問題。船隻只能擱淺在那 裏，慌張地等待漲潮拯救自己。這肯定破壞美好的 周日，倘若船員們是在去有利或滘西灣的路上！

在白沙灣拱頂上柔軟的地基——說來慚愧，通常都 是輕微的事故，損壞情況亦比較輕微。以下貼士助 您避免在東面遊艇會碼頭附近出入的船隻，以及由 南面碼頭駛往紅綠兩色船塢的船隻擱淺。

首先，花時間研究潮汐狀況。您可瀏覽E Sea App、 政府潮汐表、香港天文台，又或留意遊艇會船排及 梯級周圍的水印。例如，海水淹蓋花園酒吧後方舢 舨梯級的話，代表水位高漲，在白沙灣一帶的大部 分有威脅性的海牀維持良好水深。

Second, ensure that Navionics and the depth sounder are on before slipping moorings. Be aware that a common trait amongst Skippers is to leave the navigation system off until beyond the swing moorings or at the end of the navigable channel abeam Trio Beach. Setting the depth alarm to the zero nulls of the vessel's draught (and a wee bit more) allows some contingency. Typically, at speeds of less than five knots, (the maximum speed in the ‘Hebe basin’ until after Trio,) the signal cone of the average commercial echo sounder will be a boat length ahead of the vessel. Enough time to react if the alarm sounds.

Finally, take time to review paper charts of any area that you may be uncertain of. Ensure you know the value of the contours, (metres, feet, fathoms or even cables), where the shape of the seabed changes under your intended track, and if shortcuts and diversions present potential threats.

• Would a shorter passage south of Gin Island and North of the Cham Pai light marker be safe with a low spring tide and a 0.6 m contour? • What does that 0.6 signify? • What are those green blobs and are they relevant to you?

A safe passage means there must be no doubt that the terrain below your keel presents a threat. If it does, you need to change the route and review your next intended navigation metrics.

Illegal moorings up to the Southerly Cardinal and the light (3 Fl, six seconds) at Man Tau Tsui, (or the Trio Beach Headland Light), have caused the channel— especially at night and in dense traffic, to be restricted enough to push boats underway to use the gap closer to Sha Tsui and the Leadership Training Camp. Here, the water drops to 1.1 m (mean low) and though the ground or benthos is mostly shale and mud, that contour is not a constant and is not all soft ground.

第二，在滑動繫泊裝置之前，請先確保已打開導航儀 和測深儀。請注意，不少舵手的共同特點是在離開繫 泊處前，或在前往三星灣的通航海峽的盡頭將導航 系統暫時關閉。將深度警報設置為船舶吃水深度的 零位（或再多一點點）可應對一些意外情況出現。一 般來說，時速小於5節，（白沙灣盆地至離開三星灣的 最高速度）一般商用迴聲測深儀的信號錐將標示在 船隻前方一艘船長度距離。如果警報響起，亦有足夠 時間反應。

最後，花時間複習任何您不確定的海域圖表。確保您 了解等高線的數值（米、英呎、英尋甚至電纜），顯示 在您的預期航道下海床的形狀的變化，以及抄捷徑和 改道時會否存在潛在威脅。

• 在低潮和 0.6 m 等高線的情況下，在吊鐘洲以南 和杉排燈塔以北的短道是否安全？ • 那0.6代表甚麼？ • 那些綠色圓點是什麼，與您是否相關？

安全通道代表船的龍骨下面的地形毫無疑問會構成危 險。如是，您將需要更改路線並重新審視下一個擬定 的導航指標。

Southerly Cardinal 和文頭咀燈塔（3 Fl，6 秒）一帶（ 或三星灣赫蘭燈塔塔）的非法停泊的燈光已導致航 道——尤其是在夜間和擁擠的情況下——被逼至僅 能讓船隻利用靠近沙咀和領袖訓練營的間隙駛過。 在這裏，水深降到1.1米（平均低），即使地面或海底大 多是頁岩和泥沙，但這種地形並非不變的，亦非全部 為軟地。

Old concrete mooring blocks, anchors and chains litter the seabed and a keel strike in this area, even at five knots, would prove interesting to say the least. A so-called ‘hard strike’ on a fin keel could potentially create a number of problems and invariably mean a lift out to inspect the damage. This may include all or part of the following:

• Leading-edge keel damage to build material; • Keel bolt stress or separation; • Rudder damage; • Prop damage; • Keel bullet mass damage.

A hard strike at higher speed would be serious and as we know, lost keels mean inversion and loss of hulls. A 15-tonne sailing yacht at a speed of eight knots struck the Cows Tail Reef on Shelter Island. The engine mounts separated, and the impact also caused the water and fuel tanks to move forward a matter of inches. Irrespective of the damage to its cutaway long keel, the boat was an insurance write off. Imagine the damage to a boat and crew if an impact was made at 18 knots or more?

Take a look at the water around Lak Lei Tsai, Trio Island and the rocks of Pak Pai on the western side of Port Shelter’s entrance. In low visibility or at night, this area is a high risk should you decide to hug that coast— without a good briefing to the crew, reference to the charts, a review of the ground and notice of tides, and would be folly enough for Maurice Griffiths to make a further comment ... and he passed away in 1997!

Know your ground, your tides, and the contingencies of your passage and remember, no matter how familiar with the area, plan your passage and sail that plan.

舊的水泥繫泊沉塊、錨和鏈條散佈在海底，在這裡 如有一首船的龍骨碰撞，哪怕只是5節的風速，後果 將十分嚴重。鰭狀龍骨受所謂的「硬擊」可以造成 許多問題，而且通常代表要抬起船隻航檢查損壞情 況。這可以包括下列全部或部分的損壞情況：

• 龍骨前緣受建築材料損壞； • 龍骨螺栓受壓或分離； • 方向舵損壞； • 配件損壞； • 龍骨前端大規模損壞。

以更高的速度進行猛烈撞擊將十分嚴重，正如我們 所知，失去龍骨意味著船身翻倒和失去船體。一艘 15噸帆船以8節的速度航行，撞上了牛尾洲的牛尾 礁。引擎支架分離，撞擊更使水箱和油箱向前移幾 吋。不論切割的長龍骨是否受損，這艘船都已被保 險註銷了。試想想，如果撞擊速度超過18節，對船和 船員造成的傷害會有多大？

你可看看於癩痢仔、三星灣及牛尾海入口西側的白 排岩石一帶的水域。在能見度低或夜間，如果您決 定前往該海岸，將有很高的風險——如您沒有向船 員們做好簡報、沒有參考圖表、沒有檢閱地形和潮 汐通知，那麼也不用 Maurice Griffiths 多說……他 於1997年去世！

了解你的地形、潮汐和航行中可能出現的偶發情 況。亦要記住，無論你對這一地區多麼熟悉，都要規 劃好你的航程。