When it is not calm and you need to find protection for the elements for the night, choosing an anchorage becomes a very important skill! Last time we talked about dropping the hook in the middle of nowhere in calm weather where the only important factor is tide/current and bottom conditions. With stronger conditions, all the other important factors come into play.
When choosing an anchorage in strong weather, all of these points become a priority:
- Length of time anchored
- Protection from wind
- Protection from waves
- Tides and currents
- Bottom composition
Now imagine you are sailing along in a stiff breeze and the sun is starting to set. In the distance, you see huge storm clouds coming right at you and you know you are going to be in for a rough night! Finding protection is crucial for anchoring because you will not get any sleep otherwise!
Length of time anchored: At least the entire night, maybe into the next day because you won't want to pull up the anchor to start sailing during a gale!
Protection from wind: Wind load on the rigging and spars is almost inevitable. The total windage from the hull, spars, and sails will try to pull the anchor out of the bottom and send it dragging along the bottom during the storm. To hide from the wind, you need to stay in the wind shadow of something else. To stay in a wind shadow, you need to know which way the winds will be coming from. If the wind will most likely blow from only one direction, you only need to have one feature causing a wind shadow. If it could be blowing from more than one direction, you need to have more features providing protection from different wind directions.
Wind shadows cast by an object are twice as high as the object and extend out a distance of 20 times the height of the object. That means that if the object is 100 feet high, the wind shadow is going to be 200 feet high and 2000 feet long! This means that hiding behind a small island can actually provide protection from the wind.
This is one example where we were looking for an anchorage to protect us from a strong wind. It was slack low tide when we arrived, so the current was going to be flowing North overnight (in the same direction as the wind) and slack high tide by morning when we would be leaving. We draw 6.5 feet, dropped anchor in 7 feet of water and slid back into 9 feet of water while leaving out 180 feet of chain. If we were to drag, we would slide into deeper water and I wanted it to set again! The bottom was hard where we dropped the hook and the deeper water was softer. We did drag anchor that night, but it quickly re-set.
The small island provided protection from the wind (we know this to be true because the wind died down as we arrived in the lee). The shallow water around the island also broke up the seas, providing us a less windy and less wavy nights sleep.
While it might seem that we were anchored in the middle, we were actually very protected from the weather, hiding in the lee of the island.
Another important point to remember when anchoring during strong weather is the shock loads on your anchor and the scope of the rode. A 2:1 scope will pull more vertically as compared to a 10:1 scope which pulls more horizontally. When the boat rides up on a wave, the boat pulls back and this puts additional stress on the anchor, trying to pull it out of the sea bed. With a nylon rode, these forces are somewhat dissipated by the stretch of the rope. Chain is much stronger than rope, but offers zero stretch! This is where having extra chain out helps to provide some form of elasticity because of the chains catenary.
Catenary is the sag that the chain will have as it comes out of the bow roller and leads to the anchor. The weight of the chain will help form catenary and act as the absorbing force. When the boat is pushed back, the force is translated into a lifting motion on the chain. The heavier the chain, the more force it will take to lift the chain, translating into more shock protection for you.
When we anchored in the lee of the island, we had 180 feet of chain out in 7 feet of water with an effective depth of 11 feet (6 foot bow roller) giving us a scope of 16:1. Overkill, I know! I did this so that I would have enough scope if I dragged into deeper water and to give more catenary in the chain. While the lee of the island provided some protection, I believe the mast was in the wind because the boat was being pushed back quite strongly. With 180 feet of chain, the rode exited the bow nearly vertically. This means that we had plenty of chain that would need to be pulled off the bottom before the anchor would be subjected to a shock load. In addition to this, a nylon snubber was tied to the chain to give even more elasticity and to protect the ground tackle gear on the deck from shock loads.
In this example, we were sailing North when we needed to anchor for the night. The wind was from the South, so anchoring right next to the land provided us protection from the wind and the very short fetch between shore and boat provided us protection from the seas.
This river also experienced very strong currents when the tides would change. The current runs parallel to the shoreline, so I wasn't worried about being beached by the current. Current strengths are depth dependent, so staying in the shallower water was very beneficial. The difference was rather dramatic, 1.4 knot current in the shallows along the waters edge and 5 knots in the deeper water. The additional current adds strain to the anchor which adds even more benefit to hiding in the shallows in the lee of the land, hidden from wind and currents in completely open water right by the bay!
Both of those examples involve short, over night anchoring to escape some short term weather. What if you are hiding from a large storm and don't know which direction the winds will blow from or how long you will need to stay hidden? You will need much more protection!
In this occasion, a series of large thunderstorms were going to be coming through the next day and we needed to seek more protected waters. We didn't know what direction the winds would be coming from, so we searched the chart book for a protected anchorage. What I was looking for was high land encircling the water on all sides to protect us from the wind.
While looking over the charts I came across St. Mary's River. The topography surrounding the river is composed of high cliffs providing plenty of protection from the wind. Since the wind was blocked, there were no seas to speak of at anchor!
When we arrived in the anchorage, the wind was starting to pick up. We were expecting to work our way up the river into one of the creeks to find protection from the wind, but as soon as we entered the mouth of the river, the winds died and the sails dropped. We ghosted along and dropped the anchor in the sandy bottom and settled in for the night in preparation for the horrible storms that were to come.
The clouds passed overhead, but the winds and waves never came. This photo was taken during the time when heavy thunderstorms were passing over.
We had a very peaceful nights rest anchored in the protection of the river, where the high lands (even far away in the distance) protected us from wind in all directions. By staying in the mouth of the river, we found it easier to get out of the river and into the wind when it was time to leave. We left the safety of the river with the ebbing tide, once outside of the wind shadow, the winds returned and we were on our way once again.