Tuesday 10 January 2023

Anchoring To Reduce the Roll at Anchor

 Control rolling at anchor, Through  trial and error we worked through several ideas others gave us and these are the ones we found we used the most often.

You have dropped anchor in a snug harbour protected from the swell behind a headland. Later the wind picks up and the swell starts building outside the anchorage. The wind continues building from the same direction and before you know it the swell starts to wrap around the end of the point coming beam on creating that nasty roll that not only can keep you awake but it may cause you to spill your drink. 

Pivot point and with the ballast below and a mast above once the swing starts it can continue 

The beam on wave motion may be amplified if you have a keel boat due to the pendulum effect of mast and ballast. I have been told this rolling is not as noticeable on a cat, however having seen first hand the effects on several cats when anchored on the north end of Great Keppel during a well developed south easterly, clearly this beam on swell effects us all. 

Swell wrapping around a headland

We have been in anchorages where we have almost been thrown out of bed thanks to the waves wrapping around the headland or bouncing off the other side of the anchorage before sweeping back, making us roll uncontrollably.

So what are your options? Here are a couple of tried and tested remedies we put to good use over the last couple of years. 

Once the roll has set in and being in bed is something akin to trying to ride a mechanical bull. A quick remedy we have found is to sleep across the bunk, this can get a bit uncomfortable depending on how wide your bunks are. The next method is to make a nest on the cabin sole. There is no need to go up on deck in the wind and rain or deploy more ground tackle. We have found a sweet spot on the cabin sole aft of the galley partly under the companion way steps on the centre line.  We put our cockpit cushions on the floor and along with blankets and pillows slip into this bed and get a reasonable night sleep. 

People have commented on why we sleep on the floor and have asked why we don’t use our sea berth with lee cloths. We have found that we don’t get the same level of comfort in the sea berth; it appears the sweet spot is the roll pivot point on the centreline. If you’re a cat owner this option may mean you need to move up from the hulls into the centre of the vessel, the draw back is you may find the roll pivot point is lower than the floor.  However it will stop you falling out of bed, once you’re on the floor your not going to fall any further.

Set a stern Anchor 

Setting a stern anchor

An old favourite method we have used is to set a stern anchor to bring the bow into the swell. This has mixed results, it can work well, however a change of tide can see a change in the angle to the swell and before long you may find your rolling again, adjustment of the stern anchor rode can make a change for the better, or perhaps you may find buoying the stern rode and dropping it to retrieve it in the morning is the answer.   We have found that setting a stern anchor is some thing that’s best done earlier in the day. We use a light weight anchor at the stern and set it using the tender, but there are lots of options to deploy a stern anchor you just need to find what works for you. 

Change the angle of vessel by adjusting the geometry of the anchor rode

The second method of bringing the bow into the swell is to change the angle of the boat by changing the geometry of the set anchor rode. This is very similar as used in laying too with a sea anchor.  A couple of things are needed for this to work; you need a good breeze to apply tension to the bridle setup, so if the anchorage shields you from the wind this may not work. Of course tide and current can also be a major factor in whether this will work or fail. We had anchored where the current was strong enough that we were not able to get the boat to ride into the swell. 

Breeze and swell angles don't have to be at right angles smaller angles work well

1. Using a spring line, the one you would normally use when tied to the dock.  Ideally it will be little longer than the length of the boat. Using a rolling hitch or shackle if using chain, tie one end to the anchor rode just at the front of the boat. 

Springer line from the anchor rode to the stern cleat or a winch 

2. Let out some anchor rode up to about half the length of the boat or even a little more and tie it off (use your usual Springer if using chain), experiment with length here.  Now take the other end of the (spring) line back to a winch on the weather side of the boat.

Deploy more anchor rode or start to winch in the springer

3. Winch in your (spring) line until your bow is pointing into the annoying swell.  Crikey, the roll is greatly reduced. Now things have settled down doubling the rum ration shouldn’t result in spilt drinks. Now that you have set up the boat so you can sleep comfortably, give yourself piece of mind set the anchor watch alarm on your GPS or phone app.  To make the safety circle real, keep in mind 0.01 nautical miles is 18.52 metres so dialing in .1 nm (185.2 metres) isn’t going to do you much good. 

The angle of attack with the swell should greatly reduce the roll. 

As I said earlier there are certainly times when the current is strong enough that none of this works, as we have found on several occasions. We have either put up with it or moved on, sometimes we felt is was better to be out in it with the sails up than rolling around like we were in a barrel going down a hill.  

Fair winds,
Matilda Crew.

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