Calculations of Environmental Forces at Anchor

I posted this on the thread about the possible rupture of a pipeline by an anchor.

A charterer required "Critical Wind Velocity " to be calculated anytime we anchored. We used a Excel spreadsheet to make the calculations each time. I found it to be very useful.

Here are some sample calculations

I’ve several times put out extra chain on small vessels and had the dragging stopped but on the larger ships not so much.

The formula used to calculate the amount of force from the wind is relatively simple. The holding power of the anchor and chain is a little more work.

Not included are forces from swell or current.

From the table the Critical Wind Speed for a PCC with a full load is shown as 12.8 m/sec which is about 24 kts. In my experience the PCTC would start to jack around (also called horsing) at about that wind speed but not start dragging.

Our calculations critical speed usually came up to around 35 kts. The ship would start to pull the anchor back at around 35 or 40 knots of wind.

This is from the P&I bulletin above:

Here is a screen shot from the INavX / Ipad I was using:

Dragged about 1/4 miles than was OK the rest of the night.

Off Brixham - avoiding having to anchor in ports exposed to stronger north winds off the North Sea while awaiting a berth.

Question - how can you calculate chain length vs holding power without knowing water depth? Or is there some sort of “standard depth” used for these calculations?

I moved your post here.

The formula uses the height of the hawsepipe from the sea bottom in meters “y” in the diagram.

Ha here being holding power of the anchor and Hc being the holding power of the chain.

Ahh - that looks more familiar :slight_smile: Lots of recreational anchoring advice neglects the “hawsepipe height” term, incidentally. We invested in a new anchor this year so I’m very refreshed on the subject :slight_smile: Thanks!