Windage

Some facts:

MV was anchored in light airs for 2 to 3 days prior to the wind increasing to F6/7 from the east; The F6/7 winds were forecast up to 4 days previously; MV ran onto the rocks after (supposedly) having dragged on her anchor; MV has a 400 kg Bruce for normal anchoring and a 600 kg storm Bruce. It is assumed that 400 kg anchor was deployed. (Editor’s Note: See The Sequel)

Some unknowns:

What depth was MV anchored in; What the scope deployed was; What the bottom holding was.

Other unknowns:

What size and weight of chain is used on MV (38 Ø @ 32 kg/m is assumed)

Calculations:

Mirabella’s windage does NOT conform to that of a Bermuda rigged cutter in that the cross sectional area presented to the wind is approx 125m², with about 45% of that being mast and rigging. (In a ‘small’ cutter ie with say a LOA of up to 40 m the ratio tends to be Hull = 3 to mast, rigging and furled sails = 1)

With the extraordinary height of the mast, windage conforms to the height factor formula, so that what may appear as F6 at deck level can be blowing at over 45 knots at the top of the mast. This significantly affects the overall windage. For a F6/7 fairly gusty wind, the windage on Mirabella would be in the order of 210,000 Newtons (ie ~21 tonnes force), when presented bow onto the wind. (Veering & yawing have been discounted as Mirabella’s moment of inertia would preclude this looking at the site photographs, which show a gusty wind of very short fetch.)

The anchor is a 400 kg Bruce pattern. It has a theoretical holding power in good holding of 960,000 Newtons (ie ~96 tonnes) based on the formula H = KM^1.4 where K is 5 for very poor holding to 22 for very good holding.

Photos: Mast by Tom Walsh. Anchors by Manson Marine

A Bruce pattern anchor if set properly and with say anything greater that 10 : 1 scope cannot pull out.

If the scope is insufficient and the windage on the boat increases the force on anchor chain is such that it pulls the stock to an angle of say 12º to 14º; then the anchor will begin to pull out when the horizontal force on the bow roller exceeds 180,000 Newtons (ie 18 tonnes)   Equally if the anchor is just lying on the bottom it will drag if a) the scope is insufficient or b) it is the wrong type of anchor for the bottom.

So what does this mean for Mirabella V?

First of all, the above calculations are based on some fact and some assumptions. Nothing should be read into the numbers as they cannot be cross referenced to the designer’s own calculations. However, what the numbers do show are either insufficient scope, or that the anchor was not set and was just lying loose on a hard bottom so that when the forecast wind sprang up the boat dragged both chain and anchor onto the rocks. If the bottom was suitable for a 400 kg Bruce pattern anchor then the scope should have been at least 7 : 1 for an all chain rode to safely ride out a F6/7.

Another possibility is that a Bruce pattern anchor is not the right type of anchor for the bottom in question.

Nigel Charlesworth

[BYM News Home] [Mirabella V] [Mirabella on the Rocks] [Windage] [Consequences] [More Questions] [Sequel] [Some Facts] [Rumour] [IOM Report] [Comment]