Don't miss out on new posts and newsletters, Follow Us by Email

Friday, 20 July 2018

Helpful Sail Calculations

Area conversion

Square Feet to Square Metres
Square Feet X 0.09290304 = Square Metres

Square Metres to Square Feet
Square Meters / 0.09290304 = Square Feet
---------------------------------------------
Working out sheet loading
Calculating the load on a sheet :
SL in Kg = SA x V² X 0.02104
SL = Load on the sheet (Kg)
SA = Sail area of the Jib (m²)
V = Wind speed in knots squared (Knots²)

Sail area of the Jib (m²) x (Wind Speed in knots²)  X 0.02104

The load of a sheet for a jib - sized 20m² is about 515.5kg with a wind strength of about 35 knots.
With a 30m² and a wind of 20 knots this would be 252kg.

Of course the accuracy of these values is limited, however if you need to buy sheet or a turning block you have a ball park figure to work with. I sourced the formula when I need a block for my spinnaker halyard, while not a sheet it is subject to the same forces.

For those who work in old money: feet pounds etc
Load in Pounds:

SL in lb = SA x V² X 0.00431
SL = Load on the sheet (lb)
SA = Sail area of the Jib (Ft²)
V = Wind speed in knots (Kn)

Sail area of the Jib (ft²) x (Wind Speed in knots²)  X 0.00431

---------------------------------------------------------------------------------
Choosing the right size Asymmetrical Spinnaker

The luff length is the important measurement when choosing the right size Asymmetrical Spinnaker. This is the info the sail maker who made our sail gave us.  For a reasonably accurate measurement, raise a tape measure up using the spinnaker halyard until it’s at the full hoist position and measure to where you would like the tack of the spinnaker to be. The luff length of the spinnaker should be the measurement taken earlier x 1.03 (giving or taking 300mm either way).  For example, if your measurement is 16.00 Mtrs then the luff should be 16.00 x 1.03 = 16.42 +/- 0.30 or between 16.12 and 16.72.  When ordering a sock you don’t require the full length 1 - 1.5 metres short of the Luff length is ok as we have found.


Matilda with our small Cruising Spinnaker up off Bali.


No comments:

Post a comment