Sailing Charter in Orange Beach, Gulf Shores, and the Gulf of Mexico

What Percentage of the Roller Furling Main Sail is Out?

By on Nov 7, 2014 in Blog | 0 comments

Have you ever wondered how much to retract a roller furling main to reduce the sail area by a specific percentage? Say, how far to leave exactly 50% working?  We can all agree this would be very important to know in a storm situation.  And obviously, if it is pulled in half the length of the foot, the remaining sail area would be much less than half of the total.  So, how can we determine how much to retract?

Let’s start with Total Sail Area = ½ base x height.  But rather than show you three pages of notes made while remembering my high school math classes, let’s jump right to the simplified formula reduced down to:

X = A ( 1 – square root of Z )

“X” equals the distance the main sail needs to be retracted.   “A” equals the length of the foot of my roller furling main.    “Z” equals the percentage of sail you want to remain working, ie. 0.25, 0.50, etc.   Amazingly, we only need to know this one sail measurement “A” to find the reduction of the sail area.

As an example, let’s say the foot of the roller furling main is 15’, and we want to know how far to pull in the drum line to leave 50% sail area:

A = 15’     Z = 50%

X = A ( 1 – square root of Z ) = 15 (1 – 0.71) = 15 (0.29) = 4.35’

You will need to pull the mainsail in 4.35’ to halve the sail area.

I think most of us will be surprised how little we need to retract the main to halve the main sail area.  We can all agree, it is certainly good information to know.  I plan to mark my roller furling drum line so I can easily see when I’ve reduced the main by 50%!

This formula will also work for conventional mainsails by changing “A” to equal the length of the luff of your main.

Please contact me at  if you are interested in seeing how I came to this mathematical conclusion.

Letting her run!

Letting her run!

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