For the readers understanding mathematics here a numerical example of why a sloop rig is favoured when overpowered.


The boat :

mast height = 100%
mainsail centre of effort = 45% up the mast.
mainsail sail area = 100%
hound fitting = 75 % up the mast

jib sail area = additional 25%
jib sail centre of effort = 1/3 of hound fitting height = 25% up the mast



Uni-rig :

heeling moment = 100% * 45% = 4500 (no % as it will have no meaning here) = say 100 %
drive = 100 %

Sloop rig (added jib) :

heeling moment = 100% * 45% + 25% * 25% = 5125 translates into 5125/4500 = 114 % = say 115 %
drive = 125 %



Now lets say you keep the jib fully power-up but depower the mainsail to correct for the extra heeling moment.

Basically you want to find X in the equation : X % * 45% + 25% * 25 % = 4500 ( = 100 %)

We find X = 86 % ; basically you need to depower the mainsail down to 86 % drive over its full length to total compensate for the added heeling moment of the jib.

But if we now add all the drive precentages together we find that :


Uni-rig drive = 100 %
Sloop rig drive = 86 % + 25 % = 111 %


We can redo this calculation for any new (overpowered) drive percentage (smaller then 100%) and each time the sloop rig will come out increasingly better.

example :

take uni-rig mainsail drive = 80 %
Heeling uni rig then becomes : 80 % * 45 % = 3600
Mainsail drive on sloop rig : X% * 45 % + 25% * 25 % = 3600 <=> X = 66 %

Total Sloop rig drive becomes : 66 % + 25 % = 91 % <=> 91%/80% = 114 % that of the uni-rig.

With each lower percentage for the uni-rig mainsail drive (more overpowered) the advantage of the sloop rig grows larger. For 50 % uni mainsail drive we find a ratio between created drive to be 61%/50% = 122 %

And there you have it. Adding a jib to an existing mainsail creates more drive then just having the mainsail even when sailing overpowered in a squall.



And with this numerical example we also have found what the minimal gains of adding a jib are over a wide range of conditions.

In light laminair winds : no net gains (Jib is suituated too close to watersurface to catch much wind)
In light but turbulant winds : 25 % additional drive
Medium winds (fully powered up) : 11 % additional drive (lowest value reached when in turbulant winds)
Strong winds (depowering mainsail) : up to 25 % additional drive


Wouter