Small fathead mainsails, where the roach of the sail is increased due to a considerably wider head of the sail are becoming a new trend on the race course and they are proving to be a winner. Getting extra sail area on your boat can be really hard without taking a big rating hit, but increasing your crane size will give you that extra bit needed without too much effect on your rating.
Some yacht designers are already doing this in their new designs like all the new J/Boat: J/70, J/88, J/99 etc. These are a great example of a boat getting extra area without having a very tall mast. You get the same amount of power, while also reducing tipping weight and decreasing drag aloft.
The extra area gained is only from the top quarter of the mainsail, in the illustration to the right you can see the pink line just above the draft stripe. This means that the only rating change is on the MUW measurement which only moves out a tiny bit. Your headboard increases from 0.15 m out to 0.45 m or larger depending on the crane size you can add. This increase in sail area at the top of the sail is almost free from a rating perspective.
The extra area makes a marginal speed increase upwind, but it does reduce drag a fair amount. In the illustration to the right, you can see the predicted drag off the pin-head main that is considerable compared to a slightly fatter head main. Importantly, a wider head helps the airflow stick to the top of the sail longer creating less drag and generating more lift.
Wider head mainsails really come into their own downwind and reaching. The draft at the head of the sail will move slightly aft as you twist off the sail. A wider head also allows you to open and close the leech of the main quickly—letting you power up the boat or dump more quickly as the sail twists more. See the J/88 with sail number 007 to the right.
Normally, increasing the crane size from 0.15 of a meter to 0.4 is a big structural job. You will need to check with your mast builder to see if it can be done. The primary area to consider is the mast’s bend characteristics, which will change, making the luff curve of the main slightly different. On a fractional rig, a longer crane applies more load on the top of the mast and bends it more above the forestay. You will need to confirm your forestay can take the additional load. The side plates of the crane will move further down the sidewall of the mast to try and take this extra load, but it’s best to check with your mast builder to ensure you don’t break the crane off.
With a fathead mainsail, it’s important to add a 2:1 halyard. This allows the head of the sail to articulate and not point load on the sheave. It also balances the main going from a beat to a run as the very top corner of the mainsail luff tape is pulled away from the mast on a beat. The 2:1 halyard will take this load and balance it out so there is no point loading. The 2:1 halyard will pull the head of the main into the mast at all times. The mainsail designer will move the head ring aft to allow for this.
Adding extra area to the top of your mainsail will give you extra power, less drag, and a more efficient mainsail. However, it’s totally dependent on your mast builder saying it is possible and practical. Trust me, it works really well on most modern yachts.