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.
Ratings
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.
Design
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.
Mast
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.
Halyard
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.
Conclusion
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.
Can I assume that by referring to a 2:1 halyard you mean some sort of tackle aloft to increase the mechanical advantage? I can picture a halyard led to the usual sheave at the aft end of the head of the mast, then to a sheave (or block) at the aft end of the headboard, then back to a block at the head of the mast, then to the usual position at the luff (forward) end of the headboard. Is this correct? I often feel that some of the terminology used might be understood by sailmakers and riggers on the cutting edge but over the heads of older boatowners like me
Can I assume that by referring to a 2:1 halyard you mean some sort of tackle aloft to increase the mechanical advantage? I can picture a halyard led to the usual sheave at the aft end of the head of the mast, then to a sheave (or block) at the aft end of the headboard, then back to a block at the head of the mast, then to the usual position at the luff (forward) end of the headboard. Is this correct? I often feel that some of the terminology used might be understood by sailmakers and riggers on the cutting edge but over the heads of older boatowners like me
Your description is right, there is a block attached to the mainsail headboard to give you a 2:1 advantage. It’s really noticeable on the picture of the boat with the pink chute and reef in the main. The block is closer to the leading edge of the sail. Look for the white reinforcing tapes, the block is attached there and the fixed part of the halyard is pretty much vertical from there.
Your description is right, there is a block attached to the mainsail headboard to give you a 2:1 advantage. It’s really noticeable on the picture of the boat with the pink chute and reef in the main. The block is closer to the leading edge of the sail. Look for the white reinforcing tapes, the block is attached there and the fixed part of the halyard is pretty much vertical from there.
love to see a close up of the block arrangement
love to see a close up of the block arrangement