The Square Head Mainsail–on to advantage side of the equation:
The previous few essays have focused on the limiting issues surrounding full length battens including, for the vast majority of the normal boats that most of us sail, the backstay.
Briefly I have proposed that:
- There is not really solid empirical evidence that one sail with FLB is faster than the SAME sail with leech battens on the same class of boat and assuming that both boats are prepared to be as identical as possible.
- The Cost of FLB may outweigh the Value a lot of the time
- I will get to the “sail handling” aspect of FLB further along in the series. This area is in fact one of the areas that does offer increased value for the owner via ease of handling the sail: Hoisting lowering reefing.
For now I am going to concentrate on the most obvious advantage and a much lesser known aspect of full battens in general and the Square Top mainsail in particular.
In my experience virtually all discussions with sailors regarding sail shape is one sided, in that it revolves around the sail’s shape and so the implied concept is aerodynamic lift. I cannot immediately recall any discussion where the other side of the lift equation is even mentioned let alone discussed: DRAG.
Drag is everywhere on a sail boat:
The actual hull topsides
The sail’s surface
The width of the roller furler or furled headsail when sailing with a staysail
The furling drum
Rails, life lines stanchions
The dinghy stowed on the bow or in Davits
Life raft on the cabin top
People standing up- This is why most good race boats have the guys all sitting together in breeze or laying low in light air.
Radar either tower astern or on mast
Radar reflectors…..You get the picture.
What is missing from this list?
It is one of the reasons why the square head sail has emerged over the past few years.
We have discussed the usual limiting factor for the size of the roach on most boats is the backstay, closely followed by adherence to a handicap racing rule.
Enter the “open” class boats, in particular the solo offshore race boats. This cohort encompasses the Mini 650 class, the older open 40’s and the much more successful, as a class, Class 40’s, the open 50 and 60 foot mono-hulls and their multi-hulled cousins and in some parts of the world open class skiffs like the Aussie 18 footers.
None of these classes (I am not 100% certain about the 18’s) have any restrictions of sail size or shape, only number and type depending on the individual class and the race.
If Bigger (more area) is Better, so the square top sail is born.
The one element missing in this discussion so far is the mast.
The mast, the square top sail and full length battens are all interconnected.
The Twitter version:
The mast is drag
The square top sail minimizes that drag
The ST cannot work without full length battens
ESPECIALLY in this case, the FLB need a low friction track because of the great compression generated by the ST sail.
The NPR version:
Because it is sticking up in the air, the mast is 100% drag, at least for the purposes of this essay-Ignore the wing masts and wing sails please.
Over the span (the fore and aft width of the sail-the girth.) the drag from the mast is reduced because the air is smoothed out by flowing across the sail.
As the sail ascends into the air on 99% of boats it gets narrower, again almost universally due to tradition as manifest in the backstay.
At a point that varies for all sorts of reasons this reduction in drag is reduced. The drag from the mast starts to increase.
The point is that usually within a few feet of the top of the spar and for a rule of thumb it can be where the girth of the sail is less than about 4 or 5 times the local for and aft length of the mast, the amount of drag over comes the amount of lift generated by the sail.
For instance, let’s say the mast is 6 inches fore and aft. 4 or 5 times 6 inches is 24-30 inches. So in this example the drag starts to increase, dramatically, at that point on the sail where the girth is less than 24-30 inches wide fore and aft because there is not enough girth in the sail to smooth out the turbulence created by the wind hitting the mast.
Enter the Square Head sail. This sail profile minimizes the drag from the spar as well as being much more sail area.
It is functionally impractical for any boat with a backstay. Unless of course you want to lower the mainsail every time you tack which may sound like a pain but again find out what the customer is trying to do with his boat sail goals plans etc. I did do two offshore cruising boat sails that were exactly that big roach that would not clear the standing backstay. In one case, the image below, it was a bit difficult to get through in light air although he reefed in about 14 knots of wind, so the roach was easier to deal with the first reef in. I did another offshore cruising mainsail where the owner specified that he would sail with the first reef in if lots of tacking was going to be involved. The roach in this sail was even more aggressive than the first one.
For boats with such sails, very large roach OR square head, enter the twin topmast running backstays, generally referred to as “the runners”.
As the name implies, they are running backstays that attach to the masthead and are adjusted by a two or three part purchase led to a winch.
Upon contemplation it will be seen that this is not something to be undertaken lightly. Many things need to be contemplated, not the least of which, in no particular order are:
Boat & Deck hardware lay out
Standing rigging configuration
The degree of sweep of the spreaders
The skill of the operators and
Their willingness to put up with this added task when tacking
All these factors contribute to the reason why most “cruising” boats do not have square head sails.
Next up running backstays, batten compression and hardware for the battens.