Aero benefits of a straight forestay

this is true, whether a rod or cable, if it could be made perfectly straight there would be an infinite amount of tension in the forestay. That is catenary theory, and it works that way in practice too.

You are really talking about a taught or slack forstay, and if there is any differnece. In a perfect world with sails optimized for both I suspect there will be little difference, the tighter one would be slight better.

In the real world a forstay that flops around in rough conditions will compromise the sail shape no matter how well designed for a constant tension.

therefore a tighter forstay would be more efficient in terms of aerodyanmic conditions when there is the possibility of the forestay whipping and waving due to air/water turbulence.

This is true in all sailing conditions, maintaining the best sail shape, camber, twist and angle of attack, is always part of the struggle. having things tight rather than loose and sloppy will always yield better performance. That is why a racing sail cost much more than a cruising sail, more effort and materials go into mainting the shape.

This should answer the question, a rig that keeps it shape will perform better. That means a tight (and straighter) forestay.

 

Tight rigs are definitely NOT always faster, as proven by some of the most competitive classes around.

Slacker forestays are faster in light winds, generally, because they allow the headsail to take up a deeper, more draft-forward shape. That's why classes from 420s to Etchells and J/24s to Farr 40s etc ease the diagonals, lowers and/or adjust chocks in light winds; it allows the mast to bend to the correct shape for the main while also allowing the forestay to sag. For the same reason, rigs with in-line spreaders and runners ease runner tension in light airs.

So tight is not fast, and loose is not fast - what is fast is the correct tension for the wind at the time.

 

If you are around sailboat racing long enough you will see cycles in this philosophy that repeats ever so often.

"Slacker forestays are faster in light winds, generally, because they allow the headsail to take up a deeper, more draft-forward shape."

Now, that is a new one for me since I have always found that a loose luff moves draft in a sail aft, not forward. Loose luff = more draft, sure, but forward?

 

The finest example of that would be a spinnaker I imagine, but not too useful close to the wind . . .

 

Depends some of the AR of the sail...

 

now you are talking about a different issue as I understand it.

sails have to be adjustable for varying conditions, which is what you are discussing with regards to light winds/spinnakers, etc. But if you were to optimized the sail for a tight forestay you can achieve the same, or better shape for light winds, it just will be not a good shape for high wind conditions. so you either need to change sails or have some other means to reduce the camber. But it does not mean that a slack forestay is "best" for light winds, it only means that is how that particular sail maker chose to make the same sail perform well for light winds and heavy winds (i.e. use forestay tension to change the camber of the sail)

 

We seem to be talking at cross purposes. If you had an infinite number of headsails, which could be changed in an infinitely short time with no need for anyone to move to the bow or adjust anything while the sails were being changed, then "having things tight rather than loose and sloppy will always yield better performance" perhaps.

Reality is different. In many well-sailed boats, you may ease rig tension in each lull or when punching through each larger set of waves to ensure that the forestay is "soft" (i.e. sagged) enough to maintain proper headsail depth.

Rigid stays may not always be faster even if sails are cut correctly for them. When a boat is bouncing in a slop a looser rig seems to act as a shock absorber and shake the sails less. One of the reasons the early '80s experiments with Mylar spinnakers were abandoned was that the "cloth" was too rigid and therefore the sail shook too much.

And of course in most boats it's a moot point, because adjusting rig tension is not just about getting the forestay tension right - it's about getting the mast bend right as well, and in a swept-spreader fractional rig the two are inter-related.

 

Yep, there are circles in philosophy, normally driven by changing technology.

Sagging the forestay and loosening the luff are of course different things some of the time. Yes, loosening the luff will allow draft to move aft (esp in strong winds) but sagging the stay is different.

There's a good article at http://www.sailingworld.com/experts/sag-for-speed

 

Thats what Ive always learned.

Mast Tuning Aft swept spreaders ,that genearte headstay tension, has always been a mystery to me .

I read in Seahorse mag this month that for the next Volvo rig , the mastmaker, designer and sailmakers are still trying to figure out the optimum sag for the boat.

 

> When a boat is bouncing in a slop a looser rig seems to act as a shock absorber and shake the sails less.

Yet another variable - the Young's modulus of the stays.

 

One of the problems I see here is how the type of boat and style of sailing plays a role. Racing -involves all kinds of tricks of the trade that cruisers just don't bother with. Racer Cruisers are caught in limbo in that they are the worst/best of both worlds. Heavy Cruiser/ quazi motorsailers as a rule just want a rig thats strong, fairley efficient and simple.It is in this latter catagory that I fall -- My rigs have always been built with an oversize/strength tapered mast section, Oversize /strength upper stay wire, spreaders and chainplates. Oversize forestay wire and full double backstays. My backstay tightener is just the simple old KISS pully and rope system that pulls the two backstays together as it is sheeted downward. Thru experimenting (watching the log) I have a light, medium and heavy air mark on the tightener, Very basic, very simple, and seems to work just fine.

 

Race...cruise...its makes no difference.

In the end you must inform the sailmaker of what haedstay sag.. or what mast bend..you normally sail with.

If you dont then you spend good money and receive a bog standard sail that is not suited to your mast or sailing style .

To the original poster. Dont get distracted with the theoretical correct headstay sag...better to record..with pictures..your actual sag, and your actual mast bend.. then instruct the sailmaker to cut the sail to fit.

My experience is that I sail ...cruise...with a soft headstay...great sag.

 

My reference guru on sails/running rigging, Wallace Ross (Sailpower) states (page 38) that any sag in the forestay has a negative effect on the shape of the sail and it is important in order to offset this the sailmaker take it into account. Logic tells me one should design the standing rigging to minimize sag and have some means of adjustment to keep it within the design specs of the sail. In the case of cruising boats mast sections are generally heavy and stiff with limited top half bendability. (why I like some tapering in the top section) I would think initally if you owned or planned a vessel in this class you would aim for a rig with min. sag or at least sag that could be easily offset when the sailing condition requires. Keep in mind I am referring to a cruiser. In the case of a cruiser/racer or a racer one can pratically use their masts as spring poles for rabbit hunting. Possibly I am old school here but this is why I think it important the type of boat and operating conditions have to be considered if one intends to make use of mast bend to control or offset forestay sag. I.E. In this case I don't think,one size fits all.

 

Sailpower must be close to thirty five years old and there has been an enormous amount of development in sailcloth since then. Even in the '70s, though, racing boats had hydraulic backstays so that they could ease the backstay in light winds to allow the forestay to sag.

With all respect to Ross, if he is right then just about every top sailmaker today is wrong. I was lucky enough to sail the (then) latest Farr 52 with a sailmaker who had won the AC on the water and as head of the design team as well as winning the Farr 40 worlds. He thought correct control of forestay sag was so important that he had a chart that showed different sidestay tensions for each 2 knots variation in windstrength. He got on board the 52, handed the AC and Volvo vets on board his rig tension chart, and within quarter of a second there were 20 guys aloft each twirling a turnbuckle, all to get the right combination of forestay sag and mast bend.

The most popular (or so 'tis claimed" International keelboat class of all, the Flying 15, eases rig tension by about 1/4 to 1/8 in strong winds, sacrificing a straight forestay in order to get more mast bend.

As you said, of course, this does vary a lot from boat to boat.

 

Agree 100% and Ross does go into more detail on the subject. My point was heavy cruising boats are seldom if ever designed and built with the sag/sail control systems you refer to. Most of us are just happy the dam thing can even go to windward, ah but we do love our space and comforth and especially her heavy displacement in a blow. Something to be said for a 53 Buick