Loose-footed mainsails

[Dean Abramson]

Rather than continuing to divert Rick's thread specifically about the outhaul on his 25D, I am hoping to open up a more general discussion regarding the pros and cons of loose-footed mains.

Briefly, I just bought one, have not used it yet, and could still have slugs added.

Melissa pointed out to me that Bob Emmons broke his boom, and I take it that was thought to be related to the loads imposed by the loose-footed sail. And I found an older post where Rich implied that this was also related to having a single attachment point for the mainsheet. I could not find any description by Bob of his incident.

Bob, if you see this, could you jump in and elaborate, or direct me to more info.

I am sure that there are many others out there who are contemplating new sails; since the loose foot is very much in vogue, no doubt there are others wondering if it is a good choice.

My particular boat has three mainsheet attachment points on the boom. Does having just one, mid-boom, make for a very distinct stress point (just aft of that)? In my case, wouldn't that point just move to just aft of my aft-most attachment point?

I would very much like to see any comments related to loose-footed mains in general; and comments regarding compression loads and stresses on the boom.

Thanks,

Dean

[ricks]

With a mainsheet attachment at the end of the boom how are the stresses exerted by a loose footed main different?

Here are my thoughts. The majority of the force is still exerted between the tack and the clew and is mostly compression - in the x axis inward towards the center along the length of the boom. The force exerted by the slugs along the boom don't seem great. That may or may not seem obvious but check it out next time you are on your boat. With the mainsheet attachment on the end of the boom the slug forces would tend to pull the center of the boom and make it even more likely to stress it - once there is some flex in the center of the boom causing it to bend, the compression forces from either end are now misaligned and have force vectors in the y direction making it more likely for the boom to bend.

The theory is that loose footed mains when properly trimmed are more powerful so the total force applied is larger, so is the argument that a loose footed main generates to much power?

More questions than answers just thought I'd see if I can generate some further discussion.

[Andy Denmark]

Hi Dean,

To address the questions you raise:

1. Converting a sail designed to be loose-footed back to foot slides is not a matter of simply putting slides along the foot. The foots of the two sails are cut substantially differently. Conversely, you also cannot simply remove the slides from a slide-footed mainsail and have a decently shaped loose-footed mainsail. The camber, foot shelf, and depth of draft varies much more on a loose footed sail that the relatively flat slide-footed sail. It is this variation that makes a loose-footed sail much more versatile than a slide-footed sail.

2. With mid-boom sheeting it is always best to distribute the mainsheet load along the boom in several points rather than a single point. A single point attachment creates such a high loading that that point becomes the natural place for a stress failure. While the downward force is the same, spreading it out reduces the load by the number of attachment points used. (For example, 100 lbs of load at one spot is 100 lbs of force while distributing it between three points means each point carries only 33-1/3 lbs of force) Whether the mainsail is loose footed or slide-footed does not alter this. See #3:

3. 98% of the stresses on a triangular mainsail are at the corners. The foot slides (and the luff slides) carry scarcely any load at all. If they did the grommets in the sail where the slides attach would be reinforced like the multilayered corner patches at the head, tack, and clew. The slides between the three corners are there to simply hold the sail shape and little more. Just because there is only one slide on a loose-footed main doesn't change the loading on the boom to any significant degree.

In summary, what I'm saying is (A) that there is no significant horizontal loading change between a loose-footed and slide-footed mainsail because the major forces are resolved at the corners and not along the length, and (B) sheeting of the main, whether it be single or multiple attachment points, has no relationship to whether the sail is loose-footed or slide-footed.

Before the issue is raised, the larger (longer/stronger) single slide at the clew of a loose-footed main is to afford easier adjustment, not because it has a higher linear load but because it is subject to more side loading than a normal slide as used on a slide-footed sail. Due to the nature of the loose-footed sail (the travel of a loose footed clew is several times further than a slide-footed sail) the loose-footed sail is "baggier" when eased (increasing side load)while the load on a slide-footed sail is almost purely linear along its travel. The longer side tends not to "cock over" like a shorter, standard slide. This increased pressure is why outhaul tackles on a loose-footed main are more powerful than those on a slide-footed main. Methinks the single slide is also bigger/stronger because it has a high single point failure potential thus it needs a corrsespondingly larger safety factor.

I didn't mean to get into such a complicated explanation but I hope this helps clarify the questions you raised.

Hope this helps,
________
Volcano Digital Vaporizer

[Dean Abramson]

Rick and Andy,

Thanks so much for taking the time to write such thoughtful and informative replies.

I am looking forward to trying the loose-footed main, since the sail-shape advantage seems clear. Of course, I am pretty excited about having all new sails this season!

Your answers make me feel better about going loose-footed. Most twin-headsail CDs have loose-footed staysails on booms, and I have not heard of problems with breaking the staysail boom; of course, the loads are less, and probably the staysail boom's thickness/length ration is higher than a main boom's. My sailmaker, Hallett, is pretty experienced, and there are lots of Cape Dorys up here, so I doubt they led me astray.

I hope we can generate more discussion.

Dean