Forces on mast foot

When a sail is stretched between boom and mast, then the luff is curved out by lifting forces, the pull of the sail would likely be backwards against the mast, and upward against the boom, as well as forward against the tension of the leech between the peak and the clew. Overall, the greater force is forward and laterally causing heeling and headway. Twist at the top of a sail would change move some of those vectors away from aftward, but modern racing boats try to reduce twist as much as possible. It seems to me, from this very casual thought experiment, that the greatest forward vectors would be at the top of the sail and on the boom, placing a lever effect upon the mast in the forward and leeward direction. Keep in mind too, that with a jib's slot interacting with the mast and its sail, the direction of the wind and consequently the wind's forces have changed direction relative to each sail's luff.

Here's some images I've found of jibs at work.


Full, outwardly rounded luffs.


This jib is clearly pulling backwards at the luff, but doesn't it appear also to be pulling forward at the peak?

-Will

 

As @tlouth7 pointed out, the tension in the jib fabric primarily acts backwards. However, I don't see any evidence of the jib pulling forward at the peak.
And at the risk of appearing pedantic, I would like to point out that the examples you presented are of Code Zero sails, not jibs. Furthermore, the point of sail in these examples is a broad reach, with the true wind direction aft of the beam, rather than upwind.

 

Yes, you are right about that. Google was not understanding me very well, so I took what I could get.

However, I don't think the difference in the dynamics on their respective stays should be all that much. I am not arguing that tlouth7 is wrong. I've never thought about the pull of a jib's luff on its stay until it was pointed out here. I found the information a bit of a revelation. But then I started to think in more detail about the various parts of a sail, stays'l or otherwise and how it relates to the mast and what that would translate to in terms of forces on the mast step, and so I spoke or rather wrote my thoughts out here.

When I look at the action in those photos and think about the direction of forces attached at the masthead and the boat stem, I see the mast being pulled with a forward component from the forces on the forestay. It is not pushing backwards. Yes, at the tack there is certainly a backyards vector from the stay caused by the sail. But if you took away the backstays and released the main sheet, that stays'l will bend the top of that mast forward, not backwards.

Again, I'm not nay saying here, only trying to expand upon these concepts as I see and understand them.

I encourage any counter arguments and thoughts, that's how I learn the best. Thanks.

-Will

 

I did a little graphic to identify the forces generated by the mainsail on the boat:


And yes, I was wrong in my earlier post: the thrust of the mainsail is not transmitted to the boat by the mainsheet: it is via the boom at the gooseneck.
This realisation came to me yesterday afternoon as we were beating upwind in 16 kts.

 

When the boom is far off the centreline then the clew will be forward of the mainsheet attachment point, so the force transferred from the sail to the hull via the mainsheet will be forwards and leewards.

We see on some boats with particularly bendy masts that it is desirable to directly counter the forwards thrust on the mast at the gooseneck. Many National 12 dinghies for example have lower shrouds at gooseneck height:

 

Yes, indeed, but to get that you have to be running very deep. (Why aren't they running a spinnaker?) In that case, some forward force will be applies from the sail to the mast and transferred to the boat either through the backstay, swept shrouds, or on an un-stayed craft, at the base of the mast.
But that's not what makes the boat progress upwind, with the driving force being directed forward through the sheet of the jib or the boom of the mainsail. Quite a different scenario.

 

Because they don't want to be instantly disqualified and possibly face a Rule 69 hearing.

Spinnakers have never been allowed in National 12s.