Loose footed mains

Steve, gonzo or anyone else -

How much do you think the goal of maintaining a foil shape along as much of the mast length as possible is responsible for transitions to the loose footed main? Can't help but wonder if it has to do with looking at aircraft wings: The Wright brothers never transitioned to a flat foil section at the wing root, and I can't think of a current ragwing hanglider or sport aircraft that does either.

 

Chris,
I think the mainsail itself would rip well before the boom were forced to bow due to the clew being attached above the boom section centroid. (That's an opinion, not based on any calcs yet.)
However, since, apart from the foregoing, you have restated my case, I guess I have to agree. ;-))
The mast bowing transversely due to boom loading? Could happen, but it is usually such a short lever from the partners that it is unlikely to be noticable. I have never seen it, and I've sighted many a mast with a loose-footed sail.
Steve

 

Chris - the foil shape thang is why the foot-shelf was introduced (among some other considerations), but it's really hard to sun-bathe on a loose-footed main.
Steve

 

Boom Compression

Booms are compression members. The only way to eliminate the boom compression upwind is to have the mainsheet angle aft, aligning through the CE of the sail and the clew (trimming like a jib). If the boom was not in compression there would be no need for it, would there?

In the past most booms were built out of mast extrusions. By definition these were far stiffer transversely than was necessary to do the job. New, purpose built carbon booms are much thinner athwartships. Apparently the bending loads are not that significant.

Sailing upwind, in the maximum load mode, you don't usually have much vang tension, if any. The mainsheet and traveller do the job. This will change if you run out of trav and begin vang sheeting. This is when you can get some real vertical bending in the boom. The vang is holding down at maybe 30% of the "E", with the leech pulling upward at the end of "E". Then you heel/ dump in a puff and the boom hits the water, more vertical load. This is also one of the few instances where you might see some transverse bending of the mast due to the boom.

In practice, the horizontal load path for loose foot and shelf footed mains are the same. Both go from the tack (above the boom) to the clew (above the boom). One will allow you to lie in it and sunbathe (if that's your thing), the other won't, but they trim the same. Once the outhaul is "ON" they both close the gap to the boom equally, bi extra ventilating.

You'll find most boats do well if the outhaul can be maxed early. If you have some outhaul off and you try to get the leech tension required for good uphill height you will get what I refer to as "return". That is, the bottom of the main starts to point to weather instead of straight back. That is not fast.

I interveiwed a very famous sailmaker/world champion a couple of years ago. I asked what was the most surprising thing he learned about mainsail trim recently. His answer was how flat he would make his main in light air (light displacement one-design). I guess drag reduction is more important than lift in light air and flat water. Worked for him, he won the NAs and Worlds.

 

Logged out again.

The above is my post. Since the logout is not due to the site it must be my provider.

 

Would not the horizontal load (the compression load) on the boom be the same as the load in the out haul, disregarded the friction in traveler, blocks etc.??

 

It should be, unless you are using the vang or have a mainsheet that angles forward.

 

Booms are not only compression members. The position of the mainsheet at the traveller may take up some of the compression. Only in the recent past have mast sections been used as booms. It was a reflection of designers and builders economizing. For centuries spars have been designed and build for their purpouse. Before using mast sections, rectangular, oval, tee and I-beam sections were common. We should also adress the drawbacks of not having a vang. In boats with huge beam like multihulls is not a problem, but in monohulls it must affect downwind performance.

 

Chris,
Thank you for doing such a detailed job of defending my position against Paul's rantings, which position I perhaps did not make clear.

Personally, I prefer loose footed mains, they are simpler to set up, and easier for me to get the shape I want. However, that does not negate the points I was trying to make:

1. If you ignore the shelf-footed mains that were popular a couple of decades ago, and go back to the flatter-cut captive foot that was popular previously, some interesting things can happen: With mid-boom sheeting and a moderately flexible boom, as the leech tension increases in a gust, the outboard end of the boom bends upward. This eases the tension on the leach (a good thing in a gust), and increases the vertical tension in the sail cloth along the foot. Increasing the vertical tension in the sail cloth along the foot has two benefits: it flattens the sail (good in a gust), and it resists (not necesarily prevents) further bending of the boom. Since there are significant compression loads in a boom, this provide some protection against the P-delta effect that, along with the bending forces can cause it to fail. Designed properly, this is an elegant solution; the sail induces the bending, but it also reduces excessive bending.

Current thinking is to build what is effectively a rigid boom that does not bend under the various forces, and control sail shape through outhaul tension, and (primarily on fractional rigs) mast bend. Note that masts have much higher compression loading than booms, and the horizontal tension in the sail cloth along the luff induced by mast bend provides some protection from the P-delta effect.

2. A mainsail is not constructed like a foresail. The context of this discussion is the FOOT of a mainsail. The FOOT of a loose-footed mainsail is constructed similarly to the FOOT of a genoa, both can have considerable "roach". The sail drawings I've seen on North Sail's web site bear this out.

Finally, Paul, I appreciate the points that you make, and taking up the devil's advocate position. However, I must agree with Steve (Sail Design); what effectively amounts to name calling is counter-productive. It will only tend to make people unwilling to air their ideas. You are obviously well versed in this rather esoteric art/science; I think you can make your point quite well without the harsh attitude (your most recent post is a good example of how it can be done).
-Steve

 

Boom Bend

Without addressing your other jabbering, let's discuss the boom bend.

When the boom bends, do the tack and clew get closer together?

What other sail control makes the tack and clew get closer together? Easing outhaul?

Does easing the outhaul flatten the sail?


I have seen masts break from overbend. The mainsail didn't help much in stopping that. I know of one big boat boom broken by overzealous vang sheeting. The sailcloth surrendered there too.

As I stated earlier, if you made the main from aluminum plate and welded it to the mast and boom it would help resist the boom bending forces. But you wouldn't do that. You also wouldn't make a really flat main without a shelf and with a really strong foot rope, with all sorts of kevlar strapping from the foot to the head and luff, just so you could insignificantly resist some boom bend. All these hypotheticals are useless in the discussion of the loads a boom sees whether the foot is loose or not.

 

Guest (we assume PaulB with too much securoty set up) says:

"Without addressing your other jabbering, let's discuss the boom bend."

Can we assume by "other jabbering" you are referring to our attempts to bring you into civilised society and it's behaviour patterns?

Steve

 

I said the sail "resists" bend; it doesn't stop it. Designed properly, it can be part of the solution. The welded aluminum plate analogy doesn't quite do the job, because then you have something stiff, that can be visualized as adding stiffness to the main member (in this case, the boom).

The sail is a membrane. A membrane can only have stresses in the plane of its surface at any discrete point. Let's assume a very strong membrane (visualize very heavy, woven kevlar with no ability to stretch on the bias). Now bend that boom end upward, alot. the leach will fall off and the middle of the sail will have a hard, tightly stretched region radiating from the center of the foot toward the head. The shortening of the distance from the clew to the tack will result in the boom end falling off to leeward to take up the resulting "extra" material in the horizontal direction.

Steve (SailDesign) thanks for your attempts to civilize this forum.

 

We've been making a lot of educated guesses here regarding the interrelationship of structures and and aerodynamics. I'll be the first to admit the static example I posed to Steve is pretty narrow, and not very practical in the context of a rigged mainsail subject to constantly varying loads.

It would be cool to set up an experiment that holds the following constant:

1) Mainsail plan, including fabric type and panel arrangement.
2) Mast section and material
3) Mainsheet system (pick end boom or mid boom)

Then parametrically vary:

A) Boom section (maybe simplify to 2 - stiff (big circle/oval/box and flexible (scaled down circle/oval/box)
B) Apparent wind velocity (assume constant from tack to head or vary linearly from tack to head). Range from close hauled to running downwind.
C) Sail Foot Type (3 types - loose foot, loose foot w/ shelf, captive foot. I suppose we'd have to allow for the change in sail cut at the foot for loose vs. captive to be fair).
D) outhaul tension (the only thing the operator gets to vary to flatten sail - all other sail shape is due to properties of the spars or the tensions within the sail membrane due to wind velocity)

I know we could vary vang tension, traveller position, sail material, battens vs battenless, et al; but this would be one complex computer problem or wind tunnel experiment as listed.

We'd be looking for the magnitude and direction for total lift and total drag at all runs. We could then speak with some authority about performance of equivalent systems at different points of sail. Anyone know of a published study along these lines?

 

From the oiginal post:

"A lot of the recent Open 50's and 60's seem to have loose footed mainsails.
Whats the benefits / down side?"

Isn't it amazing what can happen to a thread? ;-)

Steve

 

I was thinking that myself!