A yawl has an itty-bitty second mast waaayyy back at the back of the boat, about half to a third the size of the big main mast in the middle of the boat. So what’s the deal with that? Does it make the boat easier to steer, or what? What are the advantages? It can’t be to make the boat go faster, since it’s too small.
According to this site, the mizzen sail (the small sail at the rear)
[note: the linked site didn’t display too well using IE. In Netscape 4.7, it looks fine, though. Go figure.]
I haven’t much experience sailing with a mizzen. However, I’ve heard it helps with tacking the boat. The helmsman can easily control the mizzen from the stern, even “backing” it by hand. In light winds, this is a great help.
So, in plain English, the answer is, “Yes, it makes the boat easier to steer”?
If it’s such hot stuff, huccome more boats don’t have them?
Wow, I reread my post and realised how incomprehensible it probably is to non-sailing types.
I’ll try to elaborate (probably poorly: while my sailing knowledge is pretty strong, my formal understnding for physics is close to nil). When a sailer “tacks” a boat, the sails are pulled in tightly and the boat is steered across the wind. That is, if the wind was coming from the left (“port”) side of the boat before tacking, it will be coming from the right (“starboard”) side of the boat after a successful tack.
Therefore, the direction the boat is heading shifts from one side of the wind to the other side of the wind. In between, of course, the boat will be at a point where it is heading directly into the wind. This is the most difficult point - since the boat cannot sail directly into the wind, the helmsman must rely on the momentum of the boat when it is going into the tack to “force” the boat to complete the turn.
In light winds (and especially with certain boat designs), this can be difficult. Before the tack, the boat just isn’t going fast enough. When it is turned into the wind, there may not be enough momentum to complete the turn. The boat will instead stall, stuck facing directly into the wind. To overcome this problem, a helmsman may reverse the direction of his/her turn and try to “back” the sails.
Imagine I wanted to make a port (left) turn across the wind but failed. I would be stuck facing the wind and starting to drift in reverse. Since I would be going backwards, I would reverse the direction I’m steering. It’s like a driving a car - steering right when you’re going backwards moves the car to the left.
Here’s when the mizzen sail comes in to play. While letting the wind push me backwards and left, I would grasp the mizzen and force it to the opposite side (left) of the boat (called “backing” the sail). Hopefully, it would catch the wind and fill with air. This will give the boat further momentum to turn it left. Once we had sufficient force in the sail I would then once again steer left and complete the tack.
Simple, huh? 
Sigh. And again my “explanation” is as much gobleygook.
Sorry DDG, my knowledge of boat design and history is only a trifle better than my didactic ability.
The yawl also makes sense from the boat builder’s viewpoint. As the area of sail cloth grows, so does the size of the mast necessary to hold it under load without breaking. Also, the taller the mast is, the heavier the keel must be to keep the boat upright. Very tall masts are very expensive. So…you can build a lighter, cheaper boat with the same sail area if you put part of the sail on a second mast.
By the way, if the mizzen mast is ahead of the rudder, the boat is called a ketch instead of a yawl, y’all.
Oh, please. :rolleyes: This is the level I’m at, people: “The rudder is the thing that sticks out of the back of the boat”. Okay?
So, if the “mizzen” is ahead of the “rudder”, then the boat is a “ketch”, but if the “mizzen” is behind the “rudder”, then the boat is a “yawl”?
Hah, I’m not stupid–this is one of those trick questions, isn’t it?
You can’t have the “mizzen” behind the rudder, because then it would be in the water. So there. 
Okay, so you’re sitting in the back of the boat, trying to tack (I followed that part–besides I saw Wind with what’s-her-name and that other guy), and while you’re (a) steering the rudder with one hand and (b) hauling on the rope attached to the big main sail with the other hand, at the same time you’re also supposed to (c) haul on the rope that’s attached to the mizzen? And do something with that? Just how many arms do yawl-designers think the average helmsman has, anyway?
Or are you supposed to let go of something, at some point in the proceedings?
It sounds complicated–maybe that’s why all boats don’t have them.
I have always believed that a yawl has the mizzen aft of the helm, not the rudder. The helm in this case being the steering station.
You recall we’re talking about a boat with three sails? This aint no Suzuki Swift. A yawl is a big boat and one seldom sailed alone. A canny skippper - or, a skipper with two or less arms - will coerce/hire/bribe people, called “crew”, to do some of these jobs.
Oh, and I think AskNott meant if the mizzen is ahead of the tiller (cf rudder - the tiller is the bit of wood/metal you hold to control the rudder, which is the thing in the water) then we have a yawl. I think.
Only if you’re using standard linear temporality.
First of all, this link: http://www.sailboatstuff.com/glos_bt_sl.html It has the definitions. Now, DDG and others are thinking of boats with a flat transom (back end) and a rudder hanging off that. Many boats have a long rounded back end hanging way beyond the rudder. Those have lots of room for a mizzen mast back there.
What people mean to say when they say “abaft of the rudder” is “abaft of the rudder POST”, ie the helm, as suggested. Likely, a boat of this class has a wheel, not a tiller.
DDG, you’re forgetting the jib sheets. You’ve got to handle them, too, if you’re single handing. Actually, on tacking a typical small sailboat, you can pretty much ignore the main sheet. You’re supposed to ease it slightly during the maneuver so that it fills more rapidly after the switch, and you will tighten it up again. For a small boat, if there’s any kind of air at all, it won’t be a problem, and you can get away with simply letting the main take care of itself to swing a couple feet from one side of the boat to the other. The jib has to switch sides, though. This means uncleating the jib sheet on the one side, waiting until the jib starts luffing, releasing it (may have to unwrap it from a winch drum), allowing the jib to cross, and tightening it and cleating it again on the other side (may involve winching it down).
The jib procedure would still be there, as well, on your yawl.