by GreenLake » Sun Jan 22, 2023 8:18 pm
First, I love "Practical Sailor". It's a great publication, even if many topics only apply to larger boats. For main sail rigging, the effect of the different options on the cockpit is somewhat different. Whether you use center boom or end-boom sheeting, the main sheet swivel is always in the same location. Some people use a traveler in the middle of the cockpit with center boom sheeting. This configuration will definitely make the cockpit a more crowded space, contrary to big boats with center boom sheeting that have a traveler on the coach roof, in other words, forward of the cockpit itself.
About the physics: of primary importance is the direction the main sheet pulls, measured as the angle from the vertical. The smaller the angle, the more the pull is downward. For a vang, the pull is down (and forward, but let's ignore that aspect for now). A vang only affects leech tension, not the (horizontal) angle the boom has. That is controlled by the sheet. However, sheets aren't horizontal, so they also affect the leech tension (and with it, the twist of the sail). Different rigging options will have different effective angles and therefore a different distribution between downward and inward pull.
A center boom arrangement is easy to analyze because everything happens along a single line between boom fitting and mainsheet swivel. It's easy to see the angle of pull, which gets shallower as the main sheet is let out. As you ease the sheet, not only does the boom swing to leeward, the change in angle means that less of the pull contributes to leech tension. That's in addition to the pull being reduced overall as you ease the sheet. Therefore, you are adding twist as you ease your sheet.
If you also have a vang, you can use it to maintain a lower twist. That's beneficial because it allows you to keep your main flat as you ease it, so the sail doesn't power up as it might with the leech being more slack.
How is end-boom sheeting different? There are several configurations, but let's pick one that is easy to take apart: the fixed triangle. A traveler rope forms a triangle with the apex connected to the sheet and the two other corners at the corners of the transom. (Sometimes the traveler is run through blocks at these corners, but its position is fixed by some other means, so that the distance between the apex and one of the corners is always a fixed length).
In this setup, the angle of sheet pull will be initially towards the apex, and both sides of the triangle are under tension. The apex sits higher than the main sheet swivel, so as you let out the boom, the angle of sheet in the rear becomes more horizontal more quickly than for the part that continues forward and then goes to the main sheet swivel. This affects the relative influence of sheet tension on leech tension vs. sheeting angle. You find that you affect sheeting angle more than leech tension (the pull is more horizontal).
As you let the boom out further, the imaginary line from end of boom to transom corner will dip below the highest point on the fixed triangle: the windward side will go slack and the apex will drop to be in line. From that moment on, the traveler acts as if you had sheeted directly to the transom corner. Your angle is still more horizontal as you are not pulling towards the center of the boat, but to its windward side.
The effect of the traveler is that you can have more twist at the same sheeting angle. If you combine it with a vang, the range over which you can change the ratio of leech tension vs sheeting angle now goes from more twist, with vang off to less twist, with vang on. If you used center boom sheeting you would be limited in how much twist you can achieve with the sail sheeted in.
A typical setup for end-boom sheeting doubles up the part at the end of the boom; because it's also twice as far out from the gooseneck, that part contributes four times as much as the remaining part that is led to the center and from there to the mainsheet swivel. For all practical purposes, you can ignore the contribution of that part, even if it's pull is always at a slightly different angles from that at the end of the boom.
Finally, the discussion assumed that the boom is perfectly rigid. No boom ever is. The article mentioned that pulling at the center of the boom can break it, if it isn't strong enough, and it can certainly bend it a bit. Bending the boom, like bending the mast, will pull at the edge of the sail, with a tendency to flatten it. However, mainsails are either loose footed, that is only connected at tack and clew, or they have a foot shelf sown in, a reserve of fabric, which will therefore absorb any effect due to bending the boom before it can affect the shape of the sail. Well, technically, bending the boom makes it shorter, and thus could add a bit of belly in the sail, but I'm convinced that effect is on a small enough scale to be noticed.
In terms of rigging a DS, you'll get a lot out of adding a powerful vang: in addition to the effects described, it also pushes the boom forward, helping to put a bend into the main. This bend, in turn, helps flatten the sail by pulling on the luff along the length of the main up to the spreaders. (Look for a discussion on pre-bend in this forum). It thus adds twofold to keeping a flat main, and that makes it a powerful instrument in sailing at the upper end of the wind range or in gusts, where without it, you'd be forced to reef earlier or feel less in control. (Both valuable even for cruisers).
End-boom sheeting extends the range across which you can control the relative contribution to twist and sheeting angle; at the cost of a small chance that the sheet tangles at the end of the boat (motors, cleats). Something that is generally kept under control by remembering to sheet in at the start of a gybe.
Mounting a traveler car on a track in the middle of the boat allows you likewise to change the angle of pull (by position of the car), but it is really inconvenient in the middle of the cockpit, and using a vang (with the technique called "vang sheeting") makes it somewhat redundant.
~ green ~ lake ~ ~