Designing a stayed sail rig for the PDRacer Sailboat
Most of the production sailboats out there used stayed masts. They are very cheap, easy to make, use light materials, and are very strong. The trade off is that you end up with a bunch of wires about your boat, and all sorts of hardware attached to your mast which may damage your sail if not attended to.
Forces at Work
They way a stayed mast works is very different than an unstayed. With an UN-stayed mast, we have a basically a strong stick that sits in a hole or tube of sorts, and the structure of the tube holds the mast up. With a stayed mast, it typically stands on a knob, so there is nothing to hold the mast up except the stays.
So the wind presses against the side of the mast, and the stay keeps it from falling down. Now going further, that sideways pressing force is actually converted into 2 other stresses, first the stress is converted into a tension load on the windward stay, sort of like the mast and the hull is pulling at the stay trying to break it apart. And if you watch, the leeward stay usually starts to go slack. The second thing that happens is the mast also takes that force and converts it into a compression load, pressing down on the deck.
Basic 3 Stay Mast
The most basic stay pattern is to have 3 stays, one on each side of the mast, and one from the bow. The picture to the right is Tim Cleary and #58, Mary E.
Math for stays
Placement of the stays is very important. There is a lot of force generated by the wind against the mast (lever arm), and you countering that force by hiking out over the side. I don't have a good enough grasp on the math to tell you exactly how to calculate it, but from what I understand, all of the angles need to be equal to or greater than 12 degrees. Some designers push that and have stay angles as little as 10 degrees.
So if you start with a standard PD hull that is only 4' wide, we have 24" from the mast to the gunnel. No you want to have your side stays swept back a little, all of my fiberglass boats seem to use a 30 degree angle for this. Seems adequate, there is room to swing your boom back and forth, but looks like it is back far enough that it won't let the mast fall forward. So we have 27.5" from the base of the mast to where we mount our chain plate.
With that chain plate mounting spot, we can attach the stay to the mast 10.5' in the air and the angle the stay comes back to the deck will be 12 degrees. If we raise the mounting on the mast to 12.5' in the air, we end up with a 10 degree angle.
Keeping The Mast In Column
With all that force on the mast, and the stays attached to a single point, the mast will start to flex. The lower part of the mast will flex to leeward, and the part above the mast will point to windward. As the mast flexes, if you have enough compression load pushing down, that mast can break. To prevent the mast from flexing (keeping it "in column"), you can do a couple of things.
One solution is to add an extra set of stays. The picture to the right is Greg Stoll's Venture 21, with PD #38 "Red Racer" towed behind. The Venture has 2 sets of side stays, and they work together to keep the mast in column.
These are the little sticks that are along the side of the mast. They provide a couple benefits. First is that they allow you to use a really tall mast, and since they spread the width of the stay up high, the angles are all easy to keep greater than 12 degrees. Second is that they help transfer some of the load to the leeward stay, and at the same time help keep the mast in column.
The picture to the right is of an International 14, and as you can see it has a very elaborate stay pattern.