First of all, let's review the function of each of the wires on the Potter's spreaderless fractional rig.
The primary function of the upper shrouds is to stabilize the mast sideways. Since they attach to the top section of the mast, they have the longest lever arm, and bear a greater load than the lowers.
The lower shrouds assist the uppers in stabilizing the mast, but have a lesser role because they have a shorter lever arm, and carry less of the load. They are usually a little looser than the uppers, sometimes almost as tight, but never tighter.
The lower shrouds have two primary functions. The first is to keep the middle section for the mast from bending sideways; that is, to provide lateral stability to the middle of the mast. The second function is to prevent the middle of the mast from bending too far forward in the middle section under load. When the mast is perfectly straight fore & aft, the lower shrouds may be a little loose, but they tighten up when you flex the mast by tightening up the backstay or vang or tighten the mainsheet as tight as you can when you're close hauled (pointing as high as you can).
The side shrouds on the Potter 19 have one more additional function. Since they are anchored aft, they offset the tension of the forestay, and are therefore part of setting the mast rake.
The length of the forestay controls mast rake, not the backstay, especially on a fractional rig. This is very important to remember when doing the initial rough tune in your driveway.
Rough tune at dock or driveway...
Note 1: when you adjust the turnbuckles, you have to use a small wrench to hold the wire to keep it from twisting when you turn the turnbuckle. Put the wrench on the swaged fitting at the bottom of the wire. Don't turn the turnbuckle without holding the wire.
Note 2: clean and lubricate turnbuckles before starting. Otherwise the SS threads will cut up the bronze body of the turnbuckles and maybe "gall" them -- i.e. get fused together. If that happens, you must replace the turnbuckles)
Note 3: before starting, get the boat as level as you can fore and aft, and athwartships. Use a level on the cockpit seats for fore and aft. Use a level across the seats (on a 2x4?) to level her athwartships. Stand back 100 feet or more and view from the stern to check her. Use the gudgeons and a straight edge to visually check.
1. Slack all shrouds so they're just barely "floppy loose." Release the topping lift and let the boom rest on the cabin top. (Wait for a day when the wind is pretty light. Don't do this in really high winds, you won't get good results.)
2. Getting the mast straight vertical athwartships: In addition to eyeballing it, we'll use the main halyard to measure whether or not the mast is vertical.
Measure back from the bow on each side and make a mark on each side near the side chainplates. (The chainplates may or may not be perfectly symmetrically installed. That won't matter measurably in terms of performance, but you need symmetry to measure).
Tighten the top shrouds until you feel some resistance by hand, eyeballing the mast until it appears vertical. Stand back as far as you can to check.
Pull the main halyard tightly to the chainplate/mark and mark it. Use the main halyard to measure from side to side to a mark near the chainplates. Adjust the "long" side until the halyard confirms the mast is vertical. Tighten the top shrouds hand tight.
Then tighten the uppers more, equally on both sides. Count the number of turns to be sure you tighten them an equally each side. Test after every few turns, until they'll deflect "easily" about 2 inches when you pull them about 5' above the deck with two fingers-- easily means that at first they'll be easy to pull sideways, then you'll feel them suddenly start to resist further deflection quite noticeably.
3. Setting the rake of the mast. We want somewhere between 0 and 3 degrees of mast rake. Mast rake controls weatherhelm. After we set the mast rake on land, we'll take it out on the water to check how much weather helm we have (given how the weight is distributed on the boat)
Remember that the forestay is the primary control for mast rake, not the backstay. Make sure the backstay is loose while you do this next procedure.
Hang a weight from the main halyard, almost down to the deck. Adjust the forestay until the weight is about 4.5 inches aft of the back face of the mast. That's about 1 degrees of rake for a 22-foot mast. (If you want 2 degrees, it's about 9 inches. 3 degrees is about 14 inches. -- but I'd recommend you start with about 1 degree)
Tighten the lower shrouds hand tight, checking to make sure the mast is straight from side to side in the middle. Then tighten them additionally, until they deflect 2-3 inches with your fingers, as above.
5. Setting tension on the forestay:
First, with the mast positioned after steps 1-4, tighten the backstay until hand tight. Tighten the forestay with a wrench and screwdriver a few turns until it deflects about 2.5- 3 inches. The weight at the end of the halyard may move forward.
5b) Go back and check the upper shroud tension. They should be adjusted until they deflect about 2 inches and the weight is at 4.5 inches.
5c) Tighten the backstay if necessary with the wrench until it deflects 3-4 inches and the weight is at 4.5-inch mark.
5d) Go back and check the forestay. It should now deflect about 2 inches, no more.
Repeat the steps in five, making small adjustments, until you get no more than 2 inches of defection on the forestay.
If you have a CDI, adjust the forestay until the weight is at 4.5 inches (1 degree of rake), the side uppers and lowers deflect about 2-3" (between 15 and 20% of breaking strength), and the backstay deflects 4-5 inches. The forestay will be automatically be at approximately the right tension, since the sidestays are pulling the mast aft.
The final tune is done after sailing the boat. Only the rough tune is done in the driveway. Hopefully, you'll have enough wind to see what's happening.
On the water, check that the boat is approximately level in the fore and aft plane, like you did on land. Move gear and crew until she's level. Most folks put way too much weight in the back end. The boat will slip excessively to leeward if the bow is too high (the chines can't work properly to give you that extra lift), and you will have trouble making it through a tack because the wind blows the bow around too much. It's best to concentrate the weight close to the keel, so she doesn't "hobby horse" alot.
For the on the water tuning, you'll need enough wind and enough sail area to heel about 10-15 degrees while on a close reach.
First we will check the amount of weatherhelm, then the tension of the sideshrouds.
The term "weatherhelm" is often used to mean the tendency of the boat to turn her bow into the wind when you let go of the tiller, or to describe the amount of "pull" you feel in the tiller when you're holding her on a straight course while sailing above a reach. But we're going to measure it in terms of the angle of the tiller.
What we want is about 2-3 degrees of tiller deflection from the centerline at about 10-15 degrees of heel and travelling in a straight line. That means that your boat is angled about 2-3 degrees above your course. That will give you the best lift to windward from your keel,
You should make a mark on the cockpit floor with tape, showing the centerline and also 3 degrees off centerline, underneath the tiller.
For the on-the-water tune, sail on a close reach (with your sails properly trimmed -- let's not go into that right now....) with the boat heeling about 10-15 degrees, and on a straight course.
Look down at the tiller. You should be holding it about 2-3 degrees to windward, above the centerline. If you let go of the tiller, the boat should start to round up, bow into the wind within a second or two at the most. That's the right amount of weatherhelm.
If you're holding it higher than 2-3 degrees above the center in order to keep her on a straight course, that's too much weatherhelm.
If you are holding it below the centerline, you don't have enough weatherhelm.
If the weatherhelm isn't right, go back to the dock and change the mast rake after you check the side shroud tension.
Later, when you go back to the dock: To increase weatherhelm -- increase mast rake and/or move more weight forward. To decrease weatherhelm, decrease mast rake and/or move weight aft.
Adjusting side shroud tension. We don't want visible slack in the side shrouds. If they're too loose, the mast will shift violently when we tack, which puts a shock load on all the components of the rig -- the mast, the tangs, the chainplate, the hull anchoring points.
The upper leeward shroud will loosen while you sail upwind, but it should not sag visibly. Observe it carefully. Tack to the other side, and observe the leeward shroud again.
Assuming you have the mast straight up and down athwartships before you went sailing, you'll need to tighten the uppers an equal number of turns until the visible slack is gone.
If the mast leans more to one side than the other, go back to the dock and check your rough tune, using your hands to ensure the stays are equally tensioned, and using the halyard to check that the mast is on center vertically.
If the mast bows sideways in the middle (sight up the sailtrack with your face right up against the mast), you'll need to adjust the lower shrouds either on the water or back at the dock. Make them just tight enough so that the mast is still straight at the dock, and doesn't bend sideways in the middle while sailing.
To check forestay tension
While sailing upwind on a close reach, with enough wind and sail area to heel the boat 10-15 degrees, take a careful look at the forestay. If it's too loose, you'll see the forestay sag to leeward under the load of the fully powered up jib.
When the forestay sags in high winds (or the higher "apparent wind" it experiences when sailing upwind), the draft in the jib gets deeper, and the sail becomes more powerful -- which is exactly the opposite of what you want in high winds.
When you get back to the dock, repeat step 5, but this time, get the forestay tighter, with even less deflection when you pull it with your hand than before.
1) You need enough wind to heel 10-15 degrees while close hauled. For the genoa and full main, that's about 8-10 mph (I'm just guessing, I rarely sail in such light winds). For lapper and full main, you'll need about 12 mph. For small storm jib (AKA standard) you'll need about 15 mph.
2) You don't need an inclinometer to measure heel -- just think of the hour hand on a clock. 1 o'clock and 11 o'clock represent 30 degrees. Half of that is 15 degrees. Pretend your mast is the hour hand, and look at the horizon to find the horizontal. It'll probably feel like a LOT more heel than it actually is, so check visually against the horizon.
3) Another way to estimate weather helm by looking at the rudder. A tiller that is x feet long will be x inches off centerline when it's deflected 5 degrees. A 3 foot long tiller, with the end 3" off the center line, is deflected 5 degrees. A four foot long tiller, with the end 4" off center line is also deflected 5 degrees.
How do you know your sails are trimmed properly when you're on a close reach?... well, that's a little tricky -- It requires some experience sailing a well-tuned rig.
An experienced skipper can recognize what's wrong with a badly tuned rig immediately, but a novice will have to do a lot of experimenting.
If your rig were already tuned correctly, you could trim the sails for whatever course you intend, put the tiller just a hair above the center line and let the boat go where she wants. If she sails the course , your sails are properly trimmed for that course.
If she won't sail the course you intended, then you should re-trim your sails. Ease the mainsail and harden the jib to sail a lower course (or ease the jib and harden the main to sail a higher course), holding the tiller 2-3 degrees above the center line.
Your rig is properly tuned when she'll hold whatever course your sails are trimmed for, with the tiller positioned 2-3 degrees to windward above the centerline.
Fair winds, Judy B
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