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Old 07-10-2010, 02:49   #1
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Downwind Sails and Rolling

Would just like to get peoples view on this subject, This year did a fair bit of downwind sailing, the most common rig we used was poled out Genny and prevented Mainsail. Works fine but rolls like a dog with an itch

Other times on difffernt boats we have use the twin headsails etc, but still a lot of rolling.

I once met a sailor in Las Palmas, who gave me his setup that he said did not roll, but I was too drunk to remember.!

Any ideas, whats works on single stick rigs. ( avoiding the use of spinnakers and gybing downwind etc)

Dave
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Old 07-10-2010, 03:41   #2
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What kind of boat are you on? Some pitch more than others.

I never liked running double headsails because you need to sail in such a narrow slice that even a moment's distraction will have you all messed up.

If you get into the specifics of the motion you're feeling (pitch/yaw/roll/sway, etc) there are some generally applicable "do x to solve y" kind of approaches. Sounds like you're pitching though (bow and stern are taking turns bobbing up and down).

Yaw, pitch, and roll - Wikipedia, the free encyclopedia

Some boats are notorious pitchers, some can be controlled primarily by centralizing as much weight as possible amidships / athwartships. The reasoning goes that the more buoyancy (less weight) at the bow and stern, the less pitching force can be generated. It's related to the "moment of inertia" and again the more weight you have in bow and stern the higher the moment will be resulting in more pitching.

Your sail plan can affect this as well, whereby too much lift from the jib is causing the bow to ride out and then slam back down, or it can be caused by riding the waves (since downwind you're taking the seas on your quarter / stern).

There's a few things it can be but I wouldn't overlook the weight displacement. I replaced my anchor chain recently and with the ground tackle off my bow rose 3" out of the water. Not insignificant.
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Old 07-10-2010, 03:59   #3
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no not pitching, one crossing was one a HR 42, using genoa and main, didnt pitch at all rolled like a dog...

others were in production boats , Beneteaus 393 , 411, 44, 50 and Jeaneauu, 42, 42DS, 39 and a Bavaria 50. ( amongst others).

None were in particular pitching that bad, some rolled +-40 degrees

Dave
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Old 07-10-2010, 04:02   #4
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Trade her in for a catamaran . No rolling.
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Old 07-10-2010, 04:18   #5
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So a roll across the longitudinal axis. An easy easy way to knock it out is get a little more wind on the beam, causing your mainsail to generate some heeling induced stability which should greatly reduce your roll.

Even when motoring around I try to keep my main up simply to minimize roll.

How do you think something like that would affect your downwind sailing technique? With a poled out genoa you should be plenty in the realm of still having your main in action. I'm surprised it's not doing enough to keep the roll in check.
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Old 07-10-2010, 07:14   #6
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All yachts will have a natural roll period that will be greater or lesser depending upon hull shape, cross sectional area of the apendages, etc. (e.g., heavily ballasted lean hulls with shallow draft tend to have short periods.) If the period of the cross seas approximates the natural period of the yacht, rolling will result (just as rythmically pushing someone on a swing will get them flying along) unless one can introduce a damping, which is what the sails do when one is sailing up wind. One can use a paravane connected to an outrigger and trailed from the bow although that does introduce drag or one can use a flate sheeted "riding sail". Our IOR era boat does not like sailing dead down wind but on the few occassions when we have had to do so, I have found that dropping the main to the 1st or 2nd reef and hard sheeting slightly off centerline will effectively dampen roll while allowing the headsails to have clear air although thats tough on the sail. For long distance down-wind sailing I would use twin jibs tacked abaft the stem with some dihedral angle on the poles and a flat cut riding sail (to save the main) aft of the mast.

FWIW...
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Old 07-10-2010, 07:29   #7
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yeah I was wondering about the riding sail idea. Wonder would a flat sail on the inner forestay help anything. Maybe switch to a ketch!!

Dave
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Old 07-10-2010, 07:43   #8
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Originally Posted by goboatingnow View Post
yeah I was wondering about the riding sail idea. Wonder would a flat sail on the inner forestay help anything. Maybe switch to a ketch!!

Dave
David--

One wants a riding sail as close to the lateral center of pressure as possible to obtain damping effect without inducing yaw, hence abaft the mast is likely preferable. An inner forestay arrangement might work if you can flatten the sail but I suspect it may have the yacht wandering around her course, particularly if you're relying on a wind vane. Ideally one wnats as little rudder action as possible to eliminate drag. Unfortunately, with issues like this each, yacht is unique and what works on one boat may not prove particularly effective on another.

FWIW...
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Old 07-10-2010, 07:43   #9
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Riding sails or staysails will have little effect without a beam breeze. They offer no resistance when the roll velocity is low...not until the roll velocity is high do they fill with air and offer resistance...but it's too late then...

Unlike the pitching problem, you want the weight outboard as far as possible. Or deep and high. This boat has a very tall rig and a heavy deep keel. The roll period is quite long. Disturbances like wakes induce little roll. Downwind she's much faster and thus more stable with the wind well forward. And well forward is easy because of the easy boat speed. Works out nice.
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Old 07-10-2010, 07:49   #10
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The HR was as safe as houses dont get me wrong and yes the tall rig and deep keel were probably a big contributor, just was wondering , as I had similar do not quite as excessive issue with ligther production boats

dave
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Old 07-10-2010, 07:50   #11
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yeah I was wondering about the riding sail idea. Wonder would a flat sail on the inner forestay help anything. Maybe switch to a ketch!!
Dave
Having a ketch and watching friends on their sloops, ketches are much worse with downwind rolling. They normally have shorter masts and sails that don't have the tall reach into "clean air" to minimize rolling.
- - But basically the determinant is the keel - a 14 ft keel will roll a lot less than a 5 ft shoal keel.
- - Also personally I have found that the larger the genoa the less the roll. I also use a technique of not letting the mainsail out when running downwind. I keep it close to amidships and let it out enough to "funnel" the wind into the genoa like a large windscoop. Keeping the main amidships helps reduce rolling as the sail acts like a dampner. Using it to funnel wind to the genoa increases the effectiveness of the genoa which "pulls" boat through the water versus the mainsail that "pushes" the boat.
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Old 07-10-2010, 08:01   #12
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Sailing directly down wind (wing-on-wing) is not only uncomfortable but it is slow. You will get better VMG to your waypoint by heading up and sailing a deep reach.
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Old 07-10-2010, 08:30   #13
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Unless the wind is less than 8 knots, we get better VMG wing-on-wing, and it gets to be much better when the wind picks up to 15 knots and above. Trying to sail deep without the pole means your genoa will collapse occasionally, leading to chafe on the sail and sheets.

It seems to me that the worst for rolling is downwind with a jib and no pole and no main. I remember a study somewhere which showed that the sails stabilized the roll on the wind, but destabilized it going downwind, especially spinnakers.

Edit: Thinking about it, pretty sure that CA Marchaj was the author--either Aero-hydrodynamics of sailing (later edition) or Sail Performance--techniques to maximize sail power.
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Old 07-10-2010, 09:20   #14
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Trade her in for a catamaran . No rolling.
or a tri.

Why don't you want to gybe downwind? It's faster and a lot more comfortable.
Fast is fun. I also have a Walder boom brake for sale in the classifieds, and I'll take $100 + shipping.
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Old 07-10-2010, 09:22   #15
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I like this idea:

The Twizzle Rig or Twistle Rig for Downwind Ocean Sailing
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