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Go Back   SailNet Community > Skills and Seamanship > Learning to Sail
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  #1  
Old 02-08-2003
jbr jbr is offline
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Depowering the sails and twist

I sail a 17 foot dinghy which has a vang and outhaul but no traveller or cunningham on the main. The jib is roller furling also without travellers. I sail on a lake which can be quite gusty and where the wind can really shift direction drastically. When the wind picks up above 10-12 kts I can and usually do put in a reef in the main and also partially furl the jib. In these conditions, I max out the main halyard, vang, and outhaul to flatten the main sail. I also feather to weather, ease the jib a little to widen the slot, and ease the mainsheet in gusts. Under these conditions, as you would expect, I''m sailing with the forward part of the main luffed to reduce heel. However, whenever I ease the main slightly, the entire main immediately luffs which causes me to lose power and forward momentum. The unopposed jib causes me to fall off further slowing me down, inceasing my heel, and requiring me to compsate with the rudder to control heel and maintain my heading. I''d appreaciate input from experienced sailors on two topics: 1) any suggestions for what I''m doing wrong and/or what else I should be doing (ie, is the trim on my main too critical because my slot is still too narrow)? 2) I''d appreciate input and clarification on the use of twist via vang sheeting to depower. What confuses me is that tightening the vang flattens and depowers the main although this by definition reduces twist and I had read that adding twist depowers the sail by spilling air from the leach. So by definition twist and flat sails are contradictory. A flat sail can''t spill air so it''s unforgiving to gusts and wind shifts; a sail with twist isn''t flat so it has more power! To make matters worse, if the flat main is unforgiving to gusts and windshifts and the main''s trim is overly critical as is mine I''m forced to ease more often than I should have to but am always damned when I do! The only thing I can think of is that maybe the use of twist depends on one''s heading (although I haven''t read that anywhere): a flat main without twist is what might be max depowered when close hauled and/or feathered and that a sail with twist (but flat otherwise) might be max depowered when reaching?

Thanks for any help and sorry for the lengthy question!
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Old 02-10-2003
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Depowering the sails and twist

Whew! Your question is so complex, I''m only going to try to answer part of it.

It sounds to me like you are trimming your sails pretty well, up until the point when you "ease the jib a little to widen the slot." Don''t do that.

You said you don''t have a cunningham, but you can use your mainsail downhaul to put more tension on the luff of your mainsail, moving the pocket forward.

As a general rule, the mainsail causes more heeling moment than the jib. Both the main and jib are depowered by flattening them, and/or by reducing their surface area. (There are other ways, but I don''t think we need to get there yet.)

If you are trying to point as high as you can in strong wind, you need to keep the jib trimmed in flat. When you ease the jibsheet, you put a deeper pocket in the jib, which gives it a more powerful shape, and forces you to fall off the wind a few degrees. That''s the opposite of what you want to do when you''re overpowered while beating to windward.

Try tensioning the downhaul and keeping both sails flat as I suggest and see if that helps. If you are still overpowered, try reducing your sail area a bit more. There might be some tweaking you can do, but you need to get the basics right first, and I think you are getting the sail trim out of kilter when you ease the jib.
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Old 02-11-2003
jbr jbr is offline
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Depowering the sails and twist

Sailormon6, I appreciate your suggestions. Unfortunately I don''t have a downhaul/cunningham but may talk to a sailmaker about adding one. Regarding flattening the jib, what you suggest makes sense although I''ve read one contratictory issue: In heavy air I''ve read that if the slot is too narrow it jets air onto the back of the main which may luff it or make it luff with only a tiny change in mainsheet trim which is what I seem to be experienceing(ie, "overly critical main trim"). I think I read that widening the slot slightly reduces this. Also, I don''t have a jib traveler so I can''t control jib twist.
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Old 02-12-2003
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Depowering the sails and twist

You control jib twist by adjusting the Genoa cars. Moving the car forward decreases leech tension and allows the top of the sail to twist off. Moving the car aft tightens the leech and brings the top of the sail in line with the rest eliminating twist
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Old 02-12-2003
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Depowering the sails and twist

Actually, Sailmc, I think you have that backwards.

With the car forward, the jib sheet is pulling more downward than rearward, putting tension in the leech and allowing the foot to develop more draft. With the car aft, the sheet pulls the foot taut and allows the leech tension to diminish, thus allowing more twist at the top.
*******

Sailmc wrote:
Moving the car forward decreases leech tension and allows the top of the sail to twist off. Moving the car aft tightens the leech and brings the top of the sail in line with the rest eliminating twist
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Old 02-12-2003
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Depowering the sails and twist

If you have genoa tracks and adjustable genoa cars on your boat, then I agree with sailmc and Duaneising that you can move your jibsheet leads aft. If you don''t have genoa tracks and cars, and if you are overpowered, then I think you need to reduce your sail area more.

A boat that is overpowered will heel excessively and can''t point to windward. You can reduce the power that is generated by your sails by using sail trimming devices to change their shape, or by reducing their area.

As a sailboat moves forward, it gulps huge quantities of air in the opening between the luff of the jib and the mast, and it directs that air aft through the narrow slot between the jib and the mainsail. When the jib is forcing too much air through the slot, it causes the luff of the mainsail to lift and flutter. You can use sail trimming adjustments to reduce that effect to a certain extent, but there is a point at which you have to reduce sail area in order to stop the mainsail from luffing.
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Old 02-12-2003
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Depowering the sails and twist

Thanks everyone for the great advice. In terms of reducing sail area, there are several options (and assuming you have a roller furling jib): reef the main first, reef the jib first, reef both proportionately, sail under full main alone, sail under reefed main alone, sail under full jib alone, or sail under reefed jib alone (whew!). The question is what order to work through that list to match the sail area and helm to the prevailing wind conditions (it may be different for larger boats but I''m talking about in a planing dinghy)? Some boats don''t sail well under jib and/or main alone but fractionally rigged dinghys usually do. I''ve tried tailing under reefed main alone in very heavy (for me!) winds (20 + kts ) when I got caught in a storm but I found it impossible to tack. I admit I was trying to tack from a feathered heading and may not have had enough momentum to do it. Should I have tried tacking from more of a reach to build some momentum fist (ie, close haul or close reach)? I was so intent on controlling heel I didn''t feel comfortable trying that since it contradicts the general advice to feather to weather.
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Old 02-13-2003
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Depowering the sails and twist

jbr,

As I reread all your posts in this thread, I''m wondering if you need to be hiking out to counter some of the heel. It has been over 30 years since I sailed on small dinghies, and I don''t know your boat, but moveable human ballast was always a vital part of sailing in higher winds when I did it.

Duane
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Old 02-13-2003
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Depowering the sails and twist

Sailboats work their best when the sails are balanced. When Iím trying to decide whether to reduce sail area on a boat, I ease the less powerful sail [the jib on a fractional rig, or the mainsail on a masthead rig] while beating to windward, until the sail is completely luffing. If the boat is overpowered and heeling excessively in the puffs while sailing on the most powerful sail alone, then I know it is time to reduce the area of the most powerful sail. When I reduce the area of one sail, I also reduce the area of the other sail proportionately, to keep the sailplan balanced.

If you have reefed the mainsail as far as you can, and are flying your smallest headsail, and the boat is still overpowered, then you have stayed out too long. In that case, if my headsail was hanked on, I would probably lower the headsail and use the fully reefed mainsail alone to get back to shelter. If my headsail was roller furling, I would lower the mainsail, and roll out a very small headsail to get to shelter. You should avoid being in that situation in the first place, however, because any sloop (fractional or otherwise) that is sailing on only one sail (either the jib or the main) is hard to maneuver, and doesnít always go where you want it to go (as you have seen).

You said you were unable to get up enough speed to tack the boat across the wind, because you were feathering to windward, and the boat heeled excessively when you bore off to leeward to gain speed. That means the boat was grossly overpowered. If you bear off to gain speed, the boat might heel excessively and capsize. If you don''t bear off and gain speed, you can''t tack the boat across the wind. There is no solution to that dilemma. The boat is not designed and rigged to stand up to that much wind.
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Old 02-13-2003
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Depowering the sails and twist

Thanks, again, eveyone for your great input. In regards to hiking out, I wish I could do this more effectively (in appropritate aka non-extreme conditions) but it may acutally be dangerous in my boat (Hunter 170) since there are no hiking straps and sitting on the gunwale one couln''t even reach the toerail (your feet would be on the cockpit seats with nothing to allow you to lever out against). Also, Sailormon6, I want to thank you for the clarity of your thinking regarding a strategy for reducing sail area on a dinghy. Just brilliant! My rig is fractional with roller furling jib. It sounds like in these conditions you would first roll the jib completely, asses heel, and if not severe, reef the main and partially unfurl the jib. If heel was still excessive you would drop the main and seek shelter under reefed jib. However, I''ve heard that fractionally rigged dinghys such as mine may not be able to tack under jib alone whereas they can with main alone (assuming you''ve not yet past the "point of being totally overpowered"). So my question to you is: have you found that on dinghys such as mine when moderately overpowered and needing to seek shelter under one reefed sail that you could tack with jib alone? Related: if you can partially roll the jib, why would you prefer reefed jib alone to reefed main alone (ie, is it because you can generally reef to a smaller sail area than the reefed main which imposes limits on how much you can reef)? One last related question (a bit off the wall!): I''ve noticed that in high winds my boat is blown around significantly even with the jib furled and the main tied to the mast. This is due in part to the high gunwales and light weight of my boat and to the fact that when the main is quickly lashed to the boom it''s never perfectly flaked and there is still some surface area exposed to the wind. So, my question is: it has occured to me that it might be possible in these circumstances to "trim the boom" (ie, with main lashed to it) as the ultimate reef and try to sail the boat to shelter (ie, if my motor was inoperable). I doubt I could tack but the boat might be somewhat sailable from a "close" beam reach to a run? Have you ever heard of this working? Caveat: probably wouldn''t work in a heavier boat or in huge waves. Anyway, thanks again for the excellent advice.
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