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Sail trim question.

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Ben
Master Marine Consultant

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USA
1234 Posts

Initially Posted - 05/12/2005 : 14:39:26

Hello again, ya'll. I went out yesterday. Tons of fun. Winds were from (probably)5mph to 10mph for most of the time, with about 30 minutes of 15mph (maybe). Lots of fun.

Anyway, I have three telltales on the back of my mainsail. And I'm pretty sure I read somewhere that you know you have your sails trimmed properly when they're all flying strait out.

But I also thought I read somewhere that you're supposed to have the mainsail completely full. Yesterday as I was sailing, particularly in the stronger wind, with telltales were flying strait out, but I had a "bubble" of sail area right at the mast and boom that was luffing. So I pull in on the main sheet, the bubble disappears and the sail is nicely shaped again, but the telltales stop flying straight out. Also, it seemed I lost speed. Not sure about that speed things, because winds were a bit flukey.

Can anyone tell me why what I've read conflicts with each other?

Thanks again!

Ben
Beneteau 361
Viking Kitty
Columbus, Ohio

Edited by - on

MattL
Admiral

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USA
990 Posts

Response Posted - 05/12/2005 : 15:39:52

1. Expensive fix: Get a new main, old one is a little blown out

2. Inexpensive fix: tighten up the lower forwards and loosen the lower rear shoruds. This will put a bow in the mast pulling the center of the sail out and flatter. Also keep the out haul tight.

At least this is what I have been going by.

Edited by - on

Leon Sisson
Master Marine Consultant

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USA
1893 Posts

Response Posted - 05/12/2005 : 16:49:11

What he said. Plus tighten vang and cunningham/gooseneck downhaul. The idea is to flatten the sail as wind speed increases by pulling on all three corners and bowing the middle of the mast forward to aid in reducing draft. This decreases heeling forces while improving angle of attack at the luff.

However, don't rule out the possibility that your mainsail is too blown out to do what you're wishing for. You may need to choose a compromise in control settings and broader point of sail that will give you best velocity made good to windward. Pinching that last degree of apparent wind isn't always the fastest way to get there.

-- Leon Sisson

Edited by - on

Steve Milby
Past Commodore

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USA
5908 Posts

Response Posted - 05/12/2005 : 17:10:06

My best guess is that the bubble that caused the sail to luff near the mast and boom was there because the angle of attack of the sail was too open, creating turbulence at the luff of the sail. When you pulled in the mainsheet, you corrected the angle of attack, and the bubble disappeared.

When the telltales on the leech of the mainsail are all flying, that means the laminar flow of air is staying attached to both the windward and leeward surfaces of the sail until the air flows off the leech of the sail. When the telltales started to flutter, that means the air was not flowing smoothly across both the windward and leeward surfaces of the sail all the way from the luff to the leech of the sail. Somewhere between the luff and the leech, the air separated from the surface of the sail, creating turbulence. As the turbulent air flowed off the leech of the sail, it caused the telltales to flutter instead of flying straight. The most likely cause is that the sail shape was too full for the wind strength. In light air, you want the sails to be full, but as the windstrength and boatspeed increases, you need to flatten the sails. You could have flattened the shape of the mainsail by putting more tension on the backstay adjuster, or the outhaul, or the cunningham, or the downhaul, or a combination of them (in that order).

It doesn't sound like the wind was strong enough to require a reef, but, a reef not only reduces the area of a sail, but it reduces the sail area by taking it out of the foot of the sail. Since the foot of the mainsail is the fullest part of the sail, a reef also flattens the sail considerably.

Edited by - on

Frank Hopper
Past Commodore

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Pitcairn Island
6776 Posts

Response Posted - 05/12/2005 : 18:33:54

Bet a dollar you were flying the 150. If so then when you trimmed the main to get rid of the luff you lost your slot. With the 150 the slot is far back on the main and quite frankly the luffing main was annoying but mostly irrelevant.

Edited by - on

ronrryan
Admiral

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USA
561 Posts

Response Posted - 05/12/2005 : 19:15:31

You dont mention what point of sail you were on. As you go more and more off the wind, to a very broad reach or a full run, it becomes more and more difficult to keep the flow on the main attached. Visualize a dead run: the wind is really just "pushing" on the sail, one might say, not creating laminar flow. I am assuming you are reporting conditions when "on the wind", or at least on a "close reach"---????Fair winds, ron SRSK Orion SW FL

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dlucier
Master Marine Consultant

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Virgin Islands (United Kingdom)
7583 Posts

Response Posted - 05/12/2005 : 19:18:51

That bubble appears when the mainsail is "backwinded" and, as previously stated, is generally caused by either an over trimmed headsail, a headsail that is too big for the conditions, or an under trimmed/mis-shapened mainsail.

I'm not a racer, but I seem to recall the commentators for the America's Cup saying that having a slightly backwinded mainsail (small bubble) is not necessarily a bad thing.

Edited by - on

Ben
Master Marine Consultant

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USA
1234 Posts

Response Posted - 05/13/2005 : 10:27:43

Thanks ya'll for your input. It sounds like some folks don't think the bubble is necessarily a problem, while other folks say I need to flatten my (blown out) sail. I definitely agree that I could do a lot more than I am to flatten my sail, and I'll print this out so that I can make a check list of how to do that.

The point of sail I was on was close-hauled, going into the wind.

Frank, you'd win that bet.

Steve, thanks for your very descriptive explanation. Another question then. Since the telltales were all streaming back while the bubble was present, could I have attained greater speed by flattening the sail to eliminate the bubble? The way I understand it is that the wind was behaving perfectly on the sail WITH the bubble present, as was indicated by the tell tales.

I'm hoping to sail after work today. maybe there will be good wind and I'll be able to try these things.

You guys are my heroes!

Edited by - on

Steve Milby
Past Commodore

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USA
5908 Posts

Response Posted - 05/13/2005 : 11:39:40

<blockquote id="quote"><font size="1" face="Verdana, Arial, Helvetica" id="quote">quote:<hr height="1" noshade id="quote">Another question then. Since the telltales were all streaming back while the bubble was present, could I have attained greater speed by flattening the sail to eliminate the bubble? The way I understand it is that the wind was behaving perfectly on the sail WITH the bubble present, as was indicated by the tell tales.<hr height="1" noshade id="quote"></blockquote id="quote"></font id="quote"> That depends on the size of the bubble. There's a limit to the amount that you can flatten the sails, and, when the windspeed is too high, you can't eliminate or reduce the size of the bubble any more by flattening the sails.

When you said you were closehauled, that made it clear that the bubble was caused by the jib backwinding the mainsail. That is also consistent with the windspeed, because the genoa will usually start to backwind the mainsail when the windspeed gets above about 12, unless you start using your sail trimming devices to flatten the sails. As Frank and Don pointed out, if the genoa backwinds the mainsail a little bit, that's not really a problem. If it backwinds the mainsail a lot, and you have already made all the adjustments that you can to flatten the sails, then that tells you it's time to start thinking about furling the genoa a bit (if you have a roller furler) or tucking in a reef, or changing to a smaller headsail.

When the windspeed is between about 12-15, you're starting to reach the point when you should think about reducing sail area. In the strong puffs, you'll be overpowered, which will slow you down. In the lulls, you'll be really fast. The question you should ask yourself is, "What percentage of the time are we overpowered?" If you're overpowered a small percentage of the time, then your overall average speed will be higher by leaving the sail area as it is. If you're overpowered a large percentage of the time, then you'll improve your overall average speed by reducing sail area.

If you're cruising, you should reduce your sail area sooner than if you're racing. It's physically exhausting to sail overpowered, and a cruiser needs to save his strength for the long haul, because you might be out there fighting the tiller for a long time. For most club racers, the race is only going to last for an hour or two, so you can afford to wear yourself out a little longer.

It sounds like you're doing things just about right!

Edited by - Steve Milby on 05/13/2005 11:40:58

bbriner
Captain

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349 Posts

Response Posted - 05/13/2005 : 12:22:08

When your sail is blown - it is blown - and all the 'flattening' (depowering) tools will have very limited effect. I have tightened my fwd lowers, tigtened the downhaul(aka cunningham), the vang, and the outhaul, and I would do the same with a backstay adjuster if I had one... all to no avail... the bubble is still there. However, I am still going at or near hull speed!! My jib is a 120 and with the right balance (a combination of the above factors with jib adjustment, including the fairleads) it is possible to get a good balance even in 15 kts - a balance is very little or zero weather helm. The boat will not have excessive heal at this point so all is good. The C25 is a **great** heavy weather little pocket cruiser!!

So when you have excessive heal and/or a lot of weather helm - depower all you can and if that doesn't work put in a reef on the main. If that doesn't work reef or change the foresail. If that doesn't work.... well take them all down, get the motor going and head for shore!!

Bill B
Wind Dancer
SF Bay

Edited by - on

Ben
Master Marine Consultant

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USA
1234 Posts

Response Posted - 05/13/2005 : 13:18:10

Thanks ya'll. Your expertise and experience is much appreciated.

Edited by - on

Bruce Baker
Captain

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USA
402 Posts

Response Posted - 05/13/2005 : 16:26:14

America's Cup experts said that the old 12-Meters always had backwinded mains. They said "sail by the front of the jib and the back of the main".

Edited by - on

tinob
Master Marine Consultant

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USA
1883 Posts

Response Posted - 05/14/2005 : 14:04:03

Ben,

What Bruce said...'s how I do it...with big jib full main.

Val on Calista # 3936

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