Jib Furling Direction

[Don Guillette]

Mates: I get all kinds of questions by phone & email from sailors all over the world. Normally, I can take a shot at an answer but this one stumps me. It came from a Dallas, TX sailor.

Jackdaw-- what do you think??


Don,
I have a 2007 Catalina 350 on Lake Lewisville in north Texas. She's not that accomplished
as a true sailor, but she's comfy! (In Maine we have an Ensign which we race, a gaff
rigged all teak and mahogany Dark Harbor 17, and a Tartan out of Bucks Harbor Yacht Club
on Penobscot, when we need a little more grace!)

My question is this: will the direction of the roller furl on the forestay, when
partially deployed as a reef, affect on which tack the boat points more efficiently? My
contention is that the faster laminar flow and greater negative pressure will be on the
side where the jib comes off the furl, ie the "outer" side if you will, or flat
side. Thus if the furl is clockwise, and the sail comes off the furl to starboard, she
would point better on a starboard tack. If counterclockwise, better on a port tack. This
seems to be the case, but I'm not sure of the physics.
Any thoughts?

 

[Jackdaw]

Mates: I get all kinds of questions by phone & email from sailors all over the world. Normally, I can take a shot at an answer but this one stumps me. It came from a Dallas, TX sailor.

Jackdaw-- what do you think??


Don,
I have a 2007 Catalina 350 on Lake Lewisville in north Texas. She's not that accomplished
as a true sailor, but she's comfy! (In Maine we have an Ensign which we race, a gaff
rigged all teak and mahogany Dark Harbor 17, and a Tartan out of Bucks Harbor Yacht Club
on Penobscot, when we need a little more grace!)

My question is this: will the direction of the roller furl on the forestay, when
partially deployed as a reef, affect on which tack the boat points more efficiently? My
contention is that the faster laminar flow and greater negative pressure will be on the
side where the jib comes off the furl, ie the "outer" side if you will, or flat
side. Thus if the furl is clockwise, and the sail comes off the furl to starboard, she
would point better on a starboard tack. If counterclockwise, better on a port tack. This
seems to be the case, but I'm not sure of the physics.
Any thoughts?

VERY interesting question.

But the original poster makes a mistake in his question:

If the furl is clockwise, and the sail comes off the furl to starboard, she
would point better on a starboard tack

A clockwise furl has the sail coming off on PORT.

But he thinks the roll is faster to leeward, with the smoother edge to windward. His reason is clever thinking, but wrong.

The answer lies not in faster laminar flow, but in ATTACHMENT, and the key to sail working well is keeping the flow attached on both sides on the sail, but in particular on the leeward side. That is why a headsail will work better with any rolled portion on the windward side; the flow attaches (and stays attached) to to that side easily. Once detached to the leeward side, its hard to get it back.

But all this is less important then you might think... when you are reefed, you typically have to much pressure anyway, and only the highest performance boats will really notice.

But a great question!

 

[Don Guillette]

Jackdaw: As always, thank you. I've forwarded your reply to the sailors.

 

[Stu Jackson]

Don,

You might also add: "What does it matter even if there IS a reason?"

As an engineer, I've learned that if you can't measure it, or even if you can and it amounts to a drop of water in the ocean (not a bucket), then it is a meaningless parameter, if not a meaningless question.

In addition to Jack's very good answer, this has been a subject that first came up when furlers and foils were introduced and was debunked back then. I was maybe six years old then!

 

[Jackdaw]

Don,

You might also add: "What does it matter even if there IS a reason?"

As an engineer, I've learned that if you can't measure it, or even if you can and it amounts to a drop of water in the ocean (not a bucket), then it is a meaningless parameter, if not a meaningless question.

In addition to Jack's very good answer, this has been a subject that first came up when furlers and foils were introduced and was debunked back then. I was maybe six years old then!

Stu,

Interesting point, but I'd add this:

First that is IS measurable. Like I said in my last paragraph, perhaps not on cruisers but on highly turned boat.

The second is that it promotes a deeper understanding on how sails actually work. Thinking about things at a theoretical level makes us better, even if we cannot experience it on our boat at a practical level.

 

[Warren Milberg]

I've found that when you need to reef the the headsail, you are already doing hull speed and have too much heel (unless you reefed the headsail much earlier, like one is supposed to do....). After you furl up some headsail, you should also still be doing hull speed, but with less heel and weather helm -- unless you've furled too much headsail. You really can't go faster than that, unless you are in a planing hull, and I don't think a Cat 350 is in much danger of planing....

 

[TeddyGVO]

I got it wrong, too... I thought the "pretty, smooth side" of the furled sail would be "better"... I might benefit from a description(+ Drawing?) of the term "attachment"(Maybe as opposed to "laminar flow", which I am not up with, either - sigh...). Always nice to know as much as possible about how things work. Thankx, to all!

 

[Jackdaw]

I got it wrong, too... I thought the "pretty, smooth side" of the furled sail would be "better"... I might benefit from a description(+ Drawing?) of the term "attachment"(Maybe as opposed to "laminar flow", which I am not up with, either - sigh...). Always nice to know as much as possible about how things work. Thankx, to all!

OK a picture!

Top-down view of three sails on startboard tack. A fully unrolled sail, then two partially rolled ones.
BLUE is the sail
RED is flow; solid is attached, wiggle is distured.
GREEN is lift.

Lift ONLY happens where flow is attached to BOTH sides. Flow is hardest to keep attached to the BACK (leeward) of the sail. It breaks off easily. Likewise flow on the front (windward) attaches easily.

Left image is happy; full lift. Narrow leading edge allows quick attachment on both sides.

Center is OK, breeze detaches from windward luff due to roll but quickly re-attaches.

Right is not so good; flow due to sail-roll detaches on leeward side and takes a LONG time to re-attach.

EXTRA CREDIT.... Knowing what you know now, why do mainsail luff telltails get sucked back along the back (leeward) side of the main???

 

[Davidasailor26]

So, based on the picture, if you were on the un-favored tack and had that long area of separation to leeward, would you be better off pinching a bit to move the Angle of attack forward? It seems to me that doing so would introduce some separation along the windward luff, but may get the flow re-joined to leeward sooner. Or are there other problems that would then be introduced?

 

[Jackdaw]

So, based on the picture, if you were on the un-favored tack and had that long area of separation to leeward, would you be better off pinching a bit to move the Angle of attack forward? It seems to me that doing so would introduce some separation along the windward luff, but may get the flow re-joined to leeward sooner. Or are there other problems that would then be introduced?

No, best to drive best point of sail and follow your tell tails as well as you can. Just remember that the outer-forward one might dance, which normally would indicate 'ups' but in this case just means detached flow from the roll! If your sailmaker is good you'll have a row of 3 tell-tales on each side, and hopefully the back two will be in an area of re-attached flow. Steer to those.

 

[TeddyGVO]

1. Diagram great - thankx!

2. Extra credit: Researching - Thinking about it.

 

[Capt jgw]

Taking this idea one step further, wouldn't it be a good argument for a continuous line furler where you could roll the sail up on either side just by turning the crank the other way? And when you tacked you could unroll the sail and roll it back up the other side to keep the roll on windward? I've never used a continuous line system so I don't know if they can do that.

 

[Jackdaw]

Taking this idea one step further, wouldn't it be a good argument for a continuous line furler where you could roll the sail up on either side just by turning the crank the other way? And when you tacked you could unroll the sail and roll it back up the other side to keep the roll on windward? I've never used a continuous line system so I don't know if they can do that.

Well that would indeed be better. But remember this is more of a theoretical discussion than a practical one. As has been noted, when you are confronted with breeze that makes you partially furl (aka reef) your headsail, you have more pressure than you need and the need for 100% efficiency goes away.

 

[Scott T-Bird]

Is it knowing

EXTRA CREDIT.... Knowing what you know now, why do mainsail luff telltails get sucked back along the back (leeward) side of the main???

... what I know now or now know ... I'm always confused by that

doesn't that detached airflow create turbulence that causes a rotational flow at the leeside leech? So the pressure is negative on the lee side.

(I perform window design calculations using an ASCE manual - the greatest pressure on windows and building components on the walls is at the lee-side corners of buildings and the pressure is actually negative)

 

[Jackdaw]

... what I know now or now know ... I'm always confused by that

doesn't that detached airflow create turbulence that causes a rotational flow at the leeside leech? So the pressure is negative on the lee side.

(I perform window design calculations using an ASCE manual - the greatest pressure on windows and building components on the walls is at the lee-side corners of buildings and the pressure is actually negative)

Scott,

My question was related to the fact that it is hard(er) to keep flow attached to the back of the sail. Indeed, when the wind is very light, or the sail is over trimmed, the flow will detach from the trailing edge off the sail. This disturbance actually sucks the tell-tails back along the lee side of the sail.

It means one of:

1) You're over-trimmed
2) You have too much depth in the sailshape.
3) If a headsail, your sail is too wide (switch down if you can, like to a windseeker)

 

[Scott T-Bird]

Yep, pretty much the same thing ... buildings are pretty miserable as a sail shape!