Sail trim quiz question

[Jackdaw]

We all know that when sailing upwind in optimal conditions, the boom should be on the center-line of the boat, which creates a chord line for the mainsail (tack to clew) of 0 degrees. The headsail however, is much wider, at typically 8 degree of chord angle. OK, See drawing in comment #6. OK Why? What are some possible ramifications?

I'll let a few comments pop up..

 

[Rick486]

On the boats I have sailed we typically wore the boom a bit on the leeward side, as it turned out. The further you can cheat it down to leeward, the bigger the lift component in the forward direction. We were constantly sub-optimizing the draft of the main and the position of the main sheet on the affect on the slot and the jib. We usually carried the tiniest bit of a luff in the main. On boats with a Cunningham we had one more adjustment to fiddle with I think most of these type adjustments are very boat dependent. I wouldn't recommend defaulting to the boom on the centerline without experimenting with the main outhaul, cunningham, the traveler, and the main halyard tension. On the other hand I can recall spending way too much time fiddling with sail trim and have another guy pick up a shift that we missed. I painfully recall one of the crew saying, "...he just tacked....", and I'm saying why? Aaarrgghh.

 

[Simon Sexton]

Jackdaw,

I am not sure of how your boat points upwind, but I found on the sonars that I could often sheet out on the main and travel over to windward to get a better shape on my sail; this would get me about 10-15 degrees closer to the wind than everyone else (who all sail the same boat). In that case, it either came to getting the edge in sailing closer to the wind, or getting more speed by falling off just a few degrees. To address your jib question, the luff of a mains'l is run up the mast, which is pretty much a vertical spar. The jib, however, is run up the forestay, and even if on a furling pole, the luff is run at an angle to meet the mast (even more of an angle if you're sailing a fractional sloop). This difference in luff angles changes the area of pressure on the sail, and thus provides the sail itself less rigidity. Another thing; if your main is loose-footed, then the only reason it comes close to the centerline of the boat is because the foot is attached to the boom, which is fitted with the proper rigging (triple purchase sheet and traveler, for example) to bring the foot to the centerline. The jib is only hauled close to the center by the port or stbd. sheets, which can only pull the sail within one or two points from the centerline of the vessel. The only way I've ever seen a jib being hauled anywhere close to centerline was a Island Packet 35 that had a self tacking boom for the stays'l.

I hope this helps!
God bless,
S.S.

 

[justsomeguy]

Ayup, Mr. Sexton. The staysail boom was the first thing to pop up in my mind.

 

[DayDreamer41]

I would venture an educated answer, with 8* of chord on the head sail while maintaining lift it also offsets the center of effort to allow for a balance of forces between the head and main sails.

 

[Jackdaw]

Just to be clear, why is the boat on the left the ideal upwind configuration, and not the boat on the right?

 

[JRacer]

The one on the right is going to close the slot and cause the main to backwind while the one on the left gets more work done from the main because of the airflow coming off the jib which will smoothly move across the main.

 

[JRT]

Does it have anything to do with potential air flow turbulence across the main from the headsail?

 

[kloudie1]

Cool stuff to ponder (good cigar and good rum)
https://cdn2.hubspot.net/hub/209338/news/Ad_aerodynamics/index2.htm

 

[BobbyFunn]

Im in the there can be many sails, but there is only one wing camp.

The jib is the wing, the main is a flap. The jib should have the best trim in the wind and the main should continue the jibs shape right? If they are parallel then the main is not completing the upper lift curve, but contributing to turbulance.

 

[shemandr]

The main is sailing in a lift with respect to the jib. Without the jib up the main would be stalled trimmed like that.

 

[Jackdaw]

The main is sailing in a lift with respect to the jib. Without the jib up the main would be stalled trimmed like that.

Think you meant KNOCK. ;^)

But otherwise correct!

 

[Jackdaw]

Bunch of people closing in or outright getting it right.

The main sails in a relative knock (header) with respect to the jib, and has to be trimmed with a higher AOA because of it.

 

[Jackdaw]

Follow up #1: Based on this - in what order should you trim your sails when heading upwind?

 

[Jackdaw]

Follow up #2: Based on this - what effect might this also have on VOR65 boats flying several head or code sails?

 

[Owen Thistle]

A commonly held view is that the headsail (especially an overlapping one) and the mainsail end up working together, more like a single foil. Looking at it that way, its like the headsail is the leading part of the foil and the main is the trailing part so it makes some sense that they aren't at the same angle of attack. Another way of looking at it is that the headsail has already accelerated the flow on the leeward side so it carries on along the tightly sheeted main without separating (the main doesn't "stall" because the headsail is fanning it in a sense). Either way I think the bottom line is that they are not two independent foils, but a coupled system of foils. There always seem to be many differences of opinion on these fluid dynamics questions, so I'm interested to hear them.

 

[Owen Thistle]

So I would trim the headsail first for the right angle of attack and then the mainsail for good flow off the leech.

 

[Owen Thistle]

My VOR65 racing experience has all been from the couch so far. As far as I can tell that inner headsail is just there to carry more advertising .

 

[Rick486]

Bunch of people closing in or outright getting it right.

The main sails in a relative knock (header) with respect to the jib, and has to be trimmed with a higher AOA because of it.

Yes, but the main is sailing in higher wind speed on the leeward side due to the Bernoulli acceleration effect of the slot. If the main is over trimmed, the flow will separate and stall. The main must be trimmed to maximize the slot effect. Sooo, what we do is place tell tales on both sides of the main and trim so that the tell tales closest to the mast signal lift, i.e. lay down. So it is difficult to look at the drawings and answer the question asked because so much depends on the particular configuration. For instance, in the drawings the jibs are shown as 100% or less, rather than overlapping genoas. In every boat I have either sailed or observed which does not set an overlapping jib, Shields, Ensigns, Etchells, 420s, the main is always carried off the centerline to maintain flow over the leeward side and maximize lift on the main.

 

[BobbyFunn]

I gonna throw some broscience out there and say when many headsails are used together the benifit is from the correction for turbulance. When fluid goes through a wind tunnel its all fd up due to the massive fans. The air passes tbrough a grid first so that the flow is straight again.

Or. If slot affect is good why not the slotter effect