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2 minutes ago, EarthBM said:

Is there a chance that a bowsprit + asymmetric spin + jib might outperform a poled out symmetrical spin downwind? A displacement 40’ cruiser/racer (Bene First40). 10-15kt TWS.

Same size kite? 

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8 minutes ago, EarthBM said:

As large as it gets in both cases. Or, put another way, what relative kite sizes might result in a similar PHRF rating (I know, cue rating committee jokes).

Around these parts, the kites would need to be the same size, on the same size pole, to have the same rating.

An Assym with a 5 foot bowsprit (imagine it with a 5' longer penalty pole) and tacked all the way down at the sheer is going to be quite a bit larger than the standard Symmetric. PHRF will ding you for that.

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Okay, what about same size a) assy+jib vs b) symmetrical? VMG downhill? Is there an established truth? Big$ ocean racers seem to use a), but they are reaching, short handed, and not in displacement mode (even without foils).

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9 minutes ago, crashtestdummy said:

Jib is not going to help you at vmg angles.  

spin staysail will had 1/2 knot when you put  it up and 1/2 knot when you take it down!

if kites are same size and vmg running in 10-15, symmetrical will always win in any boat unless you can break free n that low TWS

Unless you can pole back the Assym. If you can, the cut of the Assym will still have "flow" vs just being drag, which will generate more power.

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Tis a good question, but to be equivalent the assy needs a pole, and even then, when the vpp says sail down to 165 twa for run vmg, it appears (for boats like Bene 40) the sym is just slightly quicker.

To be fair tho, have not yet seen a truly serious effort at the 'vmg run A1' as that would be a highly specialised sail, unlikely to make the inventory for offshore under 3 or 4 spin limitation?

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  • 1 month later...

True passage race 4 spin inventory for medium displacement like Bene40 would be:

1. Masthead zero

2. 0.75 oz full size S2

3. 95% midgirth masthead A3, strong enough for > 30tws

4. Fractional A5

A3 luff length so that it can use pole, at lifeline height.

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  • 6 months later...
On 12/9/2020 at 10:41 AM, stealingisacrime said:

D/L is the key to making the A-sail work.
My guess is the Ben 40.7 is too heavy to gain enough speed from the A-sail to make the extra distance sailed worth it.  

This is your answer.

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  • 2 weeks later...

Why not do both - options are key.

Random leg with the right angles go with the asym.

deeper angles use the symmetrical.

 

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On 10/15/2020 at 4:26 PM, Raz'r said:

Unless you can pole back the Assym. If you can, the cut of the Assym will still have "flow" vs just being drag, which will generate more power.

^^^This is the ticket - fly the assym on conventional pole so you can square it back!!!

I owned a similar boat with five kites (S1.5, S2, S4, A2, A3) and squaring back the assyms was a potent weapon. 

The A2 was the kite we used 90% of the time because it generated more useful lift than the S2 of the same size because the thrust vector was oriented more in a forward direction. 

The only time I would favour the S2 over the A2 is on a DDW course with lots of gybes (e.g. in a narrow bay or channel) because the S2 is quicker to gybe with a full race crew. Pretty much all other conditions favour the A2. 

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5 minutes ago, gspot said:

^^^This is the ticket - fly the assym on conventional pole so you can square it back!!!

I owned a similar boat with five kites (S1.5, S2, S4, A2, A3) and squaring back the assyms was a potent weapon. 

The A2 was the kite we used 90% of the time because it generated more useful lift than the S2 of the same size because the thrust vector was oriented more in a forward direction. 

The only time I would favour the S2 over the A2 is on a DDW course with lots of gybes (e.g. in a narrow bay or channel) because the S2 is quicker to gybe with a full race crew. Pretty much all other conditions favour the A2. 

Planning PacCup and the A2 will be the 20hour a day workhorse. Maybe we get lucky and see the A4 on occasion 

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1 hour ago, gspot said:

^^^This is the ticket - fly the assym on conventional pole so you can square it back!!!

I owned a similar boat with five kites (S1.5, S2, S4, A2, A3) and squaring back the assyms was a potent weapon. 

The A2 was the kite we used 90% of the time because it generated more useful lift than the S2 of the same size because the thrust vector was oriented more in a forward direction. 

The only time I would favour the S2 over the A2 is on a DDW course with lots of gybes (e.g. in a narrow bay or channel) because the S2 is quicker to gybe with a full race crew. Pretty much all other conditions favour the A2. 

How did you determine that the A2 was more optimal than the S2 in most cases? Did you look at boat speed vs wind speed, how you did relative to other boats or did you use some other technique? I don't think there is any way I could tell how thrust vectors differ between a sym and asym just from sailing the boat.

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1 hour ago, marcklim said:

How did you determine that the A2 was more optimal than the S2 in most cases? Did you look at boat speed vs wind speed, how you did relative to other boats or did you use some other technique? I don't think there is any way I could tell how thrust vectors differ between a sym and asym just from sailing the boat.

For me, my designer said they are. He’s got the design and the VPP. 

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12 hours ago, marcklim said:

How did you determine that the A2 was more optimal than the S2 in most cases? Did you look at boat speed vs wind speed, how you did relative to other boats or did you use some other technique? I don't think there is any way I could tell how thrust vectors differ between a sym and asym just from sailing the boat.

The polars for our boat were produced with a symmetric spinnaker and we could exceed them with an asymmetric squared back. 

The reason is that the thrust produced by a sail is proportional and perpendicular to the curvature of the sail, as shown in this diagram from North Sails.

image.png.99f35331d30231b8e7afc149420008f0.png

With an asymmetric cut as show there is more lift in a forward direction, and if you square back an asymmetric spinnaker much of this will be in the same direction of travel as the boat, versus a symmetric spinnaker which will produce more heel. 

It's a simple fact of physics that asymmetric sails generate more forward lift for a given sail area, otherwise your sailmaker would design all your other sails (e.g. jibs, genoas, mains, code zeroes) with symmetric cuts as well. 

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On 12/9/2020 at 7:41 AM, stealingisacrime said:

D/L is the key to making the A-sail work.
My guess is the Ben 40.7 is too heavy to gain enough speed from the A-sail to make the extra distance sailed worth it.  

That's assuming you're flying it from a sprit and trying to do apparent wind sailing like a sport boat, which doesn't work so well on a heavier boat.

But the asymmetric sail is still a more efficient airfoil shape, and by squaring it back you can sail much closer to the rumb line and don't have to sail the extra distance. 

1 hour ago, SEC16518 said:

When you say pole back an asym, to what degree?  

Just like a sym kite keep the pole perpendicular to the apparent wind. The sweet spot seems to be about 30 degrees off the forestay in above 15 knots of breeze or more, but you do need to take wind speed and boat speed into account.

Stan Honey very successfully raced a Cal 40, and he used a rule of thumb to sail shallower angles whenever the boat speed was below around 6.5 knots, and only head down when he could maintain at least 6.5 knots. 

We adopted his strategy, and would fly the assym directly off the bow with the apparent wind at 90 until we exceeded 6.5 knots, then we would start heading down and squaring the pole back as boat speed increased above that. If the boat speed ever dipped below 6.5 knots we would head up and ease the pole forward.

In 25 knots of breeze we could sustain 10 knots of boat speed at 120 degrees AWA, very close to the rhumb line with the pole squared back 30 degrees. 

We would occasionally square back to 45 degrees, but only for relatively short amounts of time as dictated by the course and prevailing obstructions, otherwise it was more efficient to gybe.

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