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| Reefed Sail related to loss of pointing up wind? http://www.hobie.com/au/en/forums/viewtopic.php?f=71&t=30270 |
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| Author: | JollyGreen [ Mon Aug 23, 2010 10:10 pm ] |
| Post subject: | Reefed Sail related to loss of pointing up wind? |
So this weekend we were in some wind in the 20mph or so zone. I had my sail reefed till the tell tales were almost about to start being wrapped. It seemed like on one of the tacks up wind, port or starboard (not sure which), my angle upwind was severely reduced. Has anyone else experienced this? Maybe it's the port tack because the sail is wrapped around the mast leaving a several inch drop from mast to sail rather than a smooth flow from mast to sail on a starboard tack? I'm guessing I lost a good 20-30 degrees on my tacks. It didn't seem right at the time, so I would turn the boat upwind and the sail would start to luff so I would fall off till the sail filled out. When I let the full sail out, I would bury the ama quite often even when luffing the sail. JG |
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| Author: | KayakingBob [ Mon Aug 23, 2010 10:20 pm ] |
| Post subject: | Re: Reefed Sail related to loss of pointing up wind? |
I've noticed that also, and have not been able to explain it. It's not the boat as it happens at times on on side and other times on the other. Two speculations, maybe a curling or curving component to the wind, or maybe the wind near the top of the sail is a few degrees different in direction than at the cockpit area? Anyone have any other ideas? Roadrunner?? |
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| Author: | JollyGreen [ Mon Aug 23, 2010 10:52 pm ] |
| Post subject: | Re: Reefed Sail related to loss of pointing up wind? |
reconlon wrote: I've noticed that also, and have not been able to explain it. It's not the boat as it happens at times on on side and other times on the other. Two speculations, maybe a curling or curving component to the wind, or maybe the wind near the top of the sail is a few degrees different in direction than at the cockpit area? Anyone have any other ideas? Roadrunner?? Nice response, this matches exactly what I was feeling. The winds were definitely variable, and the whole time I was thinking the winds near the top of the mast must be slightly different than at the cockpit. When I got home I started to think maybe it's due to the sail being trimmed for certain kinds of wind. Maybe if I played with the downhaul or had a boom vang for the sail or something I could trim it better for these types of wind? Probably not worth doing anything about.... |
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| Author: | KayakingBob [ Mon Aug 23, 2010 11:14 pm ] |
| Post subject: | Re: Reefed Sail related to loss of pointing up wind? |
Best way I can describe it, is if I point directly into the wind, the front connection of the aka/ama is 45 degrees off the wind on either side from the seat, which is the highest the AI can point (and most efficient for the TI). Often, I can point 45 off on one side easily, but not even close on the other side. |
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| Author: | JollyGreen [ Mon Aug 23, 2010 11:19 pm ] |
| Post subject: | Re: Reefed Sail related to loss of pointing up wind? |
That's exactly what I was thinking, here are some tracks from this weekend. I have hours of the boat performing with almost the exact same tacks. It definitely seemed like one side was pointing better than the other. Maybe it's current or swell related? 
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| Author: | ugglover [ Tue Aug 24, 2010 1:45 am ] |
| Post subject: | Re: Reefed Sail related to loss of pointing up wind? |
Two speculations, maybe a curling or curving component to the wind, or maybe the wind near the top of the sail is a few degrees different in direction than at the cockpit area? |
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| Author: | ElementAI [ Tue Aug 24, 2010 4:09 am ] |
| Post subject: | Re: Reefed Sail related to loss of pointing up wind? |
Although at times it is totally necessary.... I have never really liked the sail shape we get with the half furled sail. It doesn't take much to lose the telltales and even then are they streaming accurately with air passing around that fatter mast?? I'll be glad when Matt is back from china and the r&d hobie guys |
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| Author: | chrisj [ Tue Aug 24, 2010 4:28 am ] |
| Post subject: | Re: Reefed Sail related to loss of pointing up wind? |
I wonder if the telltales are relevant by the time the sail needs furling. In light breezes, the telltales help you to optimise the trim of the sail to get the most out of the available wind. Once the sail is reefed, you are deliberately depowering the sail anyway, so the degree of weather helm and ama burying become the principal guides to sail trim. |
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| Author: | dosjers [ Tue Aug 24, 2010 5:08 am ] |
| Post subject: | Re: Reefed Sail related to loss of pointing up wind? |
I have noticed that my TI points much better on a port tack (wind is coming from the left) even with the full sail out. I think this has a lot to with the position of the center board. I never notice it in our AI but I'd bet it's the same. The other thing is that when you have your sail reefed you need to adjust your sheet on every tack if you want to maximize your ability to point. When you are on a port tack the sail wraps around the mast slightly and on a starboard tack it unwraps a like amount. It means letting the main out about 6 inches or so on a port tack and pulling it back in on a starboard tack. chrisj wrote: I wonder if the telltales are relevant by the time the sail needs furling. In light breezes, the telltales help you to optimise the trim of the sail to get the most out of the available wind. Once the sail is reefed, you are deliberately depowering the sail anyway, so the degree of weather helm and ama burying become the principal guides to sail trim. The amount of weather helm and the burying of the amas should be used a guide as to how much sail to reef not how to adjust your sheet. If you have a set of telltales far enought back on the sail they will work when reefed. Most of the time though the telltales are too far forward to be used when the sail is reefed. |
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| Author: | captain-max [ Tue Aug 24, 2010 6:32 am ] |
| Post subject: | Re: Reefed Sail related to loss of pointing up wind? |
Theoretically, when reefed (and only when reefed), the sail and mast combination presents an asymmetrically profiled foil to the wind. The starboard tack presents the best profile, due to the sail furling clockwise. On this tack, the spar forms a leading edge with a contiguous flow along the low pressure side - the windward side of the sail. On port tack however, the semi-furled spar interrupts the flow along that crucial low pressure side, creating turbulence and non-laminar flow. Contrary to what may seem logical, a sail is actually 'pulled', not 'pushed', by the low pressure gradient to windward. This is why some high-performance sailing craft, eg the Farrier 31R trimaran, use a teardrop shaped mast that can be manually swiveled to windward, thus creating a more optimum foil profile. Well, that's the theory... if anyone cares to get into that in great detail, check out C.A. Marchaj's "Aero-Hydrodynamics of Sailing". I'd love to hear if some of our experienced AI/TI sailors can confirm this: higher pointing (reefed only) on starboard tack. Bob? A challenge for you, mate...  (Tip: change the furling action from clockwise to anti-clockwise, and try both tacks again?) |
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| Author: | captain-max [ Tue Aug 24, 2010 6:47 am ] |
| Post subject: | Re: Reefed Sail related to loss of pointing up wind? |
dosjers wrote: ...when you have your sail reefed...let the main out about 6 inches or so on a port tack and pull it back in on a starboard tack That's a very good observation Jerry. This supports the theory I described above. Due to turbulence on port tack, you need to let out a bit more sail to produce the same amount of drive. The extra few inches allows the airflow to 're-attach' aft of the turbulence behind the furled mast, then leaving the same chord length of 'working' sail, or effective foil. Incidentally, when on that unfavourable port tack, you should also ease the sheet slightly to present the 'effective foil' at the same angle of attack... and that's ultimately why you can't point as high. If anyone is that interested, let me know, and I'll sketch and post a couple of diagrams to explain this better.  ps: now that you've made me think about it a bit, starboard tack has been 'hotter' for me! 
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| Author: | KayakingBob [ Tue Aug 24, 2010 6:59 am ] |
| Post subject: | Re: Reefed Sail related to loss of pointing up wind? |
I think we're talking about 3 different things here, that all display the same effect. The one original posted observation varies from side to side depending on condition, and is mostly with full sail. The other two are characteristics of each model of boat, ether the layout of daggerboard and fins, or of sail reefing. Both of these favor one side tack from the other, and is always on the same side. |
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| Author: | captain-max [ Tue Aug 24, 2010 7:23 am ] |
| Post subject: | Re: Reefed Sail related to loss of pointing up wind? |
Actually Bob, the original post described a reefed state... and on only theory (rather than experience) seems to suggest that the port tack is disadvantaged, no? |
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| Author: | JollyGreen [ Tue Aug 24, 2010 8:52 am ] |
| Post subject: | Re: Reefed Sail related to loss of pointing up wind? |
Great ideas!, I'll give them a try next time. Would be interesting to see the results between the sail being wrapped different directions! My starboard tacks are usually 'hotter' as well I'll try the idea of letting out a little more sail on port tacks and see if that helps. On port tacks when reefed I remember seeing the inside telltale streaming backwards toward the mast, I definitely remember thinking that was weird, but I was moving seemingly up wind, pointing more upwind luffed the sail. Maybe letting 6 inches out would fix all the weirdness, will try that next time. I was also trying to use the mirage fins in the down position to help with pointing |
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| Author: | JollyGreen [ Tue Aug 24, 2010 9:11 am ] |
| Post subject: | Re: Reefed Sail related to loss of pointing up wind? |
Sorry, I had port and starboard tacks flipped....ugh  So my port tacks are 'hotter' with my starboard tacks having the rolled up mast in the way of sail flow and port tacks having a smooth sail. I never thought about it, but maybe everyone wraps the mast different directions?
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