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Old 19-02-2018, 13:31   #61
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Re: Boat Speed Variation between tacks

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True wind on a boat, unlike on land, is water referenced (if it's ground referenced it's called "ground wind"). Trying to sail to ground wind in tidal water would make your brain melt down.
This is context dependent. When reporting weather or dealing with weather while on passage then true wind is ground based. If you are trimming sails, then water based is fine.
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Old 19-02-2018, 14:00   #62
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Re: Boat Speed Variation between tacks

If Coriolis effect were to have any difference, would it not totally depend on the direction you were sailing? and if the wind were to shift 180 degrees and you sailed 180 degrees differently would it not have the opposite effect?

Just thinking out loud???

I also vote for wind instrument calibration.

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Old 19-02-2018, 14:23   #63
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Re: Boat Speed Variation between tacks

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This is context dependent. When reporting weather or dealing with weather while on passage then true wind is ground based. If you are trimming sails, then water based is fine.
The true wind shown on the boat's instruments is usually relative to the water.
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Old 19-02-2018, 14:23   #64
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Re: Boat Speed Variation between tacks

Wind shear is real. On days when there is a big shear I can count on requests to recalibrate instruments, but I refuse to do it until things settle down. We get it most near the boundary of two opposing winds. However, it rarely is significant with winds over 10 knots.

TWS is a calculated number using AWA, BS and AWS measurements. One quick check on your instrumentation is to see if TWS varies between tacks, or between upwind and downwind. In this case there was pretty close agreement in TWS between tacks.
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Old 19-02-2018, 14:28   #65
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Re: Boat Speed Variation between tacks

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The true wind shown on the boat's instruments is relative to the water.
Yes,this is the normal way of calculating. I can feed either SOG or STW into my wind instruments. Since the error is often so high when.STW is used the TW calculation can.be way off.
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Old 19-02-2018, 15:38   #66
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Re: Boat Speed Variation between tacks

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Yes,this is the normal way of calculating. I can feed either SOG or STW into my wind instruments. Since the error is often so high when.STW is used the TW calculation can.be way off.
If you use SOG for TWA/S it is probably that you don't really care for TWA/S anyway. I don't care either. Until some day I have a new AP that can steer downwind using TWA, and then I will have to have a correct STW.
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Old 19-02-2018, 17:05   #67
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Re: Boat Speed Variation between tacks

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If you use SOG for TWA/S it is probably that you don't really care for TWA/S anyway. I don't care either. Until some day I have a new AP that can steer downwind using TWA, and then I will have to have a correct STW.
Actually I do care. I just feel that there are very large errors that get fed into the equation when you use a STW paddle wheel. I'm better off with a SOG based true wind calculation. Any drift and set can be added back in mentally if I think it matters which it rarely does. The sails will still see apparent wind.
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Old 20-02-2018, 01:48   #68
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Re: Boat Speed Variation between tacks

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You really should put down the textbooks and get out more. Wind shear and coriolis effect is such a tiny factor on boats with a mast of around 15m that it is not worth worrying about. Leave the worrying to the guys with 30m masts.
Hi Savoir,
I think you may be a bit off track or misguided with your suggestion. I am in fact a professional sailor and most years Iím on the water for more time than I am on land. (Although as I get older I can see that diminishing). In any case I find it interesting how people when faced with something that doesnít agree with their world view/beliefs, start to either jump to conclusions that are removed from reality (ie suggesting I get out on the water more....which if I did would mean I would barely spend time on land) or fall back on ideas that try to reinforce their world view without thinking that they could in fact learn something. I was under the impression that the reason for this forum was so that we could learn something. I could be wrong, and the reason for Naughty Catís speed discrepancy lies elsewhere or has a combination of alternate reasons. Be that as it may if you understand wind shear you understand that the effects arenít minimal at all. In the meantime, I think Iíll go for a sail...just for the fun of it....
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Old 20-02-2018, 04:31   #69
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Re: Boat Speed Variation between tacks

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Hi Savoir,
I think you may be a bit off track or misguided with your suggestion. I am in fact a professional sailor and most years Iím on the water for more time than I am on land. (Although as I get older I can see that diminishing). In any case I find it interesting how people when faced with something that doesnít agree with their world view/beliefs, start to either jump to conclusions that are removed from reality (ie suggesting I get out on the water more....which if I did would mean I would barely spend time on land) or fall back on ideas that try to reinforce their world view without thinking that they could in fact learn something. I was under the impression that the reason for this forum was so that we could learn something. I could be wrong, and the reason for Naughty Catís speed discrepancy lies elsewhere or has a combination of alternate reasons. Be that as it may if you understand wind shear you understand that the effects arenít minimal at all. In the meantime, I think Iíll go for a sail...just for the fun of it....
I think many of us, although skeptical, are entirely open to having our minds changed with facts.
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Old 20-02-2018, 04:40   #70
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Re: Boat Speed Variation between tacks

Clearly I seem to have offended you....I apologise if that is the case. I've done my best to help out a sailor with a problem. I've provided reference material to help explain....not much more I can do. I didn't do this to create an argument and I certainly don't want one....so best to leave this alone.
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Old 20-02-2018, 07:48   #71
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Re: Boat Speed Variation between tacks

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Clearly I seem to have offended you....I apologise if that is the case. I've done my best to help out a sailor with a problem. I've provided reference material to help explain....not much more I can do. I didn't do this to create an argument and I certainly don't want one....so best to leave this alone.
Is this addressed to me, or to someone else?

If to me -- I wasn't offended in the slightest. On the contrary, I am grateful that you have offered some new ideas. Just because I'm skeptical doesn't mean I'm not interested -- on the contrary. I would love to be proved wrong and learn something absolutely new.
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Old 20-02-2018, 08:38   #72
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Re: Boat Speed Variation between tacks

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Hi Savoir,
I think you may be a bit off track or misguided with your suggestion. I am in fact a professional sailor and most years Iím on the water for more time than I am on land. (Although as I get older I can see that diminishing). In any case I find it interesting how people when faced with something that doesnít agree with their world view/beliefs, start to either jump to conclusions that are removed from reality (ie suggesting I get out on the water more....which if I did would mean I would barely spend time on land) or fall back on ideas that try to reinforce their world view without thinking that they could in fact learn something. I was under the impression that the reason for this forum was so that we could learn something. I could be wrong, and the reason for Naughty Catís speed discrepancy lies elsewhere or has a combination of alternate reasons. Be that as it may if you understand wind shear you understand that the effects arenít minimal at all. In the meantime, I think Iíll go for a sail...just for the fun of it....
Hi yachdingdirect, I for one really appreciate the information and experience you bring to this discussion. As an engineer, I am always very happy to learn something new! (this was new to me)

I read a bit of the "Wind Strategy" book that you recommended on the Amazon preview page. It seems loaded with an interesting view of wind from a sailor's perspective. However, the science presented is pretty basic from what I can see. Do you know of any other references that dig deeper into the wind shear and Coriolis on the small scale of sails? I found some on the web, but as seen by the major error on the page I pointed out earlier in the thread, you get what you pay for. There seems to be little real science (with a quantitative study of real conditions) to explore the theory. (I wouldlove to find more on the subject!)
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Old 20-02-2018, 17:04   #73
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Re: Boat Speed Variation between tacks

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If you use SOG for TWA/S it is probably that you don't really care for TWA/S anyway. I don't care either. Until some day I have a new AP that can steer downwind using TWA, and then I will have to have a correct STW.
Our Simrad autopilot automatically switches to TWA steering at about 140'. For close to DDW sailing, TWA steering works very well.
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Old 20-02-2018, 18:47   #74
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Re: Boat Speed Variation between tacks

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Good morning all.



I would appreciate any views on the cause of a difference in boat between port tack and starboard tack on my catamaran. The difference is several knots and more apparent the narrower the apparent wind angle. On the port tack the boat performs well, on starboard it does not! Here is an example data set that I took from the instruments:



Port tack TWS 10.4, AWA 44,, boat speed 10.1ns



Starboard tack TWS 10.7, AWA 45, boat speed 6.2



Starboard tack TWS 11.2, AWA 45, Boat Speed 6.4.



Daggerboards were the same, the props are folded OK. I am assuming rigging? We played with the sheets but couldn't correct the differential.



Any views would be appreciated

Check that your speed paddlewheel is exactly aligned fore and aft - if it is even slightly angled to one side it will read differently on each tack. So thatís the first thing to check and easiest to fix.

If not the paddlewheel alignment, then it sounds to me that you may have the same issue as I had, which was due to a masthead wind instrument that is angled to one side and the instrument calibration was incorrect. Before calibration, when I sailed best upwind VMG my instruments showed 32 AWA on one tack and all good, but tacking over to the same 32 AWA and the sails are luffing and weíre going much slower. On that side, keeping the same sail trim, the properly sailing AWA was 44. If I tacked over using that angle then I was way faster and sails stalled on that tack. Once I calibrated the wind instrument I now can sail best upwind VMG at the same AWA on both tacks.

The easiest way to check if itís your wind instrument calibration is to not change the sheet setting when tacking. Take note of the instrument AWA on each tack while sailing your best upwind VMG. That is, keep the sail trim and boat speed the same on each tack - this is the opposite of what youíve reported with keeping instrument AWA the same on each tack and letting boat speed change.

If during the test you see that AWA is different on each tack then it is most likely your wind instrument needs aligning and/or calibration. My Raymarine instruments enable changing both settings.

If not a wind instrument problem, then it could be a much more complicated rig issue. But I donít believe it is a basic boat balance issue if your waterlines arenít visibly different - a larger cat like yours would have to be several hundred kgs on one side vs the other and visibly different to make a difference.
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Old 21-02-2018, 09:29   #75
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Re: Boat Speed Variation between tacks

Fxykty. Thank you. I am not discounting anything but similarly I will work my way down the list of causes. Weight will be last only because Catanas are know for taking a load well and I hope the wine that forced its way on board hasn't made that much difference!

How do you check the alignment of a paddlewheel. Mine slots into a sleeve with a watertight screw top - fairly standard I think. As I recall the sleeve has guides which dictate the obwrall alignment of the wheel and depth unit. Is this something you need to do hauled out or are you refering to changing the alignment of the wheel itself within its removable housing?
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