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Old 17-02-2018, 23:01   #31
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Re: Boat Speed Variation between tacks

Hard to accept at first, I agree, but once you do, you can also undstand that it was this principle that helped Glenn Ashby and Emirates Team NewZealand win the America’s Cup. Amongst other things, they were the only team able to extract advantage on Port tack....(there’s more to this story than meets the eye and it truly is hard to grasp, but is no less relevant to sailing in the modern era ...provided you have an adjustable, rigid wing sail.....a bit irrelevant to us with soft sails but fascinating all the same &#128512
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Old 17-02-2018, 23:28   #32
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Re: Boat Speed Variation between tacks

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Hard to accept at first, I agree, but once you do, you can also undstand that it was this principle that helped Glenn Ashby and Emirates Team NewZealand win the America’s Cup. Amongst other things, they were the only team able to extract advantage on Port tack....(there’s more to this story than meets the eye and it truly is hard to grasp, but is no less relevant to sailing in the modern era ...provided you have an adjustable, rigid wing sail.....a bit irrelevant to us with soft sails but fascinating all the same &#128512
So you are claiming this is the cause of a 40% delta between tacks. 6kts vs 10kts.
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Old 18-02-2018, 01:17   #33
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Re: Boat Speed Variation between tacks

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I quite agree! Until instrument bias is ruled out, I'll still give that the greatest chance of being the cause of the observation

Jim
I would go further - wind instrument error is the ONLY factor which could cause such a big difference between tacks for a boat without a really obvious list, obviously crooked mast, etc. There may be other factors contributing to this, but believing the wind instrument is definitely the main problem here.

The OP should ignore the wind instrument for a while, or even turn it off, and sail the boat by feel, finding and learning the groove on both tacks. Then calibrate the wind instrument so that it reads the same on both tacks.
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Old 18-02-2018, 04:41   #34
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Re: Boat Speed Variation between tacks

Comment here only so I can continue to follow this conversation.
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Old 18-02-2018, 04:48   #35
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Re: Boat Speed Variation between tacks

With respect Dockhead, you can regularly find well over 30% difference between port and starboard tacks and 40% isn't out of the question when there is a large amount of wind sheer. Please consider that the amount of wind sheer will change from day to day. As the atmospheric parameters change all the time...eg air temperature, water temperature, humidity, etc. so will the amount of wind sheer present.
I am often presented with examples of your reasoning as students try to grapple with this phenomenon for the first time. Perhaps you may find reading the literature will clarify your doubts.
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Old 18-02-2018, 05:26   #36
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Re: Boat Speed Variation between tacks

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Originally Posted by yachtingdirect View Post
With respect Dockhead, you can regularly find well over 30% difference between port and starboard tacks and 40% isn't out of the question when there is a large amount of wind sheer. Please consider that the amount of wind sheer will change from day to day. As the atmospheric parameters change all the time...eg air temperature, water temperature, humidity, etc. so will the amount of wind sheer present.
I am often presented with examples of your reasoning as students try to grapple with this phenomenon for the first time. Perhaps you may find reading the literature will clarify your doubts.
I can't imagine how this could possibly be true on cruising boats with masts only 15 or 20 meters high, but I will be interested to see what our experts on here say about it. And if you'll point me at some literature, I will be interested to dig deeper into it myself.
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Old 18-02-2018, 10:57   #37
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Re: Boat Speed Variation between tacks

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4. Below water line appendices and finish. Has one side more growth than the other? did one rudder get a hit and now is slightly off? It can certainly happen.
A very useful response, thank you. Your last point challenges my mind a little - would I experience that much differential if one hull was providing resistance? It is, after all, still a big old boat in a relatively light breeze, so I wouldn't be getting that much heeling. Or are you suggesting that the autopilot could over compensate for drag on one hull, slowing the boat down?

i have photos of the instruments that gave me the data above. I wish I had photographed the Sail Steer screen on the B&G.
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Old 18-02-2018, 13:16   #38
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Re: Boat Speed Variation between tacks

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Hard to accept at first, I agree, but once you do, you can also undstand that it was this principle that helped Glenn Ashby and Emirates Team NewZealand win the America’s Cup. Amongst other things, they were the only team able to extract advantage on Port tack....(there’s more to this story than meets the eye and it truly is hard to grasp, but is no less relevant to sailing in the modern era ...provided you have an adjustable, rigid wing sail.....a bit irrelevant to us with soft sails but fascinating all the same &#128512
Does the book you recommended contain any scientific data or is it more about observations that seem to match the theory? I'm not trying to be snarky, but simply very curious to find hard science, including finite element analysis or wind tunnel tests.

I have found many web pages describing the wind-shear-plus-coriolis-effect on sailing, but never FEA numbers to compare with sailors' reports. The closest to hard science I saw was from a Swedish page that had very good formulas, but I have to reject the entire entire thing.
Wind Shear - Fact or Fiction? - BLUR
Call it a typo, but the author incorrectly stated: "Due to the rotation of the earth, therefore, any air moving away from the equator in the northern hemisphere appears to drift westwards"
Ugh. Any 5th grade science student would properly call that backwards, as of course all straight-line movement in the northern hemisphere drifts to the RIGHT. (eastward in the authors example, not "westwards") That's like giving a gravity lecture and stating that objects fall up. Why the bad teaching? Because internet. I'm looking for the good stuff instead. Any suggestions on (real) hard science in this area?
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Old 18-02-2018, 13:26   #39
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Re: Boat Speed Variation between tacks

The cause is interesting, of course.

But there is a bigger lesson in here which doesn't actually have anything to do with the cause --

You shouldn't be stubbornly sailing the boat according to indicated AWA.

I guess it's yet another demonstration of the value of a dingy sailing background -- when sailing upwind, you should find the boat's groove, and stay in it, without regard to what the wind instruments say. Just because your wind instrument says you should be sailing OK, is no excuse for pinching. A decent sailor doesn't need a wind instrument at all to know that.
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I believe you refuse to go back without feeling of me;
We must have a turn together . . . . I undress . . . . hurry me out of sight of the land,
Cushion me soft . . . . rock me in billowy drowse,
Dash me with amorous wet . . . . I can repay you."
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Old 18-02-2018, 13:49   #40
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Re: Boat Speed Variation between tacks

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The cause is interesting, of course.

But there is a bigger lesson in here which doesn't actually have anything to do with the cause --

You shouldn't be stubbornly sailing the boat according to indicated AWA.

I guess it's yet another demonstration of the value of a dingy sailing background -- when sailing upwind, you should find the boat's groove, and stay in it, without regard to what the wind instruments say. Just because your wind instrument says you should be sailing OK, is no excuse for pinching. A decent sailor doesn't need a wind instrument at all to know that.


Totally agree, though even without dinghy experience, you can simply cover your instruments and tack a few times. Having a feel for trim and heading should be second nature, but easily acquired with a little practice sans instruments, in my experience.
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Old 18-02-2018, 14:20   #41
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Re: Boat Speed Variation between tacks

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Originally Posted by Dockhead View Post
The cause is interesting, of course.

But there is a bigger lesson in here which doesn't actually have anything to do with the cause --

You shouldn't be stubbornly sailing the boat according to indicated AWA.

I guess it's yet another demonstration of the value of a dingy sailing background -- when sailing upwind, you should find the boat's groove, and stay in it, without regard to what the wind instruments say. Just because your wind instrument says you should be sailing OK, is no excuse for pinching. A decent sailor doesn't need a wind instrument at all to know that.
ok you're taking this occasion to show off a little, and sure other will join you, at least you could wait for the OP's conclusion on what the problem is.
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Old 18-02-2018, 15:16   #42
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Re: Boat Speed Variation between tacks

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ok you're taking this occasion to show off a little, and sure other will join you, at least you could wait for the OP's conclusion on what the problem is.
Showing off was not intended. Rather, a different focus of the OP's attention. He is sailing to his wind instruments, and can't understand why he flies on one tack and luffs on the other. Really, he should stop doing that -- his instruments are deceiving him, distracting him from fundamental sailing -- he should turn off the wind instrument until he can feel the groove on both tacks. This is practical advice.
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"You sea! I resign myself to you also . . . . I guess what you mean,
I behold from the beach your crooked inviting fingers,
I believe you refuse to go back without feeling of me;
We must have a turn together . . . . I undress . . . . hurry me out of sight of the land,
Cushion me soft . . . . rock me in billowy drowse,
Dash me with amorous wet . . . . I can repay you."
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Old 18-02-2018, 17:19   #43
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Re: Boat Speed Variation between tacks

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ok you're taking this occasion to show off a little, and sure other will join you, at least you could wait for the OP's conclusion on what the problem is.
Hi rom, I thought all the suggestions here so far sounded quite well-intentioned, in the helpful spirit of CF. There were lots of interesting ideas and (happily) I learned something new to me. (wind-shear with Coriolis effect on sails)

I think about 8 posters so far (including you) suspected the wind instrument as a possibility. The OP has not yet described any efforts to "sail the upwind groove" WITHOUT instruments. I think Dockhead's advice was spot-on to help understand the importance of sailing by feel and sail shape, especially when instrument accuracy is questioned. Not sure why that is showing off?

Either way, I'm very interested in your experience too. I'm simply in love with your Lagoon 440 (after chartering one a while back). Did you ever experience a performance bias in sailing one tack vs the other, like the OP? I can't remember noting anything like that, but sadly I was only on the 440 for a week.
Cheers,
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Old 18-02-2018, 17:41   #44
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Re: Boat Speed Variation between tacks

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Old 18-02-2018, 18:13   #45
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Re: Boat Speed Variation between tacks

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And speaking of paddlewheels... ours works fine for the first few days after a bottom wash and then stops. We sort of think it's marine life clogging it, and if so would love some ideas about how to prevent that. But are there other explanations?

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You should lift the paddlewheel every day that you aren't sailing somewhere. Regular cleaning with a toothbrush also helps.
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