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I just completed some glider classes (that's really fun, by the way and very similar to sailing in so many ways). In gliding they have a concept for getting through areas of sink while burning off the least height that requires you to draw some "polar" graph intercepts after making corrections to the origin for the sink rate. This spits out the best speed to go though the lousy air. Cute. If you're interested, there's  a description here: http://www.5c1.net/Glider Performance Airspeeds.htm

Given the similarity to sailing, I'm mulling on a common problem on SF Bay. The wind is coming from the West through the GG at, say, 20 kts. The flood current 2kts (say - varies) comes from the same place, and you often need to beat across from relief behind Alcatraz to the relief on the beach. It's generally considered to be a good idea to drive in a fast mode across the bad current, but it's a bit more complicated because the leeway also changes the wind angle. 

As far as I understand it, best upwind speed/point is the point on the polar that intersects a hoizontal line at the top of the polar curve. Is there a way to either move the origin or rotate the boat polar to get the same effect as the glider airspeed curve to pick the right speed to go across the bad stuff to minimize the time to an arbitrary upwind mark. Does that math depend on just the bad crossing current, or the speed differential between the main flow and the current relief close to shore?

 

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Rasp you have the best instruments, that beats a telltale compass

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With a standard set of polars, and an old fashioned Maneuvering Board (Mo Board), you could easily spit out a couple iterations to come close.  Or if you have a GPS, you can just drop a mark, and then sail the course that gives you best VMG to that mark.

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The rough equivalent of the sailplane technique is called the "Wally" or "Wallying".

But for crossing the current, pretty much as Crash says, you will have to plot out the trade-off. Wind and current are roughly aligned which simplifies matters. Assuming your instruments have GPS input, they will read the correct true wind. A horizontal line from any point on a polar curve to the axis will read VMG, which needs to be corrected for the current.  If you choose to sail fast, you give up VMG and must get enough back after reaching shelter to compensate for the extra distance you will sail. And that is where assumptions are critical. Get out your maneuvering board.

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I have about 100 hours in gliders.  They say it's "just like sailing" but in reality I don't think there are too many transferable skills except for maybe some weather knowledge.   Anyway there are a million fancy ways to figure out speed to fly.  But a current could be thought of like a headwind.  A general rule of thumb is to add half the headwind speed to your airspeed.   But to answer your question I don't think it much matters just sail as fast as you can all the time.  The boat will not sink if you get too slow so I don't see any benefit to slowing down. 

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57 minutes ago, pilotdave said:

I have about 100 hours in gliders.  They say it's "just like sailing" but in reality I don't think there are too many transferable skills except for maybe some weather knowledge.   Anyway there are a million fancy ways to figure out speed to fly.  But a current could be thought of like a headwind.  A general rule of thumb is to add half the headwind speed to your airspeed.   But to answer your question I don't think it much matters just sail as fast as you can all the time.  The boat will not sink if you get too slow so I don't see any benefit to slowing down. 

"slowing down" takes you more directly upwind, but keeps you in the current longer. That's the whole point really.
It is similar to the calculus mi imisation problem from Freshman year.

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Pure headwinds or head currents  there is not a lot you can do about,  other than optimising your " angle of attack" into the current.  Flying or sailing at your minimum angle to the wind will probably slow you too much.

But glider pilots fly knowing there will be likely up currents of air in certain places,  off hills and slips,  hot spots on the ground etc. 

In sailing most of us are not sailing that far off shore,  and regularly race in one main area,.Similar to gliders you get currents and swirls along the coast. Knowing the state state of the tide , you need to learn what effects the tide has on any one place. 

I sail good part of the time on a river with tidal flow in either direction, up to 3 knots up stream 20 miles inland by river. 

So first thing on a race day check the official state of the tide, then think on what the weather will do to change that.

In our case a North or East wind will increase in inflow of tide,  slow the outflow  of tide.  High air pressure will increase the outflow of tide and reduce the inflow.

So then you sail accordingly, 

into the tide,  you hold longer along the river banks where you can,  then take a faster angle across the river curving up as you approach the bank. 

Going round bends you try to cut the inside of corners, with out running aground on the shallows, the outside of bends has a faster current against you. 

Tacking down tide you try to spend as much time as possible in the middle,  sometimes if the tide is really strong tacking well before the bank to stay in the tide. On bends you judge your tacks to spend time in the maximum tide on the outside of the bend,  but taking account of the shortest course, so that's an interesting balance to achieve. 

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well, the key in SF is that its so variable.  So if the flood is ripping, reach across it to the relief as quickly as possible.  If its only 2 knots, maybe kind of medium fast, unless the ebb on shore is getting fast. Also the size of the cone behind Alcatraz determines how far you want to go up to begin with.  Small/no cone?  Cross as soon as you can lay North Point.  Has it rained/snow melted recently?  Maybe the fresh water around the hull going out will mitigate the flood on the foils, regardless of what the tide book says. Sooooooo many variables on that racecourse.  Thats why people keep coming back for more. 

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Hi Wet Spreaders,

try this video. Might be a bit lengthy but it takes away much of the uncertainty of some of the above´s "feelings". Have a look at 19:23 which gives an astonishing view I once had experienced as a kid sailing on an inland river upstream of a barrage. Ih that environment you are mostly close to "land" to see at once the effect of what you are doing. But seeing is not understanding. So enjoy all those embarrasing experiences of sailing in current, that´s life!

 

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Here's my pretty picture of the problem. 

The variable that matters is the flood to future ebb differential. Obviously if there is no ebb at all and flood goes all the way to the beach, then best VMG is the best way to drive. If the ebb was infinitely fast, then the best option is always the shortest time across the flood. Therefore, since the ebb is finite, the answer is neither best VMG nor best boat speed.

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So, back to my original problem - is there a simple transformation (shift or rotation) on the polars that you can apply that provides guidance as to the best boat speed to set up to cross the current?

 

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It would be simpler (not simple, but simpler) if it were binary with max flood in the channel and zero flood in the relief, but of course it is more complicated than that since it is a gradient. And then of course your polars, themselves change with the increase in apparent wind in the relief. Then you have to add in the gradients in the cone of relief behind Alcatraz and the elusive but sometimes real reverse one on the face of Alcatraz. 

The best example of this is in RBBS when the PRO favors the starboard end of the line by MILES but it still pays to start at the pin and get to the relief first. Unless, of course, the PRO favors the starboard end by enough miles that the relief doesn't pay.

Good luck working it out with calculus. It is easier to just do it a thousand times in a one design fleet.

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Perhaps the second derivative is more interesting. Given the problem as stated, should you pinch or foot in the puffs?

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Since it is a human helmsman driving in actual chop with actual puffs and lulls, not a computer simulation, I like to tell the driver to sail "fat" or on the outside of the groove but not footing as if overstood. Instead of seeking an average of target boatspeed, make certain not to ever drop below target boatspeed. Similar for AWA. Since it is a long starboard tack full hiking is critical to get to the relief fast.

That is not mathematical advice, but it is advice that a helmsman can follow.

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On a free leg the polars show you to run down with the puffs and come up in the lulls. Given the problem, and having magically chosen the perfect course, do we pinch or speed in the puffs?

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2 hours ago, Wet Spreaders said:

Here's my pretty picture of the problem. 

The variable that matters is the flood to future ebb differential. Obviously if there is no ebb at all and flood goes all the way to the beach, then best VMG is the best way to drive. If the ebb was infinitely fast, then the best option is always the shortest time across the flood. Therefore, since the ebb is finite, the answer is neither best VMG nor best boat speed.

From your diagram, a practical problem seems to be whether it is actually practical to short tack inside the ebb flow on the left hand side. The diagram would suggest not and that every tack onto starboard will be taken in the tide; losing much of what you gained before the tack.

So that takes you back to going by the VMG.

But I recognise it might not be to scale.

It also raises the question of why you wouldn't have tacked as far upwind as you could have in the ebb tide to leeward of the Island

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3 hours ago, Wet Spreaders said:

So, back to my original problem - is there a simple transformation (shift or rotation) on the polars that you can apply that provides guidance as to the best boat speed to set up to cross the current?

 

No.

The instantaneous polar is a static time snapshot, it does not account for future conditions.  A routing algorithm can do that though.

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my 2cents 

1knot of current is worth 5 knots of wind .

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22 minutes ago, Mid said:

my 2cents 

1knot of current is worth 5 knots of wind .

Depends on how heavy displacement you are, with the current against a light wind  a heavy displacement may beat a light displacement. Against the current the light displacement boat should win every time..

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I think this could work for the simplified problem described, but only if you're not going to make it to the mark without getting into the eddy. The way to visualise it is to replace your vario with a device showing your upwind VMG relative to the ground. Your upwind VMG when you get out of the tide (upwind polar VMG + 0.5 knots) becomes your McCready setting. Set the ring on your "vario" appropriately, and head up or bear away in the same way you'd put the nose up or down when you're flying.

I suspect it wouldn't work in real life though. McCready theory relies on the fact that you "know" your rate of climb in the next thermal, and also relies on your not being able to get where you're going without using that thermal to climb. The equivalent is knowing how fast the eddy is flowing (which you do) and needing the eddy to get there (which, in 20 knots of breeze, you probably don't). 

I think the real World situation is more analogous to ridge flying in the mountains, where some bits of ridge are liftier than others, and with some patches of sink. The best approach there seems to be leave the electronic vario in thermalling mode, and judge how much to speed and slow down by feel. In a boat you'd instinctively foot off a bit while you're out in the tide, which feels sort-of equivalent.

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7 hours ago, SF Woody Sailor said:

It would be simpler (not simple, but simpler) if it were binary with max flood in the channel and zero flood in the relief, but of course it is more complicated than that since it is a gradient. And then of course your polars, themselves change with the increase in apparent wind in the relief. Then you have to add in the gradients in the cone of relief behind Alcatraz and the elusive but sometimes real reverse one on the face of Alcatraz. 

The best example of this is in RBBS when the PRO favors the starboard end of the line by MILES but it still pays to start at the pin and get to the relief first. Unless, of course, the PRO favors the starboard end by enough miles that the relief doesn't pay.

Good luck working it out with calculus. It is easier to just do it a thousand times in a one design fleet.

Yeah - it's obviously an entirely theoretical exercise and likely impractical in the real world with other boats around. But just because something can't be implemented to 5 significant figures, you can still get some benefit from working the trend in a slowly varying system. 

Personally, I find that taking a mode that gets me to landfall West of Aquatic Park is by far the best indicator of a good crossing. Sometimes it even seems better to press West from Alcatraz for a while before cutting over to the beach. But that's not what I'm really interested in - it's the mathematical exercise of how the polars work in a cross current with the expectation of a reverse cross current. Expedition would figure it out using a monte-carlo simulation, but that's not the game either - it's the notion that MaCready's glider observation has a sailing analogy that I want to test. 

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Hi guys, 

i fly on sailplanes since 2003. lot's of similarities between that and sailing but equally lot's of differences. oversimplifying a bit all the differences are mostly due to the different timescales entailed for both activities and the speeds relative to the surrounding environment.

ultimately both sports are energy balance/conversion games (i.e. kinetc for potential and viceversa with additional energy input provided by atmosphere and sea-atmosphere combinations). 

sailing with a mcready style polar adjuster could be done but it may add little value given the long term cycle of current flows. iterating over time optimal polar conditions given changes in energy inputs from atmosphere and sea is actually what a routing software does. de facto the use of isochrones is a different way of representing a minimization of energy loss (hence efficiency).

i use routing algos as i would use a mcready on a sailplane. and even on sailplanes electronics is really helpful in further refining the energy efficiency excercise.

my two cents.

 

 

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Sail planes.... Not gliders... A glider is a craft that CAN NOT gain altitude, basically is is always going down, converting the downward motion into forward momentum. A Sail Plane CAN gain altitude, the craft is able to stay aloft by using upwardly moving air. An example of a glider, Space Shuttle. Everyone says Glider, but the correct term is Sail Plane. Compare it to calling Wild Oats or Comanche a raft....

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A couple more thoughts:

I think the McCready approach would work if you know (from experience) that you're always going to hit the eddy and tack up it. In a glider you'd have some combination of electronic vario and glide computer to help you, and you could write something similar to do it on a boat; XCSoar, which a free open source glide computer, might be a good starting point.

If you think you're going to make it without tacking and just want to know when to foot and how much, that's more analogous to being on final glide, where you still dolphin through lift, but have a fixed arrival height. Again, XCSoar should contain the algorithms you need, and you're actually in a better position than you would be in a glider, because you should know where you're going to hit sink, and how much. Getting it wrong and having to tack up the beach is also rather less embarassing than sticking it in a farmer's field 2 miles from the finish.

In practice though, I'd go with the Monte Carlo approach; in a glider you have very data to work with (instantaneous lift/sink, expected lift in next thermal, wind velocity and glider polars) so a simplified approach works. There's a lot more data available in sailing (including a full tide atlas), and Monte Carlo seems like the easiest way to crunch that data.

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11 minutes ago, Topmast said:

Sail planes.... Not gliders... A glider is a craft that CAN NOT gain altitude, basically is is always going down, converting the downward motion into forward momentum. A Sail Plane CAN gain altitude, the craft is able to stay aloft by using upwardly moving air. An example of a glider, Space Shuttle. Everyone says Glider, but the correct term is Sail Plane. Compare it to calling Wild Oats or Comanche a raft....

"The Federal Aviation Administration (FAA) defines a glider as a heavier-than-air aircraft that is supported in flight by the dynamic reaction of the air against its lifting surfaces, and whose free flight does not depend principally on an engine. "

"A sailplane is a glider (heavier-than-air fixed-wing aircraft) designed to fly efficiently and gain altitude solely from natural forces, such as thermals and ridge waves. "

https://www.faa.gov/regulations_policies/handbooks_manuals/aircraft/glider_handbook/media/gfh_ch01.pdf

the FAA considers sailplanes to be a type of glider. so calling a sailplane a glider would be more like calling a ULDB a sailboat...... Its true and to someone outside the sport probably easier to get their head around, but its not the whole story.

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guys, can we avoid getting lost on semantics? i believe we are talking about the family of contraptions able to carry one or more men for a sustained flight without the use of an engine (hangliders, paragliders, sailplanes, etc)

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14 minutes ago, Topmast said:

Sail planes.... Not gliders... A glider is a craft that CAN NOT gain altitude, basically is is always going down, converting the downward motion into forward momentum. A Sail Plane CAN gain altitude, the craft is able to stay aloft by using upwardly moving air. An example of a glider, Space Shuttle. Everyone says Glider, but the correct term is Sail Plane. Compare it to calling Wild Oats or Comanche a raft....

They're called gliders where I come from; most of the clubs have the work "gliding" in their name, and they're affiliated to the British Gliding Association.

They all work the same way, it's just that some are way more efficient than others. At one end of the spectrum you have something like an Arcus with a 50:1 glide ratio, at the other end a Space Shuttle at something like 4:1, but they all work the same way. Even a Space Shuttle would climb if you stuck it in a big enough, strong enough thermal.

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Back to the current, IME you want to "cross" the adverse current as quickly as possible if you know that there is relief not too far away and your speed is comparable (say not more 2x 3x the current speed). Sadly I can't quantify it but I've seen footing off beyond "speed mode" (releasing a few inches of genoa sheet) paying off. If you start  pinching you are last!

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17 hours ago, Wet Spreaders said:

I just completed some glider classes (that's really fun, by the way and very similar to sailing in so many ways). In gliding they have a concept for getting through areas of sink while burning off the least height that requires you to draw some "polar" graph intercepts after making corrections to the origin for the sink rate. This spits out the best speed to go though the lousy air. Cute. If you're interested, there's  a description here: http://www.5c1.net/Glider Performance Airspeeds.htm

Given the similarity to sailing, I'm mulling on a common problem on SF Bay. The wind is coming from the West through the GG at, say, 20 kts. The flood current 2kts (say - varies) comes from the same place, and you often need to beat across from relief behind Alcatraz to the relief on the beach. It's generally considered to be a good idea to drive in a fast mode across the bad current, but it's a bit more complicated because the leeway also changes the wind angle. 

As far as I understand it, best upwind speed/point is the point on the polar that intersects a hoizontal line at the top of the polar curve. Is there a way to either move the origin or rotate the boat polar to get the same effect as the glider airspeed curve to pick the right speed to go across the bad stuff to minimize the time to an arbitrary upwind mark. Does that math depend on just the bad crossing current, or the speed differential between the main flow and the current relief close to shore?

 

This is a bit harder than the glider problem. You need to be able to model the changes in current too with location and depth. Using GPS to read VMG to a waypoint is probably best.

BTW - Did they teach you about predicting sink from terrain changes and colors yet?  If you are landing in field X that has a lot of light colored farm fields or blacktop areas farther out and a belt of dark green forested lands around the field, you're going to sink over the forest part. You speed up to not waste time in the sink ;)

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1 minute ago, Panoramix said:

Back to the current, IME you want to "cross" the adverse current as quickly as possible if you know that there is relief not too far away and your speed is comparable (say not more 2x 3x the current speed). Sadly I can't quantify it but I've seen footing off beyond "speed mode" (releasing a few inches of genoa sheet) paying off. If you start  pinching you are last!

Rowing across the C&D for lunch with a cranking current and a tanker laying on the horn, we learned real quick to just take a perpendicular heading, not worry about the current, and row upstream right next to the bank where the current was minimal. It ended up being faster and the temperamental tanker pilot calmed down with the big horn too.

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51 minutes ago, Topmast said:

Sail planes.... Not gliders... A glider is a craft that CAN NOT gain altitude, basically is is always going down, converting the downward motion into forward momentum. A Sail Plane CAN gain altitude, the craft is able to stay aloft by using upwardly moving air. An example of a glider, Space Shuttle. Everyone says Glider, but the correct term is Sail Plane. Compare it to calling Wild Oats or Comanche a raft....

I doubt sailplane tactics have anything at all to do with sailboat tactics. They lack any analogy of motion thru two different mediums. I.e. foils in both water and air. Also, I'm not a sailplane pilot, but I doubt sailplanes ever gain any altitude in the airmass. Again, because no keel in the water. However anyone who can noodle the calculus for sailplanes winning races can likely also figure out the calculus of sailboats crossing a foul currents.

I would think a non-racing sailplane would have some well-known airspeed for flying in descending air. It would seem to be a simple time-altitude problem. But change it to a timed race and it becomes much more like the sailboat problem: How long will it take to regain the lost altitude upon reaching the rising air? Requires a prediction about the weather on the other side of the foul current.

For sailboats the best tactic may be to stay in contact with the competition because knowing the variables is impossible. Or local knowledge.

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19 hours ago, The Q said:

Depends on how heavy displacement you are

my bad , I confess to not having even thought of heavy displacement ......

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58 minutes ago, Mid said:

my bad , I confess to not having even thought of heavy displacement ......

Are you talking about my weight again?:D

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5 minutes ago, The Q said:

Are you talking about my weight again?:D

that would be the pot calling the kettle .... :(

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Is the answer as simple as vector addition? A polar diagram is just a fancy name for a group of vectors that define your boats speed potential at a number of angles. Current is a vector also. For each heading on the polar, deduct the current vector and redraw the polar...for all current speeds and directions. And current also affects the wind speed (think of a buoy that can sense the wind but is not moved by it, but is drifting around in the current, what windspeed will it sense?), so pick the correct windspeed polar and do a vector addition of the current on the wind. Should probably do this first. And it will likely change tack to tack.

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54 minutes ago, Ncik said:

Is the answer as simple as vector addition? A polar diagram is just a fancy name for a group of vectors that define your boats speed potential at a number of angles. Current is a vector also. For each heading on the polar, deduct the current vector and redraw the polar...for all current speeds and directions. And current also affects the wind speed (think of a buoy that can sense the wind but is not moved by it, but is drifting around in the current, what windspeed will it sense?), so pick the correct windspeed polar and do a vector addition of the current on the wind. Should probably do this first. And it will likely change tack to tack.

Seen like this, it is quite simple, take the wind speed relative to the sea (true wind speed - current in vectors), then offset the centre of your polar (one in true wind speed not apparent as we are used to!) by the value of the current! Now you can find vmg as usual.

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12 minutes ago, Panoramix said:

Seen like this, it is quite simple, take the wind speed relative to the sea (true wind speed - current in vectors), then offset the centre of your polar (one in true wind speed not apparent as we are used to!) by the value of the current! Now you can find vmg as usual.

i tried sailracer, it it seems to me that the algo is doing just that. on longer passages i routage software does something similar. am i wrong?

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42 minutes ago, Ncik said:

Is the answer as simple as vector addition?

Yes and no. You can use vector addition to model a single track, but how do you figure out which track is the optimum?

If there aren't too many variables (uniform tide and wind across the course, or gliding between thermals) you can try to capture all the variables in a differential equation and then solve it to get the best course. That's really elegant, but only practical with a small number of variables (McCready being a good example).

For a more complex problem you go for the sledgehammer approach, which is Monte Carlo analysis. Effectively, you do the vector addition for lots of different tracks (potentially thouands) and see which one wins. It's not as elegant, but it does work, and can handle much more complex data sets such as a detailed tidal almanac or detailed wind modelling. It's not magic - it's still only as good as the data you put into it, but it is a very effective way of combining all that data. You need a lot more computing power, but computers are always getting cheaper and more powerful.

Way off topic, but for any glider pilots interested in elegant mathematical solutions to real-life problems, look up how a total energy probe works. Background for the non-pilots: glider pilots use a device called a vario to measure rate of sink/climb. This is effectively a device that measures the rate of change of air pressure; if you're climbing the air pressure drops; if you're sinking it rises. The problem is that you really want to know what the air around you is doing. If you're flying at constant speed and climbing it's because the air around you is rising, but if you're zooming along at 120 knots and then pull the stick back (which is exactly what you do when you reach a thermal) you're going to climb rapidly regardless of what the air's doing, because you're converting kinetic energy to potential energy; this makes it hard to tell what the airmass is doing because every change in speed is going to screw up your vario reading, and in a straight line you're adjusting your speed most of the time. One way to solve this problem is to measure your airspeed and then do some number-crunching to compensate for its impact on the vario readings, but gliders have been around a lot longer than computers, especially portable ones, and the basic instruments are nothing more than funky barometers. The total energy probe is a very clever mechanical device, with no moving parts, that automagically compensates for changes in airspeed and allows your vario to tell you what the airmass is doing.

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43 minutes ago, Chewbacca said:

i tried sailracer, it it seems to me that the algo is doing just that. on longer passages i routage software does something similar. am i wrong?

I have to say that I keep navigating tracing vectors on a paper map as for me it is part of the fun, so can't answer your question! I am not a total Luddite though as I like having OpenCPN showing where I am and were others are.

I suspect that there aren't many people thinking like me but when PC/tablets made their way on smaller boats doing RORC races10/15 years ago, I thought that it became less interesting. I wish I had realised 15 years ago what I wrote above as that would have helped me.

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21 hours ago, Topmast said:

Sail planes.... Not gliders... A glider is a craft that CAN NOT gain altitude, basically is is always going down, converting the downward motion into forward momentum. A Sail Plane CAN gain altitude, the craft is able to stay aloft by using upwardly moving air. An example of a glider, Space Shuttle. Everyone says Glider, but the correct term is Sail Plane. Compare it to calling Wild Oats or Comanche a raft....

I have been a pilot for decades and this is a new one on me. We have always used "glider" and "sailplane" as synonyms. Any airplane can "glide", but no one calls an airplane a glider that is simply not using their engine. For example, thanks to my CFI job I have countless hours gliding C-150s and C-172s, but they aren't gliders. An FAA license doesn't say sailplane on it, it says glider. Also your "cannot gain altitude" idea is false. A Space Shuttle has a glide ratio of 3:1, so it probably can't find enough lift anyplace to stay up, but other non-glider airplanes certainly can. I have been in ridge lift in a C-172 that would have kept me up all day if I wanted to hang around and mountain wave lift coming off the Appalachians actually was a big pain in a J-3, I didn't want to climb right then! One memorable day coming home through a line of convective activity I had to keep calling for higher, we were climbing about 1500 FPM at idle! 

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6 hours ago, Ncik said:

Is the answer as simple as vector addition? A polar diagram is just a fancy name for a group of vectors that define your boats speed potential at a number of angles. Current is a vector also. For each heading on the polar, deduct the current vector and redraw the polar...for all current speeds and directions. And current also affects the wind speed (think of a buoy that can sense the wind but is not moved by it, but is drifting around in the current, what windspeed will it sense?), so pick the correct windspeed polar and do a vector addition of the current on the wind. Should probably do this first. And it will likely change tack to tack.

this is the idea I had also, but I don't think it's a simple addition of the current vector. That might be correct for where you are now, but it assumes that your situation is infinite/stable and so VMG is still the best answer. The problem to solve relies on there being something better coming later in time - so that information needs to be captured in the math. The problem is "how much point do I sacrifice now in lousy water, in exchange for earlier arrival to the better water?"

On my boat, I can sail "reasonably" upwind in 20kts at 5.9kts to 7.0kts by changing modes. The TWA varies from about 42 degrees to 48 degrees over that speed range.  VMG is probably 6.7kts or thereabouts at 44 degrees. On a beam reach I could probably hit 7.8kts. If the flood current is 2kts of hurt, but you know that there's 0.5kts of help coming up,  how fast do you go?

What I'm becoming obsessed with is the idea that there's a transformation that can be made to the polar - probably a rotation related to the leeway differential between the two bodies of water. That would change the VMG. I'll draw a picture...

 

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I'm not trying to suggest that this is the right answer - I'm trying to illustrate the idea - in this case I rotated the polar by the sum of the leeway angles.

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7 hours ago, Dave S said:

Yes and no. You can use vector addition to model a single track, but how do you figure out which track is the optimum?

If there aren't too many variables (uniform tide and wind across the course, or gliding between thermals) you can try to capture all the variables in a differential equation and then solve it to get the best course. That's really elegant, but only practical with a small number of variables (McCready being a good example).

For a more complex problem you go for the sledgehammer approach, which is Monte Carlo analysis. Effectively, you do the vector addition for lots of different tracks (potentially thouands) and see which one wins. It's not as elegant, but it does work, and can handle much more complex data sets such as a detailed tidal almanac or detailed wind modelling. It's not magic - it's still only as good as the data you put into it, but it is a very effective way of combining all that data. You need a lot more computing power, but computers are always getting cheaper and more powerful.

Way off topic, but for any glider pilots interested in elegant mathematical solutions to real-life problems, look up how a total energy probe works. Background for the non-pilots: glider pilots use a device called a vario to measure rate of sink/climb. This is effectively a device that measures the rate of change of air pressure; if you're climbing the air pressure drops; if you're sinking it rises. The problem is that you really want to know what the air around you is doing. If you're flying at constant speed and climbing it's because the air around you is rising, but if you're zooming along at 120 knots and then pull the stick back (which is exactly what you do when you reach a thermal) you're going to climb rapidly regardless of what the air's doing, because you're converting kinetic energy to potential energy; this makes it hard to tell what the airmass is doing because every change in speed is going to screw up your vario reading, and in a straight line you're adjusting your speed most of the time. One way to solve this problem is to measure your airspeed and then do some number-crunching to compensate for its impact on the vario readings, but gliders have been around a lot longer than computers, especially portable ones, and the basic instruments are nothing more than funky barometers. The total energy probe is a very clever mechanical device, with no moving parts, that automagically compensates for changes in airspeed and allows your vario to tell you what the airmass is doing.

https://aviation.stackexchange.com/questions/7697/how-does-an-energy-variometer-work

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3 hours ago, kent_island_sailor said:

I have been a pilot for decades and this is a new one on me. We have always used "glider" and "sailplane" as synonyms. Any airplane can "glide", but no one calls an airplane a glider that is simply not using their engine. For example, thanks to my CFI job I have countless hours gliding C-150s and C-172s, but they aren't gliders. An FAA license doesn't say sailplane on it, it says glider. Also your "cannot gain altitude" idea is false. A Space Shuttle has a glide ratio of 3:1, so it probably can't find enough lift anyplace to stay up, but other non-glider airplanes certainly can. I have been in ridge lift in a C-172 that would have kept me up all day if I wanted to hang around and mountain wave lift coming off the Appalachians actually was a big pain in a J-3, I didn't want to climb right then! One memorable day coming home through a line of convective activity I had to keep calling for higher, we were climbing about 1500 FPM at idle! 

The senior instructor at my 'soaring club' is a UPS pilot whose day job is flying air freight in a 757. He like to tell our club students about leaving the West Coast heading for Louisville and encountering 'wave lift' crossing the High Sierra in which he could throttle all the way back to an idle on all engines and the big airplane fully loaded would still be climbing like a homesick eagle! 

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20 minutes ago, Rasputin22 said:

The senior instructor at my 'soaring club' is a UPS pilot whose day job is flying air freight in a 757. He like to tell our club students about leaving the West Coast heading for Louisville and encountering 'wave lift' crossing the High Sierra in which he could throttle all the way back to an idle on all engines and the big airplane fully loaded would still be climbing like a homesick eagle! 

We almost took a plane to the  shop once because a huge mountain wave was all over our route and it wasn't rough, so we were going from barely holding altitude at full power to gaining 30+ knots at cruise power. I was like "This engine is f'ed, what the hell is the issue with it" until we figured it out :rolleyes:

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4 hours ago, kent_island_sailor said:

I have been a pilot for decades and this is a new one on me. We have always used "glider" and "sailplane" as synonyms. Any airplane can "glide", but no one calls an airplane a glider that is simply not using their engine. For example, thanks to my CFI job I have countless hours gliding C-150s and C-172s, but they aren't gliders. An FAA license doesn't say sailplane on it, it says glider. Also your "cannot gain altitude" idea is false. A Space Shuttle has a glide ratio of 3:1, so it probably can't find enough lift anyplace to stay up, but other non-glider airplanes certainly can. I have been in ridge lift in a C-172 that would have kept me up all day if I wanted to hang around and mountain wave lift coming off the Appalachians actually was a big pain in a J-3, I didn't want to climb right then! One memorable day coming home through a line of convective activity I had to keep calling for higher, we were climbing about 1500 FPM at idle! 

One of my instructors ( electronics)  in the RAF could have done with that lift.

  Returning to the nearby airfield in his glider, he misjudged it and took out to electricity to the local town, and made the papers.....

 

A week later he did it again..

 

The CO banned him from gliding at that airfield ever again... 

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5 minutes ago, The Q said:

One of my instructors ( electronics)  in the RAF could have done with that lift.

  Returning to the nearby airfield in his glider, he misjudged it and took out to electricity to the local town, and made the papers.....

 

A week later he did it again..

 

The CO banned him from gliding at that airfield ever again... 

:lol:

Next time go under the power lines ;)

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1 minute ago, kent_island_sailor said:

Next time go under the power lines ;)

Bonus points for doing it at 130 knots before pulling up and flying a circuit...

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My "real" gliding was in a 172 with a blown engine.
ATC: Go to XXX airport.

ME: No, I am going over to that farm field ahead of me.

ATC: You can't land in some guy's field! Go to the airport!!

ME: Well I would if I had an engine :rolleyes:

The farmer was cool and let the plane sit until the A&P came out with a new engine.

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2 hours ago, Wet Spreaders said:

image.png.3d6077ca07c761774b4f44224fcfae83.png

I think that you want to translate it instead of rotating it (see my post from a few hours ago).

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Back to the OP  - For the most part, this is not a solvable problem without access to more accurate current data than most of us would have.
If you want to take a stab at it, make up a defined scenario and graph the VMG at 2 degree intervals and you should get a result that works if those conditions hold true.

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3 hours ago, Wet Spreaders said:

this is the idea I had also, but I don't think it's a simple addition of the current vector. That might be correct for where you are now, but it assumes that your situation is infinite/stable and so VMG is still the best answer. The problem to solve relies on there being something better coming later in time - so that information needs to be captured in the math. The problem is "how much point do I sacrifice now in lousy water, in exchange for earlier arrival to the better water?"

On my boat, I can sail "reasonably" upwind in 20kts at 5.9kts to 7.0kts by changing modes. The TWA varies from about 42 degrees to 48 degrees over that speed range.  VMG is probably 6.7kts or thereabouts at 44 degrees. On a beam reach I could probably hit 7.8kts. If the flood current is 2kts of hurt, but you know that there's 0.5kts of help coming up,  how fast do you go?

What I'm becoming obsessed with is the idea that there's a transformation that can be made to the polar - probably a rotation related to the leeway differential between the two bodies of water. That would change the VMG. I'll draw a picture...

 

To answer that question precisely, you need a proper routing software I think. Nevertheless with the transformed polar I imagine you can make a much more educated guess than "crossing the bad current as quickly as possible". Often in these kind of situations there are a lot of tactics happening (getting clean air, getting to the next mark on starboard tack etc...) so I reckon that the modified polar would be good enough as what you really want is a clear idea of the best strategy as opposed to a heading precise to 1º

Edit : Crosspost with @kent_island_sailor

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It is a tough optimisation problem. Routing software is require as well as good information as input to it. All reasonably possible tracks need to be compared and the best one chosen.

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1 minute ago, Ncik said:

It is a tough optimisation problem. Routing software is require as well as good information as input to it. All reasonably possible tracks need to be compared and the best one chosen.

What you need to do is sail in a good one design fleet and go to school on the leaders. 

For anything else, this attempted calculation is the equivalent of sitting at your chart table, playing with buttons and losing sight of the big picture.  

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9 hours ago, Panoramix said:

To answer that question precisely, you need a proper routing software I think. Nevertheless with the transformed polar I imagine you can make a much more educated guess than "crossing the bad current as quickly as possible". Often in these kind of situations there are a lot of tactics happening (getting clean air, getting to the next mark on starboard tack etc...) so I reckon that the modified polar would be good enough as what you really want is a clear idea of the best strategy as opposed to a heading precise to 1º

Edit : Crosspost with @kent_island_sailor

Please find me a sailor who can hold a heading on San Francisco Bay to 1°.  Or anywhere actually.  

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59 minutes ago, Left Shift said:

Please find me a sailor who can hold a heading on San Francisco Bay to 1°.  Or anywhere actually.  

Holding a heading to 1 degree is worse than useless anyway. But I bet I can hold AWA to 1 degree without any problem at all - when the boat is set up right and trimmed right, it will do this by itself in a steady breeze and a few feet of chop. Obviously not so much on a blustery day with swell.

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5 minutes ago, Wet Spreaders said:

Holding a heading to 1 degree is worse than useless anyway. But I bet I can hold AWA to 1 degree without any problem at all - when the boat is set up right and trimmed right, it will do this by itself in a steady breeze and a few feet of chop. Obviously not so much on a blustery day with swell.

Upwind you mean. With a name like Wet Spreaders I assume you sail a Ranger 37 so in 25 knots the apparent wind swings at least 130 degrees during the round downs. Or at least that was my experience.

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On 6/10/2020 at 12:29 AM, Rambler said:

It also raises the question of why you wouldn't have tacked as far upwind as you could have in the ebb tide to leeward of the Island

Basically you do leave the island so close to starboard that you could reach out and touch it, but there are two additional complications.

1) there is often relief (a reverse cone) on the up-current side of Alcatraz

2) there are 30 other boats with the same plan, and if you leave the corner of the island on that layline in anything other than first place you will be in dirty air ALL THE WAY to the relief. 

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15 hours ago, kent_island_sailor said:

The farmer was cool and let the plane sit until the A&P came out with a new engine.

If it had been a *real* glider you could just have taken the wings off, chucked it all in the trailer, and been home in time for dinner.

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5 hours ago, Left Shift said:

Please find me a sailor who can hold a heading on San Francisco Bay to 1°.  Or anywhere actually.  

I agree, but that was kind of my point!

Nevertheless, a good helm, will do it to 2/3º (basically 242º equals I left the needle drift toward 235/240 more than towards 245/250 , useless nowadays with GPS but that was really useful in the 80's when sailing coastal in foggy and tidal conditions!

I suspect that the modified polar will give you an answer such as "sail about 10º lower than normal VMG" which will be plenty good enough considering that there will be plenty of other contradictory parameters such as dirty air to account for.

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Fat older white men in their chairs while thinking too much (see that a lot on this forum).....Just go out and sail the damn boat for Christ sake!

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Just wanted to share this with all you pilots: 

My daily commute passes the site where Lilienthal finally crashed and died. A very modest hill with a tasteful monument. 

CD8733DF-D1CE-48D6-BA5C-E0DF2E2A1C38.jpeg

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1 hour ago, premiumunleaded said:

Fat older white men in their chairs while thinking too much (see that a lot on this forum).....Just go out and sail the damn boat for Christ sake!

Errr - OK. Not sure where you get that from. I could be a blue, emaciated female teenager as far as you know.

Also, since sailboat buoy racing usually happens in the day, educated people have time to think in the evenings when idiots that failed algebra are sitting on the sofa getting drunk, watching pulp TV shows and making il-considered forum posts. 

See - anyone can play the "make shit up based on zero facts and post it" game. 

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Try this to get a rough idea for 2 knot current and 20 knot wind from same direction parallel to shore line:

Draw a line to scale representing your best boat speed and course to steer (CTS) to windward (up page) for the distance to get into the slacker water. Adjust (down page) for leeway and 2 knots current. Work out the time elapsed (TMG).

Now draw a line representing shortest distance across to be directly downwind of the point above. Adjust (up page) for leeway and current for CTS. You know from polars or experience what the boat speed is likely to be, and that will give you a TMG.

The difference between the two TMG’s gives you the time needed to tack upwind in slacker water between the two adjusted points. Those are the two extremes. And it should give you a good idea, without too much trial and error of different CTS’s in between to find roughly what the optimum CTS is likely to be....

There will be different answers for different boats, different shore line angles and different wind/current scenarios?

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PS ....... And you should also reduce the distance upwind by the reduced time exposure to the 2 knot current....

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