Short tacks or long tacks for best VMG?

[jviss]

Hey Don,

I have a route - Westport Harbor (MA) to Block Island - 27 nm. bearing 255 magnetic. Winds steady from the Soutwest means motoring with the wind on our nose, but if we sailed - short tacks or long tacks? Catalina 36, tall rig.

Thanks,

jv

 

[justsomeguy]

The fewer, the faster, generally.

 

[jviss]

The fewer, the faster, generally.

Based on what? One tack would be the longest path through the water.

 

[justsomeguy]

I based my answer on loss of speed that you may have experienced when coming about.
I suspect there's a point of diminishing returns, however.

 

[jviss]

I based my answer on loss of speed that you may have experienced when coming about.
I suspect there's a point of diminishing returns, however.

We're talking about a 27 mile rhumb line here!

 

[kloudie1]

if it is given that the tacking angle, wind, and current, and wave action are constant, the distance is the same.. the many shorts will equal the two long legs.. The time spent tacking and the acceleration time will make the many short tacks slower.. the thing that happens sometimes (?) is that the wind shifts and if you are way out there on the end of one long , you may be way off the lay line compared to the guy who is doing shorter tacks.

 

[justsomeguy]

Where I sail, it might be faster to wait for the wind to do a 180.

 

[RichH]

The 'pedantic' answer is sail on the tack which provides the best VMG. How long do you stay on that tack? ..... time on each of the TWO tacks is the ratio of Trigonometric Cosine of the sailing angle of the first leg to the Trigonometric Cosine of the sailing angle of the second leg ... for not one inch of extra distance sailed.
Calc.: Trig. Cos. of sailing angle of port tack divided by Trig. Cos. of sailing angle of starboard tack ... 'times' .... total estimated elapsed time. .... and where the 'hypotenuse' is boat speed.
If you have integrated GPS with 'windmachine', this can be plotted 'automatically'.

The pragmatic and 'reality' approach would be to tack on the inevitable windshifts that continually give you best VMG as you go up and in 'reasonable' close proximity to the rhumb line. This is how long distance sailors lacking fully integrated instruments do it.

Simple speak: dont sail off in the wrong or least efficient direction/tack; always tack on the advantageous windshift. Dont sail too far (in miles) from the rhumb line in case you encounter an unexpected 'permanent' wind shift.

;-)

 

[jviss]

The 'pedantic' answer is sail on the tack which provides the best VMG. How long do you stay on that tack? ..... time on each of the TWO tacks is the ratio of Trigonometric Cosine of the sailing angle of the first leg to the Trigonometric Cosine of the sailing angle of the second leg ... for not one inch of extra distance sailed.
Calc.: Trig. Cos. of sailing angle of port tack divided by Trig. Cos. of sailing angle of starboard tack ... 'times' .... total estimated elapsed time. .... and where the 'hypotenuse' is boat speed.
If you have integrated GPS with 'windmachine', this can be plotted 'automatically'.

The pragmatic and 'reality' approach would be to tack on the inevitable windshifts that continually give you best VMG as you go up and in 'reasonable' close proximity to the rhumb line. This is how long distance sailors lacking fully integrated instruments do it.

Simple speak: dont sail off in the wrong or least efficient direction/tack; always tack on the advantageous windshift. Dont sail too far (in miles) from the rhumb line in case you encounter an unexpected 'permanent' wind shift.

;-)

Thanks, Rich. Yes, pedantic it may be, but it's also utterly meaningless! It's a tautology - you're saying the angle is the angle, and it doesn't provide any guidance or insight into what one should do: what angle, how long on each tack, etc. I know enough about this to know that the performance of the boat on different points of sail make a difference; so without referring to the polar chart it's rather meaningless. I also know that it's generally a good idea to not stray too far from the rhumb line, though one piece I didn't mention is that this course is to the buoy to the North of Block, and I'll be turning South by Southwest after that, to get into New Harbor, so it would be good to leave that, the endpoint, to port. If I tacked onto a starboard tack and then continued past the buoy, that would be fine.

Watching the shifts is good advice, but this is expected to blow steady from the SW all day long. On average - SW.

p.s. I don't have a gauge that displays VMG, and my chart plotter doesn't either.

 

[Stu Jackson]

T

p.s. I don't have a gauge that displays VMG, and my chart plotter doesn't either.

Our way old Magellan Blazer, the GPS with ONLY numbers, "didn't" have a VMG function either. Until I did some research and found that the TTG function was VMG! When it says zero, tack.

Long tacks are favored over short tacks - a basic concept. But if you have no current (which won't be the case for that distance, but...) and your destination lies dead upwind, you end with equal tacks. The reference sources I have reviewed (Hiscock and Calder's Cruising Handbook) both say use a 20 degree cone to call your tacks.

With currents (like sailing across the English Channel with the wind from dead ahead) the skippers calculate the effect of current on their courses and steer accordingly. so if you only use your GPS to a waypoint, you'd sail a much longer course.

 

[dugout]

JVISS;
RichH gave you the best possible answer. "steady from the SW all day long" is meaningless. There is no such thing as steady winds. There are always cycle shifts.
The farther off the rhumb you carry a tack the more you roll the dice. A lift pays off and headers are disasters. Without VMG instrumentation you calculate each tack's VMG by hand or you sail by the seat of your pants. Pre-planning your tacks to a windward mark 27 miles off, to leave the mark to port? Good luck with that plan!
I would shove off, get her into the wind, hold on the lifts, tack off the headers, stay reasonably close to the rhumb, and enjoy the sail, but that's just me.

 

[jviss]

JVISS;
RichH gave you the best possible answer.

I beg to differ. The question was "Short tacks or long tacks for best VMG?" His answer was "sail on the tack which provides the best VMG."

Can you explain that to me? Is that a sensible answer?

Then this:

"How long do you stay on that tack? ..... time on each of the TWO tacks is the ratio of Trigonometric Cosine of the sailing angle of the first leg to the Trigonometric Cosine of the sailing angle of the second leg ... for not one inch of extra distance sailed."


Can you explain that to me? Not only does the calculation not make any sense, the result is not even in units of time (ratios of cosines will have a unit-less value of from -1 to +1). I don't know what "for not one inch of extra distance sailed" means.

 

[dugout]

I'll take one more swing at this.

I beg to differ. The question was "Short tacks or long tacks for best VMG?" His answer was "sail on the tack which provides the best VMG." Can you explain that to me? Is that a sensible answer?

It is the ONLY answer. It's a conditionally based question. Stay on the tack as long as it lifts or you expect to get lifted. Tack when you get headed or expect to get headed. keep doing that till you get there.

Then this: "How long do you stay on that tack? ..... time on each of the TWO tacks is the ratio of Trigonometric Cosine of the sailing angle of the first leg to the Trigonometric Cosine of the sailing angle of the second leg ... for not one inch of extra distance sailed." Can you explain that to me? Not only does the calculation not make any sense, the result is not even in units of time (ratios of cosines will have a unit-less value of from -1 to +1). I don't know what "for not one inch of extra distance sailed" means.

I'm not much on the math but RichH is using a pencil to do what folks like me do by the seat of the pants, compare the tacks. RichH stated clearly it is all about VMG and comparison of the two tacks, port and starboard. One tack will always be closer to the rhumb line than the other at any given time. Stay on that tack. Don't try to make rocket science out of this. Even with real polars and an OCKAM system the time to stay on a given tack is based upon real time conditions and a pure tactical decision. Strategic calculated plans are just mental masturbation.

 

[Joe]

Jviss.......I think the only solution is for you to learn how to use VMG info on basic GPS. My 10 year old GPS76 offers VMG, if yours doesn't then it's time to move into the 21st century and quit criticizing people for trying to answer your question.

All you need for the VMG calculation is a waypoint.... Start sailing in navigation mode after you have entered your destination then display the VMG data. Choose the tack that gives you the best reading (the longest one, most likely) watch the indicator and tack when the vmg starts to drop..... it's as simple as that.... you don't need to know trigonometry.. just a simple handheld gps and a waypoint.

 

[jviss]

Strategic calculated plans are just mental masturbation.

That's very nice, thank you for that. You're a real gentleman.

 

[jviss]

...it's time to move into the 21st century...

What's so 21st Century about sailing? Sure, I could get an instantaneous VMG readout, but it doesn't advise as to sailing strategy.

 

[jviss]

if it is given that the tacking angle, wind, and current, and wave action are constant, the distance is the same.. the many shorts will equal the two long legs.. The time spent tacking and the acceleration time will make the many short tacks slower.. the thing that happens sometimes (?) is that the wind shifts and if you are way out there on the end of one long , you may be way off the lay line compared to the guy who is doing shorter tacks.

Thanks, that sounds right, that all things being equal the distance through he water is the same, and therefore the fewest tacks is the most efficient. The variability of the wind and tide is what I should be watching so I don't get caught far off the rhumb line with a disadvantageous wind shift.

 

[jviss]

Our way old Magellan Blazer, the GPS with ONLY numbers, "didn't" have a VMG function either. Until I did some research and found that the TTG function was VMG! When it says zero, tack.

Long tacks are favored over short tacks - a basic concept. But if you have no current (which won't be the case for that distance, but...) and your destination lies dead upwind, you end with equal tacks. The reference sources I have reviewed (Hiscock and Calder's Cruising Handbook) both say use a 20 degree cone to call your tacks.

With currents (like sailing across the English Channel with the wind from dead ahead) the skippers calculate the effect of current on their courses and steer accordingly. so if you only use your GPS to a waypoint, you'd sail a much longer course.

The truth is I could use the nav station mounted GPS for VMG, but I don't think an instantaneous VMG readout is going to inform tactics very effectively.

I like the "20 degree cone" concept. Does that mean a cone extending from the target back towards the starting point, and tack when you reach the cone (for shorter tacks as you near the target)?

 

[Tsatzsue]

jviss,
Good to see a fellow Westporter. I trailer but that is my favorite departure point. I was looking at Block for this weekend as well. I was going to put in at Galilee. Sunday looks iffy for a V25 in Block Island sound although the wind would be on our back. I like the seat of the pants claim. I calculate the closest angle with the current wind and try to plan my tacks to as few as possible with the final destination in mind. It sounds like you have that down with the last one bringing you in to the harbor. Leave the math home. Mathis not fun...sailing is. Good luck and have fun. That's a good cruise. PS: I navigate with Navionics on my DroidX. Nothign fancy. Boat pic is on the beach at Horsneck Point. KB

 

[weinie]

I've got a follow up question.

Which will finish faster: A boat without a spinnaker or whisker pole sailing wing on wing versus an identical boat gybing back and forth on broad reaches?

(I was watching the local race last night and I would've place my money on the gybers!)