Up-wind sailing

kloudie1 · Apr 26, 2009

Going to be a close call.. Best way to figure this out is to look at the VGM numbers on your GPS. It should be easy to call that display. Put the upwind destination as your waypoint, then display VMG. Watch that number as you vary course, speed, and sail trim. The VMG is the speed toward the destination so this will give you the answer you are looking for on your particular boat. Currents will make a surprising change in VMG , so you may have to observe both tacks. It takes the GPS a while to work a good number, so wait about 5 minutes to see a good result. Good Luck!

jackhartjr · Apr 26, 2009

One of my many books on sailing states that no matter if you tack at 45 degrees to get there or run straight up and then turn 90 degrees that you cover the same distance. I didn't believe it and checked...sure enough it did.
Therefore do you sail faster doing the 45 degree tacks or by going up and then turning 90 degrees.
(I just know I will see a lot of debate on what I just said!)
Jack

Bill Roosa · Apr 26, 2009

Sailing and the upwind math

Not sure if I agree with jackhartjr or not. Need a little more understanding of how he came up with his numbers before I'd say "the distance is always."
I suspect he has not included the speed you go on the two different tacks verses the distance you have to travel along the two tacks.

kloudie1 · Apr 26, 2009

Jack, you are correct if there is no wind shift and no current.. but the question really was .. if I can sail at 6 kts 40 degrees off course upwind or I can sail at 8 kts 50 degrees off wind, which actually gets me to the destination the fastest.. here the answer is 8 kts at 50 degrees but the trigonometry has to be done based on an individual boat’s polar diagram, which shows best speed at any given wind angle. .. A GPS can do the trig pretty closely.

RichH · Apr 26, 2009

ditto what Kloudie states.

Follow the VMG function of your GPS. Some GPS dont have this function and you can then simply 'favor' the tack which shows the earliest ETA to your distant waypoint.

When following VMG (or earliest ETA) do some trials at slightly differing sailing angles as you watch the VMG while steering to get the 'best number'. Perfect sail trim and perfect sail shape will be needed for the 'best' VMG ..... but will probably vary during the long passage so you need to check the angles/trim/shape quite often. Also due to 'coriolis effect', the direction of the oncoming waves will be usually different (not equal) on opposite tacks .... so you need to carefully and often 'test' VMG on both tacks to insure that you are mostly keeping on the favored tack. Ditto with waves and swells generated by distant weather systems.

There are packaged instrument systems that can offer automatic VMG, etc. and calculate/plot WHEN to tack for best overall/optimum courses. Most of the top racing boats use such to optimize their 'performance'; yet. long distance cruisers can take advantage of such systems to minimize the total distance travelled. If you save 5%-10% of time during a 10-12 hour day .... thats ~1/2-1 full hour travel time saved (and NOT changing the boat speed!!!!).

Since the wind is never constant in speed nor direction, tactically you need to tack when the apparent wind direction goes forward ... and then you need to tack of the 'windshifts shifts' - good elemental sailboat racing texts describe the technique in detail. Generally - Just follow maximum VMG (or earliest ETA) on the GPS and tack if you must to keep the VMG at the highest possible value.

Scalloping (two variations)
1. Scalloping for waves .... to lessen the boats bow from plunging deeply into the face of oncoming waves (thus tending to stop the boat).
Dont let the boat 'leap' off the top of a wave crest, but rather bear off and steer at a greater angle down the backside of the wave at an angle so you sail through the trough at a greater angle (instead of allowing the bow to 'bury' into the face of the next oncoming wave); then, as the boat begins its climb/rise up the face of the next oncoming wave head up a little, so as to 'set' the boat for the next bear off down the back side of the next wave. This is a good technique for moderate to big waves that would otherwise keep stopping the boat. Summary: the boat will be a greater angle to the waves at the top of the waves and down deep in the trough .... the distance traveled looks like an "S"

2. Light wind/ small (or no) wave scalloping. Done in relatively 'flat water'.
(usually needs a small row of 'steering telltales' - a 'row' of small telltales attached to the sail at approx. your 'head height' running from the very front (luff) of the sail thence aft for about 8 feet, usually about 6-8 very small tales all in a 'row'
Once the sails are shaped and trimmed to perfection, the helmsman uses the 'steering row' as the ONLY reference to steer the boat. The helm is constantly but very lightly (fingertips on the helm) allowed to begin to head-up slightly until the first one or two of the steering row becomes 'agitated' on the leeward side. Such agitation will indicate the beginning of a 'separation stall'. Once the steering row 'begins' to become 'agitated' the helmsman gently bears off slightly until the steering row on the leeward side become 'quiet'. This will assure that 'aerodynamics' of the present sail trim/shape are working to the optimum. In following the 'steering row of tales' in this manner the boat will follow a gentle "S" course.
On most fin keeled boat you want your sail trim/shape so that the boat will have approximately 3 degrees of rudder angle to hold a beat - very 'light' weather helm pressure. Once you have set/trimmed to perfection and on a hard beat, increase your main halyard and/or cunningham tension until the 'helm' goes totally 'neutral' (no weather helm, no lee helm) ... then ease off until you have a 'very light or VERY little pressure' weather helm.
This is probably the fastest and most precise way to sail a boat on a beat .... but the sail trim and sail shape should be perfect. Such scalloping will usually result in the (fin) keel to begin to 'lift' towards windward by a few degrees .... which begins to counteract the normal 'slip to leeward' of the hull .... the boat is now pointing like a banshee and is 'fast'. This is a very effective steering method for FAST sailing in light to moderate winds (& minimal waves).

The above reply may be more complicated than what you need or desire; so, just extract what you need and what seems reasonable to you.

hope this helps

Alan · Apr 26, 2009

If you take a closer look at the polar that you posted it will show the best combination of speed/angle to sail. If you draw a horizontal line across the highest tangent point on the graph, that will be the optimal sailing angle for best VMG to weather. As you can see this angle changes slightly based on the apparent wind speed. But for example, the boat in this graph would sail a course of 44 degrees to the true wind in 8 kts of wind for best VMG.

tsmwebb · Apr 26, 2009

The math is pretty simple: VMG = boatspeed * cos(wind or course angle). You can make up a little crib sheet before you go out if you like. Just to be pedantic VMG (velocity made good) comes in two flavors (upwind and downwind) and is defined in terms of the true wind angle. If you have an integrated sailing instrument package it will probably give VMG. VMC (velocity made good on course) is what GPS's report.

A couple of things to note:

When sailing VMG or VMC always use long averages. Anytime you point more towards the wind or your waypoint your VMG/C will rise because it takes a while for the boat to slow down. If you head off or away from your WP the VMx will go down until you get up to speed. On a large boat like yours you should probably be looking at averages over several minutes (longer in light air).

When sailing VMC as you approach the layline your VMC will approach zero. When you cross the layline it will go to negative numbers. That doesn't mean you don't have to get to a layline to get to your WP! So VMC will always show that the longer tack is faster and you have to keep that in mind.

VMG from your instruments will not take into account leeway or current. In practice ignoring current when looking for target numbers is a good thing but ignoring leeway is a bad thing. But we're getting into racing stuff here so I'll drop it at that.

If you can collect average VMG data for several different true wind speeds and angles you can make a pretty good stab at getting target speeds. For any given true wind speed/sea state there is an ideal target speed for you boat. Unlike VMG you can steer towards target speed. That is if the boat is above its target speed you should head up (in the upwind case) instantly to bleed off some speed and gain some height. This, too, gets complex pretty quickly, but if you can get a general idea of your target speed for best VMG then you can use it as guidance when the seat of your pants is a little confused.

--Tom.

PS.

I notice from looking at the link that they have already figured out target speeds -- look at the tables below the graph. Up and downwind VMG are the little squares on the graph so they've done that, too. The downwind numbers assume you set a spinnaker and all the numbers assume you're racing with all that implies. Bottom line: if those numbers are correct you are running a bit over target speeds (ie. footing a little) but for cruising, IMO, that is probably a good idea because trim and steering are less critical when you're footing.

--T.

Stu Jackson · Apr 26, 2009

KISS

In the real world you will almost never be able to have equal tacks because even in racing the wind is never actually perfectly perpendicular to the start line, meaning going upwind toward any given point of your choice doesn't mean the wind is exactly from that point. What this means is that your VMG will always be different on different tacks.

But, for the time being, let's assume that your destination is really exactly into the eye of the wind and that both tacks give you exactly the same VMG, AND that there's no current and leeway is exactly the same on both tacks. Got it?

All your polars are doing is showing that the more off the wind from closehauled the faster your boat will go. Period. That's the SAME for ALL monohulls - they simply go faster on a close reach or a beam reach than they will closehauled.

So, the case could THEORETICALLY be made that if you go faster a few degrees off the wind, say close reach instead of close hauled, that you'd get there faster.

That's ALL your question really is.

So, watch some racers out on a course going upwind. Ever see ANY of them pointing off and NOT going as close to the wind as they possibly can without pinching? Almost NEVER. And the "almost" is ONLY because of conditions (like, sea state, swells, wind waves, current, sail trim, etc. - all the complicating issues ARE real world issues, but don't have a danged thing to do with your question) that we have DELIBERATELY left OUT of this example.

As was previously mentioned, you should do the math the find out if that extra speed over a longer course will give you a quicker ETA.

Notice I said ETA and NOT the VMG. This is simply because as you fall off from closehauled to any destination, the VMG will DROP. Period. It will get smaller because you're not going as close to a straight line to the destination.

Get a chart or a simple blank piece of paper out, and plot the two different courses (closehauled and close reach), determine the lengths of the course you would have sailed on each of those courses, and include the two different speeds, and figure it out. You can keep it simple and make it, say, a mile away and only use one tack on each of the plots.

So, forget ALL of the other influencing factors, and pretend that none of them matters EXCEPT course and speed, and then: just "Do-the-math"

Bill Roosa · Apr 26, 2009

Soooo

What you are saying is that as long as I go "real fast" on the first leg it does not matter how slow I go on the second?????

That is kinda silly isn't it?

Once you are on a "fastest tack" as indicated by the VMG you still have to make a decision on when to tack. Do I wait till I'm almost 90 degrees to point B or at 45 or what? How do you know when to tack?
Now if the wind shifts it is even more important to know when to tack as it may be right now!!!!

As for jackhartjr's comment that the distances are always the same consider the following:
if I start off at 10 degrees to the line between A and B and go half way and tack the distance is something.
if I start off at 80 degrees to the line between A and B and go half way and tack the distance is significantly more.
In fact the distance is hardly ever the same.

tsmwebb · Apr 26, 2009

Stu Jackson said:
Notice I said ETA and NOT the VMG. This is simply because as you fall off from closehauled to any destination, the VMG will DROP. Period. It will get smaller because you're not going as close to a straight line to the destination.

I'm not sure what you mean.

If you are at max VMG and head up or down your VMG will drop (as long as you wait for the boat to settle into equilibrium on its new course). The whole point of max VMG is to find the best close-hauled course (or running course) from the many possible.

ETA has serious short comings when used for finding the best upwind course. In order to reach some point upwind you will need to cross a layline. But, when you are near a layline and sailing towards it your ETA will tend to go to infinity. At that point ETA will provide no guidance because your VMC (velocity made good on course will go negative). Your VMG (velocity made good upwind in with respect to the true wind angle) will be valid independently of your position on the course.

I suspect there may be a semantics issue here. IMO, the term VMG should be reserved for Velocity Made Good with respect to the True Wind. It is sometimes used to mean Velocity Made Good to a Waypoint this is confusing and again IMO, incorrect. Sadly though Garmin and some other GPS manufactures use it that way. Navigators have used the term VMC (Velocity Made Good on Course) for that concept for many years. In any case, the two are not interchangeable so it is important to know which you are using.

--Tom.

jackhartjr · Apr 26, 2009

Read my post again.
All I am saying is that it does not matter how many tacks you do...or simply going straight and then making a 90 degree turn to get there the distance is the same. How proficient you are determines when you get there.
Work it out on paper...or do I need to do it for you.
This is based on a destination that is dead ahead into the wind.
It is also based on most boats being able to sail at 45 degrees into the wind. If you can sail closer than that then it will be a shorter distance by tacking.
Jack

Stu Jackson · Apr 26, 2009

Kiss II

Re questions about my post #9

Forget about sailing. Completely.

Pretend you're a motorboat but constrained to the 45 degree sailboat's need to be that far off from dead ahead.

Ask this question:

What extra speed would I need to go sailing course 2 than I would sailing course 1 where course 2 is 10 degrees further off the 45 degree angle to the mark than course 1?

Pretend you're allowed only one turn between the start and the finish, one mile away.

tsmwebb · Apr 26, 2009

Stu Jackson said:
What extra speed would I need to go sailing course 2 than I would sailing course 1 where course 2 is 10 degrees further off the 45 degree angle to the mark than course 1?

cos(45)/cos(55) times the initial speed. And... ???

--Tom.

Stu Jackson · Apr 27, 2009

Thanks, Tom

That should answer the original poster's question as to whether it's faster to foot off or not.

RichH · Apr 27, 2009

Stu Jackson said:
Re questions about my post #9

Forget about sailing. Completely.

Pretend you're allowed only one turn between the start and the finish, one mile away.

Thats the optimum 'trigonometric answer'.
The newer modern 'solution computers' for VMG vs. WHEN to tack only assume ONE tack.
If when on a long passage (.... and youre not 'covering' someone,) the extra tacks will be detractive (unless there are frequent wind shifts, etc. and/or you arent 'hooking up to weather' at the end of a long tack to gain frequent 'boat lengths' gain).

... and once more for effect and clarity, the course that gives you the maximum VMG will get you to your destination the fastest .... all automatic if VMG is included in your GPS's function; or, you're good at trigonometry.

tsmwebb · Apr 27, 2009

RichH said:
... and once more for effect and clarity, the course that gives you the maximum VMG will get you to your destination the fastest .... all automatic if VMG is included in your GPS's function; or, you're good at trigonometry.

:bang:

Your GPS, unless it is part of an integrated instrument system, CAN NOT CALCULATE VMG UPWIND. It calculates VELOCITY MADE GOOD TOWARDS A WAYPOINT. That is not the same thing. For distant waypoints directly upwind it provides a reasonable approximation of VMG upwind but that approximation breaks down if you are not directly to leeward of your wp.

If you use your GPS's VMG function and always tack so that you are making the best possible reported VMG you WILL NEVER REACH A WAPOINT THAT YOU HAVE TO BEAT TO.

Really.

If you are left of the line DDW (dead downwind) of your WP your GPS's VMG will always be greater on port tack and if you are right of the DDW line your GPS's VMG will always be greater on starboard tack. So, if you follow your GPS you must always tack instantly at the DDW line. But, sailboats can't do that. So, you'll end up in irons and never get upwind at all. The closer you are to your waypoint the more apparent this effect will be.

That isn't fastest. It isn't even fast.

You can use your GPS's VMG function to help in understanding your yacht's performance but you can't just follow it to a point upwind.

When you start looking at the nitty gritty it all gets even more complex which is why MaxSea and Deckman sell well to the racing set and why Couts and Co. make the big bucks.

--Tom.

Jim Kolstoe · Apr 27, 2009

"Scalloping" is a tactic in which you slowly ease upwind until your sail becomes less effective, generally demonstrated by its beginning to luff, then falling off a little bit. Repeated ad infinitum. It gets its name because the course you follow has a scalloped look, rather than a straight line. Its use in sail trim is to continuously test whether you are at your sail's optimal angle of attack. Most sails have a optimum range of angles of attack that is between 3 and 7 degrees wide. So you drive your boat up and down trying to figure the best heading to get you optimum angle of attack. Obviously the wind is not know for cooperating with this as it changes direction (backs or veers). This is further complicated by changes in boat speed affecting the appearant wind the sail feels, irregardless of true wind direction. Scalloping is generally used when you are close hauled. The reason is that if you are not close hauled, the sail trimmers are expected to "trim to the course," meaning that they are responsible for trimming the sail to give the best angle of attack. If you are close hauled, the sail trimmers can't do anything, so the boat's course has to be changed (scalloping) to try to keep the sails in their optimal angle of attack.
As for footing off to gain speed, I agree with previous comments that if you observe racers, they normally go close hauled (staying short of pinching since slows you down a lot) when going up wind. They may foot off slightly if the waves are bouncing them around a lot, but that's to allow them to ease their sails, which opens up that optimal range of angles of attack a bit.
Play with it and have fun.
Jim Kolstoe, h23 Kara's Boo

RichH · Apr 27, 2009

Tom
You are correct as in actuality VMG via a GPS is only truley valid/accurate during the beginning of a leg wherein the boat is along the line between the beginning of the leg and the waypoint. The reason being that most GPS are calculating the cosine of the intercept angle ... and when far out on the leg (typically approaching the sine @ 90 degrees = 0, the operative word here is 'approaching'). This becomes a mathematical resolution error and the 'approach' at that condition becomes mathematically 'trivial'.

T37 Phil · May 11, 2009

My understanding is that Vmg is a compromise between close-hauled and the direction of optimum lift or boat speed. The reason is that as one falls off, though the boat speed increases, the upwind component of the boat velocity is less. The optimum theoretical course is when you've fallen off enough so that Vmg is starting to go down. That depends on the specific polar and true wind being used. I could come up with a numeric answer, if anyone was interested.