Dear Racers, Can someone enlighten me on the best use of Vmg information from a GPS for sail trim-tweaking and angle-to-the-wind when beating, as well as using it for tacking decision making? If this is dealt with nicely in some printed or online resource, please feel free to just refer me there. I have a full Raymarine NAV/AP/WIND system, but presently no PC integration. Thanks! John Rooks Barrington RI
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VMG
Velocity Made Good is the 'component' of the angle that you are sailing towards your next 'mark'. Its 'usefulness' is only valid at the early stages of a track/tack and NOT at the end of a track when there are too many 'resolution' errors in the way that VMG is calculated by the GPS. Heres how to (simply) use it: example: Put in the 'destination waypoint' you are trying to reach ... be it a navigation mark, buoy, racing course mark, an imaginary point, etc. into the GPS. Simply sail as high on a beat, etc. as you can and notice the VMG, tack and note the VMG on the other tack.... the higher VMG number will indicate what tack is 'favored' to arrive at the 'mark' faster than on the other tack. This assumes that you are going 'right up the middle' of the 'course'. Inotherwords when early on a track VMG will determing the best and fastest course to go to the 'mark' all you look for is the 'highest possible VMG number' and stay on the course that keeps the highest VMG number. If the conditions (windstrength, direction, tidal flow) remain constant then the course as indicated by the original (maximum) VMG will remain optimum. Subsequent VMGs should be taken when the boat is somewhere directly 'in-line' between the start of the 'leg' and the mark as if you are way off the line between the 'start and finish' the accuracy/usefulness of VMG rapidly degrades. In theory, when on a tack and VMG gets to "0" that is the exact spot you should tack. But in practice, and because of the 'mathematics', the approach to 'zero' is so 'slow' (resolution of trigonmetry) that you may be considerably far way from where you should have tacked/gybed .... the VMG function is NOT very precise at near the 'end' of a leg/tack; at the bginnning of a tack,etc. the precision is very valuable in deciding which way to go and whats the best tack. When gybing/tacking and you reach the imaginary line between where you started and where you want to go, recheck the VMG to see if its faster or slower, maybe do a 'trial' tack/gybe and see if the new tack/gybe is faster in VMG, etc. when crossing that imaginary line between start and endpoint. Just keep the VMG at the highest number (when tacking up the middle) and you will travel very much less distance and get to the final 'mark' faster. VMG helps you to avoid tack directions that 'get you nowhere - fast'. If you have a 'math background' VMG is the cosine of the angle to the mark.... and at the end of a tack the cosine to the 'mark' may be approaching 1 (mark at 90 degrees to the boat) and the resultant calculation approaches ZERO .... but very slowly. VMG isnt 'foolproof' but will save a lot time and distance travelled.
Velocity Made Good is the 'component' of the angle that you are sailing towards your next 'mark'. Its 'usefulness' is only valid at the early stages of a track/tack and NOT at the end of a track when there are too many 'resolution' errors in the way that VMG is calculated by the GPS. Heres how to (simply) use it: example: Put in the 'destination waypoint' you are trying to reach ... be it a navigation mark, buoy, racing course mark, an imaginary point, etc. into the GPS. Simply sail as high on a beat, etc. as you can and notice the VMG, tack and note the VMG on the other tack.... the higher VMG number will indicate what tack is 'favored' to arrive at the 'mark' faster than on the other tack. This assumes that you are going 'right up the middle' of the 'course'. Inotherwords when early on a track VMG will determing the best and fastest course to go to the 'mark' all you look for is the 'highest possible VMG number' and stay on the course that keeps the highest VMG number. If the conditions (windstrength, direction, tidal flow) remain constant then the course as indicated by the original (maximum) VMG will remain optimum. Subsequent VMGs should be taken when the boat is somewhere directly 'in-line' between the start of the 'leg' and the mark as if you are way off the line between the 'start and finish' the accuracy/usefulness of VMG rapidly degrades. In theory, when on a tack and VMG gets to "0" that is the exact spot you should tack. But in practice, and because of the 'mathematics', the approach to 'zero' is so 'slow' (resolution of trigonmetry) that you may be considerably far way from where you should have tacked/gybed .... the VMG function is NOT very precise at near the 'end' of a leg/tack; at the bginnning of a tack,etc. the precision is very valuable in deciding which way to go and whats the best tack. When gybing/tacking and you reach the imaginary line between where you started and where you want to go, recheck the VMG to see if its faster or slower, maybe do a 'trial' tack/gybe and see if the new tack/gybe is faster in VMG, etc. when crossing that imaginary line between start and endpoint. Just keep the VMG at the highest number (when tacking up the middle) and you will travel very much less distance and get to the final 'mark' faster. VMG helps you to avoid tack directions that 'get you nowhere - fast'. If you have a 'math background' VMG is the cosine of the angle to the mark.... and at the end of a tack the cosine to the 'mark' may be approaching 1 (mark at 90 degrees to the boat) and the resultant calculation approaches ZERO .... but very slowly. VMG isnt 'foolproof' but will save a lot time and distance travelled.
Vmg for trimming
Dear Rich, Thanks for your information. How about using Vmg for trimming and setting your angle off the wind? Is this practical? John
Dear Rich, Thanks for your information. How about using Vmg for trimming and setting your angle off the wind? Is this practical? John
For trimming ....
... the best is to simply watch the boat speed.... while holding to the same course. Then go to the best VMG course and retrim based on boat speed. It takes a while for VMG to 'calculate'; boat speed changes are almost instantaneous. Angle off the wind ..... Absolutely! use VMG for angle off the wind !!! ... very efficient for 'downwind' sailing (but again be near the 'center line'). If you start using VMG, you will NEVER ever sail dead-downwind again. Offwind at an angle keeps the 'apparent wind' much higher, VMG will definitely show and 'derive' that optimum downwind angle, vs. sails chosen, sea state, etc etc etc. Use/accumulate the data and construct your own 'polar plots' for best angle, etc. Accumulate the data on BOTH tracks/tacks as wind and waves are NEVER perpendicular (corolis effect) when you construct your own polar plots. If you want to get 'real finicky' develop a set of 'polars' based on 'wind direction' for you 'usual' location .... will automatically factor out the corolis effect. ;-)
... the best is to simply watch the boat speed.... while holding to the same course. Then go to the best VMG course and retrim based on boat speed. It takes a while for VMG to 'calculate'; boat speed changes are almost instantaneous. Angle off the wind ..... Absolutely! use VMG for angle off the wind !!! ... very efficient for 'downwind' sailing (but again be near the 'center line'). If you start using VMG, you will NEVER ever sail dead-downwind again. Offwind at an angle keeps the 'apparent wind' much higher, VMG will definitely show and 'derive' that optimum downwind angle, vs. sails chosen, sea state, etc etc etc. Use/accumulate the data and construct your own 'polar plots' for best angle, etc. Accumulate the data on BOTH tracks/tacks as wind and waves are NEVER perpendicular (corolis effect) when you construct your own polar plots. If you want to get 'real finicky' develop a set of 'polars' based on 'wind direction' for you 'usual' location .... will automatically factor out the corolis effect. ;-)
NEVER?
"If you use VMG, you will NEVER ever sail dead-downwind again." Rich, how about a situation when you are sailing a relatively heavy boat DDW at hull speed?
"If you use VMG, you will NEVER ever sail dead-downwind again." Rich, how about a situation when you are sailing a relatively heavy boat DDW at hull speed?
Vmg for wind-angle upwind
Thanks Rich! I was wondering though about using Vmg for fine-tuning my wind angle upwind (your thoughts for downwind are great). What I meant was that rather than just setting your sails to their proper tightest for pointing, and sailing essentially by the telltales, is it ever the case with some boats, in some winds, that a better Vmg can be had by heading-off just a touch, and re-trimming as appropriate? Thanks! John
Thanks Rich! I was wondering though about using Vmg for fine-tuning my wind angle upwind (your thoughts for downwind are great). What I meant was that rather than just setting your sails to their proper tightest for pointing, and sailing essentially by the telltales, is it ever the case with some boats, in some winds, that a better Vmg can be had by heading-off just a touch, and re-trimming as appropriate? Thanks! John
Dropping off can get a better VMG
John, I did exactly what you are proposing with my H-216. I pointed as high as I could and got my VMG with my GPS. I then dropped off the wind in steps and found my VMG increased up to a point. I then marked my Jib sheet at the fastest position. That is, when sailing upwind, I found I had a better VMG by sailing a little lower and faster.
John, I did exactly what you are proposing with my H-216. I pointed as high as I could and got my VMG with my GPS. I then dropped off the wind in steps and found my VMG increased up to a point. I then marked my Jib sheet at the fastest position. That is, when sailing upwind, I found I had a better VMG by sailing a little lower and faster.
Dropping off can get a better VMG
Dear Ranger Paul, Great, just as I suspected. Do you have any idea whether this (heading-off a bit when sailing to windward for best Vmg) is true for most boats, and most windspeeds, or just some boats, in some winds? Thanks! John
Dear Ranger Paul, Great, just as I suspected. Do you have any idea whether this (heading-off a bit when sailing to windward for best Vmg) is true for most boats, and most windspeeds, or just some boats, in some winds? Thanks! John
John, an answer
That's what polars are for. They are similar for similar types of boats, but differ some for each boat. Most boats do best sailing upwind when they are "freed" a bit instead of pinching as high as possible. Remember, just because your bow is aiming higher doesn't mean the boat is sailing higher - it might be making more leeway (keel developing less lift due to slower speed, etc.) than if you ease the sheets a touch and let her find that sweet groove where she moves faster and points well to boot. Rich, sometimes that 2% is important - my crew claims I lost a race last year because I refused to sail DDW, and he is probably right, because there was lots of wind, my boat is heavy, and another guy who did do it passed us on the downwind leg. I can tell you I won't make that mistake again. Rule of thumb: the stronger the wind or the heavier the boat, the lower you should sail. You are certainly right about reaching off most of the time (to keep flow across the sails). But if I'm heading dead for the mark winged out and at hull speed, I'm praying for you to keep reaching all you want.
That's what polars are for. They are similar for similar types of boats, but differ some for each boat. Most boats do best sailing upwind when they are "freed" a bit instead of pinching as high as possible. Remember, just because your bow is aiming higher doesn't mean the boat is sailing higher - it might be making more leeway (keel developing less lift due to slower speed, etc.) than if you ease the sheets a touch and let her find that sweet groove where she moves faster and points well to boot. Rich, sometimes that 2% is important - my crew claims I lost a race last year because I refused to sail DDW, and he is probably right, because there was lots of wind, my boat is heavy, and another guy who did do it passed us on the downwind leg. I can tell you I won't make that mistake again. Rule of thumb: the stronger the wind or the heavier the boat, the lower you should sail. You are certainly right about reaching off most of the time (to keep flow across the sails). But if I'm heading dead for the mark winged out and at hull speed, I'm praying for you to keep reaching all you want.
Definitely works upwind too......
Like the other poster stated pointing as high as possible is not usually the fastest. Thats whats REAL nice about VMG as it calculates all the 'math' for you. When beating and especially at the start of or early in a leg of a new tack you should be sailing for max vmg --- will mathematically get you to the next mark much faster; conversely, when approaching near the end or late in the tack where VMG is full of inherent 'math resolution' errors then its usually best to pinch up and sail the end of a leg heeled over on your ear and pointing up as high as possible (even so overtrimmed that the mainsail leech is well hooked to weather) .... its all in the math. If you point as high as possible even pinching up higher than normal at the end of a tack you will usually/invariably 'always' save several boat lengths or more on the 'next tack'. Racing as well as long distance sailing is a game of 'INCHES". For instance when sailing for 5 days straight and you sail a 5% better course will save you 6 hours of travel time. if you gain 5% better vmg, then deduct another 6 hours ..... Makes a difference when storms, etc. are brewing. VMG is such a wonderful tool - I used to 'hand calculate' it. When I first saw the VMG function on a GPS I thought I died and went to heaven! Incidentally for the best upwind track, when using tell tales (especiallly tell tales in "gentry fashion") you will probably find maximum vmg at the same point where ALL the tell tales (leeward AND windward sides) are streaming 'straight back' .... except near the end of a tack where vmg 'gets mathematically fuzzy'! If you dont use tell-tales get a copy of Don Guilettes "Sail trim users guide'. There is NO other way to get a boat moving at optimum speed/vmg than properly using tell-tales. Don takes a 'very complicated' subject and makes it easy to understand. The aerodynamics of sails (thin foils) is EXTREMELY complicated and totally DEFIES logic and 'intuition' .... for 'advanced work' go to www.arvelgentry.com and download and read ALL the 'magazine articles'. Gentry is the aerodynamicist who debunked all the modern sail 'theories': Bernoulli, slot-effect, etc. etc. and was the one who introduced tell-tales to sailing. Warning - His articles are a 'bit complicated'.
Like the other poster stated pointing as high as possible is not usually the fastest. Thats whats REAL nice about VMG as it calculates all the 'math' for you. When beating and especially at the start of or early in a leg of a new tack you should be sailing for max vmg --- will mathematically get you to the next mark much faster; conversely, when approaching near the end or late in the tack where VMG is full of inherent 'math resolution' errors then its usually best to pinch up and sail the end of a leg heeled over on your ear and pointing up as high as possible (even so overtrimmed that the mainsail leech is well hooked to weather) .... its all in the math. If you point as high as possible even pinching up higher than normal at the end of a tack you will usually/invariably 'always' save several boat lengths or more on the 'next tack'. Racing as well as long distance sailing is a game of 'INCHES". For instance when sailing for 5 days straight and you sail a 5% better course will save you 6 hours of travel time. if you gain 5% better vmg, then deduct another 6 hours ..... Makes a difference when storms, etc. are brewing. VMG is such a wonderful tool - I used to 'hand calculate' it. When I first saw the VMG function on a GPS I thought I died and went to heaven! Incidentally for the best upwind track, when using tell tales (especiallly tell tales in "gentry fashion") you will probably find maximum vmg at the same point where ALL the tell tales (leeward AND windward sides) are streaming 'straight back' .... except near the end of a tack where vmg 'gets mathematically fuzzy'! If you dont use tell-tales get a copy of Don Guilettes "Sail trim users guide'. There is NO other way to get a boat moving at optimum speed/vmg than properly using tell-tales. Don takes a 'very complicated' subject and makes it easy to understand. The aerodynamics of sails (thin foils) is EXTREMELY complicated and totally DEFIES logic and 'intuition' .... for 'advanced work' go to www.arvelgentry.com and download and read ALL the 'magazine articles'. Gentry is the aerodynamicist who debunked all the modern sail 'theories': Bernoulli, slot-effect, etc. etc. and was the one who introduced tell-tales to sailing. Warning - His articles are a 'bit complicated'.
Bob - That guy who passed you DDW ......
had better VMG !!!!! It matters not the angle that you're sailing as going fast in the wrong direction will never win races. Its the component angle - VMG (cosine of the angle between your boat and the 'mark' is 'the one that pays the money' and wins the pewter. Maybe VMG by GPS should be outlawed in racing, so we can get back to the 'human forms of sailboat racing'. All the electronics and gizmos favor those who can spend the most money ..... really isnt fair.
had better VMG !!!!! It matters not the angle that you're sailing as going fast in the wrong direction will never win races. Its the component angle - VMG (cosine of the angle between your boat and the 'mark' is 'the one that pays the money' and wins the pewter. Maybe VMG by GPS should be outlawed in racing, so we can get back to the 'human forms of sailboat racing'. All the electronics and gizmos favor those who can spend the most money ..... really isnt fair.
Vmg Access As A Learing Tool
In regards to restricting the use of Vmg, this would of course eliminate access to GPS itself, which may be impossible at this stage. One reason I believe full utilization of Vmg should be supported comes from the text in this message thread. Does everyone know that DDW is not always the fastest point? Vmg will tell them. Does everyone already know their best angle and trim-point, in various winds, when sailing to windward? Again, it appears that Vmg will help to articulate this. I can't believe that the economics arguement is central to this, as a Vmg capable handheld GPS is now much cheaper than an installed knotmeter; would we likewise propose that knowing hullspeed is somehow unfair? Why not outlaw new sails and bottom-scrubbing while we're at it... John
In regards to restricting the use of Vmg, this would of course eliminate access to GPS itself, which may be impossible at this stage. One reason I believe full utilization of Vmg should be supported comes from the text in this message thread. Does everyone know that DDW is not always the fastest point? Vmg will tell them. Does everyone already know their best angle and trim-point, in various winds, when sailing to windward? Again, it appears that Vmg will help to articulate this. I can't believe that the economics arguement is central to this, as a Vmg capable handheld GPS is now much cheaper than an installed knotmeter; would we likewise propose that knowing hullspeed is somehow unfair? Why not outlaw new sails and bottom-scrubbing while we're at it... John
John ---
The fuzziness is the way that VMG is calculated. What VMG actually is - is simply a trigonometry calculation between the forward speed and the present angle TO the 'mark'. At the beginning of the leg the angles are relatively small the calculation is very accurate; but. later on during the 'leg' when you reach 90 degrees to the next 'mark' (layline) the calculation becomes 'trivial' - the trigonometric cosine approaches ZERO hence the 'answer' becomes 'infinite' --- hence 'fuzzy'. Its just a 'math' anomaly. So once VMG establishes the best trim and course during the early part of the leg, simply keep the settings towards your intended tacking/gybing point ... unless the wind velocity and direction changes. Heading up/down/ etc. with no other changes (windspeed, etc) will probably result in a decrease in VMG. So dont stretch-out a tack to the 'layline' and expect VMG (VMG=zero) to tell you the EXACT point at which to tack as it cant be accurately 'calculated' by the GPS or by VMG.
The fuzziness is the way that VMG is calculated. What VMG actually is - is simply a trigonometry calculation between the forward speed and the present angle TO the 'mark'. At the beginning of the leg the angles are relatively small the calculation is very accurate; but. later on during the 'leg' when you reach 90 degrees to the next 'mark' (layline) the calculation becomes 'trivial' - the trigonometric cosine approaches ZERO hence the 'answer' becomes 'infinite' --- hence 'fuzzy'. Its just a 'math' anomaly. So once VMG establishes the best trim and course during the early part of the leg, simply keep the settings towards your intended tacking/gybing point ... unless the wind velocity and direction changes. Heading up/down/ etc. with no other changes (windspeed, etc) will probably result in a decrease in VMG. So dont stretch-out a tack to the 'layline' and expect VMG (VMG=zero) to tell you the EXACT point at which to tack as it cant be accurately 'calculated' by the GPS or by VMG.
Rich - What about hull speed??
Rich- How about this. I checked Vmg on my Raymarine Wind system, and it says that it needs hullspeed data to function properly. I know however that many handheld GPS units have Vmg function, and require no hullspeed data. Is the Wind System Vmg perhaps some 'true' Vmg, always assuming a mark directly to windward, while a GPS-based Vmg is computing speed made to a specific windward point? We seem to use the term Vmg generically, but this would be a strong distinction, yes? Thanks! John
Rich- How about this. I checked Vmg on my Raymarine Wind system, and it says that it needs hullspeed data to function properly. I know however that many handheld GPS units have Vmg function, and require no hullspeed data. Is the Wind System Vmg perhaps some 'true' Vmg, always assuming a mark directly to windward, while a GPS-based Vmg is computing speed made to a specific windward point? We seem to use the term Vmg generically, but this would be a strong distinction, yes? Thanks! John
Difference !!!!!
The wind machine when coupled to a speedo only reflects speed through the water (not corrected for drift and current) .. thus VMG by a wind instrument needs to be further corrected for the drift and current. Plus, the windmachine has NO idea of the location of the 'mark' that youre trying to reach .... sounds like a 'definitional error' to me, if I understand windmachine VMG. I dont use VMG from a wind instrument so I dont have that experience. A GPS uses SOG in the calculation thus reflects TRUE/actual VMG to a waypoint.
The wind machine when coupled to a speedo only reflects speed through the water (not corrected for drift and current) .. thus VMG by a wind instrument needs to be further corrected for the drift and current. Plus, the windmachine has NO idea of the location of the 'mark' that youre trying to reach .... sounds like a 'definitional error' to me, if I understand windmachine VMG. I dont use VMG from a wind instrument so I dont have that experience. A GPS uses SOG in the calculation thus reflects TRUE/actual VMG to a waypoint.
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