Downwind vmg
- Thread starter europe172
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Birdseye
Well-known member
Me too.
Remember Flaming, he's asking about an old Westerly bilge keeler and not the sort of thing you sail.
anoccasionalyachtsman
Well-known member
Really?
anoccasionalyachtsman
Well-known member
Being rude about someone else's boat makes you look even more silly.
flaming
Well-known member
It's your contention that using VMG to waypoint is the best way forward that I have the issue with. And that's the same for a moth and the tubbiest of tubs.
Birdseye
Well-known member
Its your contention that you can sail purely by the wind and ignore the location of the mark or the tides that I have issue with. In the final analysis, what matters is how fast you get to the next mark and that doesnt solely depend on boat speed . Direction over the ground matters too.
Let us suppose that the mark is dead downwind. For boats that can plane, there is a case for sailing at an angle to the direct course with increased speed making up for increased distance. I onced did several races in a lightweight 30 ft tri and you never sailed that boat dead downwind but then you could sail far faster than true wind speed by sailing at an angle. I dont know whether you have sailed a bilge keeler of that early generation but I did so for many years - take it from me they wont plane unless there is a very strong wind and rollers coming up behind. Even then it wont be more than a brief surge. Its a completely different world
But there is another issue anyway . Much racing is not laid courses but navigation marks so the "downwind" leg is rarely downwind. Then you have tides - you cant simply ignore say a 3kn cross tide and sail purely to the wind and for speed through the water. Thats where judgement comes in and one of the numbers you consider heavily is the progress you are making towards the mark as shown by the VMG.
The OP asked about casual racing in an older bilge keeler and not round the cans in a modern lightweight fin or the sort that , judging from your comments, you normally race.
Anyway I will call it a day at that and leave anoccasionalyachtsman to offer his childish abuse to others.
flaming
Well-known member
That is not at all what I'm saying....
I'll have one more go....
For any boat, from lightweight 30ft tris to bilge keelers, there is an optimum downwind angle to get downwind at the best VMG. These angles widen as the breeze drops. If your next leg is within the cone formed by these angles, such that you would have to sail lower than your optimum downwind VMG to get to it, then you will be faster sailing those angles (assuming good gybes etc) than you will be pointing straight at the mark, always. The slower the boat and the stronger the wind, the narrower those angles are. To the point where for more traditional boats in 15kts plus they are effectively 180. But even in traditional boats those angles will be wide in 6 knots of breeze. Look at those polars up thread, that's a 60 degree "cone" in 6kts of wind.
If you choose your downwind angle using VMG to waypoint, you are going to get it wrong on occasion. And those will be mostly when the mark is NOT dead downwind, but is off to one side, whilst still being within that cone. Under those circumstances is might look better on VMG to waypoint to sail just a bit lower and point straight at the mark.
BUT it isn't the instantaneous VMG to the mark that is the important measure, it's the total VMG to the mark over the course of the leg. If I sail a bit higher and faster than you, then my VMG to the waypoint might be a little slower initially. However, when I then gybe on the layline for the mark my entire boatspeed is now at the mark, and my boatspeed is higher than yours because I'm sailing higher, so my VMG is a lot higher. If my average VMG over the whole leg is higher, then I will arrive first. This is easy to see on the water, if I sail a hotter angle but am not going forwards relative to you such that a gybe will just take me back to you, or behind you, then your angle is better. If however I inch forward and get to a point where a gybe would take me clear across your bow, then I've sailed a better downwind angle, and despite your VMG to waypoint being initially higher than mine, I'm now ahead..
Golden rule of thumb is if your mark is within the cone, you always, always sail your best downwind angles. It is extremely rare to gain an advantage by sailing lower than your best downwind VMG over a leg of any length. Noting of course, as per my 1st post in this thread, that in reasonable breeze for the type of boat the OP is sailing, 180 degrees may well be the best angle.... But it certainly won't be in light winds.
This is simply about how to get downwind the fastest way possible. The application of tides, and which side of the course you should favour etc is a completely different topic. You are confusing how to sail the boat fast with tactics. Best upwind and downwind VMG angles are just about getting the most performance out of the boat. Deciding how to use that performance to position the boat on the race track to take advantage of tides or windshifts is tactics.
RJJ
Well-known member
Agree. See Stuart Walker's tactics primer.
It's about rungs on the ladder, upwind and down. Another boat could be closer to the mark, but on a worse rung of the ladder. Assuming as you say that you haven't yet laid the mark i.e. you are within the "cone".
The extreme example is if you have a 90 degree gybing angle, then immediately before the layline your VMG to the mark is zero. When you gybe, your VMG to the mark is your SOG. But your VMG to the wind is constant. Your goal is to optimise VMG to the mark, but over the entire leg not at any point in time. You achieve that goal but optimising VMG to the wind - at every point in time.
Laser310
Well-known member
usually when people don't understand the importance of measuring VMG w.r.t the wind.., they never do...
trying to explain doesn't work - their mind is closed to the possibility that they are wrong
perhaps if they could be shaken from their certainty.., they might, on their own, decide to delve into it
one piece of information that might cause someone to think again, is the fact that in B&G systems "VMG" is calculated w.r.t the wind.., not the mark.
is B&G just careless with their use of the terms?
no.
VMC (Velocity Made Good on Course) is the term used by B&G for velocity made good to a waypoint. VMC is also a useful number - particularly when reaching.., where it is often - but not always - maxinized by sailing the rhumb line
trying to explain doesn't work - their mind is closed to the possibility that they are wrong
perhaps if they could be shaken from their certainty.., they might, on their own, decide to delve into it
one piece of information that might cause someone to think again, is the fact that in B&G systems "VMG" is calculated w.r.t the wind.., not the mark.
is B&G just careless with their use of the terms?
no.
VMC (Velocity Made Good on Course) is the term used by B&G for velocity made good to a waypoint. VMC is also a useful number - particularly when reaching.., where it is often - but not always - maxinized by sailing the rhumb line
Daydream believer
Well-known member
I may have missed it , but I do not see if the OP was sailing a 2 sail race or a 3 sail one. If 3 sail, is it an asymetric?- Which totally changes the angles & so much of what has been said needs updating. Down wind with a spinnaker changes the angles again.
Plus boat speed. If the boat is travelling very near max knots dead down wind, does one need to start gybing? But then how many courses are dead down wind? There is usually some bias.
Plus boat speed. If the boat is travelling very near max knots dead down wind, does one need to start gybing? But then how many courses are dead down wind? There is usually some bias.
Medway Matt
Active member
This is a really interesting thread, thanks everyone.
Do you think the polar plot posted higher up be reasonably applicable to a GK29?
Do you think the polar plot posted higher up be reasonably applicable to a GK29?
anoccasionalyachtsman
Well-known member
The boatspeed are too high, but the shape won't be far wrong.
europe172
Active member
europe172
Active member
It was light winds using two sails as I use a atn tacker around my furled headsail
Medway Matt
Active member
Thanks. It was the shape I meant really, I knew the speeds would be higher.
Daydream believer
Well-known member
So you are sailing a 3 sail race. Have I got that correct?
That alters the plan a lot.If one is flying an asymetrical sail then hiding it behind the main by running dead down wind would certainly be wrong.
The angles would be markedly higher. Furthermore, With a sail that has its foot anchored high up to the forestay rather than being allowed to fly forward with the luff well forward & free will mean a different approach as well
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europe172
Active member
Just for clarity , I wasn’t sailing straight down wind rather sailing just enough off downwind to keep the headsail full , my original question was about the trade off between boat speed and distance to the mark by bringing the wind more towards the beam and how to work out the best angle, I now think that another 10deg would have given more speed at the cost of a slight increase in distance sailed. I am grateful for all the advice given
Daydream believer
Well-known member
You use the term " headsail", but earlier mention anATM asm fixed to the forestay. I assume you were using an asymetric sail down wind ( with the strop round the furled jib)
If running fairly dead down wind did you think of goose wing?
Pull the main aft a bit, such that the wind hits the leech first & the wind passes the opposite way across the sail. This then pushes extra wind into the asymetric sail to keep it full; which could be poled out on the opposite hand.
The wind then hits the sail & flows round the forestay (or in this case luff of the asym) forward
It is a technique that can be used in light winds when the sea is fairly flat & the sails are not being thrown out of balance.
The yacht can sail dead down wind & thus the shortest course.
Would this suit heavier displacement types ( relative) that can keep going through short lulls, due to their weight, but not respond so quick on the higher gybes in the gusty bits as a sportier boat might?
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europe172
Active member
I did try goosewinging but it felt slower, but I know that feelings can be deceiving hence the original post
RJJ
Well-known member
You can have funny times goosewinging especially if the main can be fully squared-off, where 15 Deg either side of a dead run(with flow over the sails in either direction) is faster and better VMg than a dead run. Similar to a Laser.
With swept spreaders, running by the Lee is less effective.
