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post #11 of 23 Old 07-17-2019
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Re: pointing, pinching, VMG

Some of this depends upon the program used to calculate vmg but the ideal is to have the best vmg with the sails perfectly trimmed and the tell tales horizontal.
Image you on a reach but need to be on a beat. While on the reach your boat speed is higher than on a beat (true for every sailing polar) and the telltales are perfect. You don’t touch the sheets and go to a beat. Now your boat speed falls but your vmg rises. This is an extreme example as in this scenario your sails will luff to to the point where drive will be lost. But there are multiple points before this occurs where your telltales will go squirrelly but vmg improve.
Suggest go on a day sail. Find a situation that reproduces your experience. Put the boat on AP course setting. Then fool with the trim watching vmg. Vmg will improve further when ideally trimmed.
The OP notes light air 7-10kts. Here you have two choices. Go for a very flat sail to prevent back eddies and maintain laminar flow across the sails. Or increase draft and cord to get the most drive from the sails. One will be faster. The sails will look very different although angle of attack is the same.
Examine your sailing polar. In general the point at which boat speed falls as you continue to head up is the point at which you are “pinching”. This is true cruising or racing. It’s built into the basics of your boat. You may not do as well as your polar as your sails age. I pinch to get around a headland or some such situation when cruising but try to avoid pinching while racing. While racing will try to stay right at the edge of pinching which requires a very alert helm. Someone reading the water (if light air) not just the wind indicators. Some one reading the natural oscillations of wind direction in moderate to light air. In heavier air although you need to consider that you also need to consider the seas trying to stall the boat so angle of attack may vary. This requires very alert trimmers keeping up with the changes the helmsman is making as necessary.
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post #12 of 23 Old 07-17-2019
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Re: pointing, pinching, VMG

Color me stupid but.. if your GPS plotter program is accurately computing VMG as max... it hardly matters how your sails are trimmed.... your are moving a fast as you can toward your destination. It may be counter intuitive... that your sails APPEAR to be less than properly trimmed... but the VMG tells a different story.

The curious thing is WHY are the tell tales telling you a different story than your computer?

Could it be that your keel will is not trimmable provides lift related to current and heel?

pay attention... someone's life depends on it
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post #13 of 23 Old 07-17-2019 Thread Starter
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Re: pointing, pinching, VMG

Hi Guys,

Thanks for all of the comments.

On the boat I normally race on I usually trim main. On that boat the owner drives. He's good at it and we are successful. However, he's not a fan of electronics. The trimmers trim and the driver drives. He steers to the tell tails but since no one is looking at VMG I don't know if the VMG would increase if he headed up a little more or down a little. Next time I will use my hand held GPS and try to pay attention to that.

On my boat for upwind work I will set halyard tension, outhaul, headsail cars, backstay, etc for the wind conditions. Then we trim the sails and I steer. On Monday the headsail was in as much as possible. The top third of the leach was against the shrouds so I can't trim in any harder. The mainsheet was on hard and the traveler was used to pull the boom to center line. Normally I would sail to keep the tell tails streaming. Monday was first time I really paid attention to VMG and that's when I noticed VMG increasing when i was 'pinching'. There was minimal weather helm and the boat sails very nicely.

The sails are in good condition. The main is dacon, bought in 2015. The headsail is a 135 laminate, bought in 2016. Bottom is clean, foils clean.

Next time I'm out for a day sail I will do some more testing and tweaking.

Barry
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post #14 of 23 Old 07-17-2019
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Re: pointing, pinching, VMG

There have many times when I am sailing back to my then mooring out in Shelter Island when the wind was very light and I decided to motor sail because I didn't want to tack. I would roll in the head sail and try to keep the main up without "losing its sailing shape" . I was motor sailing too hard on the wind I got no help from the main and worse it was more drag. I found that the main could add a bit of speed as long as it was drawing. Now way could I sail that close to the wind with two sails or even one.

Don't forget to consider the impact of current on your VMG and CMG.

pay attention... someone's life depends on it
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post #15 of 23 Old 07-17-2019
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Re: pointing, pinching, VMG

Do you get the same phenomenon in stronger wind?. How far do you need to let sails out to get them looking good? Do you trim front to back? Is the jib over trimmed closing the slot and making the main luff? Do the shapes of jib and main match?
I like you don’t understand this. Have gone faster with part of the sail out of trim and the rest good at times but not the whole thing.

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post #16 of 23 Old 07-17-2019
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Re: pointing, pinching, VMG

Quote:
Originally Posted by SanderO View Post
...The curious thing is WHY are the tell tales telling you a different story than your computer?...
They aren't. Telltales describe the flow around certain parts of the sail. But what is the best flow pattern? That varies with the boat, the condition of the sails, and the boat. Some lifting and some stalling can be correct, depending on the course.

You are seeking best VMG. Telltales are only a part of the picture, like oil pressure or engine rpm to a race car driver. Lap time is what matters. This is why trial horses were so vital before VMG computers (they still are), because not everything is visually obvious.
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post #17 of 23 Old 07-17-2019
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Re: pointing, pinching, VMG

Barry, I'm a firm believer in steering with a very light touch. A sailboat with a well tuned rig will tell you how to steer it. If you're sailing closehauled and a little puff comes along, and the boat heels a little more, and you have to tug a bit more on the tiller to hold it's heading, that's the boat's way of telling you that it will be happier if you'll let it head up to windward 2-3 degrees. A happy boat sails faster and points higher. If you can let the boat come to windward 2-3 degrees while maintaining the same boat speed, your VMG will improve. If you don't let the boat head up 2-3 degrees in that situation, you'll have to hold it off the wind with the rudder, and that will cause drag and a loss of boat speed. When the puff subsides, you'll have to bear off the wind to keep your speed and VMG optimized.

I think your mainsail is slightly overtrimmed when it's trimmed to the centerline. When you let the boat come up to windward slightly, it takes a little pressure off the mainsail, reducing rudder drag slightly, so your boat pointed 2-3 degrees higher without losing boat speed, and that's why it produced an improved VMG.

My suggestion is, when you're closehauled, ease the mainsail off from the centerline until the luff just begins to lift, and then trim it in just a skosh. Then steer with a very light hand on the helm, and, when the boat tells you it wants to head to windward a bit in a puff, let it. That will maximize your boat speed, pointing and VMG, and reduce drag 100% of the time.
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post #18 of 23 Old 07-17-2019
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Re: pointing, pinching, VMG

How does your instrument system derive VMG? Is it VMG relative to true wind? Or to a waypoint? How sophisticated is your wind instrument?

IMO, you should be, first, picking your angles based on strategy and lay lines to the mark or destination, and then subsequently maximizing boat speed at that chosen angle.

In other words, “sail to target speeds”.
Figure out how to tell the difference between direction headers and velocity headers. How you drive and trim is different between the two.

Google “velocity header target speed” and “sailboat target speeds”

Judy B
San Francisco Bay and Delta
F24 Trimaran

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post #19 of 23 Old 07-17-2019
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Re: pointing, pinching, VMG

Quote:
Originally Posted by jblumhorst View Post
How does your instrument system derive VMG? Is it VMG relative to true wind? Or to a waypoint? How sophisticated is your wind instrument?

IMO, you should be, first, picking your angles based on strategy and lay lines to the mark or destination, and then subsequently maximizing boat speed at that chosen angle.

In other words, “sail to target speeds”.
Figure out how to tell the difference between direction headers and velocity headers. How you drive and trim is different between the two.

Google “velocity header target speed” and “sailboat target speeds”
Great point. My instruments use a waypoint for VMG wind has nothing to do with it. Whatever factor improves the VMG it will show... upwind or downwind or no wind.
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post #20 of 23 Old 07-17-2019 Thread Starter
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Re: pointing, pinching, VMG

Hi Judy,

Thanks for your note.

My electronics are a B&G Vulcan plotter, Raymarine ST50 wind, speed, depth. They are integrated so the plotter can calculate true wind.

The courses we use are windward leeward. I have the marks programmed into the plotter and the plotter is displaying SOG, VMG, COG, DTW, TTW, BTW, and a bunch of other things. The Vulcan is designed for sailing and will display laylines, polars, etc.

I also have a garmin GPSMAP 78SC. It is not integrated to any instruments but does have waypoints in memory.

The Garmin and Vulcan both display VMG and it was very very close on both.

Since the course is windward leeward for the upwind leg I usually just trim the sails in as far as possible - headsail sheeted in until the leetch hits the shrouds, main in as far as possible, then traveler up to get boom on centerline, and then I steer to the tell tails.

I will do some research on velocity header target speeds.

Barry



Quote:
Originally Posted by jblumhorst View Post
How does your instrument system derive VMG? Is it VMG relative to true wind? Or to a waypoint? How sophisticated is your wind instrument?

IMO, you should be, first, picking your angles based on strategy and lay lines to the mark or destination, and then subsequently maximizing boat speed at that chosen angle.

In other words, “sail to target speeds”.
Figure out how to tell the difference between direction headers and velocity headers. How you drive and trim is different between the two.

Google “velocity header target speed” and “sailboat target speeds”

Barry Lenoble
Deep Blue C, 2002 C&C 110
Mt. Sinai, NY

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