Help me improve VMG

[Dave_S]

Originally Posted by 44'cruisingcat

No, 30' AWA isn't for slow boats. The AC 72s pointed much higher than that.

On our boat the Simrad autopilot has a VMG optimiser, and it usually settles on 30 - 31' apparent.

It really depends on how well set up your boat is for windward sailing. Few production boats will sail to windward as well as home built ones.

Some upwind sailing on our boat:

https://youtu.be/34qgJRprAlE

Thanks for the video.

I was happy to see your leeward stay that loose, mine are much the same, maybe mine were slightly looser but not by much. Your boat seems to point higher (looked like 27ish?) for that result.

I agree with all the comments that pointing as high as possible may not be the best vmg but I'm hoping if I can work the setup to point higher I may learn to setup for better VMG at a better angle. Maybe I'm wrong but I enjoy doing it. Also it seems I'm always a degree or two short of making a mark or island. I think that's everyone though.

[Jim Cate]

Also it seems I'm always a degree or two short of making a mark or island. I think that's everyone though.

Good point, Dave! There are times when simply sailing very high is more important than maximizing VMG, so learning how to do so is worthwhile. Keep up the study and experimenting... you'll be a better sailor as a result.

Jim

[fxykty]

Originally Posted by Jim Cate

Good point, Dave! There are times when simply sailing very high is more important than maximizing VMG, so learning how to do so is worthwhile. Keep up the study and experimenting... you'll be a better sailor as a result.

Jim

But on a cat with stubby keels or even daggerboards you have to keep speed up, or else you slide sideways and your heading doesn’t matter.

I couldn’t see the wind angle dial clearly on my phone, but 44’s boatspeed was around 9 knots during the video. That ensured his board(s) stayed efficient and prevented excess leeway. I expect if his boatspeed was 6 knots his leeway would be increased and the higher heading angle wouldn’t help VMG.

[Just Another Sa]

Originally Posted by 44'cruisingcat

Sorry but this is absolutely NOT fact.

The AC 72s were sailing at very similar AWA on both the upwind and "downwind" legs. There was only about 4' difference.

However the boatspeed changed by about 200%.

Which meant VMG to windward changed from about 90% TWS to around Minus 200% TWS.

What you quoted was and is an absolute fact. What you wrote is also correct. There is no contradiction.

[Just Another Sa]

Originally Posted by StuM

That is not a fact, it is a terminological inexactitude.



VMG = 50% of SOG* when TWA is exactly 60°


Are you really saying that any boat will do 15 knots VMG at 60° TWA in 30 knots of wind?


*if you assume no leeway

Wrong. It is a fact for any vessel having a constant AWA of 30 degrees including powerboats with no sails.
In other words any boat will do 15 knots VMG at 60° TWA in 30 knots of wind assuming it is able to have exactly 30 degrees of AWA in that condition. Of course most boats do not have 30 degrees awa at 60 degrees TWA, and I never claimed that to be the case.

For any boat capable of keeping AWA as a constant while bearing away, will have max upwind VMG when TWA = 2*AWA regardless of windspeed, boatspeed or value of AWA. Get it now?
If not, it's time to review basic trigonometry.

[Just Another Sa]

Originally Posted by Dave_S

Thanks for the video.

I was happy to see your leeward stay that loose, mine are much the same, maybe mine were slightly looser but not by much. Your boat seems to point higher (looked like 27ish?) for that result.

I agree with all the comments that pointing as high as possible may not be the best vmg but I'm hoping if I can work the setup to point higher I may learn to setup for better VMG at a better angle. Maybe I'm wrong but I enjoy doing it. Also it seems I'm always a degree or two short of making a mark or island. I think that's everyone though.

100% of sailboats have one thing in common when sailing with no auxiliary propulsion. They all achieve their best VMG upwind when AWA is increasing when bearing away, and AWA is decreasing when pointing higher than optimum VMG requires. They never get their best VMG when AWA remains approximately constant when TWA is changed. The only difference is what should be considered as approximately constant AWA. For a boat having AWA at constant value of 30 degrees for a significant range of TWA, it is certainly more than 2 degrees and less than 5 degrees, while for AC50 it is definitely less than 1 degrees, perhaps even closer to 0.2 degrees than 1.
For a non foiling sailingboat, optimum TWA for best upwind VMG is always less than 2*AWA, because AWA is changing, in most cases substantially less.

But as long as you do not see a difference in AWA, keep bearing away for better VMG. When awa initially begins getting wider (as in at least 1 degree change not caused by rounding effects but real ones), awa is close to the optimum, most probably withing 2 degrees. Of course the required change in TWA is more than in AWA at that point, meaning that you should bear away more than just 2 degrees.

For the other commentators let me point out that whenever leeway angle is too large you do not see AWA staying approximately constant, which is the reason it does not change the statements I made. They are valid for that case (constant AWA) and when bearing away more, and thus having even greater speed and less leeway.

[Just Another Sa]

Originally Posted by Jim Cate

Good point, Dave! There are times when simply sailing very high is more important than maximizing VMG, so learning how to do so is worthwhile. Keep up the study and experimenting... you'll be a better sailor as a result.

Jim

That is correct as long as sailing high is interpreted as course over ground, not heading, and as long as we are not talking about racing very close to other boats when racing rules might in some case mean more than performance.
For cruising situations, VMG is much more often more important than sailing high, because cruising does not most of the time involve being in the collision course with other boats or sandbanks or rocks.

[Just Another Sa]

Originally Posted by fxykty

But on a cat with stubby keels or even daggerboards you have to keep speed up, or else you slide sideways and your heading doesn’t matter.

Exactly, but that is not an issue whenever you see AWA as a constant while changing TWA significantly. Something underwater is already working efficiently or AWA would not stay constant.

[Stu Jackson]

Originally Posted by Dave_S

>>>>>>>>>>>>>
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Also it seems I'm always a degree or two short of making a mark or island. I think that's everyone though.

Being short of an upwind mark is something you eventually get better at. I found that I learned a lot by sailing the same waters and learned to ADD as many bits of information I could get to improve. These included: following the betters sailors (we raced our boat one0design almost exclusively); using landmarks as well as stuff on the water (many of the marks were immovable buoys or placed YC marks that didn't move); practicing how to deal with laylines, currents, some advantages of overstanding rather than pinching; things that happen with the wind locally (while folks always seem to think the winds on SF Bay are constant, from a particular direction, even during the standard summer breezes there are specific areas of wind direction changes making tactical decisions important, "local knowledge"). And this was all well before electronic aids were introduced to the recreational boater and eventually to our boat. After about five years of racing every month, I got better at it, much better. Couldn't do it now!

In looking at all the things that I just wrote, it occurs to me that VMG is only one, of course THE important one) of many factors affecting getting to that upwind mark successfully. You can lose more in one sloppy tack than you made in that 1 or 2 degree improvement.

Good luck, fun topic, thanks for it.

[StuM]

Originally Posted by Just Another Sa

Wrong. It is a fact for any vessel having a constant AWA of 30 degrees including powerboats with no sails.
In other words any boat will do 15 knots VMG at 60° TWA in 30 knots of wind assuming it is able to have exactly 30 degrees of AWA in that condition. Of course most boats do not have 30 degrees awa at 60 degrees TWA, and I never claimed that to be the case.

For any boat capable of keeping AWA as a constant while bearing away, will have max upwind VMG when TWA = 2*AWA regardless of windspeed, boatspeed or value of AWA. Get it now?
If not, it's time to review basic trigonometry.

Ah, I see. Or to put it another way: VMG = 50% TWS for any boat which is sailing with Boat Speed = TWS at 30° apparent. (That's the only case where AWA 30° = TWA 60°)

[fxykty]

Originally Posted by StuM

Ah, I see. Or to put it another way: VMG = 50% TWS for any boat which is sailing with Boat Speed = TWS at 30° apparent. (That's the only case where AWA 30° = TWA 60°)

What cruising catamaran can make that kind of speed at that angle?

[Just Another Sa]

Originally Posted by fxykty

What cruising catamaran can make that kind of speed at that angle?

Perhaps something like MC2 60 or a larger Gunboat might at suitable true wind speed. Or some oneoff of similar size. But it's really more about having enough sailarea and suitable windspeed, just that condition is more often met with bigger cruising cats than on smaller ones. Now if you would include racing boats ...

But not the one Dave_S has based on the information he has given about his Schionning Waterline 1480, which is why I wrote:

In practice VMG will not reduce until AWA increases, as your boat is not capable of keeping that AWA at TWA = 60 degrees or beyond that.

in my post #40.

[44'cruisingcat]

Originally Posted by Dave_S

Here is a 90degree trace from Roam. Pretty credible guy.

That's going to keep me thinking.

WOW


Quote


How's those upwind angles! Figured out how to make Roam do 90's.
Full main, no head sail, who needs a self tacking jib?
I didn't leave the wheel once... but Liss did play the traveller a bit to make sure we didn't get stuck during a couple of tacks.
No fossils were harmed in the making of this picture ��

We found another way.

About 1 1/2 knots of tide with you does the trick!

[Dave_S]

Originally Posted by Just Another Sa

100% of sailboats have one thing in common when sailing with no auxiliary propulsion. They all achieve their best VMG upwind when AWA is increasing when bearing away, and AWA is decreasing when pointing higher than optimum VMG requires. They never get their best VMG when AWA remains approximately constant when TWA is changed. The only difference is what should be considered as approximately constant AWA. For a boat having AWA at constant value of 30 degrees for a significant range of TWA, it is certainly more than 2 degrees and less than 5 degrees, while for AC50 it is definitely less than 1 degrees, perhaps even closer to 0.2 degrees than 1.
For a non foiling sailingboat, optimum TWA for best upwind VMG is always less than 2*AWA, because AWA is changing, in most cases substantially less.

But as long as you do not see a difference in AWA, keep bearing away for better VMG. When awa initially begins getting wider (as in at least 1 degree change not caused by rounding effects but real ones), awa is close to the optimum, most probably withing 2 degrees. Of course the required change in TWA is more than in AWA at that point, meaning that you should bear away more than just 2 degrees.

For the other commentators let me point out that whenever leeway angle is too large you do not see AWA staying approximately constant, which is the reason it does not change the statements I made. They are valid for that case (constant AWA) and when bearing away more, and thus having even greater speed and less leeway.

I must have read this 10 times by now and I don't think I've got it yet.

Please correct me if I'm wrong.

Is increasing and decreasing backwards ?

If 30° AWA is the highest I can point with airflow staying attached I should run at 30 then start to gradually turn down wind and the extra speed I get will bring awa back to 30° to a point where I can no longer make 30. Then I should bear away to 32 - 35°. At which point, somewhere in that range I should be at maximum VMG.

[Dave_S]

P. Ś-.

At this point hopefully AWA = TWA/2