Trimming sails for downwind (confirmation)

Bill Roosa · Sep 20, 2011

Beating vs running
When running the tell-tails have to be reinterperated. they will not be streaming backward clearly. Your intuition is correct that you would want to use the sail as a wing if you can and get the apparent wind to flow as if it was a beat. the sails are just big bags when running so unless you can develop sufficient boat speed to bring the wind forward and "beat while going downwind" you are left with presenting more sail area to catch more wind. this is problimatical because as the boat speeds up down wind the apparent wind speed drops to 0 at some point. which is why you would want to sail at an angle to the ture down wind direction. this allows the boat to travle faster (but over a longer distance point to point) and is genneraly the faster (but still slow) way to get to the down wind mark. On plaining boats and ice racers you can actually build enough speed this way to point farther down wind (boat makes it's own wind!!!)

Stu Jackson · Sep 20, 2011

And with the spread back spreaders on his Hunter, he doesn't have a whole lot of choice.

Todd Smith · Sep 20, 2011

Watch the speed-o-meter and try different trim. You will find the optimum for your boat and sails. Write down the wind speed and direction ( true and apparent both, if you have them ) and what trim seems to give you the best speed.

RichH · Sep 20, 2011

"Can I increase my forward driving force by easing the main more (maintaining shape of course), presenting a smaller cross section to the wind but gaining some lift by the airfoil shape of the main? I know he goal is to keep the tells flying but can I in this downwind course?"
ANSWER: most definitely get the wind to flow across the sails and keep the tell tales flying ... only when the wind is so great that the boat is moving at or near/beyond 'hull speed' will going straight DDW any benefit.

For 'displacement hulls' .......
Your optimum in downwind speed generated will be determined by the combination of ... A. drag (cross sectional shape + frictional drag) sectional area exposed to the wind and .... B. the amount of LIFT due to aerodynamic forces generated by the sail by having the airstreams moving 'across' the sail .... a combination of both.

This combo of downwind sailing is an 'artform' called 'ventilating' the sails. It is to keep the airstreams moving 'across' the sails thus you gain aerodynamic lift (as well as simple 'drag').
"Ventilation" is more important in light to moderate conditions, ..... less so in the higher wind ranges especially when the (non-planing) boat is near or at 'hull speed'.

If you have "VMG" function on your GPS chartplotter (and possibly integrated to an electronic wind vane at the head of the mast) this all become simple .... simply sail the angles which give you best VMG.
All this depends on how well your boat sails, the faster and the 'more easily driven' or how easily the boat accelerates will determine how one 'scallops' (constant gentle curved course) in order to keep the speed and VMG at a maximum) .... even as you are sailing a much longer distance to the 'mark'.
Sailing a 'straight line' course on such a 'leg' will usually not result in sailing at the 'optimum' (see "turbo sailing" in previous discussions for sailing on windward legs in this forum) as ideally you sail to keep the 'apparent wind' at a maximum while simultaneously following VMG to the 'mark'. The faster the boat is going the higher the apparent wind and the more 'aerodynamic lift' you get from the sails.
The famous sailing legend Buddy Melges was the pioneer of 'tacking downwind' to keep the sails ventilated, he dominated his olympic and scow classes because of this ... and soon the entire racing community adopted this method with the result that all the 'auxiliary special downwind DDW sails went to the scrap pile. The simple downwind technique was to 'ventilate' the sails so that they gave aerodynamic lift (as well as drag). You want to have the air going 'across' the sails when going 'downhill'.

hope this helps ;-)

Joe · Sep 20, 2011

You're on the right track, tacking downwind is much faster than running. I think the following link may be of interest to you. It's a discussion of polar diagrams and sailing performance. It will introduce you to the concept of VMG (velocity made good)

http://www.sailingworld.com/experts/get-your-performance-on-target

here's an excerpt:

Polars are boatspeed predictions across a range of wind angles and wind speeds. In its simplest form, a polar is a curved plot showing boatspeed versus course angle to the wind (true wind angle), for a fixed wind speed. This curved plot is called a polar because it is typically plotted using polar coordinates. Polar coordinates are simple: you have the origin (or center point of the plot), an angle radiating away from the plot axis, and a distance from the origin. Angle and distance, combined, determine a unique point on the plot.

Armed with polar info for your boat, you can determine its optimal course.

Enjoy.

Joe · Sep 21, 2011

Clark said:
Thanks Joe, I had actually found that article recently. I've even found a couple of examples of polars; one for a Cat 30 (similar layout to our Capri 22 Tall) and another for a larger boat with a design similar to our H280. I may just have to break down and pay for one for Rascal (our Capri 22). That is the boat we race now and one that we can get the main out to ~70* off center line w/o messing the shape up with the spreaders. I'm not sure if that is enough though in my example. As others have said, I'll just have to get out there and "play" with it.

The Capri 22 is a very cool boat to race. Have fun. Using the polar data will give you purpose in your down wind strategy.

DancingDays · Sep 22, 2011

Wow, I came here to ask some downwind questions, and lo and behold there's already a thread on it.

I'm also struggling with the broad-reaching vs DDW on a downwind course. Almost all the advice I find assumes a spinnaker, which I don't have. My Bene 31 takes the recent trend of bigger mains and smaller headsails to a bit of an extreme, where I only have a 105 jib. We've got a little bit of the swept back shroud issue, but not so much as a Hunter. Our summer winds tend to be light (e.g. around 8 kts)

When racing, we do reasonably well upwind, but get stomped downwind. I've tried heading up to as high as 160, but in general don't seem to get enough of a speed bump to give me a better VMG than wing-a-wing DDW. I think we should be able to do better. Any hints?

Joe · Sep 23, 2011

DancingDays said:
Wow, I came here to ask some downwind questions, and lo and behold there's already a thread on it.

I'm also struggling with the broad-reaching vs DDW on a downwind course. Almost all the advice I find assumes a spinnaker, which I don't have. My Bene 31 takes the recent trend of bigger mains and smaller headsails to a bit of an extreme, where I only have a 105 jib. We've got a little bit of the swept back shroud issue, but not so much as a Hunter. Our summer winds tend to be light (e.g. around 8 kts)

When racing, we do reasonably well upwind, but get stomped downwind. I've tried heading up to as high as 160, but in general don't seem to get enough of a speed bump to give me a better VMG than wing-a-wing DDW. I think we should be able to do better. Any hints?

Don't sail DDW, for one.... and 160 is still too deep... Try 135 True. Look around for a polar diagram for your boat... Beneteau may have one, give them a call.... and use it to set your target speed and VMG. The very most important thing is to keep your air clear.

But in general, try heading up to 135 True, and don't drop down till you're at max speed. Getting the speed on the broad reach will allow you to cheat down towards the rhumb line a bit, but you must always head back up to maintain target speed. Stay near the middle of the course if the wind is shifty, always stay in clear air... that means if someone tries to roll over you, luff them up, forcing them outside then jibe down into clear air, jibe on the lifts for a better angle to the mark, hit the layline only early enough to allow you to reach top speed when rounding the mark.

Bill Roosa · Sep 23, 2011

A couple of things about VMG
It is the speed toward the mark not your boat speed (you knew that though)
It does not account for the extra distance you have to travel to "make it back to the mark"
Jibing can overcome this but it takes a long range plan to capitalize on it.
If you look at the polars you will see a smooth curve for boat speed as a function of apparent wind direction. There is no accounting for the distance you have to cover though. ie when reaching at 90 degs to where you want to go it really does not matter how fast you go you will never get there. So the important thing to understand is there may be two bumps where your VMG is higher. Another way to say this is as you turn away from DDW you see the VMG go up and then go down, continue to turn into the wind and you might see the VMG start to go up again and actually reach a higher value. Sometimes it is actually faster to go REALLY fast back and forth across the course than to just try and do it in one jibe. alternately you can go WAY off the course on the fast tack and use one jibe but that is loaded with risk as you will not be in a position to see the winds the "pack" sees. Your winds may be better or worse and you will suffer or prosper for it.

RichH · Sep 23, 2011

DancingDays -
Just about all the previous discussions will apply to your boat, even in a non-spinnaker class with a fractional rig. Tacking down wind (gybing) on broad angles (not to exceed ~135 deg.) to maintain the 'highest' apparent wind (and following a VMG course) will give you the most optimized course on each leg.
Dont sail a 'straight line' leg when doing so --- come up a bit when the wind goes 'light' and more downwind in the blows/gusts -
Also search on this forum for 'turbo-sailing' as such will also work well for downwind as well as upwind. Keep the jib 'drawing' and the tell tales flying ... The simplicity of keeping the tell tales 'flying' will get you to the best trim/shape.
;-)

Todd Smith · Sep 23, 2011

Bill Roosa said:
A couple of things about VMG
It is the speed toward the mark not your boat speed (you knew that though)
It does not account for the extra distance you have to travel to "make it back to the mark"
Jibing can overcome this but it takes a long range plan to capitalize on it.
.../quote]

Bill, this is true if you are using a GPS's "VMG to the waypoint" function. If you use "VMG in the direction of the wind" then it does account for the extra distance. I just want to alert people about the 2 meanings of VMG.

Everything else is spot on.

If you have instruments that display "VMG towards the wind" then just adjust your course until that number is the highest you can get. That's your best angle for that wind speed.

You can use a GPS to display "VMG towards the wind" by placing a waypoint downwind at a very large distance (500 miles ?). Then watch the VMG display on the waypoint. The warning here is that if the wind shifts, then the VMG is no longer "towards the wind".

Bill Roosa · Sep 23, 2011

hey Todd
Never heard of VMG toward the wind. you learn something new every day.
I'm not sure why I would care how fast I was going toward some arbitary point though. I always thought that I was racing "to the mark" not to some point down wind that may or may not be close to the mark. I can see that it alignes the VMG with the boat polar plot and that could be useful but when racing it is about "the mark" and I will, no doubt, have different VMG on opposite tacks. which leads us back to more racing strategy and gets us off post.....
So what do you use it for?

DancingDays · Sep 23, 2011

Joe said:
Don't sail DDW, for one.... and 160 is still too deep... Try 135 True. Look around for a polar diagram for your boat... Beneteau may have one, give them a call.... and use it to set your target speed and VMG. The very most important thing is to keep your air clear.

I do have a polar diagram from Beneteau, but if I read it correctly, the downwind curves assume the use of a spinnaker. That is, for each wind speed it has discontinuous curves for up and downwind. I assume there's no reasonable way to infer non-spin downwind targets from that, correct?

DancingDays · Sep 23, 2011

I'm a bit confused by the VMG discussion. Doesn't VMG towards a mark give you closing speed to that mark, regardless of "extra distance"? And VMG to the wind not the same as VMG to a directly upwind mark?

Stu Jackson · Sep 23, 2011

VMG 500 miles away

So what do you use it for?

To figure out the favored tack or side. This, simplistically, assumes the wind direction is a constant, but is helpful nevertheless. It is used by voyagers to determine favored tacks for long distance cruising, and can be used by racers for the same reason. Racers generally take the favored tack by heading up to the wind before the start of a race and comparing the wind angle against the set of the starting line. The starting line is rarely perfectly set up against the wind, so using both VMG to the first waypoint (uphill after a start) and the 500 mile wind waypoint is handy information to have in your arsenal.

I think we should also start saying "gybe downwind" rather than tacking down wind to avoid confusion.

RichH · Sep 23, 2011

DancingDays said:
I do have a polar diagram from Beneteau, but if I read it correctly, the downwind curves assume the use of a spinnaker. That is, for each wind speed it has discontinuous curves for up and downwind. I assume there's no reasonable way to infer non-spin downwind targets from that, correct?

Make your own polar plots by careful recording of the angles/windspeeds (correct to True wind, ugh) versus observed boat speed, etc. when precisely sailing these angles/windspeeds ... this will give you an efficiency factor between YOUR performance and a 'reference ideal'. Do this all 'upwind' at first.

Then begin sailing downwind at the same angles as on the ideal polar plot ... without a spinnaker and note the differences .... and then corrected by YOUR developed 'efficiency value' to use as your 'target speed'.

Simply observe and record and plot the new numbers in comparison to the ideal numbers .... and you now have YOUR actual polar plot ....
If you are not close to the 'ideal' find out why and make corrections to your boat, your helmsmanship, boathandling, your trim and sailshape. Record everything, compare, then make corrections until you 'get it right'. 'Tell tales flying perfectly' will be the most important 'correction'.

Todd Smith · Sep 23, 2011

Bill Roosa said:
hey Todd
Never heard of VMG toward the wind. you learn something new every day.
I'm not sure why I would care how fast I was going toward some arbitary point though. I always thought that I was racing "to the mark" not to some point down wind that may or may not be close to the mark. I can see that it alignes the VMG with the boat polar plot and that could be useful but when racing it is about "the mark" and I will, no doubt, have different VMG on opposite tacks. which leads us back to more racing strategy and gets us off post.....
So what do you use it for?

Great question Bill.
I will illustrate the concept upwind first, because it is more intuituve.

When sailing ( racing ) to a mark upwind, you know that you are going to be tacking back and forth to get there. For an example, I have a desitation 20 naut. miles ( in the upwind direction), and I can make 5 knots VMG to windward. This means that it will take me 4 hours to get there. It does not matter if start directly downwind, or off to one side.

Warning, ASCII art here - oops didn't work, see image below.

It takes the same amount of time to get from (1) or (2) to the destination. But the VMG_to_the_waypoint will be very different on the port or starboard tack. By position (3), just before you take over, then the VMG_to_the_waypoint will be almost zero.

Upwind you HAVE to tack because you would just stop if you pointed straight into the wind. Downwind you will still get there, but it will take longer.

See image below

If you start at point (2), you could take the straight path ":" or the long path "*".

Again, if it is 20 nm (in the downwind direction) to my destination and I can make 5 knots VMG_downwind by following the "*" path ( which is 135 degrees true wind angle ), I will get there in 4 hours. If I can only go 4 knots DDW, then the ":" path will get me there in 5 hours.

Sorry if I ran long, or didn't answer the question.
Todd

oops, that ascii art did not come through.

Bill Roosa · Sep 23, 2011

OK, I see what you are getting at. Using the VMG to the mark will sometimes not give you any useful info as you have drawn your diagrams. And using VMG to the wind would. However, I'd never take course 1-to-end point cause "you always do the longest tack first" and I'd never do 3-to-end point because I would not want to be that far off the start line or around the mark starting that leg which increases the distance I have to cover. So while I agree with you mathmatically I don't think 1 and 3 are "smart" racing.

I can certianly see the usefulness in general sailing though.

I was always under the impression that as I near the mark i should stop looking at VMG and make a decision to tack or hold on which really depends on how good my crew is at sail handling and now we are off topic AGAIN.

Trimming sails for downwind (confirmation)

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