Downwind angles
- Thread starter RussC
- Start date
Ideally you’d have a polar diagram for your model boat showing boatspeed at every wind angle in each wind range. The next best thing is an instrument networked to your wind vane and speed sensor that can tell you your VMG. You can watch your VMG at various wind angles to see where it’s maximized. You can do the calculations yourself with just speed and direction information if you don’t have the instrument networking.
My guess is you’ll find that the best angle is either dead down wind wing-on-wing, or just high enough on a broad reach to keep your jib full. It might also be that you’re best off on a broad reach in calmer conditions and better wing-on-wing in heavier wind.
My guess is you’ll find that the best angle is either dead down wind wing-on-wing, or just high enough on a broad reach to keep your jib full. It might also be that you’re best off on a broad reach in calmer conditions and better wing-on-wing in heavier wind.
We have found that 135 degrees TWA is best when flying only the jib and main. Your instruments may have a VMG to wind which will allow you to experiment. In this case the VMG will be negative but that is exactly what you are trying to discover.
jssailem
SBO Weather and Forecasting Forum Jim & John
- Oct 22, 2014
- 23,546
The suggested angles are great places to start. Then compare the chosen angle to your VMG for the waypoint.
You can preplan this using current weather data in LuckGrib (an apple product). By entering the start and target waypoints, your polar data for your boat or a similar boat from the catalog, the program will calculate the quickest route with bearings and points of tack for the fastest sail.
I believe that PC or Android users can get similar information using Predict wind.
You can preplan this using current weather data in LuckGrib (an apple product). By entering the start and target waypoints, your polar data for your boat or a similar boat from the catalog, the program will calculate the quickest route with bearings and points of tack for the fastest sail.
I believe that PC or Android users can get similar information using Predict wind.
This is a good time to get acquainted with your GPS' VMG (Velocity Made Good) function. There are a couple of ways to use it.
1. For navigation, where you're most interested in the heading that will get you to the mark in the fastest time. i.e. effective progress to the waypoint over sailing angle for highest boat speed. In this case, the waypoint/mark will be relatively close and can be used quite effectively when sailing a route with multiple waypoints.
2. For optimal speed at various sailing angles, you can plug in a distant waypoint....an imaginary mark many miles away...use DDW for the wind direction. True wind direction and speed are important because they aren't affected by changes in boat speed or course heading. Anyway, Set your distant waypoint, turn on the VMG display, and start clocking your boat slowly through the arc from ddw up to 90 port & stb and jot down your changes every 10 deg. make notes of sail changes, sail trim adjustments, current and wave direction, etc. Of course, note any true wind speed changes.
I use the vmg for navigation ALOT!..... especially upwind to help me with lifts and headers. Be careful on the down wind legs, its too easy to look at the speed and start sailing too wide an angle which will mess up your progress in the desired direction. Watching the VMG will help with your discipline in sticking with True wind indicators and the polar data that will have been collected during these exercises. Good luck. It's fun to work on this aspect of sailing.
1. For navigation, where you're most interested in the heading that will get you to the mark in the fastest time. i.e. effective progress to the waypoint over sailing angle for highest boat speed. In this case, the waypoint/mark will be relatively close and can be used quite effectively when sailing a route with multiple waypoints.
2. For optimal speed at various sailing angles, you can plug in a distant waypoint....an imaginary mark many miles away...use DDW for the wind direction. True wind direction and speed are important because they aren't affected by changes in boat speed or course heading. Anyway, Set your distant waypoint, turn on the VMG display, and start clocking your boat slowly through the arc from ddw up to 90 port & stb and jot down your changes every 10 deg. make notes of sail changes, sail trim adjustments, current and wave direction, etc. Of course, note any true wind speed changes.
I use the vmg for navigation ALOT!..... especially upwind to help me with lifts and headers. Be careful on the down wind legs, its too easy to look at the speed and start sailing too wide an angle which will mess up your progress in the desired direction. Watching the VMG will help with your discipline in sticking with True wind indicators and the polar data that will have been collected during these exercises. Good luck. It's fun to work on this aspect of sailing.
Hello,
As you have guessed, this is not an easy answer. Maximizing VMG is the goal but you don't want to be racing downwind trying all sorts of angles to determine your best VMG.
The difficulty in determining the best wind angle is caused by the wind speed. The higher the wind speed the more DDW you can sail. The lighter the wind, the 'hotter' angle you need to sail.
You will have to do some experimenting with your boat to find your best angle(s). You will probably end up with something like
130 TWA when the wind is under 10
150 TWA when the wind is 10-20
180 TWA when the wind is over 20.
If you have a whisker pole that will also change your angles.
Last point - I have never seen polar charts that didn't use a spinnaker for downwind sailing.
Good luck,
Barry
As you have guessed, this is not an easy answer. Maximizing VMG is the goal but you don't want to be racing downwind trying all sorts of angles to determine your best VMG.
The difficulty in determining the best wind angle is caused by the wind speed. The higher the wind speed the more DDW you can sail. The lighter the wind, the 'hotter' angle you need to sail.
You will have to do some experimenting with your boat to find your best angle(s). You will probably end up with something like
130 TWA when the wind is under 10
150 TWA when the wind is 10-20
180 TWA when the wind is over 20.
If you have a whisker pole that will also change your angles.
Last point - I have never seen polar charts that didn't use a spinnaker for downwind sailing.
Good luck,
Barry
Lots of interesting comments for me to mull over. thank you! I wasn't aware that some of the more basic GPS units could give vmg info. I thought you had to get into the mega dollar systems for that. I might have to start a separate thread on recommendations. I am very tech averse however, as I'd rather drive my classic (62) car than my new one and preferred hang gliders for 18 years over sailplanes LOL. seat of the pants sailing is true sailing in my book (but maybe I can learn to cheat when needed also 
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A former poster on this forum who is very knowledgeable about racing stated unequivocally that the fastest way to the mark for a jib & main configuration is ddw. I could not find that quote so I'll leave his name out in case I'm mis-quoting him.
I assume that is with the jib poled out. There are other variables, besides the ones noted above, like a mast head vs. fractional rig, the size of the foresail and the skill of the driver.
I assume that is with the jib poled out. There are other variables, besides the ones noted above, like a mast head vs. fractional rig, the size of the foresail and the skill of the driver.
I affirm Barryl's reminder that the optimum course for a destination DDW does vary with the windspeed. And the VMG on a GPS allows us to understand what the best headings for our boat are as wind speed varies.
I would add that our asymetric spinnaker is key to our downwind tactics at lower windspeeds. With it, we can often be at optimum VMG when DDW or close to it at those low wind speeds.
I would add that our asymetric spinnaker is key to our downwind tactics at lower windspeeds. With it, we can often be at optimum VMG when DDW or close to it at those low wind speeds.
Every gps I've owned has a vmg feature. It only needs a way point. I works by measuring how fast you're moving toward that mark, neither the wind angle or windspeed is considered... True and Apparent. You may be thinking of the complicated polar charts that many, (careful not to say "serious) sailors use to determine optimum point of sail depending for various conditions. But the simple gps vmg and the more complex polar charts still share the same goal.... it's just the gps vmg only tells you what the number is... while a polar basically tells you how to get it. The gpd is like the speedometer, the polar is like the roadmap.Lots of interesting comments for me to mull over. thank you! I wasn't aware that some of the more basic GPS units could give vmg info. I thought you had to get into the mega dollar systems for that. I might have to start a separate thread on recommendations. I am very tech averse however, as I'd rather drive my classic (62) car than my new one and preferred hang gliders for 18 years over sailplanes LOL. seat of the pants sailing is true sailing in my book (but maybe I can learn to cheat when needed also
While that wasn't me I must add to your first sentence: assuming the wind is also directly to the mark. It rarely is though.
Even my ancient 1980s West Marine Vector Loran had the VMG function (I just looked it up by doing a search on West Marine loran manual and wouldn't you know but that ancient Vector manual is still there on the internet - fascinating!!!). It is basic to all navigation programs because they are calculating TTG time to go to the next waypoint. It's pretty simple math.Lots of interesting comments for me to mull over. thank you! I wasn't aware that some of the more basic GPS units could give vmg info. I thought you had to get into the mega dollar systems for that. I might have to start a separate thread on recommendations. I am very tech averse however, as I'd rather drive my classic (62) car than my new one and preferred hang gliders for 18 years over sailplanes LOL. seat of the pants sailing is true sailing in my book (but maybe I can learn to cheat when needed also