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Roger H Strube, MD

Roger H Strube, MD

Using Your GPS VMG Function

I am a member of the Punta Gorda Sailing Club based in Florida. At one of the monthly meeting one of our best racers presented elements of boat speed including hull shape, sail shape (&age), sail trim and Polar Diagram use for up wind sailing. I had discovered a way to use a small, inexpensive GPS (I have several Garmin 72s) and its VMG function to maximize upwind and downwind speed. The process described below illustrates how to use your GPS bypassing the need to develop polar diagrams during practice sails and refer to them frequently while racing. The concepts may be of more importance to cruising sailors for safety reasons. The following document was attached to an email sent to the membership of PGSC.





PGSC Owners, Skippers, Racers, Cruisers,

Jerry Haller, one of our clubs best racers, made an excellent sailing presentation focused on the elements of windward performance. The information was applicable to racing and cruising captains, even though we all know, “Gentlemen do not sail to weather”. Part of his presentation explained the use of polar diagrams to determine the best wind angle and speed to sail upwind to get there in the shortest time possible. At the end of Jerry’s presentation I added some information regarding how to use your GPS to determine your best windward gain (Velocity Made Good = VMG) on a race course or, for that matter, anytime a cruiser is trying to beat the squall line to a sheltered anchorage. A not so short description of how this is done (“Windward/Leeward Performance for Dummies”) follows.

Virtually all sailors use a GPS if for no other reason than speed, compass heading, for most cruisers, ETA (Estimated Time of Arrival). Many sailors do not know how to use their GPS VMG function to maximize their windward performance. Most sailors will place “Way Points” in their GPS of the Marks of the Course or their harbor entrance destination. For simplicity, I will talk about the Windward Mark of the course but the discussion clearly also applies to any windward Way Point. As most GPS manufacturers use similar terminology and screens, the discussion should apply to all units. I have several cheap Garmin GPS 72 units and will use their screens to illustrate the technique.

Most PGSC members have the Charlotte Harbor fixed marks entered into their GPS Way Point list. If Drop Marks are used, the Signal Boat will post the compass direction to the Windward Mark. For this discussion let’s assume that the start is at Marker #1 and the wind is coming from 30 Degrees. You will note that Marker #2 bears 30 degrees from Marker #1. With this assumption a drop mark or posting Marker #2 (Course “D” PGSC Sailing Instructions page 6) will make no difference regarding use of your GPS. Assume the windward mark is posted at 30 degrees or “Course D” and the wind is at 10 knots. You have Marker #2 Waypoint in your GPS and select “Go To”. Your boat will do 5 knots to weather in this 10 knot breeze.

Starting at Marker #1 your GPS will read 5 knots boat speed, 2.4 nm to the way point and 30 degrees at the time you start. If you are on starboard tack, as you proceed up the left side of the course at 5 knots boat speed, your VMG (speed toward Marker #2) decreases to zero as you approach the lay line. Also, your compass heading to Marker #2 increases from 345 degrees (30 degrees – 45 degrees tacking angle) as you approach the left lay line. If your boat tacks through 90 degrees, at the lay line your GPS will read “0” VMG (you are sailing at a right angle to where you want to go making no Velocity toward Mark #2) and 75 degrees compass heading to Marker #2 (30 degrees wind + 45 degrees tacking angle = 75 degrees). Setting your GPS “Go To” at the windward mark (or harbor entrance) may help you find the mark and bang the corners of the race course for your tacking angle but it will not help get you there FAST.

What you really want to know is your Velocity Made Good TO WINDWARD, not just to the mark. This is done by setting the GPS so that the direction it sees to Windward is always parallel to the center line between the start and the mark. This is done by determining the direction of the wind (30 degrees) and then, on the “Map Page” of your GPS, moving the curser a couple of thousand miles directly up wind. Moving the cursor further faster is facilitated by pushing the “Out” button on the “Map Page” several times so a much larger map  area is displayed. Then “Go To” this phantom windward mark. With your GPS in this setting, your upwind VMG will be consistent anywhere on the course and your direction to the mark will remain consistent at 30 degrees. If your boat speed is 5 knots, your VMG should be about 3.5 knots. Now all you need to do is watch your VMG and Boat Speed and Maximize your VMG. If you sail a little higher and your boat speed drops to 4.8 knots but your VMG goes up to 3.7 knots, you are covering more windward distance in less time. By using your VMG you will be able to hit the high point (fastest windward performance) of the “Polar Diagram” for your boat in any wind. Now all you have to do is buy new sails, paint (or scrub) the bottom, apply perfect tactics and sail trim so you are able to sail up to your rating.

So, how do you set your GPS to work this magic?  On my Garmin 72, three pages out of five allow me to change size and information in the boxes at the top. These pages are: “Map Page”; “Pointer Page”; and “Highway Page”. By using the “Page” or “Quit” buttons you are able to scroll through all five pages. Once one of the three data field screens is up, click “Menu”. A list will come up including “Setup Page Layout” and “Change Data Fields”. Scroll down to “Setup Page Layout” and for old eyes select “Large (2 Rows)”. Then click “Enter”. The page will come up with two large boxes with whatever in them. Now click on “Menu” and then scroll down to “Change Data Fields”. Click on “Enter” and the page will come up with the top field highlighted. Click “Enter” again and a list of data options come up. I like to put VMG in this top box. Hit “Enter” and the page will re-appear. Use the down arrow to select the second, lower box and follow the above directions to enter “Speed” in the lower box. This can be repeated for the other two pages that allow data field options. You will now have large boxes on three pages with VMG over Boat Speed. Before the start of the race, check the line (wind direction) then select “Map Page”. Push the “Out” button several times to get a larger map view and move the curser 2000 miles up wind. Place a way point there and click on “Go To”. Now just maximize your VMG upwind and negative VMG down wind (down wind tacking angle) to give you the shortest time around the course (or to beat that thunderstorm to the safe anchorage).

Example 1: You are in the starting area at Marker #1 for Summer Series #8, August 24. You have set up your GPS as described above while sitting in your living room several days ago. The Summer Series uses fixed marks. The Signal Boat has posted “Course D” so you know your first mark is Marker #2, 30 degrees from the starting line. You check the wind (go head to wind) at the line and find it is 40 degrees. You open the “Map Page” on your GPS and run the cursor out 2000 miles at a course of 40 degrees. You hit “Go To”. What does this take? Maybe thirty seconds each for a couple of wind checks and, if you are slow, 20 seconds to set the GPS? You are going to invest 2 to 3 hours of your life in this race, isn’t a minute of setting your GPS worth the effort. Now just sail to maximize your VMG and try not to sail past your Lay Line. Starboard tack will be about 355 degrees (40 degree wind – 45 degree tacking angle) and Port tack will be about 85 degrees (40 degrees wind + 45 degrees tacking angle). Starboard tack (left side of the course) is the shortest tack. You sail to your rating and win the race.

Example 2: You are cruising back from Fort Jefferson, Dry Tortugas heading for Marko, Naples or Ft Myers. The clouds start to build and the wind shifts as the air is sucked into the line of thunder storms behind you. Marko is the closest but almost on the nose. If you try to make it to Naples you judge you will get swallowed by the monster. Your diesel is over heating at higher RPMs and you want to save it for entering the harbor. If you sail fast, you can make it into the safety of the anchorage at Marko before the squall line catches you. You have set up your GPS as described above. The Way Point at Marko Pass bears 30 degrees but the wind is, perhaps, 10 degrees to the right, about 40 degrees. You bring up the Map Page on your GPS and run the cursor out 2000 miles at 40 degrees and click on “Go To”. You now tack to windward at your maximum VMG. You are able to toggle back and forth between your phantom, 2000 mile mark and the Pass entrance by selecting the appropriate Way Point. You safely arrive and drop anchor 10 minutes before the storm hits.

On the flip side (no pun intended for multihull sailors) you may be sailing almost directly down wind to safe shelter, again trying to beat the monster your weather reports say will be there in several hours. Most boats sail faster (multihulls much faster) at some angle off directly down wind. Imagine you are returning from your cruise in the Bahamas sailing from Bimini to Miami. You are in an East wind of 8 to 10 knots and once again, the diesel is having overheating problems. You want to save the engine for docking at Miami Beach Marina. A nasty weather system develops over central Florida moving rapidly to the Southeast. The wind is predicted to shift to Northeast at 30 knots with the front. If you turn back, the monster will swallow you before you can get to safety behind one of the Bahamian Cays. It could swallow you before you reach Florida if you don’t sail fast. You don’t want to be in the Gulf Stream with 30 knots against the flow. You bring up the Map Page on your GPS and click on the “Out” button until you can see Mexico. You run the cursor out past Baha more than 2000 miles directly down wind (about 270 degrees). You place a way point there and click on “Go To”. You set a second way point at the entrance to Government Cut at Miami. You are now able to toggle between the two way points so you may sail your maximum down wind VMG on Port Tack and know when to jibe to Starboard to reach the channel entrance. As the front approaches you determine it is safer to enter the channel south of Key Biscayne and quickly anchor in its lee before the wind changes. You sail up the channel, take the North channel at the fork and sail behind Key Biscayne, head to wind dropping your main then anchor 100 yards off shore 10 minutes before the wind shifts to the Northeast at 30 knots.   

If you decide to use VMG this way, let me know how it works out for you.

Captain Roger H Strube 

                      VMG FOR DUMMIES                   CAPTAIN ROGER H STRUBE                      www.wingsailor.com

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