I dont know if this is question with an easy answer or not.

Just wondering, for training purposes if there are handheld GPS units that will calculate your speed towards a mark. What I mean by this is that you predesignate your destination (could be any mark on the course). The GPS calcualtes your speed to this.

Speed towards the mark being different from forward speed. Going directly at the mark, forward speed is equal to speed to mark, but if you are going forward at any bearing off the mark, your speed to mark will be less than forward speed.

This ultimately could help determine the ideal bearing toward the mark, and will give the fastest bearing to mark.

Anyway... just wondering if there are GPS units that will do this.

Thanks

Sure...what you are looking for is "VMG" or "Velocity Made Good". All of the garmin units that I have owned have this capability. It takes your current speed and current direction and calculates how much of that is contributing toward your destination. The tricky part is freeing up a finger to push the buttons at the mark and program the GPS to track to that position while you are racing....not easy.

Jake:

I think "Velocity Made Good" is the same as your actual boat speed minus the current and wind losses. This would not show you what your choice of tacking angle has done for you. Your GPS could show you your ideal tacking point though (if you like to bang the corners) by showing you the degree heading to your next mark. However, you have to know the ideal number of degrees that your boat performs while beating (Can you handle a 15 degree pointing angle?)

Wyatt

If you have a good GPS unit and you load your "next" waypoint (on inflatable marks, not easy, but if it's a government channel mark, you can get the lat/long off a chart) then VMG is good to that waypoint. GPS VMG doesn't care about wind or current, just your gain on the next waypoint. You can be going 10 knots at a 45 degree tack with current pushing you back...it only sees the relationship of your unit gaining on that waypoint in VMG mode. You can then try each tack and see which one is getting you there faster, but as Jake said, nearly impossible to load all that stuff while actually racing!
If you are doing a long distance race and can load the lat/long ahead of time, it is useful.

Actually VMG is your speed made good toward your destination. There is some marine terminology that defines this slightly differently but it all means the same thing. If you are traveling to a destination that is at a 360 degree heading and you are moving at 10mph at a 60 or 300 degree heading, your VMG is 5 mph (and your wasted velocity away from your destination is 5mph). Current and leeway can certainly affect this by changing the course you are sailing over ground (i.e. your heading). The GPS unit knows nothing of tide or leeway or your steering angle (unless you have some REALLY sophisticated electronics) and only knows the course over land you are traveling relative to your destination.

VMG = COS(heading difference to destination) * velocity

Timbo:

Probably a semantics question: Your GPS will always show you the shortest distance to the mark; your current speed of the vessel (as it moves in a straight line, not towards the mark): your heading (the direction you're currently going): and the bearing to the next mark (the shortest line to where you want to go). Sorry if I seemed confused, but "velocity is a rate of movement with a definite direction and is used in connection with a current. Speed is a rate of movement in any direction". p.341 Chapman Piloting

Wyatt

Jake explained it better than I did.

Wyatt

Just what I was looking for. Thanks for the help.

During a race, obviously this would be difficult, but during trraining, tihs could help develop feel fro correct angle and trim.

Gordon

Some of the Garmin units have a MOB ("Man Overboard") feature that quickly sets a waypoint at the spot the user is in when the button is pushed. It also sets the GPS to navigate to that point with one more button push. My 72 has this feature and though I've never tried it, it would be much easier to try and set a windward mark with it during a race.

One thing to keep in mind about a GPS VMG reading...as you progress along the course towards the laylines, the angle to the mark changes considerably, so the GPS unit is constantly re-calculating the VMG. Once you get the mark off your beam, VMG goes to zero and then negative as you sail beyond 90 deg. This can be handy if you know you can tack through 90 degrees (or something close to 90) as a way of "seeing" the layline.

To more accurately train for better performance, you'd do better to set the GPS waypoint very far upwind/downwind (+50 miles?) beyond the mark you are really heading at...so the angle changes are much less significant.

http://www.ockam.com/ocku1.htm