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GPS for VMG on dinghy

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dave101 View Drop Down
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Post Options Post Options   Quote dave101 Quote  Post ReplyReply Direct Link To This Post Topic: GPS for VMG on dinghy
    Posted: 31 Dec 10 at 8:31am
I have been looking into getting a GPS so I can work out the best VMG upwind (in a Laser). Some of the Velocitek units and the more expensive Tacktick seem to be able to do this

My question is that are the measurements accurate enough to pick up small speed changes due to sail trim, e.g. outhaul tension, sail angle of attack?




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JimC View Drop Down
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Post Options Post Options   Quote JimC Quote  Post ReplyReply Direct Link To This Post Posted: 31 Dec 10 at 8:55am
I suspect the issue will be not so much whether the units are able to pick up such small changes as whether you're going to be able to separate the tiny effect of such small changes from the much bigger variations caused by sailing technique, waves, wind changes etc etc...
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Post Options Post Options   Quote Neptune Quote  Post ReplyReply Direct Link To This Post Posted: 31 Dec 10 at 11:31am
I have reservations about GPS systems on dinghys.  i have seen one advertised recently that allows you to pin the ends of the start line and in the run up to the start give you feedback as to how far you are off the line - to me thats too much outside assistance.

while some classes might value you that i think its a detriment to the class overall even if initiallly they are only allowed for training.  if enough people spend the money then there comes a demand to allow them for racing.  i think the 700's are about to allow GPS to be carried while racing - the first RS class to do so, while inthe 600 we still have discussions as to whether the tactic function on a tactic is more than is allowed in the rules!

The laser think only allows an anologue compass and think you would be better to two boat train rather than rely on a compass!




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Post Options Post Options   Quote blueboy Quote  Post ReplyReply Direct Link To This Post Posted: 31 Dec 10 at 12:48pm
Originally posted by JimC

I suspect the issue will be not so much whether the units are able to pick up such small changes as whether you're going to be able to separate the tiny effect of such small changes from the much bigger variations caused by sailing technique, waves, wind changes etc etc...


Correct. I've been using GPS on yachts for getting on for a couple of decades now and there isn't a snowball in hell's chance of getting useful tuning data for a dinghy out of one. Not, at least, without AC-type resources to throw at it to allow correlation to wind data etc.
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Post Options Post Options   Quote blueboy Quote  Post ReplyReply Direct Link To This Post Posted: 31 Dec 10 at 12:50pm
Originally posted by Neptune

I have reservations about GPS systems on dinghys.  i have seen one advertised recently that allows you to pin the ends of the start line and in the run up to the start give you feedback as to how far you are off the line - to me thats too much outside assistance.


Quite apart from the question of whether you think pre-start, it's a great idea on a one or two person boat to be fixated on an instrument instead of the boats around you. Crunch, sorry didn't see you mate, I was looking at my GPS.
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Post Options Post Options   Quote dave101 Quote  Post ReplyReply Direct Link To This Post Posted: 31 Dec 10 at 1:13pm
This wasn't for use during racing, just whilst practising. However, from the comments it looks like a GPS is not going to be much use to me anyway

Is there another way of working out the best boat/sail trim, apart from comparisons with nearby boats?
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Post Options Post Options   Quote pondscum Quote  Post ReplyReply Direct Link To This Post Posted: 31 Dec 10 at 1:20pm
With differential GPS you can certainly get more than enough velocity accuracy, but such units are more expensive/complex. Normal open service single-frequency units like the velocitek etc will give you an accuracy of 0.1 kn at best and you get into needing smoothing - the infamous GPS speed spikes. GPS is reasonably useful on a yacht for seeing the effect of trim, but even then the speed varies quite a bit with the waves.
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Post Options Post Options   Quote Skiffman Quote  Post ReplyReply Direct Link To This Post Posted: 01 Jan 11 at 6:42pm
useless unless you are going to get something like the cosworth system which requires a rib with a high tech wind mast driving next to you. 

other boats are much better to compare against, when we go tuning all we do is ideally have 2 boats but max 3. Then you spend 3, 5minute runs getting comfortable with the rig and technique. Then one boat changes for an hour then after they go to there fastest setting. Then the next hour the other boat and change about with settings. Finally finish it off with both boats going on there fastest settings.

But do not tune to be fast in tuning runs, it can be very easy to make your boat the fastest in tuning but then slow around a race track.

The key to it all is to have a boat that is very easy to sail and fast enough. No point having a hard boat to sail that can go really fast when you have just gone the wrong way.
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Post Options Post Options   Quote Jimbob Quote  Post ReplyReply Direct Link To This Post Posted: 01 Jan 11 at 8:32pm
Dave 101 "My question is that are the measurements accurate enough to pick up small speed changes due to sail trim, e.g. outhaul tension, sail angle of attack?
The simple answer is NO! Details like that are not available even from two boat training. On the water you are sailing within a system which is chaotic in two dimensions with a crew which can also vary in many aspects. And feel is very much a part of good performance. So results will only ever be approximate. But, you can still gain macro type info from training with a GPS unit like a Velocitek. For example you will be able to deduce what macro settings are good and what are bad up and downwind after analysing a reasonable amount of data obtained while sailing, (and you will have to try and remember what the wind strengths were and the set ups were at the time). But that is not too difficult; I have done it. And it will certainly help your tacking, gybing and mark rounding. It is very easy to work out just how much time you lose on these from a Velocitek graph. And you know what they say. More time is lost from poor tacks on the beat than from a badly set up boat.
Lastly, put GPS into the Search function above as this subject has been done many times before.

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Post Options Post Options   Quote JeffStineSailing Quote  Post ReplyReply Direct Link To This Post Posted: 12 Dec 21 at 9:53pm
"My question is that are the measurements accurate enough to pick up small speed changes due to sail trim, e.g. outhaul tension, sail angle of attack?"

Hello Dave 101.
My answer to your question is a qualified yes.
While GPS has it's limits, it can also be useful, particularly if you don't have a training boat to sail against or a RIB with serious instrumentation pacing you.

A word about GPS Accuracy:
You've probably used GPS navigation in your car or while hiking.
It's not perfect but it's pretty good and on a single device it's generally quite consistent.
Inconsistency in GPS position generally is the result of interference with the signals arriving from the satellites. Trees, buildings, human bodies, changes in atmospheric conditions all have an effect on those signals. When we're sailing a GPS has a good shot at the sky without much interference, particularly if that GPS is on your wrist in a watch.
You won't get absolute accuracy but what you need is relative accuracy. 
Is indicated VMG increasing or decreasing ?
If you look at replays of your sailing tracks with the tracks overlayed on actual charts going around fixed marks, you'll see that even absolute accuracy is really quite amazing much of the time.

Back to your question.
The first challenge is to choose a time for your tuning / practice when conditions will be fairly stable. That will allow you to measure changes you make to the greatest degree possible.

Then, if you can read real time VMG or VMC toward some point upwind, you can play with sail trim and how high or low you're pointing and read the results as you go. Doing that with outhaul tension is harder because you have to stop and adjust, but if the conditions are stable you might get similar information.

Watching real time numbers change as you change your trim is pretty great and is useful.

Along with that you can record your track and sail a set course several times with different settings, then load that track into RaceQs and see the results. RaceQs is a pretty great playback and analysis site that's completely free. I use it a lot and have used it when helping folks tune up their trim and upwind or downwind work. You can see tacking efficiency, VMG, tacking angles etc. With that tool you can compare multiple runs up a given course with settings different for each run and see how they compare.
Again, stable conditions are important and may be hard to find but still, you're likely to learn something from recording and analyzing your sailing.

RaceQs replays multiple boats together if they're sailing in the same area at the same time and upload their tracks, so you can do complete race replays comparing boats, where they went on the course, how high they were pointing, how good their tacks were, who was faster on each leg, etc.
It's a great tool.

I looked for GPS based instrumentation that will do what you're looking for (and more). I used handheld devices for a long time and tried out a number of watch apps for GPS watches and didn't find anything that gave me what I was looking for so I wrote an app for Garmin's watches that provides me with the information I want while I'm racing. So far I've gotten feedback that's quite positive. The most common feedback I've gotten is that having real time performance data on your wrist is just amazing. You might find that it gives you much of what you're looking even if it's not the same as being on a pro team with a RIB following you with big money instrumentation :-)

It runs on very reasonably priced watches by Garmin (~$200 and up) that are built for a wet environment.

You can check out the Sail2WIN app on my web page for the app, or at the Garmin App Store.

Here's a screen shot.





Edited by JeffStineSailing - 12 Dec 21 at 10:10pm
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