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?

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!

Is there another way of working out the best boat/sail trim, apart from comparisons with nearby boats?

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.

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.

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.

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 http://raceqs.com" rel="nofollow - 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 http://www.jeffstinesailing.com/Sail2WIN" rel="nofollow - Sail2WIN app on my web page for the app, or at the http://apps.garmin.com/en-US/apps/791191d4-0593-426c-838d-ee71558a967d" rel="nofollow - Garmin App Store .

Here's a screen shot.

Thanks for that.

No, I didn't notice the year given the way it's displayed.

Maybe someone will find the post helpful anyway.

Jeff

I am sure he does , but he has something to sell and doesn’t want to buy an advert.