GPS: planing and tacking

Moderator: GreenLake

GPS: planing and tacking

Postby Interim » Mon Oct 05, 2015 9:12 am

I was playing with a new GPS app this weekend. Are they considered more or less accurate?

Assuming so, I was hitting 6.5 - 7.1 knots. This would fall into the "planing unintentionally" category. I was mostly trying to round a point for a nice downwind sail. My question is what is the top end that the day sailer will hit with some reasonable control? 8kn? 10?


--john
1979 DSII
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Re: planing

Postby GreenLake » Mon Oct 05, 2015 12:05 pm

Accuracy of GPS..

I use my GPS both on the boat and on the bike. I've had days when the GPS put my bike track solidly in the water, instead of along the shore....

One thing to make sure is not to put the GPS where you have to reach for it and pick it up to read it. Who knows what speed readings you get that way.

Generally, it's quite easy to estimate speeds based on the wake. The first time someone brought a GPS on board, I had fun testing my skills in estimation.
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Re: planing

Postby Interim » Mon Oct 05, 2015 1:04 pm

What are the indicators of different speeds, using the wake?

I was a little surprised by the compass readings as I was tacking back to windward. It seems like I was going through about 120 degrees. A bit off the ideal. Some factors were
1) 2' waves I was dealing with'
2) No traveler; and
3) luffing quite a bit (wind was 15mph + gusts, and I failed to head the advice "if you think it is borderline, reef.")

I was only making 4knts into the wind (at best).

--john
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Re: GPS: planing and tacking

Postby GreenLake » Mon Oct 05, 2015 3:44 pm

@interim: don't mind when people revive older topics to add to the discussion there, but you seem to want to branch out, so I gave you a new topic for all questions about using GPS to monitor performance.

Now, to estimating.

Just sail at different speeds and observe your wake. How visible is it, does it looks like a "powerboat wake" behind the transom, and so on. I did "measure" some of the slower speed using a "dutch log", that is, timing how long it took a bit of flotsam to pass down the length of the hull. Done a few times to give me a baseline, then correlated to the wake (also the sound the boat makes going through the water).

Now, with a GPS, you can do an estimate, then have another person tell you whether you got it right. I think you'll improve quickly.

The tacking angles are not that atypical I would say.

Waves can make a big difference. I sailed in a 14ft dinghy a while back in waves and they were not perpendicular to the wind, instead they were perpendicular to one tack and parallel (more or less) to the other. The result was really interesting. The boat felt like there was something wrong with the trim because it never really made speed against the waves while in the other direction it was as if everything had come together, because as far as the boat was concerned it was seeing super flat water. The DS is a bit heavier that what I sailed in that day, but if the waves were really 2' you'd be getting there.

On the DS, in the right conditions (more moderate than what you report) you can improve pointing with Barber haulers.
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Re: GPS: planing and tacking

Postby jeadstx » Tue Oct 06, 2015 12:10 am

Greenlake is correct about not making abrupt movements with a GPS. Picking your GPS up to look at it will show false readings as your motion is faster and often in the wrong direction. My crew observed this happening on the 2013 Tx200. After that, when checking speed and bearing, I learned to pick it up slower and hold it for a bit to let it settle in to the correct (or more likely correct) data. When sailing thru reefs on the Texas coast, I position my GPS to allow the least movement when trying to hold a course using pre-plotted waypoints, I also have crew keep a sharp lookout.

John
1976 Day Sailer II, #8075 - Completed the 2011, 2012, and 2013 Texas 200
1952 Beetle Boat Swan Catboat
Early Rhodes 19
1973 Mariner 2+2, #2607 - Completed 2014, 2015 and 2016 Texas 200
1969 Day Sailer I, #3229
Fleet 135; Canyon Lake, Texas
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Re: GPS: planing and tacking

Postby Interim » Tue Oct 06, 2015 8:24 am

When people say they are using GPS, are they typically referring to a smart phone app, or something more dedicated?

I'm a bit uncertain about the reliability of a 99 cent GPS, even though it is using a $600 phone and a multi-million dollar satellite system.

But maybe I don't want the truth. I'm happy at how easily I was hitting 7 knots.

--john
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Re: GPS: planing and tacking

Postby powpowhunter » Tue Oct 06, 2015 12:09 pm

I use a Garmin Oregon GPS, which is one of the smaller, less expensive handhelds that will accept Garmin "Blue Chart" nautical charts. I have not compared it to my phone, because I'm always afraid to have the phone out and loose in conditions during which I'm curious about how fast I'm actually going. The GPS is water proof/resistant, and my phone is not. Since I get wind forecasts in MPH and not knots, I leave my gps in statute units as well. Hull speed seems to be somewhere around 7 mph. Max recorded speed so far has been 12.1 mph, about 10.5 knots. This was during a big gust on a broad to beam reach. Coming up a little and luffing out the sail kept the boat upright, and I was amazed at how fast we accelerated and coasted along at. Felt totally in control, but also quite exciting.
Last edited by powpowhunter on Tue Oct 06, 2015 2:45 pm, edited 1 time in total.
-Tyler
1977 DS2 #8389 "Tidenaut"
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Re: GPS: planing and tacking

Postby GreenLake » Tue Oct 06, 2015 12:35 pm

Garmin 72H with USB connector.

I rarely sail where navigation is other than by landmarks, and not very often in locations where I need any form of charts. So I've ignored electronic charts and use the GPS mostly as a log (for speed) and afterwards to download the track and show me where I've been (and to weep over the tacking angles :) ). If you know you sailed some upwind stretch by tell-tale more or less perfectly, the GPS track will later show you the wind shifts.

When I want to check whether a navigational aid is actually where plotted in the charts, I put a kink in my course when I'm passing it, that let's me find it in the track later, and I don't have to take notes...
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Re: GPS: planing and tacking

Postby K.C. Walker » Tue Oct 06, 2015 1:06 pm

You can judge your speed on a Daysailer by looking at the wake, as Greenlake said. If your wake is at the transom you are sailing at the theoretical hull speed, so about 5.5 kn. If your wake is at about the cuddy, or half the boat length, you are traveling about 2.75 kn. If your wake is behind the transom you are starting to plane. That is actually pretty easy to recognize as the transom squats down in the water and you get a big wave right behind the transom. It takes a lot of power from this point to actually get planing, as the boat is sailing uphill. The heavier the boat the bigger the wake and the more power it takes to get out of the hole. If the wake is passed your transom by 2-3 feet you are probably sailing about 6 kn.

The Daysailer has a nice planing hull shape but is a bit under canvassed and a little heavy for easy planing. It can certainly do it though! I don't have a GPS but have an old handheld speedometer thing. It's a tube you dip in the water and it pushes a ball up a gauge. I've clocked in at 7 kn still in a mushy plane. It's hard to hold onto the thing even at that speed, though. I have gone much faster but I've not clocked it.

One day this summer we got a couple of really good runs on a deep reach with the UPS jennaker. The winds were between 15 and 23 kn according to my wind gauge. The wake was behind the transom by some 25 feet. My guess is that our speed was well above 10 kn. There was a rooster tail coming off of the rudder that sprayed up maybe 3 feet. I can tell you it sure felt fast!

Sailing back up wind was really wet and not near as much fun!
KC Walker, DS 1 #7002
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Re: GPS: planing and tacking

Postby jeadstx » Tue Oct 06, 2015 2:07 pm

I use a DeLorme PN-60w GPS which is accurate to about 6 or 7 feet. When I sail the Tx200 I also use a SPOT linked to it so I can compare GPS tracks and SPOT tracks later. I have a pretty good feel for the accuracy of my GPS unit. I have used the PN-60w since 2014. Prior to that I used a DeLorme PN-40 which did not link to a SPOT unit. I still use the PN-40 as backup. I save all my tracks for viewing later.

Sometimes when getting a speed reading you might get a higher reading if you are surfing on a wave. I have seen this on the Texas coast several times.

John
1976 Day Sailer II, #8075 - Completed the 2011, 2012, and 2013 Texas 200
1952 Beetle Boat Swan Catboat
Early Rhodes 19
1973 Mariner 2+2, #2607 - Completed 2014, 2015 and 2016 Texas 200
1969 Day Sailer I, #3229
Fleet 135; Canyon Lake, Texas
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Re: GPS: planing and tacking

Postby Interim » Tue Oct 06, 2015 4:26 pm

Seems like everyone is using dedicated units. I have the Sailing Speed app. John's note about the precision is helpful, because my app has a "precision" meter that varies from 5 meters to 30. I suppose I have my answer.

--john
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Re: GPS: planing and tacking

Postby kokko » Wed Oct 07, 2015 10:56 am

Most purpose-built gps devices will be accurate +/- 6 ft at a 95% confidence level. Although the sampling rate is fast, I do not know what algorithm the devices apply to smooth out the data. Still, instantaneous boat speed may not be accurate, but is very accurate with sufficient time and distance.

The genius of the smartphone apps is a cheap gps receiver is augmented with the cell phone signal. If you have a strong cell connection, your fix is as accurate as the purpose-built gps receivers. If you do not, then your fix from the cheapo gps receiver in your cell hone is not great.

Since I cruise a lot on Lake Superior out of cell phone range, I have a gps puck that works with my iphone and nav apps to give me a good fix. I have a Dual XGPS-150.
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Re: GPS: planing and tacking

Postby Interim » Wed Oct 07, 2015 11:15 am

That is helpful. I sail on a 3000 acre lake within range of at least two cell towers. I get a strong signal, and I notice that the longer I have the app open the better resolution I get. For my purposes, this will do fine.

A daysailer on Lake Superior? I wouldn't be able to get Gordon LIghtfoot's voice out of my head :D

--john
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Re: GPS: planing and tacking

Postby kokko » Wed Oct 07, 2015 11:53 am

Sorry about the confusion. Truelove is not sailed on Superior. We have friends with a 36' Albin Stratus we sail up there
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Re: GPS: planing and tacking

Postby kokko » Wed Oct 07, 2015 12:01 pm

Whether from a Garmin or an Iphone, I like to be able to keep a log of all my sails. I do prefer the Navionics app for the Iphone since it keeps the log, and overlaps my track with the lake. It has a summary of time started, time finished, distance, average speed and top speed. I can email the track to my guests after the sail. I would like to be able to add notes about the wind and weather condition, but I cannot.


btw. Truelove is on the hard, freshly scrubbed, polished and waxed, and will go into winter storage in a few weeks. I have 56 sailing tracks recorded this past season. I was able to sail Truelove one last time before pulling her with her new main.
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