We want to know how fast we are sailing. I see two alternatives ... add a knot meter to our boat and/or use a GPS device. What are your thoughts on the merits and problems of each? I know a GPS is extremely accurate at zeroing in on its location - but how about speed? Is the displayed speed/velocity updated frequently enough to be useful to determine the impact of some action (such as raising the center board on a downwind leg - or - adjusting the placement of a jib bock)? (i.e. would we see an immediate effect of such an action on the GPS - or will it take a matter of minutes to register?) For those of you who prefer the knot meter - is there any way to avoid driling a hole in the hull for the transducer? Thanks, Tom Grass Grasshopper II H 26 - #174
- Status
Limited use
Tom Although you can depend on a GPS for timely ( read actual) SOG readings, it sounds more like you'd want to know speed through the water for which a GPS will have limited utility- it's not really a matter of preference. Get your drill warmed up... Don
Tom Although you can depend on a GPS for timely ( read actual) SOG readings, it sounds more like you'd want to know speed through the water for which a GPS will have limited utility- it's not really a matter of preference. Get your drill warmed up... Don
GPS Cycle Time
Most GPS have a cycle time that can be set. Quite often, it can be set to seconds. My magellan h/h can be set to a .5 ,1 , 5, 10, 30...second increment. Thay means that it is updated a frequently as desired. Now, we know that the GPS speed accuracy is affected by such things as tide, current, and set. The true reading of a GPS is Speed over Groud (SOG), not speed through the water.
Most GPS have a cycle time that can be set. Quite often, it can be set to seconds. My magellan h/h can be set to a .5 ,1 , 5, 10, 30...second increment. Thay means that it is updated a frequently as desired. Now, we know that the GPS speed accuracy is affected by such things as tide, current, and set. The true reading of a GPS is Speed over Groud (SOG), not speed through the water.
We discussed this recently
and I looked at the spec for the timing signal from the GPS satellites. My interpretation is that the signal is transmitted every 30 seconds, but that the time 'hack' might change 5 times in that signal. So the shortest interval that the GPS could use to update its data by using new data from the satellite is 6 seconds (I believe that it is actually 30 seconds). So anything your GPS is doing more quickly than 6 seconds (or possibly 30) it is doing internally, for instance by applying a previously calculated velocity to the distance from a waypoint. If anybody has more intimate knowledge of the GPS signal specification, I will happily learn from them.
My experience is that the GPS is good for counting down to a destination, but for immediate speed data, use the log. Cheers, Bob
and I looked at the spec for the timing signal from the GPS satellites. My interpretation is that the signal is transmitted every 30 seconds, but that the time 'hack' might change 5 times in that signal. So the shortest interval that the GPS could use to update its data by using new data from the satellite is 6 seconds (I believe that it is actually 30 seconds). So anything your GPS is doing more quickly than 6 seconds (or possibly 30) it is doing internally, for instance by applying a previously calculated velocity to the distance from a waypoint. If anybody has more intimate knowledge of the GPS signal specification, I will happily learn from them.
My experience is that the GPS is good for counting down to a destination, but for immediate speed data, use the log. Cheers, BobEither is helpfull..
I use GPS for speed over ground and it is useful for measuring differences in speed from sail trim, etc. Also provides VMG and other useful data. For speed through water, I use a knot-stick... http://knotstick.com/ Inexpensive, reliabe, accurate and no battery connection or holes to drill! The only problem is that in close crossing situations (like when racing) the drag line astern could get run over. Fair winds, Tom
I use GPS for speed over ground and it is useful for measuring differences in speed from sail trim, etc. Also provides VMG and other useful data. For speed through water, I use a knot-stick... http://knotstick.com/ Inexpensive, reliabe, accurate and no battery connection or holes to drill! The only problem is that in close crossing situations (like when racing) the drag line astern could get run over. Fair winds, Tom
Hummingbird Fish Finder Measures Speed
On my H260 I use a handheld GPS for speed and find it works great. Small changes in speed and picked up and displayed instantly. Another alternative, if your looking for "speed thru water" is a fishfinder/depthmeter. When I was purchasing my boat a friend (who worked at West Marine) pointed out that while the "depth meters" were all very expensive there were available "fish finders" for a lot less money and they, pretty much, did the same thing. I purchased a Hummingbird 200, very low end unit for around $100. In addition to having a very large screen and too many features to mention, it had an accessory for boat speed. It was a little speed wheel that was mounted on the transom and was wired to the unit to display the speed. Unfortunately, the GPS worked so good I never bothered getting this accessory.
On my H260 I use a handheld GPS for speed and find it works great. Small changes in speed and picked up and displayed instantly. Another alternative, if your looking for "speed thru water" is a fishfinder/depthmeter. When I was purchasing my boat a friend (who worked at West Marine) pointed out that while the "depth meters" were all very expensive there were available "fish finders" for a lot less money and they, pretty much, did the same thing. I purchased a Hummingbird 200, very low end unit for around $100. In addition to having a very large screen and too many features to mention, it had an accessory for boat speed. It was a little speed wheel that was mounted on the transom and was wired to the unit to display the speed. Unfortunately, the GPS worked so good I never bothered getting this accessory.
GPS Rulez
Most GPS units update at least every second and they are very accurate in regard to speed. Yes, it measures speed over ground and thus leeway and current affect it, but anything you do to increase the speed of the boat will be reflected virtually immediately whereas set and leeway don't change much over the short term. Thus it is excellent for this purpose. Most logs are horribly inaccurate unless calibrated frequently. They often vary in accuracy depending on which tack you're on. They are good for letting you know when you're in a current by noting the difference between the speed it reports and that reported by the GPS. And there are no holes in the boat. If I had to go with just one, it would be GPS every time.
Most GPS units update at least every second and they are very accurate in regard to speed. Yes, it measures speed over ground and thus leeway and current affect it, but anything you do to increase the speed of the boat will be reflected virtually immediately whereas set and leeway don't change much over the short term. Thus it is excellent for this purpose. Most logs are horribly inaccurate unless calibrated frequently. They often vary in accuracy depending on which tack you're on. They are good for letting you know when you're in a current by noting the difference between the speed it reports and that reported by the GPS. And there are no holes in the boat. If I had to go with just one, it would be GPS every time.
Speed over ground VS over water
Correct me if I am wrong but to me the fact remains that it does not matter a toss what your speed is over water for as most of you have mentioned this can be effected by tides and currents. At the end of the day it is speed over ground that is the most important thing. If you are racing you race a course which is measured in nautical miles. If you are sailing from one port to another you measure your distance in nautical miles. So at the end of the day it is the speed that you are covering the ground that results in your arrival time. Nothing to do with currents and tides other than for the fact that they may speed you up or slow you down. Measuring speed with a GPS is far more accurate and will give you an exact result.
Correct me if I am wrong but to me the fact remains that it does not matter a toss what your speed is over water for as most of you have mentioned this can be effected by tides and currents. At the end of the day it is speed over ground that is the most important thing. If you are racing you race a course which is measured in nautical miles. If you are sailing from one port to another you measure your distance in nautical miles. So at the end of the day it is the speed that you are covering the ground that results in your arrival time. Nothing to do with currents and tides other than for the fact that they may speed you up or slow you down. Measuring speed with a GPS is far more accurate and will give you an exact result.
Tom Grass: try to imagine this
You are going 6 knots against a 6 knots current, your GPS is going to show that your speed is a big fat 0. But it will show you exactly where on this planet you are at while going 0. The speedo will show you are going 6 knots. Now, you are going 0 knot with a 6 knots current, you GPS is going to show that your speed is 6 knots, and a speedo will show you are going 0. However, if you have both, you can then figure out the speed of the current. So, depending on what you want to see each has their uses.
You are going 6 knots against a 6 knots current, your GPS is going to show that your speed is a big fat 0. But it will show you exactly where on this planet you are at while going 0. The speedo will show you are going 6 knots. Now, you are going 0 knot with a 6 knots current, you GPS is going to show that your speed is 6 knots, and a speedo will show you are going 0. However, if you have both, you can then figure out the speed of the current. So, depending on what you want to see each has their uses.
Boat Speed - SOG vs SOW
Tom - I assume that your primary sailing grounds are Lake Carlyle. In that case, currents, tides, etc are a non-issue for you and you could probably go with either a GPS, electronic log, or knot-stick. When I had my H26, the previous owner had an electronic log that had an impellor mounted under the boat, but there was no hole drilled through. He kept it when he sold the boat, so I never got a great look at it, but I was under the impression that it used some sort of pickup within the boat to read the data. T J
Tom - I assume that your primary sailing grounds are Lake Carlyle. In that case, currents, tides, etc are a non-issue for you and you could probably go with either a GPS, electronic log, or knot-stick. When I had my H26, the previous owner had an electronic log that had an impellor mounted under the boat, but there was no hole drilled through. He kept it when he sold the boat, so I never got a great look at it, but I was under the impression that it used some sort of pickup within the boat to read the data. T J
You CAN Have It All
On my '94 H26, the previous owner installed a Garmin GPSMAP 135. It's a little dated now, but it has a transom-mounted transducer which has a depth sounder and a speedo impeller. It is screwed into the bottom/transom with small wood screws. On the H26, there is a 1/2 inch lip where the bottom extends a little bit further than the vertical transom section. The actual bottom isn't even penetrated. Bottom line, it measures SOG with the GPS and SOW with the speedo. It also serves as a fishfinder and displays a chart from a replacable data cartridge. Down side, the display is small. BrianW
On my '94 H26, the previous owner installed a Garmin GPSMAP 135. It's a little dated now, but it has a transom-mounted transducer which has a depth sounder and a speedo impeller. It is screwed into the bottom/transom with small wood screws. On the H26, there is a 1/2 inch lip where the bottom extends a little bit further than the vertical transom section. The actual bottom isn't even penetrated. Bottom line, it measures SOG with the GPS and SOW with the speedo. It also serves as a fishfinder and displays a chart from a replacable data cartridge. Down side, the display is small. BrianW
Hand Held GPS
I use a small hand held GPS which I bought from Magellan over 6 years ago. Uses 2 AA bateries which last forever. Total cost was $79 for the unit and $ 9 for the holder(which you can remove and stow away when not sailing. It works great! No drilling, no leaks, no cables, no hassles, no brainer!!! Keep it simple you will be very happy! Phil Phil
I use a small hand held GPS which I bought from Magellan over 6 years ago. Uses 2 AA bateries which last forever. Total cost was $79 for the unit and $ 9 for the holder(which you can remove and stow away when not sailing. It works great! No drilling, no leaks, no cables, no hassles, no brainer!!! Keep it simple you will be very happy! Phil Phil
- Status