mattnj
Well-Known Member
Nasa marine make one that is way less...
Electromagnetic Log 2 (transducer and data box) - Nasa Marine Instruments
Paddlewheel Simulator, Boat Speed vs SOG, Ground Wind, Log
- Thread starter mattnj
- Start date
Nasa marine make one that is way less...
Electromagnetic Log 2 (transducer and data box) - Nasa Marine Instruments
johnalison
Well-Known Member
I had a Seafarer electromagnetic log back in the ‘70s. It actually worked very well and I was sorry to changed to a paddle wheel. One reason I was sorry was that it was Navico rubbish from 1987. I have had my current Raytheon/Raymarine one for twenty years and it has worked well. While we were cruising for three months it seldom needed cleaning and rarely picked up weed but now that we have cut back it needs a clean every few weeks. This is hardly much of a chore and I usually end up doing it while on the move.
You can get from A to B with SOG only but you don’t have to be a manic racer like flaming to actually prefer sailing a boat well.
You can get from A to B with SOG only but you don’t have to be a manic racer like flaming to actually prefer sailing a boat well.
TernVI
Well-Known Member
Like most cheaper instruments, that only has a resolution of 0.1 knot.
Once you've used a speed log with a resolution of 0.01 knot, you won't want to go back.
It sees changes in speed before you do.
We don't trust the numbers to be exact, but we trust 6.33 to be faster than 6.30 for instance, whereas with a cheap log you don't know if a change from 6.3 to 6.4 is a real change of 0.1 or 0.001 or just noise.
I find a speed log very useful if there are beginners on the boat, or just people not used to the boat.
It helps to be able to put some numbers into 'you are pointing too high' or just 'see it's speeding up now...'
And for pure cruising, it's good to know that your engine still drives the boat at 5.5knots at 2000rpm or whatever.
Laminar Flow
Well-Known Member
Absolutely.
We do not race, at least not with this boat and we will never manage 7.3kts to windward, but we do push her hard and always try to get the best out of our tub under sail and for that you need to know STW.
We also have SOG of course and using both we can make a qualified assessment of the tide or current, even when other reference points are not available. So, in combination, it also allows me to make pertinent navigational decisions which are part of plain good old seamanship.
Since I tend to keep making improvements to the sailing ability of our boat, I do like to know whether the modifications were successful or if I am wasting money and effort.
Additionally, it is a useful tool for assessing engine performance or the state of fouling on your hull.
While cruising we seem to keep moving enough so that fouling has never been a problem.
GHA
Well-Known Member
No doubt doing it all wrong, but cruising I find VMG to a waypoint way off in the distance much more use than stw. Set windvane and let it settle for a few minutes. Tweak a little and try again. Log is much slower than gps to update speed changes so guessing a lot of people might be fooling themselves....
johnalison
Well-Known Member
Your last point is one I have also made elsewhere. There have been many occasions when I have been motoring longish distances, usually North Sea, and my motoring speed has dropped from the usual 6.5 to 6.1 or 2 without a change in engine note or boat feel from the sail drive. Usually a short burst of astern has cleared it and we were able to continue at full speed at the same revs. On one occasion I saw a small branch appear as we moved off. With SOG, I would probably have assumed that the tide had changed.
mattnj
Well-Known Member
I'm with you.....all I'm interested in is the speed (over ground) to where I'm going....tweak to get that as fast as possible....
johnalison
Well-Known Member
I seldom use VMG to waypoint, though I have the option. VMG to wind I do use, but not often. This does need a working log of course. I mainly use it for practice as it is very hard work to sail to it. In order to maximise VMG I have to attend to boat speed, wind angle, boat direction, what the sails are doing, what the waves are doing, as well as the VMG meter. The results can be very instructive, and in calm water I find that I need to point far higher than feels natural, as well as paying especial care to be gentle with the helm.
GHA
Well-Known Member
.... though not always, on a long passage sometimes a little loss in vmg can make a big difference to comfort and wear on the boat. Cruising has a whole different set of priorities, speed isn't always one of them, on a long passage how many things need fixing at the other end trumps it every time
Laminar Flow
Well-Known Member
I never thought it was a case of one or the other.
Unless you carry a scientific calculator with you (or tables with sine function, if you're really old school) then your instruments need both be connected and have wind speed/angle and STW to calculate VGM to weather.
Regardless, my GPS certainly doesn't give me VGM on any course that requires tacking, but I can calculate it since I have access to wind data and STW.
Of course, anyone can do as they like, but I kinda enjoy getting the best out of my boat, even if we do "only" cruise.
GHA
Well-Known Member
Different cruising perhaps - talking long passages, days between course changes, why bother with vmg to weather - irrelevant, is the boat happy? - that's the big question.
TernVI
Well-Known Member
The log has something like 11.8 pulses per metre.
6 knots is about 3m/s, so the log is getting data around 35 times a second.
A lot of GPS update only once a second. Ours is quite old to be fair, but not as old as the log.
Both will be doing some averaging.
Boats are big heavy things which don't change speed all that quickly.
GPS indicated speed possibly changes faster than the true speed of the boat.
Depending on how good it is at averaging the errors.
A boat also changes direction a lot quicker than it changes speed so GPS speed will tend to change quicker anyway, SOG is a vector.
But as I said up thread, a lot of speed logs don't register any change at all until you've changed speed by up to 0.1knot.
TernVI
Well-Known Member
Indeed.Different cruising perhaps - talking long passages, days between course changes, why bother with vmg to weather - irrelevant, is the boat happy? - that's the big question.
Also on a passage on any length, that direction you call 'weather' will change!
jwilson
Well-Known Member
I think I've said it before, but for serious-ish racing with polars and targets a log is obviously essential, but if you can sail a dinghy (or a yacht) well you can get near-as-dammit close to similar speeds without any instruments at all except a compass. And certainly good enough for cruising, even if you do like to sail well and get mildly irritated by all those that don't. I have a log, I put a new paddlewheel in it over the winter and did a calibration run last week. But I often don't bother putting the paddlewheel in....
Laminar Flow
Well-Known Member
Currently in Brittany, before that Holland, the Baltic. While our boat is not a racer, we would be hard pressed to spend days traveling on the same course.Different cruising perhaps - talking long passages, days between course changes, why bother with vmg to weather - irrelevant, is the boat happy? - that's the big question.
I'm not sure if my boat is happy; I do not put much stake in the emotional well-being of inanimate objects.
I do however care much for the happiness of my wife; perhaps that is why I do not have to do a lot of singlehanded sailing.