How do you do dead reconning in the middle of the ocean where you don't have visuals or depth?
Knotmeter or GPS
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In the middle of the ocean without a GPS or Sextant, dead reckoning is all you have. Visual references are not used in deak reckoning Franklin.
According to Bowditch:
Dead reckoning allows a navigator to determine his
present position by projecting his past courses steered and
speeds over ground from a known past position. He can also
determine his future position by projecting an ordered
course and speed of advance from a known present position.
The DR position is only an approximate position
because it does not allow for the effect of leeway, current,
helmsman error, or gyro error.
Dead reckoning helps in determining sunrise and sunset;
in predicting landfall, sighting lights and predicting arrival
times; and in evaluating the accuracy of electronic positioning
information. It also helps in predicting which celestial bodies
will be available for future observation.
The navigator should carefully tend his DR plot, update
it when required, use it to evaluate external forces
acting on his ship, and consult it to avoid potential navigation
hazards.present position by projecting his past courses steered and
speeds over ground from a known past position. He can also
determine his future position by projecting an ordered
course and speed of advance from a known present position.
The DR position is only an approximate position
because it does not allow for the effect of leeway, current,
helmsman error, or gyro error.
Dead reckoning helps in determining sunrise and sunset;
in predicting landfall, sighting lights and predicting arrival
times; and in evaluating the accuracy of electronic positioning
information. It also helps in predicting which celestial bodies
will be available for future observation.
The navigator should carefully tend his DR plot, update
it when required, use it to evaluate external forces
acting on his ship, and consult it to avoid potential navigation
I have a Walker taffrail log and I wouldn't go off shore without it. It is so much better than nothing. Even in the gulfstream the current is less than 4 knots so you can with DR, a log, celestial sights and good charts determine how far you traveled through the water and compare that with your change in position and determine the set and drift.
Sorry...I was thinking of something else...was thinking of using landmarks to plot a position on a map. As stated above, DR does not help in determining the current.
I think we are getting off course here
Plain simple fact is, it helps greatly to have a knot meter to know the speed in the water. I also helps greatly to have a working GPS to know speed over ground. Each is a tool used for different purposes. If you have both, keep both. You don't NEED both, but it sure does help to have both.
I know one time I almost smacked into something at night because I didn't know I had a current at an angle causing sideways drift. I saw I was pointing away from the object while motoring so I went down below to get something. Took longer then expected but wasn't in a rush because nothing was in front of me and no boats around. Came up just in time to take the helm and make a course adjustment before the big smack.
Now that I have both working correctly, I check for current all the time, especially after a course change. Sometimes I'll turn just to see if there is a sideways current.
Another reason it's good to know about the currents is so you can find the favorable current (racers love that). In the Gulf we have tons of circular currents all caused by the Gulf Stream kicking off eddies and the eddies drifting west until they die off when hitting the west shore. Before going offshore, I'll print out the currents and try to catch the down slope of an eddie. It can make a difference of many hours per eddie.
The charts that show the currents aren't all that precise, so you have to determine the direction of the local current to figure out what part of the eddie you are in. Make a course change for a couple of hours and then you can start to see an increase of 2.5 knots for the next 100-200 miles. Nice huh?
The charts that show the currents aren't all that precise, so you have to determine the direction of the local current to figure out what part of the eddie you are in. Make a course change for a couple of hours and then you can start to see an increase of 2.5 knots for the next 100-200 miles. Nice huh?
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Oh, yeh, I chucked the compass too, as it would never agree with the GPS. A dockmate said somthing about leeway, but he was sober, so I immediatly distrusted him.
I have a non-functioning knotmeter and a functioning GPS. I have never missed the knotmeter (it hasn't worked in the ten years I have owned my boat), but after reading some of the responses here, I can see how it could be useful. I just leave the paddlewheel permanently installed. I wouldn't uninstall it if it worked just because I have a GPS.
sometimes that leeway is due to poor sailtrim and pointing. Sometimes it's caused by currents. Sometimes it's caused by both. But yes, I get your point...I should have checked the GPS course agaisnt the compass.
Our knotmeter has not worked for a few years. Information about speed over water or speed and direction of a current is pretty useless to me. It is the speed over land that gets me places and which allows me to plan ahead. I can pretty much deduce whats going on by knowing the speed over land and behavior of the boat. I will not remove it as perhaps a future owner may want it.
The answer is simple. You should use all the possible information available to you to make the correct decision in any situation. Eliminating a source of information reduces the probability of you making a correct decision. No Brainer!
OK
Hi Guys,
How do you clean/replace the little bugger (paddle wheel)? I sit there with a Q-Tip and wipe away as much growth as I can however there are times when it gets fouled with something during the week because it doesn't work coming out of the slip. I put some of that bottom cleaner on it last fall and more stuff came off than ever. Any help will be appreciated.
What's that selfsealing through hull all about.
All U Get
Hi Guys,
How do you clean/replace the little bugger (paddle wheel)? I sit there with a Q-Tip and wipe away as much growth as I can however there are times when it gets fouled with something during the week because it doesn't work coming out of the slip. I put some of that bottom cleaner on it last fall and more stuff came off than ever. Any help will be appreciated.
What's that selfsealing through hull all about.
All U Get
What I Do
"How do you clean/replace the little bugger (paddle wheel)? I sit there with a Q-Tip and wipe away as much growth as I can however there are times when it gets fouled with something during the week because it doesn't work coming out of the slip. I put some of that bottom cleaner on it last fall and more stuff came off than ever. Any help will be appreciated."
I use a small, thin knife blade to scrape the gunk off and don't forget the sides. I then put on two coats of transducer paint (available through WM). I check to make sure no droplettes of paint have glued the wheel before reinstalling it. Good for six months or more in SoCal. More in winter. Usually do it twice a year.
"How do you clean/replace the little bugger (paddle wheel)? I sit there with a Q-Tip and wipe away as much growth as I can however there are times when it gets fouled with something during the week because it doesn't work coming out of the slip. I put some of that bottom cleaner on it last fall and more stuff came off than ever. Any help will be appreciated."
I use a small, thin knife blade to scrape the gunk off and don't forget the sides. I then put on two coats of transducer paint (available through WM). I check to make sure no droplettes of paint have glued the wheel before reinstalling it. Good for six months or more in SoCal. More in winter. Usually do it twice a year.
Raymarine's transducers come with a thru-hull that has a spring loaded flap. As you pull the transducer out, the flap closes and waterflow reduces to a trickle. When you put the plug in, it opens the flap.Hi Guys,
How do you clean/replace the little bugger (paddle wheel)? I sit there with a Q-Tip and wipe away as much growth as I can however there are times when it gets fouled with something during the week because it doesn't work coming out of the slip. I put some of that bottom cleaner on it last fall and more stuff came off than ever. Any help will be appreciated.
What's that selfsealing through hull all about.
All U Get
I don't have to clean mine as I take it out after sailing.
Well, I use a low-tech approach as well. I have a hand-held GPs that I use only for lat and long, speed and tides. If I suspect the SOG is not accurate (or have a reason to be concerned, which is rare), I toss my log overboard and tow it for six minutes. If the distance travelled (as reported on GPS) is identical to what the log and watch say it should be, I'm good. I reel in my log and store it until needed again.
Several people I know spray the paddlewheel with McLube SailKote. It seems to help prevent the marine life from attaching/growing quite a bit. The best measure is to remove the through-hull when the boat isn't being used.
Just remember, your Wind nstrument measures the apparent wind speed and direction and relies on the input from the knot meter to calculate the "true" wind speed and direction. While it is true that current will cause these numbers to be less accurate (after all, to calculate the true wind speed and its true direction, you need the boats true speed and true direction (as well as heading (after all, we do make leeway while we are sailing)).
(Only the top end instrument systems are capable of gathering and using true SOG, true COG, and true compass heading (not the same as COG) and converting this to a true wind speed and direction).
If your speedo is too much of a PIA, remove it (if you do, I would recommend (as you indicated) glassing in the hole to eliminate the potential for a nasty surprise). Otherwise, leave it in, perform routine maintenance on it, and possibly (as suggested by another poster) pull the paddle wheel when you aren't actually sailing the boat.
Steve
Alchemist C-380
(Only the top end instrument systems are capable of gathering and using true SOG, true COG, and true compass heading (not the same as COG) and converting this to a true wind speed and direction).
If your speedo is too much of a PIA, remove it (if you do, I would recommend (as you indicated) glassing in the hole to eliminate the potential for a nasty surprise). Otherwise, leave it in, perform routine maintenance on it, and possibly (as suggested by another poster) pull the paddle wheel when you aren't actually sailing the boat.
Steve
Alchemist C-380
Just curious, what instrument system uses SOG instead of STW for true wind calculations. None that I have seen do.
really true or just locally true
The true wind can "technically" be calculated with either.
True wind using the SOG gives the actual true wind and is quite handy
relative true wind (not apparent) is used with the STW. It gives results close to the actual true wind but subtracts out the contribution of current and lee-way.
My Raymarine uses the SOG to calculate true wind as I believe any system with access to GPS data will. In the event the GPS goes down or is not present then it falls back to STW version.
FWIW
The true wind can "technically" be calculated with either.
True wind using the SOG gives the actual true wind and is quite handy
relative true wind (not apparent) is used with the STW. It gives results close to the actual true wind but subtracts out the contribution of current and lee-way.
My Raymarine uses the SOG to calculate true wind as I believe any system with access to GPS data will. In the event the GPS goes down or is not present then it falls back to STW version.
FWIW
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