Knots vs MPH

[fryedaze]

Prior to starting the Loop we always used Knots while cruising the Chesapeake. It works well with the paper charts and cruising guides. When you enter the rivers, Canadian water and ICW the charts and cruising guides are all statute miles. Its easier to shift the plotter over to Mile rather than do the mental gymnastics converting the data.

 

[Peter B]

Until the mid-19th century, vessel speed at sea was measured using a chip log. This consisted of a wooden panel, attached by line to a reel, and weighted on one edge to float perpendicularly to the water surface and thus present substantial resistance to the water moving around it. The chip log was "cast" over the stern of the moving vessel and the line allowed to pay out.[6] Knots placed at a distance of 8 fathoms - 47 feet 3 inches (14.4018 m) from each other, passed through a sailor's fingers, while another sailor used a 30-second sand-glass (28-second sand-glass is the currently accepted timing) to time the operation.[7] The knot count would be reported and used in the sailing master's dead reckoning

So says Wiki.

So also says Patrick O'Brien, in the Jack Aubrey series, of which I am now at book 10. Learning quite a lot. Many of you have read them, I know.

Also, l apologise for repeating what many had already said in my post above on the navigation and speeds thing. I didn't realise how busy you beavers had been while we downunder were asleep.

 

[BandB]

Yes I do. Ive always used cable as a description of distance at sea. Its a handy distance. Traditionally it about 100 fathoms or 200 yards but if you want to get more precise its 608 ft for the imperial measurement or 720 ft in the US navy. You'd have to wonder why we could never standardize such a measurement.

Never have used cables. And don't like fathoms for the same reason others expressed with waves in meters sometimes and feet the other. If you're use to feet and suddenly see fathoms or, more risky the opposite, you may make a critical mistake.

 

[Baker]

Lotsa fun information on this thread!!!!

 

[RCook]

Weather forecasts are now problematic, as you must listen carefully to the units, if mentioned, to determine whether the weather will be too much for your comfort. I am of course referring to the Canadian weather habit of referring to the winds on the waters of Georgia Strait in kilometers, not knots.

Weird! As of last summer, Canada weather on the north coast (near Prince Rupert anyway) still spoke of wind speed in knots. So does their marine weather web site. Just as does Alaska marine weather.

Alaska land-based weather is reported in mph, however. So when we listen to VHF weather and they talk of "local area" weather, around Craig for instance, it's in mph.

 

[Tony B]

Isn't a nautical mile "one minute of longitude"....at the equator???

Yes, that is correct. As a matter of fact, that is true for any latitude because lattitude lines are parallel.
Longitude is a different story. The distance between longitudinal lines varies as you notice on a global size chart, they are not parallel, they joint to a point at both the north and south pole and are widest at the equator.

Any, when on the Gulf, Carrib or Atlantic, I use knots because they are based on nautical miles and that is also how those charts are based. For distance, every minute of arc on latitude equals 1 NM and 1 Degree equals 60 NM's.

In the latitudes that I am familiar with, 1 minute of longitude is also close enough for me to do math in my head and consider it as 1 NM even though it os not.

Now that I am inland on the river system, so far as I have been exposed to, all the charts, river markers and GPS's show Statute Miles and so I that is what I am now using.

I go with the flow.

 

[Moonstruck]

Never have used cables. And don't like fathoms for the same reason others expressed with waves in meters sometimes and feet the other. If you're use to feet and suddenly see fathoms or, more risky the opposite, you may make a critical mistake.

When I started out fishing there was really not even Loran single track A in wide spread use. We used a printing fathometer and an RDF. We would fish the fathom lines on the chart such as 5 fathom, 10 fathom, 100 fathom, etc. So if a buddy asked on the radio where we were fishing I would answer something like this, "100 fathom line about 10 miles south of the Big Rock". With the modern piloting aids that has gone by the wayside. However there is still validity in using the curves to fish where fish are at a certain depth.

What really messes with my mind is going from feet to meters in the Bahamas. I will see a 2 on the chart and immediately alter course when it is really over 6'. Just a conditioned action I suppose.

 

[Britannia]

...

What bugs me are self-proclaimed knowledgeable folks who talk about about "knots per hour" as a speed measure. Nope.

Thread drift alert!

While we're on the topic of misusing units - another one that bothers me is when people talk about amps when they mean amp hours. I have a friend who says things like "I used 130 amps yesterday" when he means amp hours. Then he'll say that his fridge "uses 3 amps per hour." It drives me nuts!

On the other hand, once I get over my frustration I know what he means. So perhaps the problem is more mine than his?

Richard

 

[BandB]

What really messes with my mind is going from feet to meters in the Bahamas. I will see a 2 on the chart and immediately alter course when it is really over 6'. Just a conditioned action I suppose.

Where it's really a problem and I've run across a couple of boats fall victim to this is where one assumes the 2 is meters or fathom and it is feet. I ran across one who soft grounded and was easily backed off who understood quickly his mistake and took it as a learning experience. I heard the story from him first handed.

The other one was a classic "how not to." This one I heard part on radio (We were docked at Resort World, Bimini), heard the rest from the marina and from a tow captain. First he grounded soft but deep at about 15 knots. Then he tried to power it on through and off, just putting it in worse condition. Next he called asking about towing. He carried no tow membership and was quoted $1000 plus $200 an hour for any time over an hour took removing him from the ground. He ranted and cursed and refused that and had everyone get out of the boat and try pulling it off by hand. As that obviously didn't work it then reached low tide and the boat was in less water. Finally he called back regarding the tow. The towboat arrived. Told him they'd wait for high tide but with what he'd done now it was likely to be a salvage and would cost more. The man proceeded to use every expletive and derogatory term possible against Bahamians. But they did use floats under the edges of the hull to minimize any further damage and took about 4 hours removing it and price went to $2000. The tow captain recommends he go to the marina and get a diver to check things out. The owner says, he wouldn't step foot on the Bahamas if it was the last place on earth and he's turning around and heading straight back to Florida. Now all this was taking place just south of Bimini. That's when he was informed he needed to clear into the Bahamas. A few more expletives and he said no he didn't as he wasn't going to the Bahamas. He was informed he was already in and had been "anchored" in Bahamas waters for 10 hours. He received an escort to Brown's and Alice Town, paid his $300 plus $75 exiting fees and left cursing everyone there.

Now, this last part I only heard as rumor and that was that he failed to clear back into the US and got cited for that.

So, I'd say the first guy suffered from the inconsistency on charts but the second guy more from his own idiocy. I was told by people who observed him at the marina that he was very lucky he was in the Bahamas as his language and behavior toward the Bahamian authorities likely would have gotten him arrested in the US.

 

[BandB]

Danderer mentioned "knots per hour", well the other one is "RPM's".

 

[Baker]

Thread drift alert!

While we're on the topic of misusing units - another one that bothers me is when people talk about amps when they mean amp hours. I have a friend who says things like "I used 130 amps yesterday" when he means amp hours. Then he'll say that his fridge "uses 3 amps per hour." It drives me nuts!

On the other hand, once I get over my frustration I know what he means. So perhaps the problem is more mine than his?

Richard

I think "amps" implies amp/hours just like "knots" implies nautical miles per hour. So I think you might own this one!!!....

 

[Britannia]

I think "amps" implies amp/hours just like "knots" implies nautical miles per hour. So I think you might own this one!!!....

Er, nope. Amps means amps. Unit of current. An amp hour is one amp flowing for one hour. It's not at all like knots.

Sorry.

Richard

 

[Baker]

Er, nope. Amps means amps. Unit of current. An amp hour is one amp flowing for one hour. It's not at all like knots.

Sorry.

Richard

Well then I'll shut up because I am way out of my league when it comes to electricity. But I actually do understand what you are saying now that I think about it.

 

[Tony B]

The confusion with amps and ampere hours is common.


Amp is a unit of current. Current has a by product of the wire resistance and it produces heat, so in wiring, your wires are rated by amps. If you draw 20 amps on a 10 amp rated wire, the wire warms up, if properly matched fuse, the fuse will blow or the wires will burn up because of the heat.


When the layman refers to amps, he is referring to the appliance's usage which is a function of the current usage (amps) over time. Only 2 main players care about usage or ampere hours. One is the power company that bills you for usage and the other is your batteries. Your batteries are rated by their storage capability of how many amps can they produce for a given time frame which is ampere hours.


Now back to appliances. Say for instance, your fridge specs say 5 amps. That means when your fridge is operating (the compressor and fan on) you are drawing the full 5 amps continually, until the thermostat shuts it off. This means the wiring should be able to handle at least the 5 amps so that it doesn't burn up. So lets say the fridge kicks in only about 50% of the time. the usage or ampere hours would only be 2 1/2 ampere hours as far as the power company and your batteries are concerned.




There are other factors involved, but this is the short of it.

 

[JDCAVE]

So I don't guess anyone is using cables. 1 nm = 10 cables

Actually tow-boat and other commercial operators use it all the time, at least up in Canada. And...the length of seine and gillnets are usually stated in fathoms.

In another generation statute miles will mean nothing to a Canadian as they are slowly thinking in terms of kilometers for their land based measurements.

Actually nearly all Canadians describe their distances in hours. "How far is Hope from here?" "Oh it's a couple of hours." Seriously! They've already done the calculations for you. We go back and forth between measurements. Air temperatures in C, oven temps in F. Body weight in pounds. Other weights often in kilos. None of us use MPH anymore.

During WWII the fleet air arm of the Royal Navy calculated airspeeds in knots whereas the Royal Air Force used MPH. Routes were planned by navigators in RAF Bomber Command using MPH for much of the war, until 1945 when they converted over to knots and nm. It took squadrons several months to complete the transition and it was quite complicated as navigators had to work and forth between both units. Airspeed indicators (ASI) were in statute MPH whereas the charts were in nautical miles.


Jim
Sent from my iPad using Trawler Forum

 

[Britannia]

Well then I'll shut up because I am way out of my league when it comes to electricity. But I actually do understand what you are saying now that I think about it.

Please don't "shut up" - I hate to see anyone feel like they have been shut down on a forum.

Keep on posting!

Richard

 

[123]

Reading true this topic I am somewhat shocked. If you cannot take a mile on the right side of a map with a Compass, if you don't know how to use Parallel Rules, if you don't understand the difference between latitude and longitude how can you operate a "little" ship safely at sea?????

 

[Britannia]

... So lets say the fridge kicks in only about 50% of the time. the usage or ampere hours would only be 2 1/2 ampere hours as far as the power company and your batteries are concerned.
...

Good summary. The quote above isn't quite right.

If the fridge kicks in 50% of the time at 5 amps then the average current is 2.5 amps. In one hour the fridge will use 2.5 amp hours. In a day it will use 30 amp hours.

I suppose you could say that it's 2.5 amp hours per hour - but that's a convoluted way of simply saying 2.5 amps.

To be specific - amps are a measure of current - which is the flow of charge (think electrons moving). 1 amp is the same as a charge of 1 Coulomb flowing per second. An amp hour is actually a measure of charge (think charge stored in a battery). 1 amp hour is 1 amp flowing for 1 hour - which is 3600 Coulombs.

Richard

 

[psneeld]

Reading true this topic I am somewhat shocked. If you cannot take a mile on the right side of a map with a Compass, if you don't know how to use Parallel Rules, if you don't understand the difference between latitude and longitude how can you operate a "little" ship safely at sea?????

You are assuming that the person knows celestial nav too?

Otherwise positioning and plotting on an electronic chartplotter will do it all.

If electronics fail....all the plotting tools without celestial aren't that much help.

 

[Tony B]

Good summary. The quote above isn't quite right.

If the fridge kicks in 50% of the time at 5 amps then the average current is 2.5 amps. In one hour the fridge will use 2.5 amp hours. In a day it will use 30 amp hours.

I agree. When I was writing this I got caught in the trap of thinking hourly, not daily. here goes a gotcha moment: there are 24 hours in a day, so the total would be 60 Amps


I suppose you could say that it's 2.5 amp hours per hour - but that's a convoluted way of simply saying 2.5 amps.

Agree, and that is what this whole discussion is about - the confusion

...............Richard

As for your opening " The quote above isn't quite right."

When I was a younger, I was always referred to as the kid that "isn't quite right". LOL

 

[BandB]

When I was a younger, I was always referred to as the kid that "isn't quite right". LOL

Wifey B: I think it could be said about all of us here, and probably has been, "they're just not quite right." But, all is right in my little world and I'm happy so don't need to be quite right.

That's like normal? What the heck does that mean? Who is normal? Who even wants to be? If a lot of what I see around me is normal, then no thanks. Call me crazy, but call me happy.

 

[Moonstruck]

360 degrees x 60nm/degree = 21,600nm (24 856.836 077 mile [statute]) is closer to the polar circumference (24,860) than the equatorial circumference (24,902) so even at the equator, using latitude is more accurate.

Ok, your're right. First I thought I was right. Then I thought Paul and David could be right. Now I think you are right. I am so insecure and impressionable not to mention confused. It's not easy being me.

 

[Tom.B]

So how many amps in a mile and how does that convert to knots per amp/kilometer?

Puppy-monkey-baby wants to know.

 

[BandB]

So how many amps in a mile and how does that convert to knots per amp/kilometer?

Puppy-monkey-baby wants to know.

Wifey B: 7

 

[Tony B]

Very impressive. I thought I was the only one here that knew about the Pergafram-Kahagan concept

 

[Art]

I traveled 100 statute miles at 10 mph = 10 hr travel... unless of course bucking wind or current.... or maybe having wind or current with me... or maybe having one with me and one against me. One way or another it is not too un-similar as traveling X nautical miles at X knots with wind or current in similar or opposing directions... whether the directions be for or against me.

Personally I see no fault or problem using statute miles and mph or using nautical miles and knots for keeping track of distance/speed/time stats... as long as in which ever combo I begin a leg of the journey my mind/numbers remain using that particular calculation combo... throughout.

In either combo chosen the variables of wind and current can easily be factored in to produce time schedules that a particular journey may require.

Tiz not rocket science. With today's high tech assistance items it's not even a big deal to dial things in. Back in the days of compass, time clock, paper chart, boat speed to get closely dialed in was a bit harder to accomplish, and, in my opinion a lot more fun. Although it did require some work, the past-days personal-enjoyment-outcome of having been successful in calculations by arriving on time and in one piece was priceless!

 

[Paul Swanson]

Yes, that is correct. As a matter of fact, that is true for any latitude because lattitude lines are parallel.

In the latitudes that I am familiar with, 1 minute of longitude is also close enough for me to do math in my head and consider it as 1 NM even though it os not.


NO! One minute of longitude is only one NM at zero latitude, that is, at the equator. As you go north or south from the equator the meridians of longitude get closer together and therefore a minute of longitude is less than one NM. In fact it decreases as the cosine of the latitude. At 40 deg latitude, one minute of longitude is only 3/4 of a NM, not an insignificant error.

Paul

 

[Britannia]

As for your opening " The quote above isn't quite right."

When I was a younger, I was always referred to as the kid that "isn't quite right". LOL

I didn't mean to trigger you!

And you're correct on my arithmetic error - there are indeed 24 hours in a day even though sometimes it feels like there are only 12! However, the answer is 60 amp hours and not 60 amps - still got to get that unit right!

Richard

 

[BandB]

What is the key is communication. If someone you don't know well gives you a mileage or speed, ask which it is. I would suggest to be clearly understood that one when referring to distance on the water use the words "statute miles" or "nautical miles". For us with more people involved day to day we set a hard fast rule that we only use knots and nautical miles. Not saying it's better, just saying a mix might get confusing so we are always aware what is meant when talking with each other. We still always use the word nautical though and don't leave "miles" dangling out in the wind.

 

[Cuttyhunk47]

To me, anyone who has sold out and changed to knots from cables is not a real mariner. I suppose you measure your depth in feet and not fathoms too!