So, what is a "knot"

[loubetti]

What he is saying is that the ship is making 20 knots through the water. Going northbound, it makes 46 nm in two hours, or 23 knots over the ground, benefiting from the 3 knot Gulf Stream. Going southbound, it makes 34 nm in two hours, or 17 knots over the ground, since the water is moving 3 knots against the ship. Given today's preponderant reliance on GPS for navigation, the ship's speed is always given in terms of speed over ground, while the speed through the water from the Doppler sonar is a "backup" reference, and a key to the exact current strength and direction at that time.

 

Not sure what the barnacles have to do with it. That affects speed through the water, but has nothing to do with current.

 

That's basically what I alluded to above. So, in aviation we would just call it "airspeed" and "ground speed". We get affected by winds up there which either increase or decrease our ground speed.

 

The other speeds I mentioned would not apply to ships, as we are in the air, and air gets thinner as we go higher. The thinner the air gets the faster we go through the air, but those winds always have an affect. Again, not ship related.

 

As to hull barnacles, would they not increase drag on the hull, and slow the ship's speed in the water? Would deploying the stabilizers not slow the ship by a bit? I know if I extend my landing gear I'm going to lose about 20 kts of speed. It's all drag.

 

Nice conversation and, "Cheng" I am your #1 fan! Sort or my maritime counterpart! :D

[chengkp75]

That's basically what I alluded to above. So, in aviation we would just call it "airspeed" and "ground speed". We get affected by winds up there which either increase or decrease our ground speed.

 

The other speeds I mentioned would not apply to ships, as we are in the air, and air gets thinner as we go higher. The thinner the air gets the faster we go through the air, but those winds always have an affect. Again, not ship related.

 

As to hull barnacles, would they not increase drag on the hull, and slow the ship's speed in the water? Would deploying the stabilizers not slow the ship by a bit? I know if I extend my landing gear I'm going to lose about 20 kts of speed. It's all drag.

 

Nice conversation and, "Cheng" I am your #1 fan! Sort or my maritime counterpart! :D

 

Yes, barnacles and hull growth will affect the speed through the water, but will not change the relationship between speed in water and speed over ground. In truth, ships, until heavily fouled below the waterline, will just use more power to maintain the same speed through the water.

[loubetti]

LOL! Well, my good man, that is exactly what I was saying! Drag is drag. Sea or air, it is the same. Of course, with planes we're pretty much maxed out on power to begin with, so we don't have much of an option. We always plan for basically max cruise speed (within reason), then we just deal with what mother nature dishes up. Of course, in very rough air we have to reduce speed, just like ships might do.

 

I think we're totally on the same page. :D

[navybankerteacher]

 

The other speeds I mentioned would not apply to ships, as we are in the air, and air gets thinner as we go higher. The thinner the air gets the faster we go through the air, but those winds always have an affect. Again, not ship related.

 

D

 

That faster through thinner air concept makes sense - as drag would presumably decrease; but doesn't lift also decrease in thinner air, also thrust from the prop turning in thinner air? Wouldn't there be some optimum altitude for a given aircraft?

[Wol]

That faster through thinner air concept makes sense - as drag would presumably decrease; but doesn't lift also decrease in thinner air, also thrust from the prop turning in thinner air? Wouldn't there be some optimum altitude for a given aircraft?

 

We're probably getting a bit outside the intention of Cruise Critic (!) but, very simply (because there are several more layers) airspeed is measured quite crudely by the difference between "static" pressure and "total" pressure. The former is the outside air pressure as seen by the airspeed indicator through sideways ports, the total pressure is seen from a tube pointing straight ahead. The difference gives a measure of the air pressure due to forward speed.

 

As the altitude increases so the outside air pressure decreases, and this indicated airspeed no longer reflects the actual speed through the air. But since lift and drag are both related to the indicated airspeed, effectively what you see is what you want - speed in relation to stall, etc.

 

True airspeed (actual speed through the air) is only of use for navigation purposes, not control. Except that taking off and landing TAS can be rather high at high level airfields!

 

Ground speed, of course, is entirely different although related to TAS.

[annawanX2]

I'm just happy that when I walk by the map on the ship that shows where we are, that I now can figure how fast we are going !! Thank you for that !!

[navybankerteacher]

I'm just happy that when I walk by the map on the ship that shows where we are, that I now can figure how fast we are going !! Thank you for that !!

 

You will not find any map showing the ship's position -- you will, however, usually see a chart displaying that information. Don't ask why, perhaps maps get soggy at sea, but charts are what are used.

[skf]

statute miles and nautical miles. Especially since they're both such odd numbers.

I am knot an expert on nautical miles, but being a horse-racing fan I know about statute miles and furlongs.

A mile was originally the distance Roman soldiers would march, 5000 feet in 1000 paces, "mile" referring to 1000 in the Latin languages. When they got to England the residents were using the Furlong, which is 1/8 mile. Since you cannot evenly divide 5000 by 8 they stretched the 5000 ft mile to 5280 feet.

And this is why races are run in Furlongs, but not ships.

(We lost $6 today betting the horses.) Steve

[Burpelson]

Well Luddite,

 

It appears that barnacles cause 'literalness!'

[navybankerteacher]

Well Luddite,

 

It appears that barnacles cause 'literalness!'

 

Correct! Mr. Luddite is probably the soundest thinker among us on this site -- too bad it's not contagious.

[NurseDave]

Call me when it's time for knotcho cheese nachos.

[Maimou]

This is the naughtiest thread I've read on cruisecritic.

 

Or knottiest-not sure. Thanks for the laughs and info.

[Cuizer2]

Wow, I really messed that up. Thanks for catching it. I mixed the 3 minute rule with the 6 minute rule. Under the 6 minute rule, speed in knots divided by 10 = distance traveled in 6 minutes.

 

Under the 3 minute rule, you multiply your speed by 100, and that equals yards traveled in 3 minutes.

 

There is also the sixty minute rule. If you divide your speed by one you get the distance you'll travel in one hour.

Edited March 12, 2016 by Cuizer2