Coast Goard hovercraft hits unlit anchored sailboat with no radar reflector.

[TrentePieds]

Ah - there we have it.

Clearly Siyay's skipper messed up because he was scanning for oblate spheroids.

There aren't many of them in GH, so no wonder he got confused :-0)!

TP

[Lodesman]

Originally Posted by TrentePieds

he was scanning for oblate spheroids.

There aren't many of them in GH,

Have you seen the crowd in Moby's?

[Boatyarddog]

Originally Posted by TrentePieds

Ah - there we have it.

Clearly Siyay's skipper messed up because he was scanning for oblate spheroids.

There aren't many of them in GH, so no wonder he got confused :-0)!

TP

Funny 182 posts and we're ALL still guessing.
Morphed into ?
Really just a close call.
No blood, No foul
Boatyarddog

[garyfdl]

Originally Posted by Lodesman

Submarine periscopes usually give a pretty good return, because they create a "rooster tail" of water as the submarine moves through the water.

Nope. in fact, as an old sub-sailor, I'll wager that the scope mast would snap before the boat went fast enough for the scope to 'rooster tail'... if a rooster tail is even possible from a abject that extends 10's of feet below the surface, and is attached to a much larger object.

Originally Posted by Lodesman

However, only sticking up a couple metres means the radar antenna would need to be mounted nearly 600 feet above sea level.

Are you saying the scope sticks up a couple meters? Nope, again. The scope will (normally) sick up above the water just enough to see, but not be seen. Sticking up 'a couple of meters' gives away the boat's position, and makes it a much bigger radar 'target'.

[StuM]

Originally Posted by Lodesman

With the standard I grew up with, a nautical mile is 6080 feet. This discussion on how this was derived takes up a fairly significant portion of Chapter 1 in the Admiralty Manual of Navigation, Vol III.

The International NM has been defined as 1852 metres since 1929.
(That's 6076.115 ft)

The US abandoned their definition of 6080.20 ft and adopted the International NM in 1954

The UK finally abandoned their definition of 6080ft and adopted the International NM in 1970.

[Chotu]

All you need to know about what a nautical mile is in a single diagram.

[Lodesman]

Originally Posted by garyfdl

Nope. in fact, as an old sub-sailor, I'll wager that the scope mast would snap before the boat went fast enough for the scope to 'rooster tail'... if a rooster tail is even possible from a abject that extends 10's of feet below the surface, and is attached to a much larger object.


Are you saying the scope sticks up a couple meters? Nope, again. The scope will (normally) sick up above the water just enough to see, but not be seen. Sticking up 'a couple of meters' gives away the boat's position, and makes it a much bigger radar 'target'.

You really like to quibble don't you? Well, as an old sub-hunter I can assure you that you are wrong. Of course from inside the boat you're not likely to have a good vantage point. You can get an idea from the vid.

I would wager that generally in a typical sea-state the periscope must stick up above the tops of the waves to have any effectiveness at all, so yes a couple metres above the trough if having a good look around. I also kept with the poster's use of the word periscope, but let's face it, there are many other things that can stick up - not just the attack periscope. It could be a comms mast, or given the location, a snorkel. A radar blip doesn't specify.

https://www.youtube.com/watch?v=Uj3sHSv1VkM

[Lodesman]

Originally Posted by StuM

The UK finally abandoned their definition of 6080ft and adopted the International NM in 1970.

That explains a bit - why would the Navy upgrade perfectly good textbooks from the 60s

I am curious about how they could have calculated it to be 6080 ft before the widespread adoption of SI, then when they calculated it in metric they came up with a different length altogether.

It never made much sense to use metric to define a nautical mile, since we tend to use yards and feet in our mental math calculations.

[requiem]

This reminded me of another periscope example: https://youtu.be/Q0yL0ddfgWc?t=1311

Originally Posted by Lodesman

I am curious about how they could have calculated it to be 6080 ft before the widespread adoption of SI, then when they calculated it in metric they came up with a different length altogether

As a bit of trivia the length of the foot also changed in that time, so the nautical mile had another slight change in length before receding to 6076'. (The UK foot became slightly longer, and the US foot slightly shorter.)

But, to your query, Chotu's diagram represents the idea behind the meter, that it be defined as 1/10,000,000th of a quarter meridian. After much measurement and surveying, including the discovery that gravity varied* from place to place, a length was arrived at and a platinum bar created as a benchmark. Eventually as variations, errors, etc. were found, it was decided to stick with the benchmark. The actual meridional distance is slightly greater than 10,000 km, and I believe that's what accounts for the extra length.


* This nixed the idea to define it based on the seconds pendulum.

[ChrisJHC]

Originally Posted by Lodesman

It never made much sense to use metric to define a nautical mile, since we tend to use yards and feet in our mental math calculations.

Looking at the Bureau of Meteorology (Australia) site right now.
Forecast wind = 25 km/h
To roughly convert to knots, I just divide by 1.8 which my mental arithmetic tells me is around 14-15 knots.

Your way, you would first have to convert 25 km/h to mph, yards/hr or feet/hr then apply whatever factor is required to convert that to knots.
Probably not as easy to do in your head!

[Lodesman]

Originally Posted by ChrisJHC

Looking at the Bureau of Meteorology (Australia) site right now.
Forecast wind = 25 km/h
To roughly convert to knots, I just divide by 1.8 which my mental arithmetic tells me is around 14-15 knots.

Your way, you would first have to convert 25 km/h to mph, yards/hr or feet/hr then apply whatever factor is required to convert that to knots.
Probably not as easy to do in your head!

WTF are you talking about?

1/10th of a mile = 1 cable or 200 yards or 600 ft. Easy to apply radian rule. Or you could use 185 m.
Or at X kts, I cover X cables in 6 mins, X00 yards in 3 mins, X00 feet in 1 min. Do that in metric.

[Pelagic]

Originally Posted by Lodesman

Or at X kts, I cover X cables in 6 mins, X00 yards in 3 mins, X00 feet in 1 min. Do that in metric.

Lol..brings back memories of the fast calculations needed when using Reflector Plotter/plotting sheets on the old radars in the simulator exams

S=d/t

[Jim Cate]

Originally Posted by ChrisJHC

Looking at the Bureau of Meteorology (Australia) site right now.
Forecast wind = 25 km/h
To roughly convert to knots, I just divide by 1.8 which my mental arithmetic tells me is around 14-15 knots.

!

Well, you must be looking at land forecasts, not marine. The marine f/c state wind speeds in knots, not kph, so the good folks at the BOM seem to understand that mariners think in knots, lubbers in kph or (god forbid) MPH.

Channel Forecast for instance.

Jim