making turns for 15 knots

In reading some of the novels and other books, you hear the sonar
operators saying "Making turns". How did you get a blade count
converted to knots?

Thanks

MAH

mah wrote:
In reading some of the novels and other books, you hear the sonar
operators saying "Making turns". How did you get a blade count
converted to knots?

Uh, that's classified (no, seriously).

But in layman's terms, think of it like this- say you have a
motorboat. If the engine is turning the propellor a certain rpm,
let's say 1000 prop rpm for the sake of argument. For any given prop
rpm, the boat will end up at a certain speed, in this case let's say,
30mph. The prop will make more or less the same sound in the water,
today, tomorrow, and a year from now. Let's say 1200 prop rpm will
make the boat go 35mph. If you have some way of listening to and
accurately measuring that sound, then when you hear the first sound
know it is probably your boat or one very similar to it going 30mph.
If you hear the second sound then it is probably close to 35mph.

And so on...

The secret squirrel part is how do you find out the blade rade to
knots correlation of everyone else's submarines.

Jim Carriere wrote:

mah wrote:
In reading some of the novels and other books, you hear the sonar
operators saying "Making turns". How did you get a blade count
converted to knots?

Uh, that's classified (no, seriously).

But in layman's terms, think of it like this- say you have a
motorboat. If the engine is turning the propellor a certain rpm,
let's say 1000 prop rpm for the sake of argument. For any given prop
rpm, the boat will end up at a certain speed, in this case let's say,
30mph. The prop will make more or less the same sound in the water,
today, tomorrow, and a year from now. Let's say 1200 prop rpm will
make the boat go 35mph. If you have some way of listening to and
accurately measuring that sound, then when you hear the first sound
know it is probably your boat or one very similar to it going 30mph.
If you hear the second sound then it is probably close to 35mph.

And so on...

The secret squirrel part is how do you find out the blade rade to
knots correlation of everyone else's submarines.

Probably just a near approximation isn't it?...they know a heck
of a lot about the type of boat that it is from general intel,
history of the company/country/service that made it/modified
it/worked on it etc, so they'd likely come 'close nuff'. Pretty
impressive to watch 'the boys' going over the J(XX) charts
picking out the exact boat!...
--

-Gord.
(use gordon in email)

If you ever snorkel around the Keys or Biscayne Bay you get real
sensitized to 'blade count' only in this case it's more of a scream as
some cowboy comes ripping past at 50+ in his Cigarette. When you hear
'up doppler' it's time to sink to the bottom until you hear the 'up'
become 'down'. Those props can mess you up real bad! Now, big boats go
'thrum thrum thrum thrum . . . ." if they're just poking along. Still,
250 rpm is only about 16 a second on a 4 -blade prop so you can hear
and recognize the beat on say a freighter or tanker. Walt BJ (now
land-locked - Colorado is amazingly short of reefs and surf))

The secret squirrel part is how do you find out the blade rate to
knots correlation of everyone else's submarines.

Well, ATC can give a pilot a groundspeed readout based on tracking his
progress on the radar screen. Can't something of that sort be done
with subs?

Or could the Doppler effect be used for that purpose?

vince norris

vincent p. norris wrote:
The secret squirrel part is how do you find out the blade rate to
knots correlation of everyone else's submarines.


Well, ATC can give a pilot a groundspeed readout based on tracking his
progress on the radar screen. Can't something of that sort be done
with subs?

As in time-distance using two or more fixes, yes. I think the old
fashioned way was with a piece of paper, but it is nothing for a
primitive (by today's standards) computer to do it for you with two
or several fixes.

Or could the Doppler effect be used for that purpose?

Also yes, but of course if the target is not heading directly (or
near directly) towards or away from your receiver, then doppler shift
will be less pronounced. You can sanity check the "big picture"
using, among other things, doppler, fixes, and common sense.

Last, a lot of the time you only have some of the above information,
the fixes may have questionable accuracy, so then you get an
approximate solution.

Today ou can hear another ship (prop) a hell of a lot farther away than you
can get an accurate fix on it. Although in WWII the equipment wasn't that
developed, it was probably true then also.

Diamond Jim

Quote: " . . . is amazingly short of reefs and surf"

So why would you live there?


(kidding . . .not starting a flame war . . . just kidding)

Ended up in Colorado because of company move so didn't cost us a dime.
House now paid off; good place to live except dry; Rockies 30 miles
away. Wx is pretty good about 90 percent of the time (about 330 days
it's good enough for VFR flight!). Plus a lot of inertia. We do get
down to Corpus Christi/Port Aransas every year).
Walt BJ

wrote in message
oups.com...
Ended up in Colorado because of company move so didn't cost us a dime.
House now paid off; good place to live except dry; Rockies 30 miles
away. Wx is pretty good about 90 percent of the time (about 330 days
it's good enough for VFR flight!). Plus a lot of inertia. We do get
down to Corpus Christi/Port Aransas every year).
Walt BJ


Agreed that Colorado is a good place to live (and my weather is better than
Walt's).

Tex Houston