Sub physics - experts needed

Bill Nichols · 11-20-07, 10:14 AM

My drag estimate was based on Sturgeon-class SSN for size, power and speed. I would expect the Sturgeon-class to have a lower drag coefficient than the LA, because it has a lower length-to-beam ratio.

Max acceleration, of course, would mean cavitating at shallow depths. If you limit prop speed to below the cavitation threshold, acceleration to flank speed will take longer, of course. Somewhere I have notes on that and I even wrote a program. I no longer have the latter, but I seem to recall acceleration to flank from a dead stop took around 60 seconds without cavitating.

Sounds like we're both in the same ballpark.

Dr.Sid · 11-20-07, 11:16 AM

Hey Bill (I mean hail you O big sub guru) you SHOULD know about the screw. Is it variable pitch ? Will you tell ?

Bill Nichols · 11-20-07, 11:48 AM

Nope, its not variable.

Dr.Sid · 11-20-07, 12:31 PM

Good .. that simplifies some things.

Yet another problem. How fast does the screw rotate ? In DW there are numbers like 7 for TPK, with 30 knots it gives 210 revolutions per second. Seems too much to me.

goelfou · 11-20-07, 04:38 PM

Quote:

Originally Posted by Dr.Sid

Good .. that simplifies some things.

Yet another problem. How fast does the screw rotate ? In DW there are numbers like 7 for TPK, with 30 knots it gives 210 revolutions per second. Seems too much to me.

It could be true if "Turns" referred to turns per seconds.

TPK is the propeller efficiency, I don't have the real algorithm, but in game it seems to be calculated as follow :

Speed = ( Freq * 15 ) / TPK
or
Freq = ( Speed * TPK ) / 15
or
TPK = ( Freq * 15 ) / Speed

So TPK may be "Turns(per 1/4 minute = 15*Freq = rpm/4) Per Knot"

Dr.Sid · 11-20-07, 04:53 PM

Hm ! Good point .. I always thought that you obtain speed as demon frequency*TPK, but that is not true. Your formulas seems to be correct.

So in DW, 688 with TPK=7 at 30 KTS would have 14 turns per second, at 5 KTS it would be 2.33 turns per second, which sounds much better.

11-20-07, 10:14 AM	#1
Bill Nichols Master of Defense Join Date: Mar 2000 Posts: 1,502 Downloads: 125 Uploads: 0	My drag estimate was based on Sturgeon-class SSN for size, power and speed. I would expect the Sturgeon-class to have a lower drag coefficient than the LA, because it has a lower length-to-beam ratio. Max acceleration, of course, would mean cavitating at shallow depths. If you limit prop speed to below the cavitation threshold, acceleration to flank speed will take longer, of course. Somewhere I have notes on that and I even wrote a program. I no longer have the latter, but I seem to recall acceleration to flank from a dead stop took around 60 seconds without cavitating. Sounds like we're both in the same ballpark. __________________ My Dangerous Waters website:

11-20-07, 11:48 AM	#3
Bill Nichols Master of Defense Join Date: Mar 2000 Posts: 1,502 Downloads: 125 Uploads: 0	Nope, its not variable. __________________ My Dangerous Waters website:

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11-20-07, 11:16 AM	#2
Dr.Sid The Old Man Join Date: May 2005 Location: Czech Republic Posts: 1,458 Downloads: 6 Uploads: 0	Hey Bill (I mean hail you O big sub guru) you SHOULD know about the screw. Is it variable pitch ? Will you tell ?

11-20-07, 12:31 PM	#4
Dr.Sid The Old Man Join Date: May 2005 Location: Czech Republic Posts: 1,458 Downloads: 6 Uploads: 0	Good .. that simplifies some things. Yet another problem. How fast does the screw rotate ? In DW there are numbers like 7 for TPK, with 30 knots it gives 210 revolutions per second. Seems too much to me.

11-20-07, 04:53 PM	#6
Dr.Sid The Old Man Join Date: May 2005 Location: Czech Republic Posts: 1,458 Downloads: 6 Uploads: 0	Hm ! Good point .. I always thought that you obtain speed as demon frequency*TPK, but that is not true. Your formulas seems to be correct. So in DW, 688 with TPK=7 at 30 KTS would have 14 turns per second, at 5 KTS it would be 2.33 turns per second, which sounds much better.