Twin screws

goldorak · 12-03-08, 06:41 PM

Quote:

Originally Posted by Dr.Sid

Btw. where does the second picture come from ?

http://www.kaon.co.uk/kraken.asp

MBot · 12-06-08, 04:14 PM

Thise DEMON pictures are fascinating. Could any of the experts perhaps explain what exactly we are seeing there? I can not figure out how that works.

Dr.Sid · 12-06-08, 09:17 PM

I'm not really expert but this was discussed earlier here.
The thicker lines are blade frequency. It's always dominant. You see more that one because what you see are harmonics - multiplies of base frequency. Practically any sound has such harmonics. So if you look for you real blade frequency, it's the lowest frequency (23 Hz on green, 45 Hz on black & white).

Then those thinner lines between those thick lines are harmonics of shaft frequency. Either shaft makes these noises or there is difference in individual blades, so there is something which only can be heard once per turn.
Again, the lowest is the base shaft frequency, the rest are just harmonics. But what is important is how many times the blade frequency is larger then shaft frequency, or (in other words) how many shaft harmonics fits between blade harmonics, because that's how many blades the screw has.
You can see it's 3 on b&w image and 5 on the green one.

12-06-08, 09:17 PM	#3
Dr.Sid The Old Man Join Date: May 2005 Location: Czech Republic Posts: 1,458 Downloads: 6 Uploads: 0	I'm not really expert but this was discussed earlier here. The thicker lines are blade frequency. It's always dominant. You see more that one because what you see are harmonics - multiplies of base frequency. Practically any sound has such harmonics. So if you look for you real blade frequency, it's the lowest frequency (23 Hz on green, 45 Hz on black & white). Then those thinner lines between those thick lines are harmonics of shaft frequency. Either shaft makes these noises or there is difference in individual blades, so there is something which only can be heard once per turn. Again, the lowest is the base shaft frequency, the rest are just harmonics. But what is important is how many times the blade frequency is larger then shaft frequency, or (in other words) how many shaft harmonics fits between blade harmonics, because that's how many blades the screw has. You can see it's 3 on b&w image and 5 on the green one. __________________ ComSubSim forum at CADC Last edited by Dr.Sid; 12-06-08 at 09:19 PM.

12-06-08, 04:14 PM	#2
MBot Loader Join Date: Apr 2002 Location: Switzerland Posts: 90 Downloads: 3 Uploads: 0	Thise DEMON pictures are fascinating. Could any of the experts perhaps explain what exactly we are seeing there? I can not figure out how that works.