Twin screws

[XabbaRus]

OK on a real submarine how would this look on DEAMON?

How do you figure it out in real life, without going into classified stuff?

[suBB]

Originally Posted by XabbaRus

OK on a real submarine how would this look on DEAMON?

How do you figure it out in real life, without going into classified stuff?

Taking a common sense approach here… :hmm:

I like to think of this like playing a guitar through a guitar amplifier, same frequency(fundamental) but ‘twice’ gain (sound)

That said, I would think the DEMON display on twin shafts would be the same, but the freq lines on twin shafts would display 2x amplitude(brighter) since you have two shafts(twice propeller noise) spinning at the same frequency (rpm)

I would also think that since both shafts will be spinning at same (rpm) on a direct course, if not you’ll suffer inefficiency in propulsion(drag). But while going into turns, (rpms) could vary to give up better handling, if so then the frequency lines could alter for a brief moment of time.

Just a thought…

[goldorak]

Here's an interesting read :

http://www.fuzzytech.com/e/e_a_kumm.html





Example of a single screw 5 blade propeller :

[Dr.Sid]

Subb: it's not that easy. When two exactly same frequencies sounds together, phase-difference comes into play. It may double the amplitude, but it may cancel the sound completely.

On guitar this never happens as the frequencies are always little different and also they always change a bit, so there is never good real match. And sound also comes not only from the string, but from the guitar body so the phases can't really exactly cancel.

On sub it too should not really cancel, but it may lower the amplitude significantly. Of course only in case the both props are EXACTLY at the same frequency. Which may be the case if they are propelled by the same engine. If they are not they will only have similar frequencies, and you would just see two lines (I guess) or one line a bit stronger, but far from twice.

Now if such props are synchronized or not, I don't know.

Btw. where does the second picture come from ?

[goldorak]

Originally Posted by Dr.Sid

Btw. where does the second picture come from ?

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

[MBot]

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]

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.