SUBSIM Radio Room Forums - View Single Post - Why don't dieseal subs use pumpjets propulsors

aaken · 07-27-06, 03:42 AM

One thing is that pumpjets have higher inertia than propellers so they do not respond as quickly as an unshrouded propeller to speed variations. I imagine that for shallow water operations this has a certain importance.
Also, not all shrouded propellers and pumpjets are more efficient than propellers, and not in all ranges of speed.
in brief:
a shrouded propeller with an accelerating shroud (i.e. a duct in which the water accelerates from the inlet to the outlet) is generally more efficient than an unshrouded propeller. This is in part due to the fact that the propeller/duct, creating a lower pressure or suction field on the inner side of the duct itself, creates a positive thrust on the shroud. This thrust more or less compensates the additional drag that the above mentioned suction field generates on the after body of the boat. In reality the shroud itself has a certain drag so the thrust that has to be produced by the shrouded propeller may not always be smaller than the thrust that should be produced by an equivalent unshrouded propeller.
But I don't think that submarines use accelerating ducts because a shrouded prop with an accelerating duct has smaller diameter and higher rpm than a shrouded propeller with a decelerating duct and it should be more noisy.
On the other end, a shrouded prop with a decelerating duct creates a negative thrust on the shroud therefore the thrust that has to be produced can be (and usually is) larger than the thrust produced by and equivalent unshrouded prop. On the other end it usually has better efficiencies than shrouded props with accelerating ducts (working with the same outlet-inlet speed difference) because it has higher specific diameters. Also it has the advantage of having lower rpm and lower loads. Having lower rpm it is less prone to cavitate (at given depth and given boat speed) than an unshrouded prop.
But since a diesel sub operates most of the times at low speed the andvantages of a pumpjet are very limited in terms of quieting and could actually increase the Ah consumption.

07-27-06, 03:42 AM	#2
aaken Planesman Join Date: May 2005 Location: Naples Posts: 188 Downloads: 43 Uploads: 1	One thing is that pumpjets have higher inertia than propellers so they do not respond as quickly as an unshrouded propeller to speed variations. I imagine that for shallow water operations this has a certain importance. Also, not all shrouded propellers and pumpjets are more efficient than propellers, and not in all ranges of speed. in brief: a shrouded propeller with an accelerating shroud (i.e. a duct in which the water accelerates from the inlet to the outlet) is generally more efficient than an unshrouded propeller. This is in part due to the fact that the propeller/duct, creating a lower pressure or suction field on the inner side of the duct itself, creates a positive thrust on the shroud. This thrust more or less compensates the additional drag that the above mentioned suction field generates on the after body of the boat. In reality the shroud itself has a certain drag so the thrust that has to be produced by the shrouded propeller may not always be smaller than the thrust that should be produced by an equivalent unshrouded propeller. But I don't think that submarines use accelerating ducts because a shrouded prop with an accelerating duct has smaller diameter and higher rpm than a shrouded propeller with a decelerating duct and it should be more noisy. On the other end, a shrouded prop with a decelerating duct creates a negative thrust on the shroud therefore the thrust that has to be produced can be (and usually is) larger than the thrust produced by and equivalent unshrouded prop. On the other end it usually has better efficiencies than shrouded props with accelerating ducts (working with the same outlet-inlet speed difference) because it has higher specific diameters. Also it has the advantage of having lower rpm and lower loads. Having lower rpm it is less prone to cavitate (at given depth and given boat speed) than an unshrouded prop. But since a diesel sub operates most of the times at low speed the andvantages of a pumpjet are very limited in terms of quieting and could actually increase the Ah consumption. __________________ Last edited by aaken; 07-27-06 at 04:41 AM.