Forward props

[Akeswins]

Hello.
What do you think about the throughput of an ahead propeller (like Volvo IPS) respect to the backward prop system ?
It increases the efficiency? Or both propulsion system are tied?

[gonzo]

Forward props can have a smaller hub. Another reason they are more efficient is that there are no struts, shafts or drives in front to create turbulence.

[marshmat]

You do of course then have the strut/shaft behind the prop, getting in the way of the prop wash. I'd like to see some scientific tests on this, actually- the debate has been raging in the airplane world since the 1900s as to whether the prop belongs on the front or back. Abong aircraft it seems to be more a matter of which end of the desired engine the shaft comes out of; in boats, I really don't know why we always go for rear prop. It might somehow be structurally better for the shaft to take compressive forces than tensile, but I would think modern technology would be able to give us solutions to such problems if there were a reason to.

[cyclops]

"We are not spending more money to do the same job!" Per our CEO and his bonus plans.

[Tim B]

The obvious reason for having a prop behind the shaft is that it is preferable from a balance and simplicity point of view. pointing a prop forwards on a boat has been (until now) very difficult. Even now it's not easy. I'm just waiting for electric podded drives for small boats.

Propellers in clear water (ie uniform flow) undoubtedly perform better than propellers aft of a boat. However, for the increase in performance it is an awful lot of extra effort.

Tim B.

[DMacPherson]

One of the reports in out Knowledge Library gives a bit of technical background to the potential efficiency gains for azimuthing podded drives (the term for a pod-mounted tractor propeller under a ship). You can find it at

www.hydrocompinc.com/knowledge/library.htm (http://www.hydrocompinc.com/knowledge/library.htm)

Look for report 127 - Modeling Tractor-style Azimuthing Podded Drives. The report is about predicting performance for these types of drives, but it also contains some background on why they are, or are not, better than conventional drives.

Regards,

Don MacPherson
HydroComp

[Akeswins]

I have read the report 127 . The efficiency of a Pod azimutal propulsion is globally better than a traditional system, cause the wave resistence is less .
But in planing boats, when the wave resistence is less important than friction, I think this propulsion less efficient respect a traditional one.

[Tim B]

It's only azimuthing if it rotates

[tom kane]

Tractor propellers have been used extevsivley on aircraft and boats.