Podded propulsion

schottels

Hi Leonidas,

That was more of a wild guess, as I have no idea of your displacement, your target speed and the resistance.

As a quick guess, I assumed for a semi-displacement of about 45 m, that you'd want it to go something like 25-30 knots and would need about 2 x 2500 -3000 kW power. That may be way off the mark, you tell me...

From the Schottel website, the best applicable pod drive would be the Combi drive with a vertical electrical motor (max. input power 2700 kW).
That one weighs 46 tons per piece (pod alone). I don't know if they produce it already.

A pod with the electrical motor inside the pod would be the SEP2 ( 2000 kW, 40 t) or the SEP3 (3200 kW, 56 t).

They also have a mechanical drive called "twin-propeller" which also weighs 40 tons per piece.

If you add up the extra weight of the alternators, the variable speed drives, etc, your installation would have to be extremely efficient to compete with the engine-gearbox-shaft-propeller solution.

Not the mention the extra weight of an electrical engineer you'd possibly need on board

I am all for electrical propulsion but I just don't think it's an option for semi-displacements (yet). Now if you say top speed is 22 knots for example (which would still be semi-diplacement), the situation may look totally different.

Bruno

PS. allthough I live in Holland, I am from Belgium (studied N.A. at the University of Ghent). Do you have a link on the MARIN research on pods?

That 'd be great! Please send it to infoREMOVETHIS@innomare.com.
Remove the removethis, that's anti-spam protection.

Thanks,
Bruno

Interesting thought Brian. That unit sure does look like it has all of the latest technolgies in the one package. Which leads me to a few questions.

Could this unit be used as main proplosion?
It looks like a far better proposion than azipods (shrouded, rim driven, reduced draught, easier engine access).

I've seen a lot of people refering to azipods (like the units on Air/Ice) increasing the draught on a yacht.
Is this true or false, if it's true, why don't they design the yacht so it doesn't increase it's draught?

I understand that a propellor in a shroud increases it's efficiency, why arn't more propellors in shrouds?

Grasshopper Ben.

Shrouded Propeller Usage, Kort vs Rim Driven

Propellers in shrouds or 'ducted' is quite an expansive subject. A really good reference on propeller design is Dave Gerr's book. ( The Propeller Handbook By Dave Gerr ISBN 0-7136-5751-0)


I searched thru this forum and did not find much previous discussion on the subject, so I will reference you to a few on another:
Propulsion of Kort Nozzle
...couple of quotes:
* "Having said this, I'm referring to common accelerating nozzles like the marin 19A and others in that series, as this was the type referred to in the beginning of the thread. There are straight nozzles that are intended just to protect the prop and then there are also decellerating nozzles which are used to make the flow into the prop more uniform... These other nozzles don't tend to provide more thrust."
* "NO....all nozzles have more drag than an un-ducted propeller. What the nozzel does is manipulate the speed of the water flow into the wheel to change the apparent speed of advance, "J". Depending on how good you were at it, the thrust increase for a given horsepower input will be greater than the increased drag and you will see increased overall thrust. Miss the optimum point and overall net thrust will be less!"

two other threads:
another, Kort Nozzle
Bigger Props, Faster Ships

There are many other discussions that might be found on this subject of ducted propellers. As a generality, the ducted propellers we see most often in commerical usage are the hi-thrust, low speed units known as Kort units. The shroud of these units is generally of an airfoil configuration to help affect the flow. When you combine this big shroud surface area, its airfoil configuration, its support structure, and all in addition to conventional drive shaft and struts for the prop, you end up with a lot of extra drag when trying to push the arrangement to a higher speed. Plus the interference and other problems experienced between the tip speed of the prop next to a close tolerance shroud inner surface.

Note that the electric rim driven unit eliminates a lot of these negative aspects.

The electric rim driven unit's outer perimeter could be located right next to the bottom of the hull, or even recessed into the bottom hull surface. There is no longer the problem of providing a 'sufficient clearance' between the conventional propeller tips and the hull bottom.

Kort Nozzles

Another consideration is vibration/noise

Good day everyone,

Let's put aside for a while the technical specifications and let's take as an example the cruise industry which has now a young but good experience on this matter (since 1998). We can see that shipyards have lost a huge amount of money in claims from cruise companies experiencing various problems on their brand new cruise vessels equiped with azipod or mermaids! I think everyone will agree to say that it is an amazing system, technicaly speaking and when you manoeuvre a 40 000 GT cruise ship equiped with azipod, it's becoming a real pleasure! I experienced it. Just to say that it took few years to make this system perfectly reliable on cruise ships. What will be the reliability of such a system on small yacht? Appearently there is little or no experience to assess this! Even on cruise vessels, we don't know how this system will evolve with time passing? I know several engineers who are not that confident for the time life of the whole system. We can even see that cruise companies are coming back to a more conventional propulsion system for their new building : two shafts, fixed propelers and two becker rudders (even for the huge 160 000 GT new cruise vessels!), helped by three stern thrusters (should be app. 3 MW each!).
Isn't it a risky bet to use such a system on a small yacht with such a short experience of this technic?

Laurent