Pod drives

[Doryman]

Was reading an article in the latest SEA about pod drives (they have been around for a few years, I gather.) I would be interested in comments from the peanut gallery (especially Bob) about the pros and cons of these drives as well as how they compare to outdrives. I am intriuged by the idea of being able to use them as thrusters, too.

[JamesTXSD]

Hi Warren,

When we were in Florida this past winter, there was a boat with twins that they were testing. They were building a metal cage type apparatus that the boat could run over at speed to see what the effect would be. Now, that's something that would have been interesting to see! The boat had a steering wheel, but in talking with the guys who were running it (relatively tight lipped) they were impressed with the joystick control to steer the boat. This particular boat also had the tractor type prop set-up.

More draft to deal with. The tractor prop set-up would concern me in any kind of grounding.

I'll be interested to hear what any others who have more hands-on have to say.

Best wishes,
Jim

[Sea Wolf]

For clarification, is this what we're talking about?




"The SCHOTTEL Electric Propulsor (SEP) and the Siemens-SCHOTTEL Propulsor (SSP) are innovative propulsion systems which differ mainly in the kind of electric motors housed in the pods.

The two pillars of the SSP – a joint development of Siemens and SCHOTTEL – are the SCHOTTEL Twin Propeller technology and a permanently-excited motor (PEM) from Siemens characterised by its high efficiency and slim-line construction.

With the SEP SCHOTTEL offers a propulsion system with asynchronous or synchronous motors depending on the owner/shipyard request and application of the vessel.

Pod drives in twin propeller version are particularly suitable for the market sectors RoPax, double-ended ferries, supply vessels, tankers, container ships and yachts. For the offshore industry, pods with single propellers in nozzles are part of the new development."



If so, gimmie a break!

Way too complex and expensive!

Looking for a $250,000 C-Dory?

Maybe OK for a 60+ foot tug!

Note: Celebrity Cruise Lines has one of these systems on one of their cruise ships and is suing the manufacturer over the need for constant repair, replacement of bearings, and misrepresentation of the practicality and utility of the system.

An alternative: Turn the main motor off and install a foot operated electric trolling motor (bass boat style) and you get the same effect for low speed maneuvering for about $1,000!

P.S.: Did you want a "Joy Stick" with your pod for $15,000 extra?

I guess "permanently-excited" is not a guaranteed feature of ownership?

My $0.02!

Just being funny (trying anyway)!

Joe.

[Doryman]

Joe -- Not talking about those. Am talking about units made by Cummins and Volvo. They look a little like the ones in your photos, but are made for pleasure craft. Could not find any photos online, but I am sure they are out there somewhere.

Warren

[Dora~Jean]

Here is a link to Volvo's new IPS drive system. I don't thing they're practical for our type of boating, at least in the configuration it is currently. I don't have much faith in forward facing propellers!

http://powerboat.about.com/od/drivesystems/a/volvopenta_IPS.htm

[tsturm]

[Sea Wolf]

[rogerbum]

Here's a link to information on the Cummins version "Project Zeus". Also below I provide links to some images of the units. From the Cummins web site:

" The drives are designed to produce horizontal thrust so the full power of the engines are utilized to drive the boat forward, unlike standard inboard shafts that push a boat upwards. Counter-rotating propellers feature increased blade area, larger gear ratios, and no side forces and minimized cavitations. The results of the advances are impressive, 7% faster top speed and a 30% increase in fuel economy at a higher cruise speed."

Since these gains are similar to what is reported for the Volvo units, it would appear that most of the efficiency gains are due to the direction of the thrust and the prop/lower unit design that reduces cavitation (as opposed to the forward pointing props on the Volvo units).


Cutaway view of a twin application - note that these are installed with rear facing props and a more conventional skeg design.


Here's another view:



These units are not designed to turn in a complete circle:



But they do turn independently of each other and the thrust can be reverse on one to achieve sideways motion:

[C-WEED]

You already have the efficient parallel to the hull thrust with the outboard and lots of other benefits. You just need the maneuver part. If you have ever operated a small outboard that has 360 degree steering you know how maneuverable it can be. And powerful as the prop can pull in a forward motion as it was designed to do no matter what way it is pointing.

How about making a bigger version? Maybe use a ring and pinion to rotate the outboard? Any modification is going to add weight. You would need a way to limit power at extreme turn angles. IE: Too much power with the outboard pointed aft could pull a stern under causing lawsuits.

You could even make a version that rotates just the lower unit instead of the whole power head. You would then need some sort of indication of which way the prop is pointing. Imagine how easy the steering would be if you didn't turn the whole engine... You could then design the boat with more usable room if the power head didn't have to rotate. While your at it make the trim/tilt on the lower unit as well and swivel 180 up out of the water for mooring. Not having the power head tilt would save even more space aboard.

[Sea Wolf]

Lots of good ideas discussed here about the advantages, disadvantages, and trade-offs of various drive configurations.

Not surprisingly, as the systems grow more complex, the lists of trade-offs and disadvantages outweigh the advantages.

This is especially true in smaller boats. KISS is King in small boats!

That's why Rube Goldberg was never a successful Naval Architect!

The engineer's dream is to find a solution that is simple, elegant, and all encompassing.

It's much too easy to add layer upon layer of complexity while trying to use imperfect solutions to solve the original problem.

As an example, one could postulate that a modified jet drive system might be a good solution to the original issue of how to provide low speed maneuverability. So we add an albeit inefficient jet pump to an inboard engine, then add ducting inside the bottom of the hull to allow sideways thrust at the bow and stern at low speeds. To control this ducting and thrust, we'll need to have valves controllable from the helm with a lock out at higher rpms, and can add a joy stick, a computer, and a servo system to achieve the desired control.

Suddenly, our system is overly complex and takes on a life and need structure of its own that overwhelms the problem to be solved. We've led ourselves down the Yellow Brick Road again, only to wind up in the Dead End of Eternal and Endless Re-Engineering.

Such are the joys of the engineers and problem solvers. To be successful at that game, one has to recognize when you're painting oneself into a corner.

Fun thinking on a slow Saturday morning!

Joe.

[Doryman]

I agree -- the current pod drives have little relevance to current C-Dory boats. Nonetheless, we may see all or part of this technology creep into our little niche somehow, eventually, maybe.

Bob has gone on record as disliking outdrives. I am hoping he will chime in with a few words as to whether he thinks the pod drives are better or worse than outdrives.

Warren

[westward]

In my opinion, as an exclusive saltwater boater, I would never again choose a drive system where the lower unit was always submerged. The only possible exception to an outboard I'd consider might be the propeller shaft/rudder on a larger diesel inboard. In the 5 or so years I had my glasply, I had to rebuild the outdrive 3 times!!! One failure left my wife and kids adrift, luckily in protected waters. Too complicated and too much chance for seawater intrusion. On the other hand, I've enjoyed many hundreds of trouble-free hours over several seasons with outboard power. One of the private water taxi's in the San Juan's told me he keeps 2 spare outdrives to minimize his downtime when he has to ship a drive back to the midwest for a rebuild. This despite the fact that he runs what he feels are the most heavy-duty drives made. (Outboards are out of the question for his 44' power cat and he needs the manuverability outdrives offer.) Maybe this POD swivel technology will be added to the lower units of outboards eventually, but not on my boat if I have a choice. Best, Mike.

[Sea Wolf]