Outboard dual propellors, how do the work

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carson

carson

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  Now that the "Dog days of Summer" started early, I have this very scientific question:

 I checked out a large outboard motor parked near my RV. It had 2 props - in series- and my curious mind started to work. I found out that the forward and the rearward props turn in opposite direction.

 I also noticed that the pitch of each prop were reversed.

Here comes the question: How do they work? Since they are of opposite pitch yet turn in the same "water pushing" direction when under power, how does this increase the power/waterflow?

  Does the forward one push more water into the aft prop, or is there a more scientific explanation.

see pix below..

 carson FL

 
 

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These are contra-rotating propellers and are supposed to be more efficient. They have been used on aircraft also.

An explanation :  http://en.wikipedia.org/wiki/Contra-rotating_propellers
 
Lowell, great post of WIKI. I understand.

  I normally look up my questions on WIKI first...thought this Marine question would bring out a few old Salts on the forum to chime in..

  Sounds to me that the gains are marginal, maybe not even worth the extra cost. No such thing as a free lunch, I suppose.

  In the case of my example (pix) there were some severe nicks on the props. Guess that negated his power increase. Horsepower cannot be increased if the propeller is perfect in the first place.

  How about inventing the perfect propeller...  Has anyone done that? 99% efficiency. Let's wait..

Thanks for the post.

carson FL

 
LOL, Lou, why do you think I am in my cool home thinking of things to say? Maybe it's guys like you that need some stimulation? The pool outside is 94?... the inside of the home is 80?. I'm in heaven..

  I am taking Vitamin D to make up for the lack of Sunshine..

Take care,  carson FL

 
When Volvo introduced their version of this prop many years ago, they called it the DuoProp. I've never used them, but a Google search on DuoProp will return lots of results, including this one.
 
Thanks, Tom for the link. I am glad I asked the question..

  All that makes sense to me, being the only source so far. Do you think we can take that as gospel.?

Sounds like a Sales pitch, but very convincing. I just don't believe everything I read now a days.

Thanks for the link. Tom. What do you think?

carson FL


 
carson,
there is no "perfect propeller".
Depends on what kind of boat you have. And it depends on the purpose of the boat/ship.
The four propellers of the fast patrol boat I served on five years had huge blades - together with ~13,000 HP they are good for a great acceleration. Waterskiing behind the boat was nearly impossible because of the swell(?) - we tried and failed.
On the modern frigates you see completely other blades.
 
Thanks Mike...
we tried and failed.
Click to expand...

  For new comers, you are talking about the German Science. I am biased, as I was born in Germany many years ago. Science has no borders. Where would the world be without the input of all  countries?

  Thanks for that input...can you elaborate a bit more?

carson FL

 
 
Do you think we can take that as gospel.? ... Sounds like a Sales pitch, but very convincing.
Click to expand...

That just happened to be the first one that came up on Google, and I didn't pay a lot of attention to the detail.

Here's a link to Volvo Penta's explanation of the benefits of the DuoProp. With a conventional prop, there's an effect known as 'prop effect' or 'prop walk' that causes the stern of the boat to walk to one side (left/port right/starboard in the case of a clockwise rotating prop). On a twin engine boat, the props are turning in opposite directions, thereby canceling the prop effect, but not necessarily recovering any wasted energy. The claim of the DuoProp is that it cancels the prop effect on a single prop.
 
Tom said:
The claim of the DuoProp is that it cancels the prop effect on a single prop.
Click to expand...

Tom, in my experience it does, indeed, work.

I had a 24' Wellcraft single engine, single prop boat that I used for inshore/offshore fishing.  It was a bear to handle when I was the only experienced person on board because I continually had to ask someone to man the helm when I was needed to tend the lines. The boat would indeed turn drift to port if left unattended, even for a few seconds.

My second boat was a 24' Albemarle with the Volvo Penta DouProp.  That boat was rock solid and tracked true when the wheel was unattended (we're talking short reasonable times in the open ocean here, of course).  

When you have 4 to 6 lines out behind the boat (at up to 200 yards), that's very important to prevent tangles.
 
Thanks Lou. I didn't doubt Volvo Penta's claim; They have, after all, been selling DuoProps for a lot of years. I just didn't have any personal experience with them. I did, however, think "that's a great idea" when they first put it on the market.
 
Methinks I made an error in my earlier message re the direction of prop walk; I've corrected it. More on prop walk.
 
All this is very scientific and it all make sense.

   All I know is that I ran a 35' trawler for several years with a single screw. All I had to learn was how to react to stern movement going forward or in reverse, especially when docking. I had it figured out to a science... was able to maneuver in tight quarters without using the rudder. Without boat speed the rudder is useless. It is one of the things that "true Sailors" have to learn. Never had an incident in "not" hitting the dock at the right place.

   Bow thrusters, twin screws were out of my vocabulary at the time. I was proud of every successful docking. Sometimes the human brain has an advantage..

carson FL

 

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Tom, your attention to detail and accuracy constantly amazes and delights me.

I didn't catch your original reference to port thrust (of the stern) for the normal clockwise turning prop.  I only remembered what my boat did.  

Stern to the right, bow to the left (port), was what always happened to me.

Actually the condition was somewhat mitigated by the fact that the steering (on my boats) was done by the position of the outdrive rather than with a fixed prop and rudder steering.  Still, someone had to hold the wheel to prevent the tendency.

I miss those days......   Like I always say, getting old was fun, being old sucks...... hope that doesn't offend anyone...
 
Carson, I've watched you "old salts" maneuver those big boats in tight places many times and marveled at your abilities.

It's fun to watch the 50'ers, with dual engines, back down into their berths using nothing but the engines for steering.  The Captain will stand with his back to the console (facing aft) and play those throttles, behind his back, like a fine musical instrument.  That boat settles in without a bump or a rub. 

It's the little things that make a lifetime of memories...
 
PancakeBill said:
Yeah, but try it with just sails.  Been there, done that.  Motor wouldn't start. 
Click to expand...

Bill, I've never had to do that with anything larger than a "sunfish".
 
26' sloop. backed in.  Little tense, but worked just right,

Not sure I woould go try it today, but if it were today, I'll use my friends motto.  Not my boat.

 
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