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Old 01-28-2013, 08:40 PM   #21
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This is a new one on me. The direction the props turn doesn't seem to make any difference to the ability to pivot the boat. The PT boats of WWII had all three of their props turning the same direction, yet they had no problem pivoting the boat with the two wing engines although it probably wasn't as quick a pivot since the propwalk from one was counteracting the propwalk from the other with the thrust opposed. So all they had going for them was the twisting or yawing moment. Their rudders were so small as to not really be much of a factor although the Higgins was better than the Elco in this regard.

If counter-rotating props turn inboard, you still get the combined propwalk of both props turning the same direction when you oppose the thrust, same as you do with a pair of props that turn outboard. The only difference is the inboard turning props will propwalk the other direction as the ouboard turning props.

The rudders will still have the same effect if they are put over in the direction of the pivot. The only difference will be which rudder is being struck by the propwash.

In fact I've been told that outboard turning props actually do a better job of pivioting the boat because when you oppose the thrust the wash off the prop turning in reverse, which would be turning inboard, corkscrews forward where it curls up and strikes the side of the hull, helping to push the hull sideways even more.

I don't know if this is anything that's been tank tested and proven to be a hard and fast fact, but when I've looked over the side of the boat when we've had opposing thrust in to pivot the boat I can see the water from the prop in reverse bubbling and boiling against the side of the hull forward of the prop's location. So perhaps there is something to it.
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Old 01-28-2013, 08:53 PM   #22
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True story. But what part of manuvering? It makes twisting easier. It makes going fast easier. It also doesn't squat as much. Inboard props are hard to handle which is why our boats have outboard props. A boat that needs to work sideways as part of it's job will have inboard props.
explain how that works...I have always seen the opposite because prop walk works WITH outboard turning props...

Tugs have outboard turning props...so what boats are "made" to walk sideways????
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Old 01-28-2013, 09:12 PM   #23
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explain how that works...I have always seen the opposite because prop walk works WITH outboard turning props...
Agreed. It's true that propwalk works with opposed thrust props no matter which way they normally turn as long as they are counter-rotating when both are in forward (or reverse).

A. If props are outboard turning in forward and you put the port prop in forward and the starboard prop in reverse, both props will be turning the same direction (counterclockwise) and you will get combined propwalk to port, thus pivoting the boat to starboard. The pivoting or twisting force from the opposed props is pivoting the boat to starboard and the combined propwalk is also pivoting the boat to starboard. So the two pivoting forces work together.

B. If both props are inboard turning turning and you put the port prop in forward and the starboard prop in reverse both props will be turning the same direction but the other way (clockwise) and the combined propwalk will be to starboard thus pivoting the boat to port. So the opposed thrust is still trying to pivot the boat to starboard as in A but the combined propwalk is now trying to pivot the boat to port. So the speed of the pivot--- and the maneuverability of the boat--- will be reduced.

Therefore, if you want the maximum effect from opposing thrust, you want a counter-rotating twin with outboard turning props, not inboard turning props.
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Old 01-28-2013, 09:20 PM   #24
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I agree that outboard turning props are desirable..inboard for high speed craft that don't worry about docking.

Without typing way more than I can handle right now....many pro skippers I work with worry less about twisting and more about single engine maneuvering in tight quarters.

Here's the most basic example I can come up with right now. Coming to a dock starboard side to. Approach at 30 degrees...about 50-25 feet away...both to neutral...then port in reverse. It slows you to a stop and now the outboard engine starts to pull the bow parallel and the stern towards the dock...AND the prop walk from an outboard turning prop (now helping because it's in reverse) further pulls you to the dock..

Simple, clean, effective, etc...no muss no fuss and done tens of thousands of times a day by about every commercial twin operator I've ever driven with. (at least on craft under 100 feet/100 tons of conventional design.
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Old 01-28-2013, 09:26 PM   #25
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That's exactly how we do it although once the forward way has been stopped by the outboard (from the dock) prop in reverse we will often put the inboard engine in forward and the rudders hard over away from the dock to speed up the pivot and get the aft part of the hull right up against the dock quickly so the land handler can get to the dock right away with the spring line. Particularly if the wind wants to blow us off the dock or forwards.
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Old 01-28-2013, 09:27 PM   #26
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All tugs have outboard props? Really? Ever seen a crew or workboat move sideways up to a dock or platform? Why won't your boat do this? I don't know why, I am not a hydraulics engineer. Maybe Mr Roberts will chime in.
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Old 01-28-2013, 09:36 PM   #27
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All tugs have outboard props? Really? Ever seen a crew or workboat move sideways up to a dock or platform? Why won't your boat do this? I don't know why, I am not a hydraulics engineer. Maybe Mr Roberts will chime in.
Having run tugs and crewboats with outboard turning props...I don't know where you got your info and all I did was ask you to explain the basics like Marin and I have.

My sportfish would most certainly walk sideways as well as any tug or any other boat without flanking rudders/nozzels.
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Old 01-28-2013, 09:47 PM   #28
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Even after 14 years I continue to be surprised at how fast our GB pivots when opposing thrust is put in. Like every twin-engine boat in the boatyard in our marina that I've paid attention to the props on, our boat has outboard turning props. This fast-response pivot plus the use of rudder and my more recent addition of the use of power on one, the other, or both, has "saved" many a docking situation particularly on windy days.

If the props were inboard turning I cannot see how the pivoting performance of the boat would be improved but I can see how it would be reduced.
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Old 01-28-2013, 10:20 PM   #29
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True story. But what part of manuvering? It makes twisting easier. It makes going fast easier. It also doesn't squat as much. Inboard props are hard to handle which is why our boats have outboard props. A boat that needs to work sideways as part of it's job will have inboard props.
Marin, please reread the post. I am agreeing that outboard props make all those things easier. Inboard will not twist or pivot like outboard props but it will walk dead sideways. My information comes from 2 real tug captains who live on my dock who both tried to walk my boat. Different times and they both said the same thing. It has something to do with the turbulance at the keel which helps produce side thrust off the rudder and overcoming pivot. As this topic is about walking sideways and not outboard vs inboard I think we have beat it up pretty good.
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Old 01-28-2013, 10:30 PM   #30
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The move-straight-sideways-ability described by your tug captains sounds to me like a characteristic that is very specific to a very specific type of hull configuration. So perhaps not a broad attribute of inward-turning props in general.

But it would be interesting to know how their props on their boats act to make a straight sideways move possible.
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Old 01-29-2013, 06:03 AM   #31
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Many tugs have inboard turning props. Ship docking tugs will have them frequently due to the strong "Back" (reverse) that they provide. Many general towing tugs employ them as well. They due "walk" better than an outboard turning set-up.
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Old 01-29-2013, 11:21 AM   #32
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So why do inboard turning props back better and walk better?
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Old 01-29-2013, 01:26 PM   #33
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I just saw a new 42 Viking walk 90 degrees away from the dock with about 1/2 knot fair current.

They are outboard turning so I guess inboard turning for better than 90 degrees is good?
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Old 01-29-2013, 02:05 PM   #34
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I can't tell you why other than it is how the water plays off of the rudders (for walking). As far as why they give a good "back", I would assume they are pulling "clean" water when astern. I'm NOT saying that outboard turning wheels won't back or walk, what I'm saying is that inboard turning wheel boats are better in those specific ops. Watch a tug operator in any busy port and observe dockings and undockings. You will see boatmen walking not just light tugs but barges as well. To "walk" the barge, you need momentum (swing) on the tow, and the rudder gets shifted away from the intended direction, with reverse on the inboard side and ahead on the outboard wheel. Before everyone gets their thong in a wad, NOT every barge (or boat) can be made to walk.
Two VERY experienced captains that I know were attending a company required Ship handling simulator course at a VERY well known school. They asked the instructor if the Navy ship (that was on the simulator) could be made to walk. The instructor assured them rather smuggly that it could not be made to walk because it was exactly like the handling characteristics of the ship. Well , they made it walk much to the disbelief of the instructor....he didn't have much to say after that.
Now that all that has been said, should you "walk your boat" just because you can? NO. It is just another tool in the box. It is the most over used manuver when a little bit of rudder would have been just as easy.
Just like "palm Beaching it" (another show off manuever) , walking is sometimes used to impress audiences at Shooters rather than a necessary docking tool.
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Old 01-29-2013, 03:47 PM   #35
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I can't tell you why other than it is how the water plays off of the rudders (for walking). As far as why they give a good "back", I would assume they are pulling "clean" water when astern.
Not trying to be argumentative but how would a prop turning one way in reverse get any cleaner or less clean water turning the other way in reverse?

Looking at our boat out of the water I don't see how the direction the props turn would make any difference at all to how clean a bite of water they get, in forward or reverse.



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Old 01-29-2013, 07:16 PM   #36
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So what is Palm Beaching it?
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Old 01-29-2013, 08:05 PM   #37
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Since I am not a naval architect, I don't know the hows and whys of the hydrodynamics involved. Much the same way I don't know how an inverter changes DC input into AC output.

Palm Beaching it is a sport fish boat maneuver Backing down with the operators back to the helm and controls. I would assume it is derived from backing down and keeping the stern pointed to a fish being fought.
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Old 01-31-2013, 02:11 AM   #38
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I've only read about this, and a long time ago at that, but it was my understanding it worked on the larger, high powered, sport fishing boats. Of course the particular boat also played into the technique. Not all boats would do it. Some required some tweaking, some would do it easily and some not at all.

It won't work on my boat either. Maybe the single screw has something to do with it.
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Old 01-31-2013, 09:31 PM   #39
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In order for me to see the dock if I am backing in, I have to execute this maneuver, altho' I never knew what it was called. I simply cannot see the boats' stern any other way.
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Old 02-04-2013, 09:57 AM   #40
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With azimuthing pods and bow thruster, it's a "snap."
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