Fun with prop-walk :-)

[Sven]

I've been reading the rudder mod thread with some interest.

When I first saw Lew's post about Serendipity I sought out the spec sheets and admired the line drawings. I really liked the modified keel and skeg-hung rudder ... cruising course-keeping and kindly motion at sea coupled with the need for performance.

I also looked at the prop location and direction and the rudder and thought that is all good as long as you are going forward. In reverse I could not imagine how the design would work until you built up a head of steam.

During our post sale sail (the courtesy run with Lew, ever the gentleman) I spent a lot of time obsessing on going backwards to buoys. After many attempts to back up to a buoy I finally got close enough (not under directional control) to just declare victory.

Since then Nancy and I have spent more time backing up to buoys (right in front of the CG station !!) over and over again and I think I'm ready to admit defeat

I can not control our beautiful E39-B in reverse until we are moving.

Knowing that limitation is part of the solution.

The final straw was when we finally went to the fuel dock to fill up (Lew didn't want to do that earlier because we'd just end up with aging fuel) as we hoped the weather would clear and if nothing else we could power up to Catalina or Santa Barbara Island about 70-100 NM away. The lady does what she wants to in reverse ... as expected from the drawings Pam King sent us !!!

We put enough fuel in the tank to get us back even if we had to power both ways.

To make sure we were in some state of control I set up he spring lines to let us restrict where we would go and make sure we wouldn't go in the opposite direction. I moved the fenders in anticipation of needing them further aft as the spring lines would pull us around.

Amazing, the spring lines never even got tight ! The stern just walked to port as I was backing up. Fortunately, the fender was far enough aft to keep the hull from hitting the corner of the dock.

Lesson learned. Lines and spring lines will always be in place from now on because this lady will go where she wants to when backing up.

Would a bigger/deeper rudder help ? Maybe, so we're watching the thread on rudder upgrades.

Did any of you read the WWII story about the tug (mine dropper ?) where the essence of the whole story was the prop walk and how that lead to the dropped depth charge ? I appreciate that novel so much more right now.

Happy New Year to all and thanks for all the help.


-Sven

 

[rwthomas1]

I don't think that what you are experiencing is anything new. Fin keel/spade rudder boats all prop walk, especially with fixed props, AFAIK. My E38 is the same way. No directional control until the boat gets some headway. The technique I use is to keep the rudder centered, put it in reverse, give a good quick blast of throttle and then take it out of gear. With a bit of headway and in neutral the boat responds to helm and no propwalk. It can be a handful. RT

 

[treilley]

Rob's technique is spot on. I do the same with my 35 and use the prop walk to my advantage when needed.

FYI, my reverse performance got much better when I replaced my original fixed prop with a Flex o Fold 2 blade.

 

[Loren Beach]

Back 'n' Forth...

I don't think that what you are experiencing is anything new. Fin keel/spade rudder boats all prop walk, especially with fixed props, AFAIK. My E38 is the same way. No directional control until the boat gets some headway. The technique I use is to keep the rudder centered, put it in reverse, give a good quick blast of throttle and then take it out of gear. With a bit of headway and in neutral the boat responds to helm and no propwalk. It can be a handful. RT

What RT sez!
Plan B is to get more initial "straight line thrust" in reverse by changing over to a feathering prop. We did that in '95 and still enjoy the better handling in tight docking situations. As Rob points out, there is still a need to get some sort of movement backwards, relative to the water, in order for the rudder to have any effect at all.
To add spice to the whole process, we moor on a river with current from ahead, so that it takes even more (!) reversing before the rudder takes effect...

Oh well, if "yachting" were too easy, just anyone could do it!

Loren

 

[CaptDan]

Back & Fill

What RT sez!


Oh well, if "yachting" were too easy, just anyone could do it!

Loren

And they do. A summer afternoon jaunt on Lake Washington - speedboats flying from all directions, each piloted by a unit with his/her own concepts of road rules - will convince anyone that's true.

There's no free lunch with these boats, but there are plenty of tricks to make backing up somewhat controllable;

1. Picking a berth with a cooperative prevailing wind, or no wind at all.

2. Choosing the right prop with a better 'bite' when backing down.

3. Planning ahead when approaching an unknown landing; planning even further for leaving it.

4. Using the prop walk to 'kick the stern; it can be a beautiful thing.

Finniky reverse behavior is common to most all single screw vessels; virtually every one I've ever helmed requires some deft terpsichory; engine bursts, rudder maneuvers, skillfully applied shifts, spring lines for unfavorable currents, and the ever faithful 'bump the wharf with fenders and pray' technique.

That's my take; and the two-cents worth of advice ya pay for.

Capt Dan G>E35II "Kunu"

 

[Martin King]

There is a guy on my gangway who has another popular make of
boat that is able to back out of his slip with ease, turn the boat
90 and continue backing out past me and all the way down
the fairway to the inner harbor with ease! Unfortunately
I have yet to sail an Ericson that can do that, but you can make
improvements. Regarding the 39, a deeper rudder will help, but the
biggest improvement will come from a 3 blade feathering prop such
as a Max. My 39 originally came with a tiny 2 blade fixed prop that
reminded me of a bow tie. Enough to push the boat out to the
starting line but backing down took all day, and backing up?
A nightmare in tight quarters! Talk about prop walk, the boat
would spin 90 degrees or more before deciding where she was
going to go next. Sometimes left, sometimes straight, and if
there was any wind who knew? Since her racing days were over,
I went to a fixed three blade with more bite, and that improved
things somewhat and so did the larger, deeper rudder. But the
biggest improvement came from the Max prop. Hands down,
the best bang for the buck came from that piece of gear. Backing
down and backing up were vastly improved as well as increased
speeds under sail with the reduced drag.

Martin

 

[SASSY]

Prop walk

A three blade prop is the least expensive way to eliminate prop walk. My wife and I are cruising on our 87 Ericson 34 with our three month old baby so control, and manouverability outweighed the increased drag asociated with a three blade prop. Now reverse is a cake walk.

 

[Sven]

Thanks for all the answers.

We have a fixed 3-blade currently. I was surprised at Martin's observation about the feathering Maxprop being the easiest. Since he knows exactly what I'm talking about with the E39/E39B underbody I'll accept that he is right even if I don't pretend to grok the hydrodynamics that make it better than a fixed one.

A trick that I've been taught (but never tried) when starting to back up at dockside is to put the transmission in reverse while still tied up, until there is enough of a flow past the rudder from prop suction. I'd be interested in trying it but assume it works since it was S.O.P. for a charter outfit up the coast. It might be essentially the same effect as briefly gunning in reverse and then going into neutral, setting up a flow but without much headway.

The observation about always having an exit or escape plan when pulling into tight quarters is paranoia at its most useful.

Thanks again,


-Sven

 

[Ray Rhode]

Sven,

I also use the blast of reverse and then neutral to get out of my slip. It really works well. Prop walk can be a good thing if you plan for it. Twice when pinned at the fuel dock I have been able to turn Journey in about 1 1/2 to 2 boat lengths much to the shock then relief of the crews of the other boats.

Ray Rhode
S/V Journey
E35-III, #189

 

[Loren Beach]

Hydrodynamic considerations

The reason that the feathering props are a lot better in reverse is that the blades rotate and the leading edge of each blade is "facing" the other direction. I figure that like any other underwater "foil" that you have -- keel or rudder -- the efficiency drops radically if you run the flow over it in reverse.

We noticed a major difference in reverse thrust the minute that we changed from the original two blade fixed to a two blade featherer. I just wish I could have afforded to go with a three blade... everyone I have spoken to in the industry tells me that they are quite a bit quieter due to better balance and produce less vibration.

Still, the reverse thrust in tight moorage situations and the increased speed in light air under sail make any feathering prop a great addition, IMO.

Loren
(Martec bronze two blade featherer, since 1995)

 

[Martin King]

I was surprised at Martin's observation about the feathering Maxprop being the easiest. Since he knows exactly what I'm talking about with the E39/E39B underbody I'll accept that he is right even if I don't pretend to grok the hydrodynamics that make it better than a fixed one.

Sven,

I don't pretend to know about the physics involved-whole books
have been written on the art and science of prop selection. I do know
that a geared feathering prop has a big efficiency advantage. Because the blades are free to rotate around their axes, the leading edges present correctly no matter which way the shaft is turning. A fixed blade is
leading with the trailing edge when being driven backwards and will
deliver less thrust for a given blade area.

Martin

 

[Lew Decker]

Backing down was always an adventure on Serendipity. I don't have any secrets.

My first slip was a narrow affair positioned so that I had to back against a quartering wind from astern. I first learned to swear on a golf course when I was young. That vocabulary served me well backing out of the slip. I had to turn to port to escape the channel but Serendipity always turned to starboard, aided by the ill wind that blew her coo coo. I kicked up a lot of mud trying to get her bow pointed in the right direction. - A miracle happened, though, and I moved to the best slip in SD County. End of the Backdown Blues.

On my current yacht, I don't have the same kind of adventure. No motor. No reverse. No problemo.

 

[Steve]

Speed = Control

We run the OEM two blade. At the risk of repeating others, I found the best way to back down is not to be timid, but rather (controlled) aggressive.

Start far enough away from your intended target, overcome the the prop walk with speed, use more speed to over come current and wind, and just go for it, shift into neutral when your comfortable, once near, common sense kicks in as far as placing into forward to slow and control your stop.

I think we have all seen the occasional weekend sailing family try and back into a dock or slip by getting close and just tapping the throttle in reverse only to be taken by the wind and current, typically the bow comes around to some embarrassing position ... with the crew all showing that clueless look of, "how did that happened?"..

Steve E35-3

 

[Sven]

The reason that the feathering props are a lot better in reverse is that the blades rotate and the leading edge of each blade is "facing" the other direction. I figure that like any other underwater "foil" that you have -- keel or rudder -- the efficiency drops radically if you run the flow over it in reverse.

I do know that a geared feathering prop has a big efficiency advantage.

Of course, thanks ! The efficiency must be exactly what makes the difference. The prop serves as an egg-beater or paddle-wheel in either direction but with a feathering prop's increased efficiency in reverse it has proportionally more propulsive effect relative to the walk-inducing egg-beating.

Backing down was always an adventure on Serendipity. I don't have any secrets.

Fortunately she is sailing forward most of the time and then she makes up for wanting to change her mind whenever she is going backwards




-Sven

 

[Seth]

Most, but not all

Of course all of the suggestions here are good-but there are some modern boats which though a combination of underwater shapes and prop selection, are perfect backer-uppers..But so what? We have to live with our boats, and as has been said, this is manageable with the proper techniques..Happy New Year!

 

[MarkA]

Of course all of the suggestions here are good-but there are some modern boats which though a combination of underwater shapes and prop selection, are perfect backer-uppers..But so what? We have to live with our boats, and as has been said, this is manageable with the proper techniques..Happy New Year!

My 1978 34-T seems to be one of those. She backs out of the slip and around the corner under constant power while tracking straight. The prop walk is negligible, and I rarely have to play throttle tricks. She has a fixed three-bladed prop that might be oversized. I'd like a feathering 2-blader to reduce drag.