Propellers/Handling

[Nomad Willy]

Does anyone know what type of propeller produces the least amount of* " walking " while going astern ?

*** Eric Henning

 

[FF]

The smallest diameter prop with the least pitch, and fewest blades.

Walking the stern is a very very useful part of docking, esp if going to a wall.

Just Plan it on the correct side tie-up.

To reduce the stern walk the lowest rpm SHOULD BE USED IN REVERSE ,

TO MAX THE EFFECT A QUICK BLAST OF rpm WILL JUMP A BOAT SIDEWAYS A FEW FEET .

To power a distance in reverse with a small high speed rudder simply tales a bit of speed to get the rudder working.

Barn door rudders will give control at lower speeds .

 

[Forkliftt]

FF,How can you determine the correct side tie up?
Steve

 

[Marin]

nomadwilly wrote:

Does anyone know what type of propeller produces the least amount of* " walking " while going astern ?

*** Eric Henning

Eric--

The prop shop folks we use told me that a three-bladed prop generates less prop-walk than a four-bladed prop, assuming the same diameter and pitch on each.

*

As to Forklift's question about how you determine the "correct side tie up" I'm assuming that what FF meant is that you should choose to put the boat alongside a dock facing in the direction that prop walk will move the stern of your boat into the dock.*

So if you have a boat with a left-handed prop (prop turns counterclockwise in forward when viewed from astern), you would try to dock with the*dock on the starboard side of the boat.* This way, when you shift into reverse the now-clockwise-turning prop will "walk" your stern to starboard and thus up against the dock.

Obviously your ability to do this will depend on the current, wind, other boats already at the dock, and so on.

Operators of twin engine boats have it a little easier since the props on most twins are counter-rotating.* So backing down produces no significant propwalk.* But propwalk can be created when you want it by putting one transmission in forward and the other in reverse.* With both props now turning the same direction, the effect will be a high degree of propwalk moving the stern to the side.* Which way it goes will depend on which transmission is in forward and which is in reverse.

But all the owners of single-engine boats, power and sail, that I know all try whenever possible to put the boat alongside a dock in such a way that they can use the propwalk in reverse to move the stern of the boat into the dock (unless they have a bow thruster which can be used to negate the influence of propwalk while docking).


-- Edited by Marin at 21:44, 2009-02-27

 

[Baker]

Forkliftt wrote:

FF,
How can you determine the correct side tie up?
Steve

I think what he was referring to was the side that it walks to in reverse.* That way you can pull up and stop the motin of the boat all the while alking the stern towards the dock.

*

 

[Codger2]

.....or, if you have a left handed prop, you can tie up, port to, and walk the stern away from the dock (to starboard) when departing.

 

[2bucks]

I'd say that most owners of single engine boats will tie the side of the boat to the dock that is most advantageous for entering AND leaving the mooring. If a particular dock has a current or wind problem that is known, then one might put the "wrong" side to while docking with a warm engine, help on the dock, and everybody is awake and ready to help. Because leaving at o'dark thirty, with foggy windows, trying to be quiet, and half your crew still asleep is not a good plan if you can't get away from the dock cleanly.

My boat has a left hand prop which means it tends to back to starboard. If I have a choice when pulling into a double slip I'll moor on the port side which is not the side that the stern wants to pull to in reverse. As I pull in, I have momentum and steerage to slide right up to where I want to be. And if I've done a good job, I need very little or no reverse thrust. Then, when leaving, the natural motion of the boat is to pull me off the dock and out into the fairway.

The biggest mistake I think new single engine boat owners make is not thinking in advance how they will get out of whatever situation they put themselves into.

 

[Marin]

2bucks wrote:

The biggest mistake I think new single engine boat owners make is not thinking in advance how they will get out of whatever situation they put themselves into.

I think that's a mistake a lot of boat operators make, single or twin.* I've seen twin engine operators go up to a two-sided*fuel dock and use the wind to help them get to the dock apparently not realizing the same wind will pin them against the dock so they cannot get away without a lot of effort.

Sometimes, of course, one has no choice.* We got pinned (and knew it was going to happen) at a customs dock in Ganges in BC.* Fortunately we were able to use the bow-line-and-fender technique to hold the bow in while we powered the stern out so we could back clear.* But a lot of boaters do as 2bucks describes, and think only about the docking but not the un-docking.

 

[Nomad Willy]

Great copy guys,

**** As Ken says* .. it's important to think ahead**.. and as Marin says. When one really wants to know something this topic drift really gets in the way of the mother lode. I'll restate the question more directly*** what propeller blade style produces the least prop walk? Marin, prop walk is not created with one engine in fwd and one in reverse. Oh oh. I think I just steped on my ( whatever ).* With counter rotating props,* ..* indeed there would be as much prop walk as a single screw, since both screws will be rotating the same direction.*Is that how a twin screw boat can leave a dock such that the bow and stern leave the dock at the same rate and no way is created in either*direction? The bow is levered and the stern is prop walked . *Of course one needs to know which way to turn the screws. Have I got it right ? It seems to me that I saw someone do this and I thought* .. impossible* ..* he must have thrusters. Have I got it right Marin ?** When you were pinned against the dock why couldn't you just lever the bow out* with fwd/reverse thrust? This is new ground for me as I've never operated a twin screw vessel.

**** Eric Henning

*

 

[Chris Foster]

Hi Eric -

I think FF had the right answer in his first sentence:

The smallest diameter prop with the least pitch, and fewest blades.

I'm trying to find a source to verify that, but I seem to remember that's the right answer.

 

[Keith]

If you're pinned against the dock, you can either use a spring line to swing the stern out, then back away, or put the rudder hard over, give it a big quick kick and let the rudder push the stern out. Prop walk isn't going to help you get off a wall when you're pinned there.

 

[Marin]

nomadwilly wrote:

With counter rotating props,* ..* indeed there would be as much prop walk as a single screw, since both screws will be rotating the same direction.*Is that how a twin screw boat can leave a dock such that the bow and stern leave the dock at the same rate and no way is created in either*direction? The bow is levered and the stern is prop walked .

Eric---* To your question*"is the bow levered out and stern is prop walked to make the bow and stern leave the dock at the same rate?" the short answer*is "no."

The long answer is this:

I have never seen anyone move a twin-engine, no-thruster boat directly sideways to or from a dock.* I have read numerous explanations of how this can be done IN THEORY, but I've never seen it done, and when I've tried it, even under the direction of a very experienced twin-engine, ex-USCG skipper, neither one of us could make any of the theories work.* At least not on our boat, which has a conventional semi-planing-hull-with-keel configuration with the props pretty close-in to the keel.

Putting one transmission in forward and the other in reverse causes both props to rotate in the same direction, so their prop walk will move the stern sideways.* We have 23" diameter, 4-bladed props so the prop walk force is pretty strong.**

In a conventional twin-engine configuration with both transmissions in forward*the port prop rotates counterclockwise and the starboard prop rotates clockwise when viewed from behind.* If the dock is against your starboard side and you want to move the stern out to port away from the dock, you would put the port transmission in forward and the starboard transmisison in reverse.* This will cause both props to turn counterclockwise and thus "walk" the stern to port.

BUT...... putting the port transmission in forward and the starboard transmission in reverse will also pivot the boat to starboard, or if*viewed from above, clockwise.* This will lever the bow to starboard, or into the dock in our example.* This is why, as we did at Ganges last year, my wife dangled a large fender between the starboard*bow and the dock as I moved the stern out to port*away from the dock and into the wind.

Of course, prop walk alone is not enough to walk the stern out against a wind or current.* The main force moving the stern out in this case was the fact I put the rudders hard over to starboard.* The thrust from the port prop in forward against the rudder was doing most of the work of moving the stern to port.

You could ask "Why put the starboard engine in reverse, then, since it's not contributing much to pushing the stern out?"* More about that in a moment.

With the exception of the one-in-forward-one-in-reverse prop setup, we would have used the same technique if the boat had been a single.* We'd have been using the prop's wash in forward against the hard-over rudder to move the stern to port away from the dock, and we would have used a line from the bow to the dock to prevent the boat from moving forward along the dock.** And we still would have needed the fender between the bow and the dock.

As to why we put the starboard engine in reverse to help move the stern away from the dock.......** The water*being thrust*by*a turning prop has a bit of a spiral to it, and this spiral will turn or spin the same direction as the blades.

Remember that in most installations when viewed from behind, the port prop turns counterclockwise (left-hand) and the starboard prop turns clockwise (right hand).* This means the ouboard blades of each prop are descending when the transmissions are in forward.

When the transmissions are shifted into reverse, the outboard blades of each prop are now ascending.* This causes the*spiraling movement of the water coming off the prop to*turn in and push against the side of the boat and keel forward of the prop.

With both transmissions in reverse, the sideways force of the water hiting one side of the hull is countered by the force of the water from the other prop*hitting the other side of the hull.* Same as the propwalk force*from one prop is negated by the propwalk force from the other prop.

But when you put one transmission in forward and the other in reverse, not only will both props be "propwalking" in the same direction, but the prop in reverse will be spiraling water*toward the front of the boat against the hull and keel which will push the hull in the same direction that the props are trying to "walk it."

So when moving the stern of a twin-engine boat out away from a dock on your starboard side you actually have three forces helping you do it.* 1.* With both props turning counterclockwise, their combined propwalk will move the stern to port.* 2.* With the rudder*over to starboard, the propwash from the port prop in forward will move the stern to*port.** 3.* The water spiraling forward from the starboard prop (in reverse) will push against the starboard side of the hull and keel and try to move the hull to port.* Of these three forces, the strongest one is the thrust of the port (forward) prop against the deflected rudder.*

Note that none of these forces will move the bow out from the dock.

This has all been about*what happens at a dock.* But our boat will pivot 360 degrees*quite quickly in open water simply by putting one transmission in forward and the other in reverse and leaving the rudders at zero degrees.* It does this partly*because of*propwalk and the spiraling water against the hull.* But the main force pivoting the boat is exactly*what you mentioned in your previous post--- the leverage from one prop thrusting backwards and the other prop thrusting forward.* In open water, the boat pivots around.* With a dock next to the boat, this leverage is still being generated but the dock prevents the boat from pivoting*and the bow is simply*forced against the dock.*

Of course Newton's action-reaction law means that pushing the bow against the dock will move the stern in the other direction, so this, too, will contribute to the forces moving the stern out.* So I guess there are really four forces moving the stern out, not just three.

The only way I have experienced to move our twin-engine boat sideways is not much different from what you would do to move your single-engine boat sideways.* You have to "walk" it in.* In the case of a single-engine boat you use inertia, momentum, and alternating the direction of thrust (rudder) to move the boat to the side.* In a twin, you use the same forces although you have the additional "pivoting" force from putting one transmisison in forward and one in reverse to "twist" the boat.* But this alone will not move it sideways.

As I said earlier, I've read*and tried a number of techniques to move a twin directly sideways up to a dock.* Outboard engine in forward, inboard engine in reverse, rudders toward the dock, rudders away from the dock, you* name it, none of them have worked as their proponents said they would, at least not in our boat.* The only way has been to alternate the forces acting on the boat to keep it walking over to the dock.* But there is no "set the controls like this and it will move straight sideways"* that I have found to work under actual docking conditions.

There is also the additional tool of diferential power between the engines on a twin*that can be used to affect how the boat is moving but then it starts to get complicated.* The "one-in-reverse-one-in-forward-use-the-rudders" technique is pretty basic and quickly becomes instinctive to the point where you don't even have to think about it to do it.

Of course you can get a bow thruster and all problems are solved....

Sorry for the long answer but you asked a good question and this is my understanding of the issue.



-- Edited by Marin at 04:30, 2009-03-01

 

[FF]

"indeed there would be as much prop walk as a single screw, since both screws will be rotating the same direction.

Not so ether the engines are rigged so one operates "backwards",

or a tranny setup is selected that allows one prop to spun in the opposite direction."

A 2 engined boat where both props rotate in the same direction is not efficient and not a product from a engineered builder.

 

[Forkliftt]

Marin,That is a great answer. A very accurate, well explained response. I suspect it will be searched out in years to come as the best explanation available.
Steve

 

[CamanoFolks]

Just had to throw another 2 cents into the mix.* Last summer we added wedges to our rudder.* The improvement to slow speed handling was eye opening.* Properly designed a wedge will deflect water at about a 45 degree angle w/ the rudder hard over.* For us, this means one short, hefty blast of forward w/ the rudder hard over will kick the stern out from the dock almost to the 35/40 degree angle.* A second blast will get you to the 80/90 degree point.* If done quickly the forward way is minimal.* I'm not making any claims that this will work when pinned to the dock by the wind for that we'll go back to the bow line and fender approach.* We're still learning but so far this mod is a real plus.

 

[RED]

Here is how I "walk" my boat to or away from a dock. This works for a twin, and to a much lesser degree (sometimes not) with a single. It also depends greatly on pitch.

A short, simultaneous application of forward and reverse on the two engines, and then back into neutral, will swing the boat before it begins any forward movement. A slightly longer "burst" with, say, the port forward and the stbd reverse will move the bow to stbd and slightly forward. Then reversing the application moves the stern to stbd and slightly aft.

Think of it like doing the twist. Hold your arms forward and move your body to simulate moving the throttles, and your butt will point the direction of the stern swing.

It's not fast, but I have used it in crowded situations to raft up to another boat, and if I have to "parallell park".

Alongside a dock, doing the same thing will swing the bow into the dock and move the stern out slightly. allowing me to then back out with the dockside engine. Some times it takes more than one "application" to rotate the boat enough to, for example, clear a baot moored behind me.

I'm not saying this is perfect; if I am pinned to a dock by both wind and current, I would use a breast line and a fender to get the bow in and the stern out.

 

[Nomad Willy]

Krogenguy,****
**** Iv'e heard of the wedges before and have always thought of them like thrusters* ..* if one is skillfull* ..* one shouldn't need them. By asking the question I suppose I've counterdicted myself so perhaps I should pay more attention. I've always assumed there was considerable drag, especially being in the propwash, so Iv'e not really thought about them. Among Willard boat owners there is considerable interest in articulated rudders. The results are reported to be amazing here too but my bull head still keeps telling me good boat handling skills should negate the need for any of this stuff. I'm glad you mentioned the wedges though Tom and at the moment they are at the top of my list of possibilities.
Marin,
**** Thats a lot to swallow but when I read it slowly enough it all added up into a fasinating trip through hydraulic and mechanical physics** ..** all except for the spiral part. Of course the propwash rotates downwind of the blades and in the same direction but the prop wash of both air and hydralic screws cones inward just aft of the blades. The prop wash is largely fwd or aft but lots of water is slung outward, radially off the ends of the blades.*On a*clockwise rotating screw water is slung up, down,left and right. The up/down does not move our boat laterally. The water*slung to port moves the boat to the to*stbd*because of action and equal reaction*and the water slung to stbd moves the boat to port but it's not*equal because of the drag of the water moving laterally across the hull directly above and to*stbd of the propeller. The*friction of the water on the hull wants to move the boat to stbd and the action/reaction of the water being slung to stbd wants to move the boat to port*but the ability of the stbd moving water is impeeded or reduced by friction caused by the hull bottom. This reduces the velociticy of the water slung to stbd so the total force availible to move the stern to stbd is greater than to port hence* .. a left hand propeller rotating* clockwise in reverse will sling more water to port causing the stern to move to stbd. *The water being thurst*fwd by the reversing*propeller causes friction*that will ( correct me if I'm wrong )* amplify*prop walk. The spiraling water will tend to be moving outboard and the resultant friction will ( to some degree ) *move the boat to stbd, increasing the tendency to prop walk to stbd. What I think I learn from you Marin, is that the spiraling water has a lateral component and when it's in contact with the hull it will cause enough friction to cause some lateral movement.
**** Why have the outboard sides of the propellers decending? It would seem to me if one reversed that custom leveating off the side of a float would be a reality* .. not just possible in theory. There must be a very good reason as the ability to move a boat sideways would be a very positive side effect of twin screws.

**** Eric Henning

*

 

[CamanoFolks]

The wedges add about one inch of width on each side of the trailing edge of the rudder.* I doubt that they add much drag.* A secondary effect is much less auto pilot action to maintain a straight course.* Before you get too excited about articulating rudders check out the effect of backing with one of those hard over.

 

[Tidahapah]

When you are talking about wedges on your rudder are they the similar to an a fish tail trailing edge.
After a lot of experience with Schilling articulated and fish tail rudders I fitted a piece of angle bar to the trailing edge of my rudder when I fabricated it some 15 years ago.
Increasing the angle of turn of your rudder also helps with this set up as it directs the rudder wash (flow) at a more perpendicular direction to the hull hence more side thrust, reacts better with short bursts.
Overflow and underflow plates also assist in this thrust concentration.
I have attached a picture to explain what I am talking about.(will attach photos when I have better link)
Benn

 

[Tidahapah]

Picture of rudder with fishtale and over & underflow plates

 

[Keith]

I've got the drawings to do this on my Krogen, but haven't yet. One question occured to me... to any of you who have this setup, does it push the rudder around in reverse? I know that if I get too much way on in reverse, the rudder will be pinned hard over and I have to stop or slow down to move it again. Does the fishtail rudder make this worse, or have any other negative effect in reverse?

 

[CamanoFolks]

Keith,
I haven't seen any but then I've not backed a long ways.* Just into a slip and the usual hop/skip/jump approach seems to still work well.* Wheel hard to strb/back slow/quick blast fwd & continue.

 

[RickB]

Hi Eric,

Gotta make a couple of observations on your comment* below:

"... but the prop wash of both air and hydralic screws cones inward just aft of the blades. The prop wash is largely fwd or aft but lots of water is slung outward, radially off the ends of the blades."

The wash doesn't cone inward. Due to what is called "tip rolloff" a vortice is created by chordwise flow rolling off the tip and that flow moves the wake back toward the hub but only by a few percent of the blade span and then it quickly stabilizes in less than a blade span to form a cylinder. That is what you see in all those neat pictures of the spiralling*trail of bubbles behind a propeller in a test tank.**That cylindrical*flow*spreads out as it leaves the influence of the hull wake.

There is very little*(virtually no)*spanwise flow on a marine propeller. What little there is is very localized, mostly*in the root, and is reduced to zero at the blade tip. That is what creates the tip vortice you see in all the pictures.**It moves inward as I described above, not outward at all so there is no water slung radially outward from the blades to cause any effect at all. There are pressure pulses as a blade approaches the hull, that is what causes noise and vibration when the*blade tips are too close to the hull but there is no outward flow.

 

[Nomad Willy]

Morning Rick,

*** I agree with a lot of what you said but if there is pitch there must be spanwise flow. The " tip rolloff " you mention must be a result of the spanwise flow that causes the tip vortice. There are many very wide blade tips on many commercial propellers that leads me to agree with you about the spanwise flow being small. I'm buying into that to the extent that I'm leaning closer to a propeller with large circular blades. I belive they are smoother and have better backing manners. The inward cone prop wash recolection may have come*from my aviation past. So you may have caught me posting a falsie* .. shame on me.

Eric Henning

 

[Marin]

nomadwilly wrote:

*

all except for the spiral part.

Eric---

The bit about the spiraling propwash and its effect on a hull I got from an MIT study, some of which was quoted on the Grand Banks Owners forum a number of years ago. It was the same study that also laid to rest the business about whether a freewheeling prop has more or less drag then a locked prop.

All I remember about the spiraling propwash explanation is that the movement of the "column" of water being thrust back from a prop has an overall twist to it that is imparted by the rotating action of the prop.* I do not recall any discussion about what happens to the water right at the prop blades, only that the*"column" of*water being moved back by the prop has a slight spiraling motion.

When*a boat is moving through the water*at whatever speed the prop is going to move it, there is probably*not much, or any, "column" of water coming off of the prop.* But when a boat is*more or less stationary, as it is at a dock, the props will move water*quite a distance behind or in front of it depending on which way they're turning.** For example, when we run our boat in the slip with the transmissions in gear, even at only 1,000 rpm, the prop in forward moves a lot of water all*the way across the fairway to the slips across from us.

If winter winds conspire with my work schedule to prevent us from taking the boat out for four weeks, we will run the boat in the slip for about half an hour.* We put the port transmission in reverse and the starboard tansmission in forward and run the engines up to about 1,000 rpm to put a load on them so they'll get up to operating temperature.* With one in forward and one in reverse there is mot much load on the docklines.*

If I stand on deck and look down at the port side of the*hull at the waterline, I can see the water that's being moved*toward the*front of the boat*"piling up"*against the hull and then*"ricochetting" (sp?) off forward*at a very*shallow angle.* So there is obviously some sideways force being generated there.* And since our boat has a relatively prominent keel for this type of boat, I presume the same thing is happening with water being spiraled into the side of the keel ahead of the prop.

I'm no physicist or hydrodynamics expert, so all I can do is repeat what I have read about this spriraling water business.* But I've read about it in enough places that so far I have given the idea credibility.

Of all forces acting to move the boat sideways with one prop in forward and one in reverse, the spiralling water effect is undoubtecly the weakest, even with the boat relatively stationary as during a docking or undocking.* But it seems to be there, at least from what I've observed by watching the water against our hull.

As I said, I believe this is a force that's*present only when the hull is relatively stationary. The props are moving water but the hull hasn't been accellerated yet*to match the thrust being generated by the props.* Once the hull starts to move, I suspect this spiraling force rapidly diminishes since the props are now moving the boat, not moving the water.

*


-- Edited by Marin at 15:11, 2009-03-03