[Dianamate]

I recently purchased a 1991 Pacific Seacraft Crealock 37. We love her but find that backing her out of our tight slip can be daunting as she insists on going to port while in reverse in any kind of condition. I have read blogs herein that suggest getting enough speed up, then putting the engine into neutral before attempting to steer, but that is not always practical. I am fending off pilings to port well before gathering any speed. The three blade Campbell Sailor fixed bronze propeller probably came with the boat. I have read magazine claims about folding props significantly reducing prop walk. Are they believable? They are a significant investment, so can anyone who might have switched to such prop designs on a similarly keeled boat share their experience and make a recommendation? They are not cheap, so I look forward to hearing from you.

 

[brokesailor]

I have a maxi prop on my Crealock 37. At idle speeds in reverse coming out of the slip I have no noticeable prop walk. When I come into the slip and gun it to stop the boat in reverse is a different story.

 

[MikeOReilly]

First off, as a fellow full keeler (not quite in your case, but close enough), I'd say prop walk is your friend. You can use it to maneuver in tight places that would be difficult with just rudder control. So don't be too quick to abandon it.

Can you go slow? You might be able to reduce the walk by moving in very slow speed backwards. How about the use of dock lines to hold line? When we leave our dock we usually have one person walk our boat backwards and then hold the bow while we're gathering speed in reverse. You might not have those same options, but it's worth studying your arrangement to see what could work.

For my money, prop walk is a blessing, not a curse.

 

[Dianamate]

Thank you both for two different insights.
Guess I need more time on the boat to see if I can get to be thankful for prop walk. Having raced J-Boats for years with spade rudders and straight keels, It's quite an adjustment.

 

[gptyk]

I use a long springline (2x the length of the boat). I back out, holding straight then turning to starboard with the line, then pull it back into the boat (it's looped over a cleat on the dock) - Easy and stable always. Can be done singlehanded. Line is cheap poly stuff from the hardware store so it floats.

My boat is an old C30 that won't back to starboard at all.

 

[Jaramaz]

prop walk is an effect of prop design + prop position. The latter is difficulöt to change, the former easy - but may be expensive.

Generally there are only some few dimensions to play with when getting a new prop - typically diameter, pitch and number of blades.
One way of decreasing prop walk is to go down in pitch, which usually results in increasing the diameter, which in its turn can be compensated by increasing the number of blades.

As you already have a 3-blade the possibility to increase the number of blades is more or less exhausted.

Modern props usually has a better blade design, resulting in less slip.

Folding props are not better than fixed, but very often one gets a more modern prop design when buying a folding prop.

So, what can you do?

1) make note of the dimensions you have today. either measure or see if there are any markings on the prop (usually are). Without knowing you probably have something like 16 x 15 (dia x pitch, all in inches)
2) measure the space for the prop - is it possible to increase the diam with ½" or 1"? - note that there should be ample distance between hull and prop tip.
3) if it is possible to put on a larger prop then you can have a look on any of the well-known prop manufacturers. They usually have a prop "design" or selection guide - go for higher dia, and then correspondingly less pitch.
4) Modern blade design may also help, even with the same dimensions. In that case I propse you get in touch with a retailer who offers to take back the prop if it doesn't work as intended - in that case they usually wants to design it as well.


Finally, is prop walk good or bad?
Prop walk is good when manoeuvring in a harbour with speed. As when entering a slip, with a good prop walk it is possible to park the boat as if it was a car - actually even better.
But in all other situations prop walk is bad.
With no, or little, speed, prop walk will move the boat in a way that can be difficult to handle.
Prop walk also lowers top speed, or in other words decrease efficiency / increase fuel consumption.

Myself, when I have had pronounced prop walk ... I have learned to live with it. But when buying a new prop, I have taken care to lower the prop walk.

/J

 

[capecodda]

I've got a Maxprop on the current boat with a similar bottom configuration to your boat. At least for me, it walks in reverse like crazy.

I do seem to be able to counter it but using relatively high reverse thrust and starting with the wheel hard over opposite the walk direction. As soon as the rudder "catches", we rapidly center the wheel.

I don't know if this is universally true or not, but any of the boats I've owned or sailed with the prop in an aperture seem to walk, and boats with big spade rudders don't.

BTW/really like your boat!

 

[PorFin]

Dianamate,

I feel your pain -- our baby has a full keel and barn door rudder, so she walks a lot.

After getting the advice to go with short, powerful bursts when moving astern, our lives have improved. That said, we still aren't gonna be trying any slalom runs in reverse.

For what it's worth, the Campbell Sailor is supposed to be the cat's meow for fixed 3-bladed props. I'm going to replace our current 3-bladed prop (of undetermined heritage) with a Campbell eventually.

 

[Jaramaz]

I do seem to be able to counter it but using relatively high reverse thrust and starting with the wheel hard over opposite the walk direction. As soon as the rudder "catches", we rapidly center the wheel.

This, and/or the "short bursts" are the usual ways to handle prop walk.

I don't know if this is universally true or not, but any of the boats I've owned or sailed with the prop in an aperture seem to walk, and boats with big spade rudders don't.

Nope.

It is the propellor causing prop walk.
The amount of prop walk depends on two (2) major factors:

a) how deep the prop is places
b) how twisted the prop is (pitch).

(a) is easy to understand. If the prop is close to water surface, then the side pressure on the prop (or if you prefer, the pressure caused by the prop) is "much" lower on the upper half of the prop circle than on the lower half.
If the prop is, say, 1.5 m below the water surface these pressures are about the same.

(b) is easy as well: the more twisted blades the more surface will be turned perpendicular to the boats lenth, causing the mentioned pressure.

A long keeled boat is slower to turn. In particular at low speeds. Then, with a working prop there is a force at the end of the boat pushing sideways ... yes, result is clear.
If there is a barn-door rudder it is to compensate for the long keel, at low speed it will of course not help much.
Most long keeled boats are heavy as well. => not so easy to accelerate.

Often, the engine in a long keeled boat is relativity high, engine is mounted above the "knee" (it may of course vary).

Just chaning prop will not improve things if you do not see to that it has some other dimensions than the prop you have today: lower pitch, better blade design.

/J

 

[Sailormon6]

The problem you're describing is one of the main reasons why I back into my slip, rather than pulling in bow-first. I usually singlehand my 35' boat, and, when I leave the slip, I use the docklines to pull it to windward, so it has a little space to move to the left without hitting the portside pilings. Then I put it in forward gear and give it a fairly strong shot of throttle for about 1 1/2 to 2 seconds, and then shift into neutral. That gets the boat moving forward slowly, but, shifting it into neutral stops the prop walk effect. That allows it to drift straight out of the slip. As soon as I am able to clear the pilings, I shift into forward gear and steer the boat into the fairway. If the fairway is so narrow that the boat won't make the turn, then I use the prop walk effect to make the boat turn by backing and filling.

The prop walk effect, pulling the stern to the left, is much more pronounced in reverse than in forward. Thus, it is much less of a factor when pulling out bow-first than it is when backing out.

The problem you have in maneuvering a single prop boat is that it doesn't have steerageway until it reaches a certain minimal speed. Thus, when starting from a dead stop, the rudder is ineffective to steer the boat until it gains steerageway. Unless the boat has a bow thruster, the only way you can make a tight turn in those slow moving situations is by using prop walk. Mike said "prop walk is your friend." I couldn't agree more. Learn how to use it when it can help you, and how to avoid or minimize it's effect when you don't need it.

 

[Rockter]

Diana :

Same problem on my Polaris 36.
Really, they do not respond to the helm when going astern until you have built up so much speed that it can get more than a bit frightening.
Also, when going astern, the rudder gets pulled hard to whatever side you are steering.
Be careful going in to shallows when going astern... it is a good way to tear your rudder off.

If your 3 blade prop moves the boat well, I really would not change it for that reason. The designer typically put a lot of thought in to choosing it.

Is there any way to use warps from shore to help the exit from the slip?

 

[sailak]

I too have a Crealock 37. While prop walk will limit how acutely you can back to starboard it isn't impossible with a little practice.

I use as little throttle as possible. A "shot of throttle" to get the boat moving then back to idle. Once the boat is moving shifting to neutral eliminates the prop walk. Shoving the stern out as you board also helps to get the boat headed in the direction you want. Once out of the slip you can "back and fill" to pivot in place and complete the turn.

I back into my slip most of the time and the prop is very helpful in that case. I dock starboard side to with the slip to port as I come down the fairway. About one slip width away I select reverse, idle power and it slows the boat and slews the stern to port. As the boat nears begin parallel with the slip prop walk helps "slide" it sideways to lineup with the slip, then ease back until the Admiral can grab a cleat.

I spend such a small percentage of my time in reverse dealing with prop walk that it is more than offset by the joy of sailing my Crealock.

 

[bobperry]

Your prop is in an aperture. You will have prop walk. End of story. It has nothing to do with the prop and everything to do with the aperture which sort of acts like a "Kort nozzle" directing the flow off the prop. Boats with nothing hanging down aft and a nice beg spade rudder back beautifully. My Valiant 40 backs great, like a car, but it has no aperture. I think going to a Max prop can help but it will not eliminate the problem.

 

[aa3jy]

Your prop is in an aperture. You will have prop walk. End of story. It has nothing to do with the prop and everything to do with the aperture which sort of acts like a "Kort nozzle" directing the flow off the prop.

With all do respect to Bob Perry..but as a former owner of an Allmand vessel with 'no' aperture..it backed down liked s&!$ not without incurring prop walk..

 

[bobperry]

aa:
I can believe that. Your boat has a skeg, right? My Esprit 37 had a skeg and you could hardly tell the difference in design geometry, i.e. skeg, rudder and prop from the V-40. But while the V-40 was perfect in reverse the Esprit was not. It was not awful but it was clearly not as docile as the V-40. I never figured out why that was. Remains a mystery. Note that I said in my post "nothing hanging down aft except a big spade rudder", gives the best results for god backing control.

 

[capecodda]

Don't want to cause I fight, but I'm honestly curious as to why.

Bob's statement that props in apertures walk certainly fully aligns with my direct experience. I've had 3 boats with with props in apertures, and they walked like crazy, 2 with spade rudders and they backed up like they were on rails. This experience extends to driving dozens of friends boats and charters.

On the other hand, I thought at least logically that Jamaraz had a reasonable explanation based on how close the prop is to the surface, and prop pitch. Maybe there would be more pressure down deep, more bite, and certainly the sort of paddle wheel effect would be enhanced by increasing pitch (if you took pitch to the extreme, your prop would look like a paddle wheel and push the boat sideways?).

I've heard other people say that it depends on shaft angle.

I've heard other people say that the blade that's closest to the hull behave's differently...sort of like a ground effect in flying.


What is the actual mechanism, and can you predict which boats will exhibit it in general?

Again, please just intellectual curiosity from a nerd sailor

 

[Jaramaz]

Well, Bob is a boat designer, so he designs the apertures.

Of course he then thinks that apertures are the answer.

But ... it is only a part of the answer. There are many factors influencing prop walk, especially at (very) low speed.

We have already discussed the propeller, and its position wrt water surface.
What about prop apertures then?
First, not all have an aperture, more like the prop is working rather free - hull is usually not far away but bends nicely upwards.
Second, if there is a real aperture then that means flows are really restricted. Now it becomes important how restricted the flows on the upside and downside are, never the same that is for sure. It is difficult to generalize here, as it depends on the size and shape of aperture, prop position in the aperture and so on. Tempting to say that lower part of prop circle usually gets better grip than upper ... but, no there are always exemptions.

It is however clear that prop walk is a result of the blade area exposed along the boat (and rotating perpendicular). A prop must have such, after all it is a screw.
It is possible to play around with some of the few "dimensioning" parameters available (sometimes called design space).
With a pronounced aperture it is possible to decrease prop diameter, either by increasing pitch or increasing number of blades. (or combine). only possible within limits, and depends on other parameters as well.

A long keeled boat is (nearly always) difficult to reverse, in particular when starting. As mentioned earlier they are often heavy, rudder is often pushed & tilted forward, prop flow doesn't hit the rudder ... the list is long.
Even if one may reduce prop walk in itself, it doesn't change the major issue - boat is low in turning at low speed, especially reversing.

/J

 

[Stumble]

One of the major causes of prop walk has been missing. Keep in mind that prop walk is caused by an imbalance of forces acting thru the prop as it rotates. The most severe of the forces is caused by the angle of the drive shaft relative to the water surface. A higher angle will result in more walk than a more parallel angle. This is because of the extra cylindrical distance the up rotating blade has to travel.

So a perfectly parallel shaft acts to decrease prop imbalances and thus to eliminates the forces that cause prop walk.

My guess is that an aperture increases this because of the outflow water direction. The downward rotating blade (the one with less thrust) hits the bottom of the aperture and sheds water mostly downward. While the upward turning blade sheds water onto the hull, which gives a plate effect and forces water to the side. The reason that this effect is worse on a boat with an aperture is that normally the prop would shed water evenly throughout its rotation (with the exception of the upward blade hitting the boat bottom).

So the ideal would be a prop on a parallel (to water surface) drive shaft located as far from any obstruction as possible (including the hull). The further you get from this the worse the effect will be. I should also point out that a nozzle will mostly eliminate prop walk, but thru different methods. Specifically by evening out the imbalance around the prop.

 

[bobperry]

Try this experiment at the dock if you have your prop in an aperture. Put you engine in gear and add some power. Now dangle a line over the stern down to near JUST NEAR where you aperture is. Watch what happens.

But I'm out of this debate. Too many experts and too few answers.
For the record almost all props are installed with a tip clearance from 10% of dia to 15%. This is primarily done to reduce vibration caused by prop wash.The geometry of most installations prevents great tip clearance than 15%.

 

[Barquito]

I'm really suprised there is not more consensus on the hydrodymamics of this problem. Seems like it would be a lot easier to do observations on the flow of water around the bottom of a boat than it would around the sails in the air (not that we all know how that works). Come on scientists, we are looking at you!