Topics

[Amel Yacht Owners] Re: Flexofold Prop


Patrick McAneny
 

Bill, I appreciate your assessment , and I am sure you are correct in that the small difference between most of the props would not add up to much.
Thanks,
Pat


-----Original Message-----
From: greatketch@... [amelyachtowners]
To: amelyachtowners
Sent: Sun, Nov 26, 2017 12:42 am
Subject: [Amel Yacht Owners] Re: Flexofold Prop

 
Pat,

I have limited experience with Flexofold props.  They were on saildrives on a catamaran in a fleet I maintained.  They worked... but had a very curious tendency to fall off!  We could never figure out why.  I think it was a saildrive thing rather than prop related, but when the owner changed to fixed props, they never fell off again....

 Like all folding props they had poor response in reverse.  They needed a LOT of RPM to really unfold and give decent thrust.  I don't think it would be an issue on an Amel, you would just give more throttle than you are used to, but when trying to prop steer a cat, it gave a very unequal response between the forward and reverse props that took a lot of getting used to

If I was you, I wouldn't sharpen my pencil too much in the analysis of the differences between various folding/feathering props.  In the real world the very minor differences between them can vary quite a bit between different boats. By way of example, when Yachting Monthly did their testing they said that a MaxProp had MORE prop walk than a fixed prop.  One the one boat I drove with a fixed and then a maxprop and that was absolutely NOT true.

The drag differences are so small they are truly trivial, and hiding partially behind the keel like they do on an Amel, that difference would be even smaller. The other parameters vary a lot from one installation to another.  Not suggesting that there is anything wrong with your choice just that it would be hard to make a BAD choice--they are all going to be WAY better sailing than a fixed prop.

My 2 cents worth...

Bill Kinney
SM160, Harmonie
Fort Lauderdale, FL



Patrick McAneny
 

Dean, The only reason that I am considering to go with Flexofold is the test results conducted by Yachting Monthly . You can find a link on Flexofold website . Overall they surpassed every other prop in the majority of areas tested. Only in backward thrust did they end up in the middle of the pack. Forward thrust which is far more important , they were top rated. The drag is so low it could not be measured , but that would be true for all folding props and nearly true of all feathering props as well. I am going to see if I can find any other independent  comparisons of props , but check it out and draw your own conclusion .
Pat
SM#123


-----Original Message-----
From: trifin@... [amelyachtowners]
To: amelyachtowners
Sent: Sun, Nov 26, 2017 12:26 pm
Subject: [Amel Yacht Owners] Re: Flexofold Prop

 
Interesting thread. 

I'm also changing out my fixed prop, but was leaning towards the Autoprop which was specified by Amel on other 54's.  Was there any compelling reason you chose flexofold over the Autoprop Patrick?

The Autoprop is priced around 2700GBP+VAT. Not sure how that compares to the flexofold?

Cheers
Dean
SY Stella
Amel 54 #154


ngtnewington Newington
 

Hi I have an Amel 54 with Brunton Autoprop. I am happy with it and have experience of Maxprops, and Featherstream as well as fixed props. I also read the prop comparison article. My view is that the test showed the Autoprop would achieve 6 knots at 2100 rpm verses 2500 for the others. Looking at my own boat fuel consumption stats, it shoots up steeply with rpm. The difference between 2100 and 2500 is massive. Also it is better for the engine to be more highly loaded at lower rpm’s. Furthermore lower rpm is less noise and wear. 
Nick SY Amelia 


On 26 Nov 2017, at 22:57, Patrick Mcaneny sailw32@... [amelyachtowners] <amelyachtowners@...> wrote:

 

Dean, The only reason that I am considering to go with Flexofold is the test results conducted by Yachting Monthly . You can find a link on Flexofold website . Overall they surpassed every other prop in the majority of areas tested. Only in backward thrust did they end up in the middle of the pack. Forward thrust which is far more important , they were top rated. The drag is so low it could not be measured , but that would be true for all folding props and nearly true of all feathering props as well. I am going to see if I can find any other independent  comparisons of props , but check it out and draw your own conclusion .
Pat
SM#123


-----Original Message-----
From: trifin@... [amelyachtowners] <amelyachtowners@...>
To: amelyachtowners <amelyachtowners@...>
Sent: Sun, Nov 26, 2017 12:26 pm
Subject: [Amel Yacht Owners] Re: Flexofold Prop

 
Interesting thread. 

I'm also changing out my fixed prop, but was leaning towards the Autoprop which was specified by Amel on other 54's.  Was there any compelling reason you chose flexofold over the Autoprop Patrick?

The Autoprop is priced around 2700GBP+VAT. Not sure how that compares to the flexofold?

Cheers
Dean
SY Stella
Amel 54 #154


greatketch@...
 

I read the test report in YM, and like most of their work, it is a useful and well thought out piece of work.  

But (you knew that was coming, right?) they use bollard pull at full throttle as the primary measure of the prop's performance. I certainly understand the appeal of the measurement.  It is easy to measure with precision.  It is repeatable. It eliminates a lot of other variables. It is easy to report and rank. It has a lot going for it.  

But... it is a not a good measure of the real world performance of the prop in use (unless you are a tugboat).  For example, slight differences in pitch can result in fairly significant difference in engine RPM at full throttle.  If the engine is running at a different RPM it is producing different horsepower, resulting numbers would be skewed quit a bit in ways that have nothing to do with the design of the prop.  Our props typically run at about 30% slip, this test measures the thrust at 100% slip, a very different flow regime.

I especially think that measuring "side pull" at full throttle while tied to the dock  is a very poor stand-in for prop walk, to the point of being almost worthless.  Especially when you consider that props don't really produce side thrust on their own, it is the interaction of the slip stream with the hull that makes the boat go sideways.  A prop that had low "slip" (that's good) when the boat was moving would come out much poorer in ranking on this test than a prop with higher "slip" even though it would produce less prop walk when actually moving the boat.

It's easy to throw rocks at a published test like this and really, really hard to come up with a simple measure of prop performance. It is almost as bad as testing anchors! To be fair to YM they do acknowledge the limitations of the testing at the end of the article.

My take-home story is that all feathering and folding props have so much lower drag than a fixed prop that whatever is left is totally insignificant on a cruising boat.  On our Amels there is no reason to discriminate between them based on this measurement.

Performance under power is really, really hard to measure in a way that you can actually compare apples to apples.  The interactions between engine, transmission, mounting geometry, gear ratios, etc, etc. will totally overwhelm the differences from one prop to another. Then there is the question of what performance measure is really important. Do you care about maximum speed?  Highest speed at cruising RPM?  Match of prop power curve to engine power output?  Maneuvering?  Motorsailing?  Fuel efficiency at cruising speed?  Overall fuel efficiency?  Different boat owners come up with different things of highest value to them.

 On one boat Prop A will be best, on another it will be Prop B. Without actual, concrete testing data on your exact hull with your exact drive train, the differences reported not really useful tools to discriminate between them.

Best to select based on whatever weighted combination of cost, robustness, maintenance needs, reputation, and advertising claims that feels right to you.  Certainly, do include published testing results in your evaluation, but understand if they used your boat in the testing the rankings might be very different in ways that would be unpredictable and, almost certainly, of very minor impact on the performance of your boat in the real world.

Bill Kinney
SM160, Harmonie
Fort Lauderdale, FL


James Alton
 

Bill,

   A great post!  Your points about the problem of load matching a particular prop to various conditions is on point I think.  I was thinking about what you said about the fixed bollard test and prop walk possibly being almost useless data and am wondering if that is always correct?  I thinking this because generally when I shift into reverse, the boat will usually have little or no forward motion so there will be a period of time where there is almost no flow over the hull in either direction.  So in effect would this period of reversing when the boat is reversing direction not be quite similar to being tied to a bollard?   

   Can you tell me if there is a significant difference in the amount of prop walk (sideways) between a prop shaft that is angled such as on my Maramu versus the shaft on the SM which appears to be in line with the WL?  

   Again, interesting discourse, thanks!

Best,

James
SV Sueno,  Maramu #220

On Nov 27, 2017, at 10:44 AM, greatketch@... [amelyachtowners] <amelyachtowners@...> wrote:

I read the test report in YM, and like most of their work, it is a useful and well thought out piece of work.  


But (you knew that was coming, right?) they use bollard pull at full throttle as the primary measure of the prop's performance. I certainly understand the appeal of the measurement.  It is easy to measure with precision.  It is repeatable. It eliminates a lot of other variables. It is easy to report and rank. It has a lot going for it.  

But... it is a not a good measure of the real world performance of the prop in use (unless you are a tugboat).  For example, slight differences in pitch can result in fairly significant difference in engine RPM at full throttle.  If the engine is running at a different RPM it is producing different horsepower, resulting numbers would be skewed quit a bit in ways that have nothing to do with the design of the prop.  Our props typical ly run at about 30% slip, this test measures the thrust at 100% slip, a very different flow regime.

I especially think that measuring "side pull" at full throttle while tied to the dock  is a very poor stand-in for prop walk, to the point of being almost worthless.  Especially when you consider that props don't really produce side thrust on their own, it is the interaction of the slip stream with the hull that makes the boat go sideways.  A prop that had low "slip" (that's good) when the boat was moving would come out much poorer in ranking on this test than a prop with higher "slip" even though it would produce less prop walk when actually moving the boat.

It's easy to throw rocks at a published test like this and really, really hard to come up with a simple measure of prop performance. It is almost as bad as testing anchors! To be fair to YM they do acknowledge the li mitations of the testing at the end of the article.

My take-home story is that all feathering and folding props have so much lower drag than a fixed prop that whatever is left is totally insignificant on a cruising boat.  On our Amels there is no reason to discriminate between them based on this measurement.

Performance under power is really, really hard to measure in a way that you can actually compare apples to apples.  The interactions between engine, transmission, mounting geometry, gear ratios, etc, etc. will totally overwhelm the differences from one prop to another. Then there is the question of what performance measure is really important. Do you care about maximum speed?  Highest speed at cruising RPM?  Match of prop power curve to engine power output?  Maneuvering?  Motorsailing?  Fuel efficiency at cruising speed?  Overall fuel efficiency?  Different boat owners come up with different things of highest value to them.

 On one boat Prop A will be best, on another it will be Prop B. Without actual, concrete testing data on your exact hull with your exact drive train, the differences reported not really useful tools to discriminate between them.

Best to select based on whatever weighted combination of cost, robustness, maintenance needs, reputation, and advertising claims that feels right to you.  Certainly, do include published testing results in your evaluation, but understand if they used your boat in the testing the rankings might be very different in ways that would be unpredictable and, almost certainly, of very minor impact on the performance of your boat in the real world.

Bill Kinney
SM160, Harmonie
Fort Lauderdale, FL



greatketch@...
 

There have been many explanations for prop walk written over the years.  The best that can be said about most of them is that they are "creative."

The idea that it was caused by the angle of the prop shaft was a popular one that has appeared in a number of popular sailing books and instructional materials. If that was the reason for why we have prop walk then an Amel SM, with its horizontal prop shaft, would have none.  But of course it does.  Not a lot compared to some boats, but enough to prove that explanation is just plain wrong or, at the very least, incomplete.

Here is how I explained it to my students when I was teaching big boat maneuvering:
An ideal propeller in a perfect universe would move through the water like a screw moves into a wood plank. A propeller with a 14" pitch would move a boat forward 14" for every turn.  But there are no perfect propellers, we do not live in a perfect universe, and water is not wood, so propellers "slip".  A propeller with a 14" pitch will move a boat (very roughly) 10" forward for every turn, and some water will be thrown in the opposite direction.  This is the "slipstream."

Because a propeller is spinning, its slipstream is also rotating as it move away from the prop.  Now comes the hard part without drawing on the napkin...  

Imagine you are behind a boat in reverse. the prop is spinning counterclockwise as you look forward. The slipstream is moving forward, and while it moves forward, it also is spinning counterclockwise.

The top of the slipstream is moving from your right to your left while moving forward.  It hits the hull and/or keel. Very close to 100% of its horizontal momentum is transferred to the boat, pushing the stern to your left (the boat's port).

The bottom of the slipstream hits your boat hull and/or keel moving from left to right.  Less than 100% of its momentum gets transferred to the boat because some of it escapes by going under the hull and/of keel.  It does push the boat somewhat to the right (starboard) but it is less efficient, so the stern of the boat experiences a net push to port.
There are probably other things going on here too, but this model seems to explain all the variations I see.  For example, when  you first shift into reverse, the boat is not moving.  Slip is close to 100%, and prop walk can be dramatic. As soon as the boat begins to move, slip decreases, and prop walk decreases even if you don't correct at all with the rudder.  Maybe you could argue that this is the time you are most concerned with and the YM test is a reasonable model.  I would argue I prefer to know if the boat can be steered once it starts moving, because that is more important to me.  So...  maybe I was a bit harsh to say "almost useless" :-)  At least I didn't write "totally useless!"

In a typical sailboat propeller installation the slip is about 30%.  Yes, it will be higher in reverse, but close enough for argument.  The way YM ran their tests the boat was tied down and could not move.  So the slip must be 100%.  The slipstream is roughly 2 to 3 times stronger than it would normally be.  The difference between the amount of propwalk in the real world (where boats actually move)  with different props is caused by the changing ratio of thrust to slip. The test they ran takes that completely out of the equation. In the test as they ran it the prop that moves more water will always show more propwalk.  

There are so many variables, that I would hesitate to ever predict the differences in prop walk magnitude between two different boats. But I'll say this, if there is a difference between a Maramu and a Super Maramu, it is not because of the angle of the prop shaft.

Bill Kinney
SM160 Harmonie
Fort Lauderdale, FL

---In amelyachtowners@..., <lokiyawl2@...> wrote :

Bill,

   A great post!  Your points about the problem of load matching a particular prop to various conditions is on point I think.  I was thinking about what you said about the fixed bollard test and prop walk possibly being almost useless data and am wondering if that is always correct?  I thinking this because generally when I shift into reverse, the boat will usually have little or no forward motion so there will be a period of time where there is almost no flow over the hull in either direction.  So in effect would this period of reversing when the boat is reversing direction not be quite similar to being tied to a bollard?   

   Can you tell me if there is a significant difference in the amount of prop walk (sideways) between a prop shaft that is angled such as on my Maramu versus the shaft on the SM which appears to be in line with the WL?  

   Again, interesting discourse, thanks!

Best,

James
SV Sueno,  Maramu #220



James Alton
 

Bill,

  That is the most reasonable explanation that I have ever heard for why props walk.    You have knack for explaining things in an understandable form.  So if i have processed the information correctly, the only way to have a single prop that does not walk is to have infinite draft so that the two slip streams fully cancel?  If I am on the right track, does this also explain  the reason that the more shallow draft boats that I have handled seem to have more prop walk than the deeper keeled ones?  I always just thought that this was due to difference in the force required to move more lateral plane sideways but now you have me rethinking this…   

Best,

James
SV Sueno,  Maramu #220

On Nov 27, 2017, at 4:07 PM, greatketch@... [amelyachtowners] <amelyachtowners@...> wrote:

There have been many explanations for prop walk written over the years.  The best that can be said about most of them is that they are "creative."


The idea that it was caused by the angle of the prop shaft was a popular one that has appeared in a number of popular sailing books and instructional materials. If that was the reason for why we have prop walk then an Amel SM, with its horizontal prop shaft, would have none.  But of course it does.  Not a lot compared to some boats, but enough to prove that explanation is just plain wrong or, at the very least, incomplete.

Here is how I explained it to my students when I was teaching big boat maneuvering:
An ideal propeller in a perfect universe would move through the water like a screw moves into a wood plank. A propeller with a 14" pitch would move a boat forward 14&qu ot; for every turn.  But there are no perfect propellers, we do not live in a perfect universe, and water is not wood, so propellers "slip".  A propeller with a 14" pitch will move a boat (very roughly) 10" forward for every turn, and some water will be thrown in the opposite direction.  This is the "slipstream."

Because a propeller is spinning, its slipstream is also rotating as it move away from the prop.  Now comes the hard part without drawing on the napkin...  

Imagine you are behind a boat in reverse. the prop is spinning counterclockwise as you look forward. The slipstream is moving forward, and while it moves forward, it also is spinning counterclockwise.

The top of the slipstream is moving from your right to your left while moving forward.  It hits the hull and/or keel. Very close to 100% of its horizontal momentum is transferred to the boat, pushing the stern to your left (the boat's port).

The bottom of the slipstream hits your boat hull and/or keel moving from left to right.  Less than 100% of its momentum gets transferred to the boat because some of it escapes by going under the hull and/of keel.  It does push the boat somewhat to the right (starboard) but it is less efficient, so the stern of the boat experiences a net push to port.
There are probably other things going on here too, but this model seems to explain all the variations I see.  For example, when  you first shift into reverse, the boa t is not moving.  Slip is close to 100%, and prop walk can be dramatic. As soon as the boat begins to move, slip decreases, and prop walk decreases even if you don't correct at all with the rudder.  Maybe you could argue that this is the time you are most concerned with and the YM test is a reasonable model.  I would argue I prefer to know if the boat can be steered once it starts moving, because that is more important to me.  So...  maybe I was a bit harsh to say "almost useless" :-)  At least I didn't write "totally useless!"

In a typical sailboat propeller installation the slip is about 30%.  Yes, it will be higher in reverse, but close enough for argument.  The way YM ran their tests the boat was tied down and could not move.  So the slip must be 100%.  The slipstream is roughly 2 to 3 times stronger than it would normally be.  The difference between t he amount of propwalk in the real world (where boats actually move)  with different props is caused by the changing ratio of thrust to slip. The test they ran takes that completely out of the equation. In the test as they ran it the prop that moves more water will always show more propwalk.  

There are so many variables, that I would hesitate to ever predict the differences in prop walk magnitude between two different boats. But I'll say this, if there is a difference between a Maramu and a Super Maramu, it is not because of the angle of the prop shaft.

Bill Kinney
SM160 Harmonie
Fort Lauderdale, FL

---In amelyachtowners@..., wrote :

Bill,

   A great post!  Your points about the problem of load matching a particular prop to various conditions is on point I think.  I was thinking about what you said about the fixed bollard test and prop walk possibly being almost useless data and am wondering if that is always correct?  I thinking this because generally when I shift into reverse, the boat will usually have little or no forward motion so there will be a period of time where there is almost no flow over the hull in either direction.  So in effect would this period of reversing when the boat is reversing direction not be quite similar to being tied to a bollard?   

   Can you tell me if there is a significant difference in the amount of prop walk (sideways) between a prop shaft that is angled such as on my Maramu versus the shaft on the SM which appears to be in line with the WL?  

   Again, interesting discourse, thanks!

Best,

James
SV Sueno,  Maramu #220





Patrick McAneny
 

Bill , Prop walk is probably less importance to me than drag and thrust. The test concluded that drag is greatly reduced , to the point of a non issue compared to a fixed prop , with folding props virtually zero drag and feathering only up to 5 % of a fixed prop. So then that leaves thrust and maybe emergency stopping. Assuming that all 15 props tested were pitched for the test boat , and they test them at the same RPM , then cannot one draw a conclusion about  the amount of thrust produced by various props. 
When one spends 3k - 4k on a prop , you of course want the best overall prop for your money. Its only logical to assume that all props are not created equal . So the question is , if you disregard independent testing / evaluation , what do you base your choice on ? Ask TOM , Dick & Harry and they will all tell you their prop is the best . I guess you can always throw a dart . Back to prop walk , your explanation helped me understand what causes it , I had a theory and it was wrong.
Thanks,
Pat 
SM#123


-----Original Message-----
From: greatketch@... [amelyachtowners]
To: amelyachtowners
Sent: Mon, Nov 27, 2017 3:08 pm
Subject: Re: [Amel Yacht Owners] Re: Flexofold Prop

 
There have been many explanations for prop walk written over the years.  The best that can be said about most of them is that they are "creative."

The idea that it was caused by the angle of the prop shaft was a popular one that has appeared in a number of popular sailing books and instructional materials. If that was the reason for why we have prop walk then an Amel SM, with its horizontal prop shaft, would have none.  But of course it does.  Not a lot compared to some boats, but enough to prove that explanation is just plain wrong or, at the very least, incomplete.

Here is how I explained it to my students when I was teaching big boat maneuvering:
An ideal propeller in a perfect universe would move through the water like a screw moves into a wood plank. A propeller with a 14" pitch would move a boat forward 14&qu ot; for every turn.  But there are no perfect propellers, we do not live in a perfect universe, and water is not wood, so propellers "slip".  A propeller with a 14" pitch will move a boat (very roughly) 10" forward for every turn, and some water will be thrown in the opposite direction.  This is the "slipstream."

Because a propeller is spinning, its slipstream is also rotating as it move away from the prop.  Now comes the hard part without drawing on the napkin...  

Imagine you are behind a boat in reverse. the prop is spinning counterclockwise as you look forward. The slipstream is moving forward, and while it moves forward, it also is spinning counterclockwise.

The top of the slipstream is moving from your right to your left while moving forward.  It hits the hull and/or keel. Very close to 100% of its horizontal momentum is transferred to the boat, pushing the stern to your left (the boat's port).

The bottom of the slipstream hits your boat hull and/or keel moving from left to right.  Less than 100% of its momentum gets transferred to the boat because some of it escapes by going under the hull and/of keel.  It does push the boat somewhat to the right (starboard) but it is less efficient, so the stern of the boat experiences a net push to port.
There are probably other things going on here too, but this model seems to explain all the variations I see.  For example, when  you first shift into reverse, the boa t is not moving.  Slip is close to 100%, and prop walk can be dramatic. As soon as the boat begins to move, slip decreases, and prop walk decreases even if you don't correct at all with the rudder.  Maybe you could argue that this is the time you are most concerned with and the YM test is a reasonable model.  I would argue I prefer to know if the boat can be steered once it starts moving, because that is more important to me.  So...  maybe I was a bit harsh to say "almost useless" :-)  At least I didn't write "totally useless!"

In a typical sailboat propeller installation the slip is about 30%.  Yes, it will be higher in reverse, but close enough for argument.  The way YM ran their tests the boat was tied down and could not move.  So the slip must be 100%.  The slipstream is roughly 2 to 3 times stronger than it would normally be.  The difference between t he amount of propwalk in the real world (where boats actually move)  with different props is caused by the changing ratio of thrust to slip. The test they ran takes that completely out of the equation. In the test as they ran it the prop that moves more water will always show more propwalk.  

There are so many variables, that I would hesitate to ever predict the differences in prop walk magnitude between two different boats. But I'll say this, if there is a difference between a Maramu and a Super Maramu, it is not because of the angle of the prop shaft.

Bill Kinney
SM160 Harmonie
Fort Lauderdale, FL

---In amelyachtowners@..., wrote :

Bill,

   A great post!  Your points about the problem of load matching a particular prop to various conditions is on point I think.  I was thinking about what you said about the fixed bollard test and prop walk possibly being almost useless data and am wondering if that is always correct?  I thinking this because generally when I shift into reverse, the boat will usually have little or no forward motion so there will be a period of time where there is almost no flow over the hull in either direction.  So in effect would this period of reversing when the boat is reversing direction not be quite similar to being tied to a bollard?   

   Can you tell me if there is a significant difference in the amount of prop walk (sideways) between a prop shaft that is angled such as on my Maramu versus the shaft on the SM which appears to be in line with the WL?  

   Again, interesting discourse, thanks!

Best,

James
SV Sueno,  Maramu #220



greatketch@...
 

Pat,

For a boat with an effective bow thruster, I agree, prop walk is rather minor consideration.

I don't entirely dismiss the independent testing, it tells me that the performance of the props are all essentially the same, and that is useful information.

Once I get to the point that all the props are pretty similar in performance, then I would check to be sure they would fit my boat.

After that, it's tough. Which one is easier to work with?  Do I feel I will need to be able to adjust the pitch underwater easily?  What kind of support is available?  Do any of them look better built than the others?  

For someone with an Amel, using an Autoprop or a MaxProp can cut several corners.  You can find out exactly the model, size, pitch settings, etc that work for other people with your exact boat and engine.  That's significant but not overwhelming value.

Otherwise, I don't have a magic answer for picking the "best" other than saying I have never heard anybody voice really serious  valid complaints about any of them, with the possible exception of the smaller two blade Varifold models, but they aren't suitable for an Amel anyway.


Bill Kinney
SM160, Harmonie
Fort Lauderdale, FL


Danny and Yvonne SIMMS
 

Hi Bill,

I can't disagree with your input but I offer my thoughts. When a boat is stationary, or changing from forward to reverse initially the prop is turning in the water and the linear flow of water over the blades has not established the propeller just acts like a wheel and walks through the water. That's why prop walk is most pronounced when reverse is engaged from forward, (and on fishing boats with huge diameter props). Then as the flow settles down over the blades the effect diminishes. The auto prop has much less walk than other props because of its free blades adopting optimum angle for the forces applied, torque, rpm, load etc. Not so like a wheel. That's my reasoning anyway.

That aside. If you want to turn a super maramu  on her axis try this. At idling speed forward put your helm hard to port. Apply full throttle fast. As the boat gets forward momentum, cut the throttle, click to neutral, click to reverse. Apply full throttle in reverse.  The bow will swing fast to port. As it slows, cut throttle and click click through to forward, again apply full throttle fast. The helm is held hard to port all the time and you must allow her to move a bit forward and reverse before you change gear.

Repeat the process until you have turned to where you want. This method won't overcome a strong cross wind but works very well.

The spectators are always very impressed.

Regards

Danny

SM 299 Ocean Pearl

On 28 November 2017 at 09:07 "greatketch@... [amelyachtowners]" <amelyachtowners@...> wrote:

 

There have been many explanations for prop walk written over the years.  The best that can be said about most of them is that they are "creative."


The idea that it was caused by the angle of the prop shaft was a popular one that has appeared in a number of popular sailing books and instructional materials. If that was the reason for why we have prop walk then an Amel SM, with its horizontal prop shaft, would have none.  But of course it does.  Not a lot compared to some boats, but enough to prove that explanation is just plain wrong or, at the very least, incomplete.

Here is how I explained it to my students when I was teaching big boat maneuvering:
An ideal propeller in a perfect universe would move through the water like a screw moves into a wood plank. A propeller with a 14" pitch would move a boat forward 14" for every turn.  But there are no perfect propellers, we do not live in a perfect universe, and water is not wood, so propellers "slip".  A propeller with a 14" pitch will move a boat (very roughly) 10" forward for every turn, and some water will be thrown in the opposite direction.  This is the "slipstream."

Because a propeller is spinning, its slipstream is also rotating as it move away from the prop.  Now comes the hard part without drawing on the napkin...  

Imagine you are behind a boat in reverse. the prop is spinning counterclockwise as you look forward. The slipstream is moving forward, and while it moves forward, it also is spinning counterclockwise.

The top of the slipstream is moving from your right to your left while moving forward.  It hits the hull and/or keel. Very close to 100% of its horizontal momentum is transferred to the boat, pushing the stern to your left (the boat's port).

The bottom of the slipstream hits your boat hull and/or keel moving from left to right.  Less than 100% of its momentum gets transferred to the boat because some of it escapes by going under the hull and/of keel.  It does push the boat somewhat to the right (starboard) but it is less efficient, so the stern of the boat experiences a net push to port.
There are probably other things going on here too, but this model seems to explain all the variations I see.  For example, when  you first shift into reverse, the boat is not moving.  Slip is close to 100%, and prop walk can be dramatic. As soon as the boat begins to move, slip decreases, and prop walk decreases even if you don't correct at all with the rudder.  Maybe you could argue that this is the time you are most concerned with and the YM test is a reasonable model.  I would argue I prefer to know if the boat can be steered once it starts moving, because that is more important to me.  So...  maybe I was a bit harsh to say "almost useless" :-)  At least I didn't write "totally useless!"

In a typical sailboat propeller installation the slip is about 30%.  Yes, it will be higher in reverse, but close enough for argument.  The way YM ran their tests the boat was tied down and could not move.  So the slip must be 100%.  The slipstream is roughly 2 to 3 times stronger than it would normally be.  The difference between the amount of propwalk in the real world (where boats actually move)  with different props is caused by the changing ratio of thrust to slip. The test they ran takes that completely out of the equation. In the test as they ran it the prop that moves more water will always show more propwalk.  

There are so many variables, that I would hesitate to ever predict the differences in prop walk magnitude between two different boats. But I'll say this, if there is a difference between a Maramu and a Super Maramu, it is not because of the angle of the prop shaft.

Bill Kinney
SM160 Harmonie
Fort Lauderdale, FL

---In amelyachtowners@..., <lokiyawl2@...> wrote :

Bill,

   A great post!  Your points about the problem of load matching a particular prop to various conditions is on point I think.  I was thinking about what you said about the fixed bollard test and prop walk possibly being almost useless data and am wondering if that is always correct?  I thinking this because generally when I shift into reverse, the boat will usually have little or no forward motion so there will be a period of time where there is almost no flow over the hull in either direction.  So in effect would this period of reversing when the boat is reversing direction not be quite similar to being tied to a bollard?   

   Can you tell me if there is a significant difference in the amount of prop walk (sideways) between a prop shaft that is angled such as on my Maramu versus the shaft on the SM which appears to be in line with the WL?  

   Again, interesting discourse, thanks!

Best,

James
SV Sueno,  Maramu #220


 


 


greatketch@...
 

That's how I taught people to do it for almost ten years.  For most beginning big boat skippers it was the most intimidating of the lessons.  Putting an expensive 40 foot boat into a space less than 50 feet wide, and requiring them spin it on its axis to get out of the dead end. Certainly terrifying to start but very rewarding when they figured it out!

One of the main reasons we spent so much time on the "back and fill" lessons was to give the novice skippers the idea that prop walk doesn't have to be a bug, it can be a feature if you know how to take advantage of it!  

I have helmed full keel boats with keel hung rudders that had poor steering in reverse, and almost no prop walk.  They were truly nightmares in reverse. Almost brownian motion. Give me moderate, predictable prop walk any day!

Bill Kinney
SM160, Harmonie
Fort Lauderdale, FL


---In amelyachtowners@..., <simms@...> wrote :

Hi Bill,

I can't disagree with your input but I offer my thoughts. When a boat is stationary, or changing from forward to reverse initially the prop is turning in the water and the linear flow of water over the blades has not established the propeller just acts like a wheel and walks through the water. That's why prop walk is most pronounced when reverse is engaged from forward, (and on fishing boats with huge diameter props). Then as the flow settles down over the blades the effect diminishes. The auto prop has much less walk than other props because of its free blades adopting optimum angle for the forces applied, torque, rpm, load etc. Not so like a wheel. That's my reasoning anyway.

That aside. If you want to turn a super maramu  on her axis try this. At idling speed forward put your helm hard to port. Apply full throttle fast. As the boat gets forward momentum, cut the throttle, click to neutral, click to reverse. Apply full throttle in reverse.  The bow will swing fast to port. As it slows, cut throttle and click click through to forward, again apply full throttle fast. The helm is held hard to port all the time and you must allow her to move a bit forward and reverse before you change gear.

Repeat the process until you have turned to where you want. This method won't overcome a strong cross wind but works very well.

The spectators are always very impressed.

Regards

Danny

SM 299 Ocean Pearl



greatketch@...
 


James,

There are so many variables... there are always exceptions to the simple rules...

If I wanted to design a boat with as little prop walk as possible I would put the prop deep on a horizontal shaft (like an SM) so the slipstream stayed as far away from the hull as possible.  I would put the prop as far back from a very narrow keel as I could to give the slip stream time to lose energy before it interacted with the hull.  Kind of like some of the ocean racing monohulls that use saildrives.

Or...  make the keel so big, deep and broad that the momentum transfer was the same on both sides.  I drove a Tayana 37 that was like this. Full keel, prop in an aperture between the keel hung rudder and keel.  Prop walk was not really noticeable. 

Some boats that have a reputation for very limited propwalk actually just have very large and responsive spade rudders that get a good bite at low speeds and overwhelm the walk as soon as the boat starts moving.

This is one of the reasons that twin screw boats almost always have counter rotating props: All the off axis yaw forces cancel out.

Bill Kinney
SM160, Harmonie
Fort Lauderdale, FL

---In amelyachtowners@..., <lokiyawl2@...> wrote :

Bill,

  That is the most reasonable explanation that I have ever heard for why props walk.    You have knack for explaining things in an understandable form.  So if i have processed the information correctly, the only way to have a single prop that does not walk is to have infinite draft so that the two slip streams fully cancel?  If I am on the right track, does this also explain  the reason that the more shallow draft boats that I have handled seem to have more prop walk than the deeper keeled ones?  I always just thought that this was due to difference in the force required to move more lateral plane sideways but now you have me rethinking this…   

Best,

James
SV Sueno,  Maramu #220



Dean Gillies
 

Great discussion, but somewhat bewildering for mere mortals like myself!
I've just placed an order for an Autoprop on the basis of 
1. good customer service from Bruntons,
2. I know it fits and work because many (maybe most?) other 54's have it,
3. I could see no compelling reason to choose differently.
It won't be fitted until next year. In the meantime I need to consider whether I need a rope cutter too.
Cheers all
Dean
SY Stella
Amel 54 #154