Re: [Amel Yacht Owners] Re: Flexofold Prop

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

Re: [Amel Yacht Owners] Re: stern dimensions

Chuck_Kim_Joy

Hi All,
Just a bit to add. We recently ordered our lettering from 'letteringonthecheap.com' for a very reasonable price. The front end is very easy to get around with lots of fonts and ability to add images. If in the U.S we had our stuff within a week.

Best Regards!
Chuck
s/v Joy
SM2K

On Mon, Nov 27, 2017 at 10:39 AM, James Cromie jamescromie@... [amelyachtowners] wrote:

Thanks Ryan.  Very helpful and exactly what I needed.

Fair winds.
James

On Nov 27, 2017, at 10:55 AM, Ryan Meador ryan.d.meador@... [amelyachtowners] <amelyachtowners@yahoogroups.com> wrote:

Hi,
Attached you'll find the measurements of my transom that I put together for my designer when I was having the new name made (which I haven't actually put on the boat yet).  I hope it helps.

Ryan
SM 233 Iteration (ex Monarae)
Boston, MA, USA

On Sat, Nov 25, 2017 at 11:08 PM, James Cromie jamescromie@... [amelyachtowners] wrote:

Thanks Denise.  This is really helpful!  No need for you to measure, because the dimensions of your lettering are on your photo, and I can scale the rest from there!

Thanks again.

On Nov 25, 2017, at 10:36 PM, Denise McGovern Mcgovern.denise@... [amelyachtowners] <amelyachtowners@...om> wrote:

[Attachment(s) from Denise McGovern included below]

I can measure tomorrow for you.  We have a handle and davits (not permanent) that I was working around.  Can you post a photo of your stern? Maybe from the listing?

Here is my lettering order with dimensions and with them installed.

Re: Bow light wiring gotcha

Dean Gillies

Hi Kent,
I think the 54 is significantly different than the SM. We have a single Port/STBD LED unit in the centre of the pulpit. The wire needs to run inside the tubing horizontally to the stbd side of the pulpit, then turn aft through 90 degrees, then to the aft leg of the pulpit, then downwards and through a hole in the glass underneath the aft leg.
The 90deg bend is the real challenge !
Cheers
Dean

Re: [Amel Yacht Owners] Re: Flexofold Prop

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

Re: [Amel Yacht Owners] Re: stern dimensions [1 Attachment]

James Cromie

Thanks Ryan.  Very helpful and exactly what I needed.
Fair winds.
James

On Nov 27, 2017, at 10:55 AM, Ryan Meador ryan.d.meador@... [amelyachtowners] <amelyachtowners@...> wrote:

Hi,
Attached you'll find the measurements of my transom that I put together for my designer when I was having the new name made (which I haven't actually put on the boat yet).  I hope it helps.

Ryan
SM 233 Iteration (ex Monarae)
Boston, MA, USA

On Sat, Nov 25, 2017 at 11:08 PM, James Cromie jamescromie@... [amelyachtowners] wrote:

Thanks Denise.  This is really helpful!  No need for you to measure, because the dimensions of your lettering are on your photo, and I can scale the rest from there!

Thanks again.

On Nov 25, 2017, at 10:36 PM, Denise McGovern Mcgovern.denise@... [amelyachtowners] <amelyachtowners@yahoogroups.com> wrote:

[Attachment(s) from Denise McGovern included below]

I can measure tomorrow for you.  We have a handle and davits (not permanent) that I was working around.  Can you post a photo of your stern? Maybe from the listing?

Here is my lettering order with dimensions and with them installed.

Re: [Amel Yacht Owners] FYI

Danny and Yvonne SIMMS

Hi. I advise delete immediately. DO NOT OPEN

Danny

SM 299 Ocean Pearl

On 28 November 2017 at 05:35 "'sailormon' kimberlite@... [amelyachtowners]" <amelyachtowners@...> wrote:

Hi,

What is this?

Fair Winds

Eric

Kimberlite Amel Super Maramu #376

From: amelyachtowners@... [mailto:amelyachtowners@...]
Sent: Monday, November 27, 2017 5:31 AM
Subject: [Amel Yacht Owners] FYI

Good Morning,

Nick

Re: [Amel Yacht Owners] FYI

Danny and Yvonne SIMMS

I advise delete immediately. DO NOT OPEN

Danny

SM 299 Ocean Pearl

On 28 November 2017 at 05:41 "Kent Robertson karkauai@... [amelyachtowners]" <amelyachtowners@...> wrote:

Does anybody know what this is?  It smells like phishing.

Kent
S/V Kristy

On Nov 27, 2017, at 05:31, ngtnewington@... [amelyachtowners] <amelyachtowners@...> wrote:

Good Morning,

Nick

<docu(1).html>

Dean Gillies

Ok, no sooner typed and I found the problem.
For the record, yes it is a setting which needs reset.
The Radar Source field had some strange text in it, but when I dropped the box down there was another option of "DRS_Radome". When this is selected then the radar starts to work again.
Tricky!

Dean Gillies

Hi all,

I have committed the cardinal sin of upgrading my Furuno MFD software when it was actually working ok before.

Ive always preached "if it ain't broke ...", however too late I didn't practice my preachings.

So yesterday I had revision 2.05 software on my MFD12 displays, and everything worked just fine.  I've since 'upgraded' to rev 2.07, and now my radar does not work. When I go to the radar display I just get a message on the screen saying "No Radar Detected".

I'm guessing someone else may have done this upgrade and struck the same problem. Presumably it's some setting somewhere which needs adjusted - hopefully!

All wisdom gratefully accepted.

Cheers

Dean

SY Stella

Amel 54 #154

Re: [Amel Yacht Owners] Bow light wiring gotcha

Danny and Yvonne SIMMS

Hi Kent,

was it a steel measuring tape you used? Mine would be too wide for the job?

Regards

Danny

SM 299 Ocean Pearl

On 27 November 2017 at 19:34 "Kent Robertson karkauai@... [amelyachtowners]" <amelyachtowners@...> wrote:

Hi Dean.  I don't know how the 54 bow pulpit compares to the SM 2000, but on Kristy, I was able to use a steel tape starting at the exit by the starboard light and retrieving it at the bottom of the stanchion.  I attached a small line to the end of the tape and retrieved it at the stanchion without getting the steel tape out.  I then pulled a larger line through, then pulled the new cable through using some lubricant designed for pulling wires.  I felt really lucky after doing exactly what you did.

Kent
S/V Kristy
SM243

On Nov 25, 2017, at 11:40, trifin@... wrote:

When I bought my 54 in August, the bowlight was not working.  Anyway, Lopo were great and sent me a new light free of charge.  Today I have been attempting to connect the new light, which has ended in a little disaster!

It was quite a challenge to remove the old cable which runs inside the pulpit rail with a 90degree corner at the front another maybe 70 deg corner at the aft end of the rail before going into the cockpit locker.

Eventually I succeeded in pulling the old cable through with a 3mm mouse line attached, leaving the mouse line to pull the new cable through. All good so far!

I then tried to pull the new cable through and only succeeded in breaking the mouse line before getting the new cable past the first 90 deg corner.  😡  The break was at the foot of the rail, where there must be a sharp edge which cut through the mouse cord. Maybe I should have a used a wire mouse line!

So, where does this leave me ?  Is there any clever way to recover from this and get the new cable through the pulpit rail?

Time to open that bottle of wine Wolfgang. At least it's warm in here now :-)

Cheers

Dean

SY Stella

Re: [Amel Yacht Owners] FYI

paul schofield <pwschofield@...>

Classic virus email. Open the attachment and the embeded macros start their dirty work. Dont open the attachments. Just delete.

Paul
Sm trillium 215

On Nov 27, 2017 11:35 AM, "'sailormon' kimberlite@... [amelyachtowners]" <amelyachtowners@...> wrote:

Hi,

What is this?

Fair Winds

Eric

Kimberlite Amel Super Maramu #376

From: amelyachtowners@... [mailto:amelyachtowners@...]
Sent: Monday, November 27, 2017 5:31 AM
Subject: [Amel Yacht Owners] FYI

Good Morning,

Nick

Re: [Amel Yacht Owners] FYI

karkauai

Does anybody know what this is?  It smells like phishing.

Kent
S/V Kristy

On Nov 27, 2017, at 05:31, ngtnewington@... [amelyachtowners] <amelyachtowners@...> wrote:

Good Morning,

Nick

Re: [Amel Yacht Owners] FYI

eric freedman

Hi,

What is this?

Fair Winds

Eric

Kimberlite Amel Super Maramu #376

From: amelyachtowners@... [mailto:amelyachtowners@...]
Sent: Monday, November 27, 2017 5:31 AM
Subject: [Amel Yacht Owners] FYI

Good Morning,

Nick

Re: [Amel Yacht Owners] Re: stern dimensions

Hi,
Attached you'll find the measurements of my transom that I put together for my designer when I was having the new name made (which I haven't actually put on the boat yet).  I hope it helps.

Ryan
SM 233 Iteration (ex Monarae)
Boston, MA, USA

On Sat, Nov 25, 2017 at 11:08 PM, James Cromie jamescromie@... [amelyachtowners] wrote:

Thanks Denise.  This is really helpful!  No need for you to measure, because the dimensions of your lettering are on your photo, and I can scale the rest from there!

Thanks again.

On Nov 25, 2017, at 10:36 PM, Denise McGovern Mcgovern.denise@... [amelyachtowners] <amelyachtowners@yahoogroups.com> wrote:

[Attachment(s) from Denise McGovern included below]

I can measure tomorrow for you.  We have a handle and davits (not permanent) that I was working around.  Can you post a photo of your stern? Maybe from the listing?

Here is my lettering order with dimensions and with them installed.

Variprop

rossirossix4

Hi Bill!  I also took a look at the report (2009).  I like your alalogy to the anchor testing!  BTW, they tested several props with "Vari" in the name but not the Variprop.  Also, they did not test a 4 bladed prop.   Anyway, I will write SPW GmbH and ask them what the current price is for the "Blue Water".  I know they have our SM on file.  I will also ask them if they have provided props for other Amels.
Bob

Re: [Amel Yacht Owners] Depth of life raft locker

Dan Carlson

We have 4 @ 5gal diesel fuel cans at the bottom of the locker and the raft sits on top of a spacer resting on top of those.  There is room on top of that for a few other small things.   Congratulations, welcome aboard and hope to see you our there.

Dan & Lori Carlson,  sv BeBe, SM #387.

On Sun, Nov 26, 2017 at 9:45 AM, isaac_02906@... [amelyachtowners]
wrote:

Hello All,

I am fairly new to the forum and very new to Amel ownership having just purchased SM #391 in FLL.  I don't have access to the boat just yet, but would like to know the depth of the port side locker so I can begin looking into a life raft replacement that will fit in that locker.

Thanks to all for this great resource!

Mark Isaac
SM #391
Ft. Lauderdale

Re: [Amel Yacht Owners] Re: Flexofold Prop

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

FYI

ngtnewington Newington

Good Morning,

Nick

Re: [Amel Yacht Owners] Re: Flexofold Prop

karkauai

I replaced my auto prop with a max prop when I repowered. The auto problem is a great prop, but requires regular maintenance to keep it functioning properly.  The Maxprop has been flawless for 4 years with only one removal/greasing when I hauled out for bottom paint and new prop shaft seals.

Kent
S/V Kristy
SM 243

Re: [Amel Yacht Owners] Re: Flexofold Prop

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