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S&S Swan Maintenance - Comments on propellers
08 May 2010 - 19:12
#1
Join Date: 03 March 2007
Posts: 197

Comments on propellers

Hi Lars,

 

I for one would like to hear your comments on propellers.  Especially the drag and efficiency of the different types, fixed folding and feathering.

Thanks

 

Mike

 

 
     
 

 
     

10 May 2010 - 22:19
#2
Join Date: 30 January 2007
Posts: 32

Dear Mike,

I have very good experience from the Kiwi feather prop which I bought when I installed a new engine 4 years ago  (a 44 hp Vetus) in my Swan 40 (Sunniva V). It gives little vibrations and very little resistance during sailing. Furthermore I have never before been able to drive in reverse so easily. I got the recommendationof this prop from an Brazilian owner of another Swan 40 (Fuga II).

You are most welcome to contact me if you want more details from my experience.

Best regards

Leif (Sunniva V, Swan40 #36)

11 May 2010 - 10:06
#3
Join Date: 03 March 2007
Posts: 197

Hi Leif

Good to hear there has been success with the Kiwi props on 40s. I was not sure if I could fit it but the 15.5 inch prop fits nicely. I like that you can adjust the pitch easily. What pitch do you have and have you adjusted from that recommended by the Kiwi folks? Do you have a 15.5 as well?? I will be putting her in the water shortly and looking forward to trying it out. Your comments about the efficiency in reverse mirrors those in other forums. One thing that worries me a bit is that due to the pitch in reverse they have a tendency to stall small engines... like mine.
Any advice or comments would be much appreciated!! Do you put antifouling on the prop??

Cheers
Mike

11 May 2010 - 17:31
#4
Join Date: 30 January 2007
Posts: 32

Hi again Mike,

I have a 17" Kiwi prop which fits without any problems with 21 degree pitch as was recommended by the Kiwi folks. (I think Fuga II has 22 degree but also 17") Reduction gear 2:1, the shaft has 30mm diameter. My engine is a Vetus 42 SAE hp rpm 3000 and functions nicely. (I found the Yanmar 25hp too weak and could not take me forward against hard wind and waves) This is not a problem with the new engine. However, the reason for installing a new engine and prop was as a matter of fact that a crankpin broke although it did fortunately not crash the crankshaft so the engine could be renovated, and sold)

The efficiency of the prop in my case is "overwhelming" already at low rpm i.e. 1500 I am reaching 5-6 knots so it is seldom I run the engine "at full speed" in ordetr to reach 7 knots.

As a matter of fact I do not put antifouling on the prop but after all I am mostly sailing in the Baltic. I enclose a fig of the prop

Good to know that you are in the water again. I am too since one week. I always feel jealous of our italian friends who can have their boats in the water all year around.

I wishg you a very sice siling summer.

Leif

12 May 2010 - 11:57
#5
Join Date: 03 March 2007
Posts: 197

Hi Leif,

Interesting!! I thought a 17" would not fit so did not go for the bigger prop. Obviously I was wrong. I wish I had have talked with you earlier!!! I will let you know how my prop motor setup works out. I will paint the prop with a hard antifouling paint and see how it goes. We have been having problems with fouling the last couple of years so I hesitate to neglect doing so!!! I am located just south of Svendborg if you get to this area we should have a Swan 40 meeting. There are two 40s here. Would be nice to put a picture on the website with all three rafted up. Where are you located??

All the best

Mike

12 May 2010 - 12:05
#6
Join Date: 29 January 2007
Posts: 803

Dear Friends

looking forward to have the pics of you four together to get online...!

fair Winds!

Matteo (38/067 Only You)

13 May 2010 - 12:50
#7
Join Date: 02 January 2008
Posts: 1410

Mike asked for advice on propellers, here some information to start with. Resistance matters can be found in various test reports, will come back on this.

Lars

The difference between flat and twisted blades

Let us start with Leif's comment that the performance in reverse improved with a feathering propeller.

Leif's observation is correct, feathering propellers usually have flat blades, and this provides good performance in reverse. Actually the performance is equal in forward and reverse for these propellers.

Why is this then not used on all propellers? The reason is that flat blades do not have optimum pitch distribution, and this reduces their efficiency some 20% in comparison with twisted blades.

Ordinary propellers have twisted blades for getting equal pitch along the blade, and the performance is better in forward than for the flat blades, but worse in reverse.

With twisted blades higher rpms are required in reverse to achieve the same thrust as in forward, and this is experienced as a lack of power.

Some reduction gears even have a lower reduction ratio in reverse to compensate for this, enabling lower engine rpms to be used.

The difference between flat and twisted blades can be summed up as follows: if best forward performance and efficiency is a priority go for twisted blades, and if good reverse performance is important choose feathering with flat blades.

A comment about manoeuvering with a feathering propeller - when engaging reverse the blades have to turn 180 degrees on the hub before they work in the new direction, and they need some time for this. After engaging reverse gear do not advance the throttle to high rpms immediately. If the blades did not have time to turn reverse power will not be available, and the consequences can be expensive.

Feathering propellers can feather into sailing position only from forward position, and not from reverse. It is suggested that you verify this the next time the boat is on the hard. Therefore forward gear shold be used when attempting to feather the propeller.

It is important for feathering that the blades turn easily, check this now and then. The hub also needs greasing, the newer propellers have grease nipples for this. In cold waters a thinner grease is needed.

How to determine if the pitch is right.

Some manufacturers of feathering propellers advise that the best pitch can be found by trial, and encourage the boat owner to do this himself. This approach relies on the fact that by increasing the pitch the cruising speed will improve, and from the owner perspective this propeller is then very good, as it brought more speed.

This generally works well in smooth water, but if the boat has to power against a strong headwind the available power is dependent on the maximum rpm the engine can reach. Increased pitch will lower the rpm, particularly in a headwind, and consequently the engine can not any more produce the rated hp, the speed drops, and the engine may overheat.

The recommendation is to set the pitch so the engine can reach its rated rpm in smooth water. There is also another aspect on this - engine manufacturers require that the rated speed is achieved, otherwise the engine is overloaded, and warranty may be voided.

If performance against a strong headwind is the first priority, the pitch should be set so the engine reaches rated rpm with the boat held stationary.

With 2-bladed propellers there may be a different situation in strong headwinds - the engine is keeping up high rpms, but the speed drops nevertheless. The reason for this is too small propeller blade area, and 3 blades would improve the situation. The recommendation is to try 3 blades before replacing the engine.

This problem can also occur with 3 blades if the propeller is too small for the engine power. 4 blades are available from some manufacturers.

Most feathering props can be adjusted to right and left handing, and to a selection of pitches, this makes them very adaptable. For Max-Prop the pitch is measured in degrees, and it can be set in 2 degrees steps for the standard model, or in 1 degree steps for the model with external adjustment. Propeller calculations for flat blades can be performed in the same way as for twisted blade propellers provided that the mean effective pitch is determined. The calculation requires that the blade outline is defined.

There are many aspects on these things, and you are encouraged to ask questions, and suggest new topics within this area

 

 

14 May 2010 - 09:56
#8
Join Date: 03 March 2007
Posts: 197

Hi Lars,
Thanks this was quite informative!! I have also checked a few articles on this in particular the one Christophe mentions in YM which was sponsored by Volvo ( see Christophes last message in the engine thread or http://flexofold.com/upload_dir/docs/Test_YachtingMonthly_low.pdf ?? Interestingly their props were not included. Being the eternal skeptic I scratched my head a bit with this review. Fixed props are not as good at going forward as other a folding prop (Flexofold)??? Are all those shipping companies wrong? Seems odd to me. Lots of different diameters and pitches---- Was the YM test done in controlled conditions so all props had similar conditions and were set up the same way? Were they comparing apples and oranges?? Gori seemed to have some problems with the review...see the comment at the end of the article. Having investigated and just bought and installed a Kiwi prop I was shocked to see how this prop ranked given all the other info I had seen on it. SOOOO I contacted Alan Pollard at KIwi here is his comment on the article.


Quote from Alan re YM tests:

"I was present at one day of the two day YM tests and was frankly very unhappy with their methods and location. We were asked to make a prop which would enable the engine to run at 3600 rpm and we did so, though we would have preferred to increase the pitch to a setting for best performance as we would for a customer. Then we would have seen better power performance all round from our prop. (Note the comment from the Prof on loading)

The stop tests were done in Lymington harbor using a stop watch, but how they decided when a boat had stopped in the ever changing tidal flows I do not know. Very hit and miss.

The bollard pulls were done tied to a marina finger, and for our slot at the bottom of the tide when there was so little water below the prop it was moving clouds of mud. With sixteen props to test, so sixteen lift outs and relaunch and some tedious installation times, it all took 2 days and so many competitors had better conditions for their tests. They also managed to quote the wrong pitch and the wrong price, and yet ignored externally adjustable pitch.


The French sailing magazine VOILE , tested 12 props in a 13 colour page article in their April edition . The French Tests were in non tidal waters. Our Kiwiprop came out with one of two five star ratings. The summary says “it is a very innovative propeller with high performance and an unbeatable quality-price combination”.

Mike again
Being a scientist (Oceanographer with a bit of background on flow) I recognize the problems with experimental design and statistics and to be honest the YM article seems to be flawed. Kiwi has a test on their web site(OK they are a manufacturer) but the test seems to be professionally done i.e. a thesis study by an engineering student and as such should be rigorous. I appreciated the effort to keep conditions similar. I won't say more on that except that I have a feeling that one should be critical when reading these reviews. These mags have advertisers and sponsors to appease so they have to tread a fine line. The only boating magazine that I believe gives an unbiased viewpoint is Practical Sailor. No ads no sponsors and they tend to be critical when necessary and rank the products. (NO I don't work for them). They are expensive.I will see if they have a prop review.... I have decided to give up my other subscriptions and get this one.

Anyway, thanks again LARS I learned again from your response!!!


Cheers
Mike



14 May 2010 - 11:29
#9
Join Date: 02 January 2008
Posts: 1410

Mike

Thank your for your comments.

Referring to propeller tests I would like to point out a very big deficiency in all yacht magazine tests I am aware of - the actual propeller pitch has not been measured, meaning that apples and oranges are compared. It is quite obvious that there are pitches declared by some suppliers that are not the actual ones.

Earlier the pitch had to be measured manually, which was very tedious already if a few blade sections had to be measured. Today there are methods available based on 3D digital image processing enabling the local pitch values over the entire blade area to be established with an accuracy of 1/10 of a millimetre. To use such accuracy is perhaps not in the interest of propeller suppliers, because most yacht propellers are finished by hand, and deviate from the proper shape and have uneven surfaces. Finishing by numerically controlled equipment is a prerequisite for a well performing propeller, but not a guarantee unless the shape is correct.

In spite of this some conclusions can be drawn from the published test results. In addition to the two tests mentioned (UK and France) there is also one done in Germany. Will comment on these shortly

Lars

 

15 May 2010 - 16:24
#10
Join Date: 02 January 2008
Posts: 1410

Comments about the Propeller Test in Yachting Monthly May 2009

Propeller thrust is depending on dimensions and rpm, but such information is not given in the article. This makes it impossible to make even rough calculations for evaluating the reasons for the differences.

Based on the information presented recently on the Forum propellers with twisted blades should perform well in forward, and flat blades be good in reverse.

The presentation confirms this.

The propellers providing high bollard pull astern also give high prop walk. If the actual side forces are compared the two Max-Props are at the top of the list.

Some props are reported to provide better speed than others at cruising rpms, if you have read the Forum you know the reasons.

Lars

15 May 2010 - 16:31
#11
Join Date: 29 January 2007
Posts: 803

Dear Mike and All (Lars, please do not read this message!),

I found Lars' comments and Mike's quotation about magazines and their articles very very interesting, I totally agree with Kiwy people, you have to be very careful when reading some articles, they indeed have advertizers...

Anyway, this is my experience: When I bought Only You I found a three blades 16" Max Prop, that was a good, reliable but very old prop, quite noisy and in need of a totally overhaul, I then decided on my own to try the four blades 16" German built VariProp, that was quite an expensive item (at least in Italy), with a fantastic mechanism to micro-set the pitch when the boat is in the water (but not snorkeling), but the drum is very big and I must say it was a very noisy prop. Maybe the blades were not perfectly balanced? I don't know.
I asked Lars' suggestion on a three blades, Max Prop 15", he said this should be the perfect prop for my boat (with the correct clearance -look at the second pic below)) and my engine (Yanmar 3YM30, 29 hp), and told me to set the pitch at 19°. As I was a bit stubborn, and the manufacturer suggested 22° (look at the first pic below), I tried 22° (...!!!...). After sea trials I found I couldn't get all the revs on the engine and had to agree with Lars' suggestion and setted a new pitch at 19° (luckily the newer MaxProp props can be setted underwater and in two/three seconds, so you can do the job snorkeling), that was absolutely perfect, I have now minimal noise, the engine is running free up per manufacturer's suggested 3.600 RPM, Only You can get 6.5 kn at 2.800 (about 56% of the engine power).

so?

Listen to Lars' words, he's a genius, that was not a suggestion, it was a wizard's word!

Fair winds!

Matteo (38/067 Only You)

16 May 2010 - 07:06
#12
Join Date: 03 March 2007
Posts: 197

Hi Matteo et al,

Nice setup on that propeller!!  The kiwi requires an Allen key and good lungs to change the pitch but it is also pretty easy. 

As for the Professor.  We are extremely lucky to have his advice!!! As I said earlier I am a skeptic but with him I have no reason to question his advice.  His knowledge is incredible<!!!!!!!!! The Professor is the right title!!

Mike

 

 

 

17 May 2010 - 10:08
#13
Join Date: 02 January 2008
Posts: 1410

Thank you for the kind words

Here a link to the Voiles April 2009 test.

http://toerzeilen.web-log.nl/toerzeilen/files/v_m.pdf

I sent the authors some comments, actually in French assuming that would help, but they did not bother to answer. Here the comments in english.

I would suggest that another diagram is done, ranking the performance also at 3000 rpm, because it changes the positions very much.

Those needing help with the French could put their questions on the Forum.

Lars

----------------------------

Dear Sirs

Thank you for an interesting test report.

Unfortunately there appears to be an error in the unit for the measured propeller thrust, it is now stated as Newton, but is likely to be kg.

Also there seems to be something wrong with the Gori information, particularly in overdrive. By drawing the range curve for each propeller it can bee seen that the Gori overdrive curve behaves very strangely. For Gori in standard pitch it is also odd that the point at 1500 rpm is below all the others.

Earlier made fuel consumption measurements with Gori in overdrive have shown increased consumption and reduced range compared to the standard pitch.

Pls find the range curves appended for your information. It should be noted that some of the numbers have been picked from your small size diagrams, and they are not as accurate as your measurements

The test report now presents the test summary in a consumption diagram for calm weather at low engine output, about 1/4 of the maximum power. As propellers perform differently under low and high load it is suggested that another condition also should be considered - that of having to power against a strong headwind. The measured thrust at 3000 rpm could be used for evaluating this, and it is suggested a similar diagram showing the order of merit in this condition should be presented also.

The propeller with the lowest consumption in calm weather might not be a good choice if high trust is important.

The report correctly points out that two propellers stated to have equal dimensions performed very differently, but there appears also to be the opposite - some propellers with stated different dimensions performed rather similarly. It is suggested that for future tests it should be considered to measure the actual pitch, there are advanced methods available for this today.

It would also be possible here to calculate the effective pitch from the test results if that is of interest.

It is likely that the gearbox reduction ratio is 2.4:1 based on the results, but this important information is not mentioned in the text, pls confirm.

The average yachtsman tends to compare propeller performance based on the highest speed achieved at cruising rpms, and if the propeller pitch is adjustable he will increase the pitch because the cruising speed goes up. Finally this may overload the engine. Suggest you should explain more about the importance of matching the propeller to the engine output, and how to determine the maximum recommended pitch with adjustable propellers.

20 May 2010 - 13:02
#14
Join Date: 02 January 2008
Posts: 1410

Are you ready for the next propeller test?

Can be found at www.segeln-magazin.de -> Korrektur Propellertest

For thouse not fluent in German there is an english translation at www.propelspecialisten.dk.

Pls make sure that you have the updated version, there was a rather big mistake in the first issue.

When you have digested this I will post some additional information pointing out differences based on the detailed masurements.

Lars

20 May 2010 - 22:50
#15
Join Date: 16 May 2009
Posts: 252

Professor -

went looking right away to the site you posted for the English link but got something else - ??

Geoff

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21 May 2010 - 04:32
#16
Join Date: 16 May 2009
Posts: 252

bizarre - went to the link from work, got the page above, then went from home and saw the results, so all is well. is the max prop as much of a dud as the report says, or were they testing an old model? I am pretty happy with mine so far....

Geoff
Corazon
411 #41

23 May 2010 - 13:57
#17
Join Date: 02 January 2008
Posts: 1410

Dear all

Continuing on the subject of propeller testing it is suggested you study the file SVABerichtNr.3492, can be found on the Segeln-magazin.de website.

This test report gives some very useful test results, particularly Tabelle 6, which enables propeller efficiency to be compared. Propeller thrust and shaft power have been measured, and by dividing thrust (Kraft) in N by power (Motorleistung) in kW the quotient N/kW can be calculated for the variuos propellers.

Have done this for you, here the results. The bottom line shows the quotient N/kW, and the interesting number is emphasized in bold.

Bollard pull forward (Pfahlzug vorwärts)

1 2 3 4 5 6
1632 2464 2173 1762 2477 1546
10.5 13.2 12.1 8.5 13.2 6.5
155 187 180 207 188 238

7 8 9 10 11 12
2069 2360 1971 1787 1918 2345
11.7 13.2 12.9 12.3 12 12.4
177 179 153 145 160 189

It needs to be pointed out that although the numbers each side of the bold one are bigger, this is due to those propellers absorbing much less power, and producing much less thrust, this increases their apparent efficiency.

The same for bollard pull in reverse (Pfahlzug rückwärts)

1 847 1279 1815 882 1835 1599
12.2 11.5 13.2 5.4 11 6.7
151 111 138 163 167 239

2235 1030 2013 1929 977 1910
13.3 10.6 13.2 13.2 8.1 13
168 97 153 146 121 147

Same comment as for forward.

Cruising speed 6.4 knots (Marschgeschwindigkeit)

548 548 548 548 548 548
4 3.3 3.2 3.8 2.6 3
137 166 171 144 211 183

548 548 548 548 548 548
3.6 3.5 2.9 2.8 4.8 3.2
152 157 189 196 114 171

Here the bold number is clearly the biggest one.

Full speed 7.6 knots (Volle Fahrt)

1630 1630 1630 1630 1630 1630
13.3 11.7 11.1 15.6 10.6 12.7
123 139 147 104 154 128

1630 1630 1630 1630 1630 1630
15 14.5 15.3 16.3 20 12.9
109 112 107 100 82 126

Also here clearly the biggest.

The calculated quotient thrust per engine power shows that one propeller is consistently better than the others, number five on the list of brands is in question. It can even outperform the fixed reference propeller in some conditions, which Mike already noticed earlier, but found hard to believe.

Careful shaping of the blades is the reason for such performance, but unfortunately this can not be verified by eye alone.

It is suggested that you compare your own favourite brands with the help of this information. This is the best of the propeller tests presented so far, and gives useful measurement results.

Pls feel free to ask questions

Lars

 

 

 

 

 

 

 

23 May 2010 - 16:07
#18
Join Date: 16 May 2009
Posts: 252

Lars -

I am probably blind, or slow, but cannot find the indicated file on the magazine website after poking around quite a bit and using search function also - could you please post a little detail on just how to grab that file?

Thanks for that and all your help and work on this interesting topic, as well as all the others - it is appreciated.

Geoff
Corazon
411 #41

23 May 2010 - 19:37
#19
Join Date: 02 January 2008
Posts: 1410

Geoff

Suggest you try the following approach:

On the magazine web site choose  Test & Technik -> Bootstests along left edge

In the search area upper left write 'propellertest', and push Suchen.

This should display the file 'Korrektur Propellertest'

Click to open, this will display amongst others the required file SVA-Prüfbericht Nr 3492. Click to open.

If you do not succeed pls let me know, and I will mail you the file as an attachment

Lars

 

24 May 2010 - 00:26
#20
Join Date: 16 May 2009
Posts: 252

Lars

Got it, thanks -

Geoff

25 May 2010 - 14:40
#21
Join Date: 03 March 2007
Posts: 197

Thanks Lars,

This is an amazing amount of information!!! I am now asking myself did I buy the right prop. The answer clearly is there is no perfect prop each one has its pluses and minuses. So we are all looking for the best compromise. At least I know for sure...I do not want a fixed three blade prop (;-).

All the best

Mike

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