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S&S Swan Maintenance - Comments on propellers |
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Join Date: 03 March 2007
Posts: 240 |
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Comments on propellers |
Join Date: 30 January 2007
Posts: 32 |
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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) |
Join Date: 03 March 2007
Posts: 240 |
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Hi Leif
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Join Date: 30 January 2007
Posts: 32 |
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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 |
Join Date: 03 March 2007
Posts: 240 |
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Hi Leif,
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Join Date: 29 January 2007
Posts: 1015 |
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Dear Friends |
Join Date: 02 January 2008
Posts: 1547 |
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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 |
Join Date: 03 March 2007
Posts: 240 |
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Hi Lars,
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Join Date: 02 January 2008
Posts: 1547 |
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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
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Join Date: 02 January 2008
Posts: 1547 |
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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 |
Join Date: 29 January 2007
Posts: 1015 |
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Dear Mike and All (Lars, please do not read this message!), |
Join Date: 03 March 2007
Posts: 240 |
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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
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Join Date: 02 January 2008
Posts: 1547 |
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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. |
Join Date: 02 January 2008
Posts: 1547 |
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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 |
Join Date: 16 May 2009
Posts: 252 |
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Professor - went looking right away to the site you posted for the English link but got something else - ?? Geoff
Zanzai develops and
produces solutions for the benefit of a better sound - the air and climate quality |
Join Date: 16 May 2009
Posts: 252 |
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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....
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Join Date: 02 January 2008
Posts: 1547 |
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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)
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)
Same comment as for forward. Cruising speed 6.4 knots (Marschgeschwindigkeit)
Here the bold number is clearly the biggest one. Full speed 7.6 knots (Volle Fahrt)
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
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Join Date: 16 May 2009
Posts: 252 |
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Lars -
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Join Date: 02 January 2008
Posts: 1547 |
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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
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Join Date: 16 May 2009
Posts: 252 |
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Lars
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Join Date: 03 March 2007
Posts: 240 |
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Thanks Lars,
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