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S&S Swan Maintenance - Comments on propellers
26 May 2010 - 11:52
#22
Join Date: 30 January 2007
Posts: 440

Dear all and especially Lars,
everything is very interesting and, thanks to Lars, I am finally starting to understand the technicalities regarding the propellers and why numerically controlled milling machines are considered a top secret issue by the navy all over the world!
After reading Matteo´s letter I wonder if Lars can give me a quick advice before I launch my boat next week or so. I have a three blade 21" Max-Prop mounted on a 411 with original motoring (Perkins 47HP at 3600 rpm, 2.9:1 gear ratio). Before hauling the pitch angle was set at 20° but as I thought it was not enough (at 2200 rpm the boat would hardly sail at 6 kn) I presently set it at 22°. Now I am starting to doubt my decision and I am still in time to change.

Daniel, 411-004

26 May 2010 - 20:01
#23
Join Date: 02 January 2008
Posts: 1428

Daniel

Yes, I can do it, but for accurate results would appreciate if you measure the diameter in mm. Max-Props are manufactured with metric dimensions, and inches are usually an approximation.

3600 rpm is the Pleasure rating for this engine, would suggest that you consider using the Commercial rating at 3000 rpm, it is much quieter, and sufficient. The difference is in high idling = full throttle in neutral gear, for pleasure it is set to 4100 rpm, for commercial to 3380 rpm, and this can be adjusted with the injection pump rpm limit screw. I think you may after all these years adjust the screw yourself without voiding warranty. The screw is probably sealed if in original condition.

Lars

 

Lars

26 May 2010 - 22:06
#24
Join Date: 30 January 2007
Posts: 440

Lars,
useless to say that I appreciate very much your help.
The diameter in millimeter is an odd number: 533 mm; this is why I guessed the inch as unit.
Regarding the rpm, I wrote the official numbers but, believe me, whenever the engine turns faster than 2100 rpm, I deeply suffer. I think I could even change the setting of the injection pump myself but letting the engine idle at 3380 rpm even for a fraction of second would give me nightmares for a week!
Can you still help me or I am just an hopelessly desperate case?
:-)

Daniel, 411-004

27 May 2010 - 16:51
#25
Join Date: 02 January 2008
Posts: 1428

Daniel

Thank you for your comments

You can sleep well, and forget about the nightmares. It is not necessary to adjust the high idling rpm, you can continue to use lower engine outputs of your preference without any adjustment.

The purpose of the adjustment is to limit the maximum rpm for a pleasure rating engine in a low rpm application like yours. The adjustment prevents the injection pump from overloading the engine when a hard fisted user (not you!) slams the throttle into full position.

Your particular case can well be used here as a continuation of the propeller discussion to demonstrate the effects of changing the pitch setting. It is clear from the previous Forum discussion that increasing the pitch will increase the cruising speed, or lower the cruising rpm, and you now asked what the proper setting would be in your case.

The engine manufacturer's recommended pitch setting for the commercial rating specifies that the engine should be able to reach 3000 rpm, and calculation shows that the corresponding pitch setting is 20 degrees.

Here it may not even be possible to try this maximum rpm in practice, as this is far over your acceptance limit.

A pitch higher than 20 degrees will prevent the engine from reaching the mentioned rpm, and reduce the available power.

You considered the use of 22 degrees pitch, and it can be commented that this will allow cruising at 2060 rpm, which is approx 140 rpm less than with the 20 degrees setting you used earlier. This will give about 6 knots of speed, and approx 15 hp is used.

Every 2 degrees increase of pitch will lower the cruising rpm roughly by 100 rpm

Let us see what happens if you set the pitch to 30 degrees, which is the Max-Prop upper limit.

For cruising at 6 knots 1670 rpm is then sufficient, assume this sounds good?

In calm weather this works well, and the engine still has some reserve left. At full throttle it can reach about 1900 rpm maximum, which corresponds to 25 hp, but full throttle must be avoided as the engine is severely overloaded by this.

The real problem comes if there is a headwind. When the speed drops, the engine has to work harder, but there is no reserve, and the engine can not cope with the demand. It might overheat very soon, and has to be shut down.

With 20 degrees pitch the available engine power in a strong headwind is 33 hp, this drops to about 15 hp with 30 degrees of pitch. The limitation with large pitches is that engine rpms are held down, and reserve power becomes severely restricted and may be unsufficient if there happens to be a serious demand.

In terms of propeller efficiency increasing the pitch does not give an advantage here. The best efficiency is achieved for the cruising condition with 20 degrees pitch.

The choice of pitch in this case is a balance between a wish to use low cruising rpms, while retaining enough engine reserve power for performing against headwinds. It is suggested that you consider the actual conditions where the boat is used to strike the right balance. If you count on sailing when it blows higher pitch settings can be used.

Pls feel free to ask more questions if something is unclear.

Lars

27 May 2010 - 22:30
#26
Join Date: 30 January 2007
Posts: 440

Thank you Lars,
your note is extremely clear and solves a question I never understood completely before.
To summarize in very few words:
1) higher propeller pitch --> lower engine rpm for the same boat speed;
2) lower engine rpm --> lower engine power produced (I understand that this is one of the main problem in engine design);
3) lower engine power produced may be not sufficient to overcome the opposing forces and keep the required speed.
The only way to avoid a conflict between 1) a 3) is finding an optimal compromise.

Another thing that I tend to forget is that low rpm do not imply always lower fuel consumption and better engine care. I tend to be more sensitive to the noise from the engine than to the actual position of the throttle; this is quite natural but not right and a compromise is also required.

Thank you again, Lars!

Daniel, 411-004

30 May 2010 - 19:53
#27
Join Date: 02 January 2008
Posts: 1428

Daniel

Good to hear that my comments were helpful

A question referring to the engine noise - do you know if there is an induction silencer on the air intake? This would lower the noise level appreciably.

These silencers were introduced around the time when your yacht was built. I seem to remember them as oval horizontal pots on the side of the engine.

Should be available from Perkins

Referring to your item 2) engine rpm and power, some newer diesels, particularly bigger units, have very peculiar power curves. Their power is constant over a big part of the rpm range, and the power curve is a horizontal line, while the torque curve rises linearly as rpms drop.

This is achieved with electronic injection control, and these engines are intended for trucks and locomotives, where it is an advantage that the speed can be kept up also uphill.

These engines must not, however, be loaded continuously at the minimum full output rpm.

Best regards

Lars

30 May 2010 - 22:30
#28
Join Date: 30 January 2007
Posts: 440

Lars,
no, my engine does not have that kind of silencer and, actually, this is the first time I hear about; it is interesting and I will inquire Perkins.

As far as you know, is there a way to have a better information on the usage of the engine? The only fast information available is the rpm gauge while the water temperature responds probably too slowly to an incorrect functioning (i.e. too low rpm for the produced power). Would, for example, a diesel flux meter or a vacuum meter inserted in a proper place give useful information?
The instant arrangement of the little pistons within the injection pump that decide the injected fluid amount could be an important information but I don´t think it has ever been made available (probably for some serious reason I ignore).
I hope I am not annoying you nor anyone else on the forum...

Daniel, 411-04

31 May 2010 - 02:07
#29
Join Date: 29 January 2007
Posts: 811

Dear Daniel,

annoying? This is one of the most interesting thread of this forum, so, many thanks to Mike to have started it, and of course to the Professor for his extremely useful help and sharp comments, and to you for keeping it running!

Fair winds!

Matteo (38/067 Only You)

31 May 2010 - 10:04
#30
Join Date: 30 January 2007
Posts: 32

Yes  Matteo, I agree that this discussion has been very valuable and informative and really show how helpful this discussion forum is.

I now start to understand why my experience with my new stronger engine in combination with the Kiwi feather prop has been so positive . The old 25hp Yanmar engine with a twoblade folding prop just had to be run at higher rpm (and noice) to give a speed of 6 knots in my Swan 40.  With this new stronger engine (42 hp Vetus) and a Kiwi prop I can reach the same speed at much lower rpm and noice and the diesel consumption is almost lower. And which is also essential: there is a leftover capacity to force the boat forward even under tough condition (I had an unfavourable experience from a situation when it was impossible with my earlier setup to advance forward  against strong wind and high short waves in one of the big lakes when for various reasons I had to take down the sails). This is no longer a problem, which is of course also a question of increased safety.  In addition of course it is also much easier to go in reverse direction with the feather prop in comparison with the folding prop, which of course is thanks to the characteristics of a feather prop, as already discussed. However one also has to learn (from experience) about the delay when changing from forward to backwards while the blades turn around, especially if the boat still has a velocity in the water when you want to  change.  From the discussion I start to understand why the pitch of 22 degrees and 17 inch diameter, which was recommended from the factory, was probably the right recommendation as verified from my practical experience.

One question which however remains to be answered, and where I have got different opinions from those asked, is whether it is deleterious for a diesel engine not to be run at high rpm during long periods  for the lubrication or not. I have got as many opinions as the number of participants in this discussion. Although the max speed is given to be 3000 rpm for my engine, there is little use of running it above 1500-1800 rpm at which I reach a convenient 6 knots and the noice is relatively low, and most essential the diesel consumption very acceptable and almost the same as I had with my 25 hp Yanmar engine.

Fair winds (which means less need of the engine)

Leif

31 May 2010 - 14:31
#31
Join Date: 16 May 2009
Posts: 252

Yes  Matteo, I agree that this discussion has been very valuable and informative and really show how helpful this discussion forum is.

I now start to understand why my experience with my new stronger engine in combination with the Kiwi feather prop has been so positive . The old 25hp Yanmar engine with a twoblade folding prop just had to be run at higher rpm (and noice) to give a speed of 6 knots in my Swan 40.  With this new stronger engine (42 hp Vetus) and a Kiwi prop I can reach the same speed at much lower rpm and noice and the diesel consumption is almost lower. And which is also essential: there is a leftover capacity to force the boat forward even under tough condition (I had an unfavourable experience from a situation when it was impossible with my earlier setup to advance forward  against strong wind and high short waves in one of the big lakes when for various reasons I had to take down the sails). This is no longer a problem, which is of course also a question of increased safety.  In addition of course it is also much easier to go in reverse direction with the feather prop in comparison with the folding prop, which of course is thanks to the characteristics of a feather prop, as already discussed. However one also has to learn (from experience) about the delay when changing from forward to backwards while the blades turn around, especially if the boat still has a velocity in the water when you want to  change.  From the discussion I start to understand why the pitch of 22 degrees and 17 inch diameter, which was recommended from the factory, was probably the right recommendation as verified from my practical experience.

One question which however remains to be answered, and where I have got different opinions from those asked, is whether it is deleterious for a diesel engine not to be run at high rpm during long periods  for the lubrication or not. I have got as many opinions as the number of participants in this discussion. Although the max speed is given to be 3000 rpm for my engine, there is little use of running it above 1500-1800 rpm at which I reach a convenient 6 knots and the noice is relatively low, and most essential the diesel consumption very acceptable and almost the same as I had with my 25 hp Yanmar engine.

Fair winds (which means less need of the engine)

Leif

Leif

In my experience you do not have to worry about running your engine at 1800 most of the time, vs let's say 3000 rpm. As you probably already know from looking into the question, it's the load (total HP demand) and not the speed alone that most people look at. In any case, unlike gasoline engines, the diesels have a greatly reduced tendency to make carbon deposits, and therefore don't need to be 'blown out' like gasoline engines that are hardly ever run hard. Low speed and low to moderate load will lengthen, not shorten, the life of your engine.

Geoff in WI

31 May 2010 - 16:22
#32
Join Date: 02 January 2008
Posts: 1428

Daniel

For older engines without electronic injection control the main parameters indicating overload are exhaust temperature, cooling water temperature, and the colour of the exhaust gases.

It is suggested that you install an exhaust temperature gauge, according to old literature Perkins recommends a maximum of 670 deg C for the 4.108.

It would also be good to calibrate the cooling water gauge, and add audible alarms for both gauges.

Suggest you also consult Perkins on this matter.

Provided that you determine the maximum rpm that can be attained in the prevailing conditions a rule of thumb is that using an rpm which is at least 300 rpm below the maximum on each occasion means you are on the safe side.

Best regards

Lars

01 June 2010 - 16:37
#33
Join Date: 03 March 2007
Posts: 197

Dear all,
I am really learning from this thread!!

An update on my Kiwi prop and my Vetus 3.10. As mentioned on the web the KIWI can stall small engines when put into reverse due to the extreme pitch. The first 10 tries at putting Storm Svale into reverse resulted in stalling the engine. Trial and error has resulting in me finding a way to keep the engine running. The results in forward and reverse are wonderful. Much more powerful in reverse ( max rpm 2200) and a higher top speed at cruising rpm.

A question to the Prof. Do the newer common rail engines suffer when run at lower RPMS? Mine is an older style system and I run her at about 300 RPM under max RPM as defined by the manufacturer which is what i get at full throttle. Seems I am following your suggestion for this engine.

01 June 2010 - 20:30
#34
Join Date: 02 January 2008
Posts: 1428

Mike

The newer engines with electronic injection control often have load sensing enabling them to effectively avoid overload conditions. The driving force behind this is the stringent environmental requirements on clean exhausts for new engines. Smoking must be avoided when heavily loaded, and this limits the amount of fuel injected, which on small engines may show up as a tendency to stall when gear is engaged.

Perhaps the suggested 300 rpm reduction needs to be clarified further. The reference for this is the highest achievable rpm in the prevailing conditions, and there may be considerable variation. For example in a strong headwind the highest rpm as well as speed will be reduced, and therefore also the safe rpm will be lower than in calm conditions.

The opposite would be ideal, i.e. to be able to use a high rpm when the resistance is high, but this requires a controllable pitch propeller.

It is not advisable to increase the pitch too much in order to gain better cruising speed.

One way to find out how the chosen pitch works in a strong wind is to keep the boat stationary, use full throttle and then drop the rpm by 300, and read off the available power from the propeller curve shown on the engine leaflet. This is not accurate, and will underestimate the power to some extent, but gives an indication. Measuring the bollard pull at the 300 rpm deduction would give actual information, but few yachts have equipment for this.

Best regards

Lars

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