Feathering Props

Discussion in 'Props' started by benjy1966, Jan 29, 2011.

  1. benjy1966
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    benjy1966 Junior Member

    Ahoy All,

    Does anyone have any experience with the 4 blade Variprop feathering prop? I have a dana 24 with a Kiwiprop fitted but it makes a fair amount of noise. The most likely reason would seem to be the fact that the Dana has a prop aperture. I'm told that the 4 blade Variprop is more suited to this kind of set up.

    Of course the Variprop is top quality and built in Germany. It looks the business and so it should be at nearly 3000€ for a 15" dia prop! Why is it that I am always drawn to the most expensive option?

    Comments and thoughts much appreciated.

    Thanks

    Benjy
     
  2. michael pierzga
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    michael pierzga Senior Member

    4 blade prop is a Bit overkill for a 24 ft boat. 4 blades are for special applications..slow shaft speed or an engine output so powerful that a three blade will not present enough blade surface area.

    and What is noise ? Vibration? Cavitation ?

    Is your prop correctly packed with grease ? Is you engine alignment correct? Is your cutlass bearing worn ? Is your flex shaft coupling still within original tolerance ? When you crank up your back stay and rigging loading does the boat bend and pinch the shaft ? Is your prop all bogged up with paint ? any dings in the blades ? are the bladed sloppy...loose fitting ? Was your prop correctly fitted to the shafts and torqued ? a typical misinstallation on all props is grease contamination on the prop shaft to prop taper. Props must be installed on a perfectly clean, dry shaft...no grease.

    Best to first identify the noise on your Kiwi before considering a change.

    Every MAX feathering prop Ive ever used becomes noisy when the grease gets blown out.

    Feathering props are inherently sloppy..the blade to hub fits cant be tight or the prop wont fold....this sloppiness is controled by grease packing . a grease dry MAX prop vibration becomes evident when the prop load changes....... when motoring...prop loaded..... when motor sailing ,wave surfing, prop unloaded.... cycle'

    Is your prop vibrating during both loaded and unloaded motoring cycles ?
     
  3. benjy1966
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    benjy1966 Junior Member

    Ahoy Michael,

    Some excellent questions here! My initial thoughts were that if I could reduce the overall diameter it might help things. The 4 blade would add area so I could reduce dia.

    The noise is cavitation. If you watch this vid, right at the end I you can clearly see the cavitation on the blades.

    The whole installation is correct, from coupling, engine mounts, engine alignment, cutless bearings, to no grease on the shaft etc etc. The boat is a Pacific Seacraft Dana 24 and it's one incredibly tough boat which doesn't flex. The prop isn't that old, has been regularly maintained, no nicks in blades etc. It has made noise from the start.

    I believe it's down to the fact that the Kiwiprop blades are very close to the hull. Kiwiprop say this has nothing to do with it and say it's all about hull shape and prop aperture. The noise is most noticeable at engine revs less than 2200 rpm. Engine is a 3YM20, 2.62 to 1 reduction. Prop has 16" dia. Original prop was two bladed 15". I wrote a post about the Kiwiprop that you might find instructive?

    Interestingly many Dana owners have Maxprops and they all report a similar noise at around 1600 rpm which would substantiate the hull form theory.

    Kiwiprop seem to think that bigger blades and less pitch will help even though it's pushing the tips very close to the hull. Much less that the 10% diameter rule.

    The Variprop is designed for small prop apertures. I know it's overkill but it would be nice to have a quiet engine installation. There's a lot to be said for it. Plus the Dana is a heavy boat (over 4 tons) so the extra blade area wouldn't hurt.

    Back to you!

    And thanks
     
  4. baeckmo
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    baeckmo Hydrodynamics

    First a question: do you reach full power rpms (3200-3500 rpm) with the pitch setting you have? (It may turn out that another pitch setting could reduce the problem).

    Second: the propeller peripheral tip speed at 2200 rpm is just 17.5 m/s. There is no way a correct propeller could generate low enough pressures to cavitate at that velocity, so there is a serious fault in shape. What you see is actually not pure cavitation, but gas release, which occurs at far higher pressures than the critical vapour pressure. Anyway that is a bad enough sign!

    Then: judging from Kiwiprops pictures, those blades have about the lousiest blade shapes you will ever find. Only when stationary (=sailing) are they reasonably adequate, hydrodynamically. In particular, the tip shape is prone to generate high levels of tip vortices, that you experience as noise. The mechanical components along the power transmission "feel" this as vibrations.

    Wether the blade shape is dictated by the function of the pitching mechanism, or just neglect, I can't tell, but there is no way this can be made an efficient propulsor; noise is telling you that!

    Yes, it may be that you need more blade area, but a four-blade is not the way to go in an aperture like yours. Two blades will enter and leave the vertical wake simultaneously, doubling the pressure pulse generated; ie more noise!

    But most of all, you need a correct shape on the blades, which you can't have with your present setup!!!!!


    BTW: what camera setup did you use for the "wet job"?
     
    Last edited: Jan 30, 2011
  5. benjy1966
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    benjy1966 Junior Member

    Ahoy Baeckmo,

    More intelligent questions. There is some talent on this site!

    I have tried all sorts of pitch settings. Nothing makes any difference. You're right, the blade shape on the Kiwiprop is, well not ideal as you say but what is odd is that a mate has the same prop on his boat and it is completely silent. Same engine, reduction etc. His boat is steel and the only other difference I can see is that he has a P bracket and the prop is out on it's own without any deadwood in the way.

    Interesting what you say about the 4 blades. Variprop say they have fitted it to many boats like mine and everyone is always very happy. This may be because there is 'some' shape to the variprop blades where there is none on the Kiwi. The original 2 blader gave some vibration as it passed the deadwood but did not make noise.

    I would like to try the Variprop but 3000€ is a lot for a prop that may not work! Any suggestions?

    Camera was a GoPro HD hero on a boat hook!

    Cheers
     
  6. baeckmo
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    baeckmo Hydrodynamics

    Now, you didn't answer my q on full power rpms; there is more to it than just pitch setting! The boat with the p-bracket and no noise is actually proof of the pudding. Without the angle of attack variation from the deadwood wake, the blades can adjust better to a "steady" inflow. But make no mistake, those blades are just slightly better than a piece of firewood, efficiency-wise.

    If you look at the "Variprop" you see that the blade leading edge is swept back, which makes the wake entrance/exit progressive, compared to the "Kiwiprop". This explains its better behaviour in a non-steady flow. The only way to get an improvement with your installation is to spend some money on:

    A/ Trimming down the deadwood to streamline shape, plus:
    B/ Finding an optimum axial position, where both the wake influence and the pulse impact on the rudder are minimum.
     
    Last edited: Jan 30, 2011
  7. benjy1966
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    benjy1966 Junior Member

    When I first got the Kiwiprop pitch was preset to 21 degrees and the engine could reach 3600 rpm (max) and it was noisy right from the start.

    I have not made the pitch finer as I have no desire to motor at cruising speed doing 3000 rpm! I tried progressively courser pitch but to no avail. Noise always present. It is possible that with a finer pitch I could eliminate the noise but only by running the engine at higher revs, making more noise and using more fuel.

    Further info. Engine replaced by Yanmar because the old one was rubbish and this one has only done 150 hrs. It has a riser but not the stupid 40 year old cast design that Yanmar insist on using, this is a proper large bore bronze riser without restriction. I can't be certain but I'm pretty sure the engine is producing the full 21 horses. Boat bottom is very smooth (coppercoat) and kept clean. Trim is usually pretty good and I'm careful to keep weight out of the ends of the boat.

    Engine has new Yanmar soft engine mounts, (yes, I know they are different front and rear and are correctly fitted) Also have a Vetus Bullflex flexible mounting although engine is well aligned anyway. Also have a PSS shaft seal fitted. Last time boat was out I checked the Original cutlass bearing (5 years old) which still has very minimal play.

    I can't think of any more info that you might need.
     
  8. Joakim
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    Joakim Senior Member

    All feathering props have "straight" blades in order to minimize drag while feathered, thus they have constant angle and not even close to constant pitch as a propeller should have. This makes them inefficient and may also be the cause of noise.

    In your Kiwiprop review you mention that the old two bladed propeller was not noisy. Why did you replace that? Was it fixed bladed? If you wanted to reduce drag, a folding propeller may be a better choice. They typically have proper pitch although the blades are rather thick for optimum performance in order to have enough weight to stay open on reverse. Do you have enough space for a folding propeller to fold? Something like 15x10 should be close.

    You don't need the extra area of four blades even at 15" diameter to avoid cavitation.
     
  9. benjy1966
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    benjy1966 Junior Member

    Ahoy Joakim,

    The original two blader was fixed, caused drag and could not stop the boat which is heavy. Sailing and stopping performance was massively enhanced with the Kiwi at the price of noise. I agree that a folding prop would probably work better but there's no room which is why I went down the feathering road.

    Still no nearer to a solution to a quiet prop!
     
  10. pjssailor
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    pjssailor Junior Member

    Not so quiet a prop

    Hi Benjy

    Greetings from Florida

    I think you have the problem pretty well figured out. The Dana 24 has a built in problem that makes it difficult to get a 3 blade prop feathering prop quiet. You know that both the maxi and the kiwi are noisy. I would suspect a 4 blade to add to the problem- but I have been wrong about so many things in my life I would not bet more that we could drink on that..

    Before I spent another nickel I would continue my search amongst Dana sailors to see if anyone has solved the problem. Nothing wrong with putting the problem to manufacturers, but I would be cautious. $4,000 UDS for a prop on a 24ft boat? I would not even think about it unless I really used the boat an awful lot and was sure I would be using it a long while. And I would not be too sure any manufacturer would tell me it it had complaints.

    As I understand it you need stopping power. Boy do I understand that. I need the same on an outboard powered light Cat. I sure like the 2 chunks of firewood I have under my 10M Catalac. They do a nice job of both power and backing down – they are tough and Cheap – and they sure help make that tank sail a lot better.. Not great but she tacks without back winding and she will make progress to windward.

    I am not sure how well the Kiwi works except for the noise. Does it power you boat decently? It is a bit of a club you know. But if it works decently and if you don't really motor a lot it might be tolerable until you really nail down something that you KNOW is better.

    I would try to find something you think is now working well and go see it and sail it and be darn sure. MY guess is there is no solution short of reworking the aperture.

    Maybe like me you can be satisfied with it being “just a boat”.

    Lots of luck. Lloyd:
     
  11. benjy1966
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    benjy1966 Junior Member

    Ahoy Lloyd,

    Satisfied with it just being a boat! Never!

    I agree that $4k is a huge amount to spend but it's less than a new engine, a set of sails or any number of other essential boat bits. My boat is the second and last boat I shall be buying, it is a 25 year plan so frankly I don't mind paying a lot for something that works as I will be using it for decades (hopefully!) And yes, sadly we do tend to motor a lot being as we are in the Med and since the rest of the set up is so sorted it just annoys me that I can't get it just right.

    The Variprop has slightly shaped blades with rounded bases and a very large blade base. They have a built in shock absorber and a 5 year guarantee. It's funny, everyone who has commented so far has said what a poor prop the Kiwi is and that the Variprop 4 blader is overkill and probably won't be any quieter anyway. Sadly, no one has yet come up with any alternatives that might work.

    Reworking the aperture is not going to happen so I need to find a prop that will work in that aperture and give me the results I want. The Kiwi performs well but makes noise otherwise it seems to perform well considering the simple blade shape.

    Sadly Variprop won't let me try a prop before committing which is not good enough as far as I am concerned. No way am I going to cough up that much for something that might not work!

    For anyone interested I did write another post about choosing the right folding/feathering prop for your boat. I've really looked into this but short of trying each prop on the market I don't see what I can do except put back the original 2 blader but it couldn't stop the boat and it stopped it sailing in light airs so that's not really an option. At the end of the day I would rather live with noise than spoil motoring or sailing performance.
     
  12. Joakim
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    Joakim Senior Member

    In your linked post you claim that three blades is clearly better than two and blade area is very important. These are only true, if cavitation is a problem due to limited diameter. In your case cavitation is not a problem (at least with 16" diameter), thus 3-4 bladed propellers will not offer better performance than an equally well designed 2 bladed. Actually vice versa, too much blade area just lowers efficiency.

    How wide blades did the original propeller have? Were you able to align it to deadwood while sailing? I don't think a 2 bladed propeller would slow you down that much on a such a heavy boat like yours.

    The blade shape of Variprop looks quite similar to Maxprop, thus I would expect it to have the same noise problem. Were the Maxprops 15" or 16"? With big blade area 3/4 bladed you should have no cavitation problems even with 14", which may help to noise.
     
  13. benjy1966
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    benjy1966 Junior Member

    Ahoy Joakim,

    Original 2 blader had fairly fine blades and caused a surprising amount of drag. yes it could have been stowed aft of the deadwood but I still wouldn't be able to stop the boat!

    The Variprop is quite different to the Max prop actually. The blade bases are much smoother and rounder on the Variprop, on the Maxprop they are cut of sharp. Could make quite a difference. Maxprops with noise all 15" but running on slightly less power. My boat has 21 to their 18 hp.
     
  14. pjssailor
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    pjssailor Junior Member

    Fast Dana

    Hi Benjy

    Greetings from Florida

    Of course it really is one of the great boats. When I say “just a boat” I do not mean “just ANY boat” I am really talking about the 2nd love of my life. Janet being the first.
    By any standard I can think of a Dana 24 is not “ just any boat”. I hope that clears that up. It is my way of keeping my perspective. It really is not quiet mortal. I have trouble that way.

    Speaking of the Med Janet and I put 2500 miles on “Serendipity” in the Med in 2006. We will be in Cervignano del Friuli around the 15th of “April for 3 months sailing and visiting that part of the world.. We will use TIM Alice for Email. If you want we might exchange Email addresses.. I believe I have indicated I will accept Email though this site.

    I suspect you may find a 2 blade folding prop that will work acceptably well. If I recall correctly, Practical Sailor did a nice study on them a few year ago. I also suspect a 4 blade prop might be worse.

    I assume you have tried to find out what Chris Humann used on his pretty fast Dana. I think there were a bit over 500 of them built. Might even contact the new builders. I gather they have some of the original employee's building them. They may even have already solved it.


    Lots of luck. Lloyd
     

  15. benjy1966
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    benjy1966 Junior Member

    Ahoy Lloyd,

    Yes, I too have email alerts on this site. Do drop us a line when you're about!

    To be honest, I think I am just Anal. I suspect every other Dana owner has some prop noise at one set of revs or another and just learn to live with it, or accept it as one of those things. I'm not like that! Often we do a canal trip where the engine is running slowly and what would be a perfectly idyllic situation is spoiled by a noisy prop.

    I have no intention of giving up. I will find a solution. I'm like a terrier in that sense!

    Let me know if you're in our neck of the woods when you visit (st Tropez)

    Fair winds
     
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