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Old 01-19-2016, 08:34 AM   #21
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"If fewer blades is better, why do the new super efficient container ships use 5-6 blade props? Is it because their RPM's are down in the 450-500 range and need that many blades to take the HP?"

Good question , as the engines operate at about 90RPM I would guess the shaft is sped up to reduce the propeller diameter.

Induced drag , the penalty of each blade moving thru the water is why a 1 blade is most efficient in producing thrust.

As noted efficient thrust may not be the basis for a prop selection , vibration reduction or a modest draft requiring a small diameter prop are other concerns.

For a production boat the "proper" prop selection will have been thrashed out by 3-5 prop installs during the first few years of production.

For a repower -up or down , 3rd time lucky seems to be the norm.
Good input Fred

Of the two paragraphs I bolded in your post copy above I also quoted here for brief commentary.

1. "Induced drag , the penalty of each blade moving thru the water is why a 1 blade is most efficient in producing thrust."

I learned the above when young in and on boats by hanging out with experts. I also learned that of the induced "drag" created by any prop blade it is the cutting edge and following edge of the blade that initiates/creates the most "drag-disturbance" in the water that affects the solid flattish area of a prop. And, IMO these drag coefficients only go to figure regarding rational thinking of a solid item (a blade) cutting through a fluid substance, i.e. in this case water. Therefore, (and, I ask this question as not an engineer expert for a contoured material creating thrust movement through water) if it is not prop vibration (due to modern prop-balancing equipment) that stops the use of a 2 bladed prop, and there is ample room for the needed diameter of a 2 blade prop... then why do we not see the efficiency of two bladed props being used on boats? Additionally, I ask... if it is the cutting and following edge of prop blade that initiate/create the drag... then why do props have blades at all? Wherein it seems via rational engineer thinking that a solid-sheet corkscrew "prop-design" would provide ultimate "water-cutting" efficiency... while also offering several other indices of improved boat use capabilities! In essence... That solid-sheet corkscrew would amount to a 1 blade prop with only a cutting edge and no "real" following edge.

2. "For a production boat the "proper" prop selection will have been thrashed out by 3-5 prop installs during the first few years of production."

Exactly! That is one reason why I feel it best to not purchase first runs of boat or car model designs (even from the highest quality manufacturers); but rather, to let these new models pass through at least a couple years of build-outs so that most is not all their kinks can get worked out.
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Old 01-19-2016, 08:52 AM   #22
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When you talk about backing efficiency as being important in boat use, part of the shape of the blade changes so that it bites the water better in reverse. I remember my prop guy talking about this, but I got lost in the fog. Props are one of those areas where the variables between blade number, blade shape, reading edge, trailing edge, diameter, pitch, hull shape, draft, desired speed, and whether or not your towing, make it necessary to seek a professional unless you want to do a lot of trial an error.

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Engineering is a creative effort wherein math is usually the deciding factor used to prove out designs initiated by postulation. The proving of the postulated designs usually takes many trial and error run-throughs until satisfactory design features have become available. In essence, reaching the point[s] to what effect the postulation was seeking to achieve. Some "new" design experiences are winning successes... some not so much... some are unfortunately abject failures.

Props on boats have had a really good run for over a century regarding their basic multi blade design. There just may be a new course being laid for boat props... could be interesting for sure! We shall see...
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Old 01-19-2016, 09:49 AM   #23
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Ted

Engineering is a creative effort wherein math is usually the deciding factor used to prove out designs initiated by postulation. The proving of the postulated designs usually takes many trial and error run-throughs until satisfactory design features have become available. In essence, reaching the point[s] to what effect the postulation was seeking to achieve. Some "new" design experiences are winning successes... some not so much... some are unfortunately abject failures.

Props on boats have had a really good run for over a century regarding their basic multi blade design. There just may be a new course being laid for boat props... could be interesting for sure! We shall see...
Ok.

Nice thing about displacement speed, what works....works. No need to go hunting for the elusive extra knots.

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Old 01-19-2016, 10:35 AM   #24
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I also learned that of the induced "drag" created by any prop blade it is the cutting edge and following edge of the blade that initiates/creates the most "drag-disturbance" in the water that affects the solid flattish area of a prop. And, IMO these drag coefficients only go to figure regarding rational thinking of a solid item (a blade) cutting through a fluid substance, i.e. in this case water.
I may have misread your description of drag, but I believe you aren't considering both form drag and surface drag. A fewer bladed prop would be likely to have less form drag but the large amount of surface area of a spiral shape would create a lot of surface drag. Higher aspect ratio foils (which are what the blades are) have better lift to drag ratios (that is why a fin keel sailboat sails better to windward than a full draft boat). So a corkscrew shaped prop isn't going to be very effective at creating lift (think Davinci's helicopter). Another consideration about the foil shape of each blade is the stall behavior, a high aspect foil is more prone to stalling and by extension a prop with to high of an aspect ratio would be less effective when changing direction, as in a docking situation. My brother's sailboat has a two bladed fixed prop with minimal surface area, this reduces the drag while the boat is sailing (not as much as a folding or feathering but cheap and simple for a low performance boat) but the boat is less effective at changing directions (forward or aft) under motor. I was involved in a human powered submarine project in college, we used a very high aspect ratio, counter rotating prop system that was about 30" in diameter, it was good for it's purpose but would be lousy for a trawler. The will always be varying benefits and limiting factors for every situation that need to be weighed out.
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Old 01-19-2016, 10:43 AM   #25
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I may have misread your description of drag, but I believe you aren't considering both form drag and surface drag. A fewer bladed prop would be likely to have less form drag but the large amount of surface area of a spiral shape would create a lot of surface drag. Higher aspect ratio foils (which are what the blades are) have better lift to drag ratios (that is why a fin keel sailboat sails better to windward than a full draft boat). So a corkscrew shaped prop isn't going to be very effective at creating lift (think Davinci's helicopter). Another consideration about the foil shape of each blade is the stall behavior, a high aspect foil is more prone to stalling and by extension a prop with to high of an aspect ratio would be less effective when changing direction, as in a docking situation. My brother's sailboat has a two bladed fixed prop with minimal surface area, this reduces the drag while the boat is sailing (not as much as a folding or feathering but cheap and simple for a low performance boat) but the boat is less effective at changing directions (forward or aft) under motor. I was involved in a human powered submarine project in college, we used a very high aspect ratio, counter rotating prop system that was about 30" in diameter, it was good for it's purpose but would be lousy for a trawler. The will always be varying benefits and limiting factors for every situation that need to be weighed out.
What if the solid-surface area of prop blades... be they any number of separated blades or even a solid "bladed" cork screw never had the water they are "screwing" through touch them. In essence a forced separation of surface metal to water contact which then removed the aspect of frictional drag regarding water contact with the solid prop material.
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Old 01-19-2016, 11:20 AM   #26
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Art wrote;
"I learned the above when young in and on boats by hanging out with experts. I also learned that of the induced "drag" created by any prop blade it is the cutting edge and following edge of the blade that initiates/creates the most "drag-disturbance" in the water that affects the solid flattish area of a prop."

If you compare the drag of a prop to the drag of a hull parasitic drag is that of water flowing over the hull's surface. Induced drag is the pushing the water aside ... the kind of drag that makes waves/wakes. If the mass of a hull was put into a flat plate 1/8" thick and moved through the water there would be almost no induced drag but very high parasitic drag. The pitch of a prop is equivalent to the wide hull pushing it's way through the water and the blade area is equivalent to the huge flat plate. Both have high drag if excessive.

With a 4 blade prop most all of them will have more blade area than a 3 blade prop. If you have a 3 blade prop and get a 4 blade (that will load your engine correctly) it will have about the same blade area per blade and hence more blade area ... so it must have less pitch to load your engine properly. In this case above your parasitic drag will go up and the induced drag will go down.

If one goes to a prop that favors either too much pitch or too much blade area thrust will be lost along w efficiency. Considering the speed of the boat there is a range of pitch/dia ratios that are most efficient. Ideally one would be approximately in the middle of this range. If you're not in this preferred PD ratio efficiency will be low. A 4 blade prop w area too high and pitch too low or a 3 blade w area too low and pitch too high both will get you in the low efficiency area of the curve.

You've got to have enough pitch to throw enough water to create the desired thrust. And the highest thrust is maximized by being in the preferred pitch/diameter ratio. And even w my slow 6 knot boat there's a big difference between max static thrust and thrust at 6 knots. Max static thrust will usually be found w max dia but there are very real limits to that also.

Many trawler owners feel a 4 blade prop will deliver more thrust .. and it will but only if the prop clearance is too small/low and the best prop won't fit. Trawlers w enough room for the optimum prop will have a 3 blade prop and those w limited clearance will have a 4 blade. There are exceptions to this but it's mostly true. However I would'nt rush out to change out your 4 blade prop. The difference is fairly small .. but it could be considerable in many cases. But if you ding the prop or find that it's too thin or pink consider a 3 blade prop. Run the numbers (Vic Prop ect) and talk to the prop people and engineers if you can. My opinion is just my opinion but I think too many trawlers have one too many blades.
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Old 01-19-2016, 11:31 AM   #27
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What if the solid-surface area of prop blades... be they any number of separated blades or even a solid "bladed" cork screw never had the water they are "screwing" through touch them. In essence a forced separation of surface metal to water contact which then removed the aspect of frictional drag regarding water contact with the solid prop material.
That is essentially what happens when a prop cavitates and it results in 100% slip. Basically the boundary layer wouldn't stay where you want it to and the prop spins in air. What is effective is surface piercing drives where shaft is at the surface and the blades exit the water each revolution. As you may have guessed they have their own drawbacks limiting trawler suitability.
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Old 01-19-2016, 01:03 PM   #28
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I have no valuable inputs on the number of blades when it comes to running at speeds around hull speeds. When it comes to cruising at higher speeds I have found that 4 blade props have increased the performance on a number of my boats from 19' up to 47". On the lightest and fastest of these boats I did expect to find some loss of high end speed but that was not the case as the speeds were just about a draw. I have seen improvements in ability to plane off, hold plane at a lower speed , fuel use and with the smaller boats a decrease in porpoising and an increase in watersports capability.
Hope this helps
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Old 01-19-2016, 01:59 PM   #29
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One thing to keep in mind is that if the blades line up with the strut, as in a 3 blade prop on a boat with a V strut or a 4 blade prop behind a full keel, smoothness can be compromised. I can look it up in Gerr's book if anyone is interested.
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Old 01-19-2016, 02:00 PM   #30
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That is essentially what happens when a prop cavitates and it results in 100% slip. Basically the boundary layer wouldn't stay where you want it to and the prop spins in air. What is effective is surface piercing drives where shaft is at the surface and the blades exit the water each revolution. As you may have guessed they have their own drawbacks limiting trawler suitability.
Howard Arneson's shop in nearby town. I've been in it and seen his work; watched his speed boats perform in SF Bay. Howard passed 10/13/2014. A genius in prop design and other things.
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Old 01-19-2016, 04:02 PM   #31
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I set mine up with a 3b 24x23. Efficiency is good, but at 2000rpm 20kts I get some pretty harsh prop noise. Also, the prop was originally stamped as 24x26, to it has been repitched three inches, too much I think.

I'd love to try a DQX 4b.

And yep, if you have a full keel and get prop noise, an odd number of blades help. Only one in the shadow of the keel at a time.

Props are 87% black art.
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Old 01-19-2016, 04:12 PM   #32
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Greetings,
We run 4 blades. Is there anyone who runs 2 blade props? Since it has been suggested 2 blades are more "efficient", other than potentially increased noise, what would be the drawbacks?
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Old 01-19-2016, 04:42 PM   #33
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To load your engine properly considerably more room will be needed for the large dia two blade prop. There may be more vibration but I think personally that would be unlikely. To get a two blade that will load your engine w the same dia you have now it would require that the blades be twice as wide w double the blade area as your present prop. Finding a two blade w double the blade area (per blade) and the same dia would be very unlikely. Not if you had an IO though as OMC and Volvo had numerous props w larger blade area and low dia. Lots of induced flow around the very wide blade tips. That's where the efficiency goes. What's your dia/pitch now? The blade tips would be so wide I doubt you would experience any increased efficiency at all. Maybe less efficiency.
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Old 01-19-2016, 05:35 PM   #34
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To get a two blade, just buy a 4b and cut two off!! Easy peasy.

Also, I think the container ships have a similar issue with our prop clearance issue, but instead of blades too close to hull, it is blades breaking the surface. So they are dia limited also. I think...
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Old 01-19-2016, 09:14 PM   #35
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To get a two blade, just buy a 4b and cut two off!! Easy peasy.

Also, I think the container ships have a similar issue with our prop clearance issue, but instead of blades too close to hull, it is blades breaking the surface. So they are dia limited also. I think...
Arneson loved his surface props breaking the surface... of course they were doing gazillion rpm pushing speeds in 120 + mph range... that's correct over 100 was just another surface-prop test cruise at 120 +/- mph for Howard.
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Old 01-19-2016, 09:40 PM   #36
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For You old pilots, I can remember as a kid flying in Piper Cubs with single blade propellers. Even then I wondered how does that work. Might have been efficient but sure was strange looking!
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Old 01-20-2016, 07:05 AM   #37
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Is there anyone who runs 2 blade props? Since it has been suggested 2 blades are more "efficient", other than potentially increased noise, what would be the drawbacks?

Sure , we have a 2 blade in our 90/90 that took 2 years as a special order from Michigan to get.

Small power (35) hp only requires a modest blade area so with a power boat sized blade (not the skinny sail boat blade) it works well.

By using a, indexing prop lock the larger diameter and blade area is hidden behind the deadwood , causing only a skin friction penalty , the cost of the prop form , and God forbid the drag of a spinning prop are avoided.

A 2 blade with large enough diameter for a 3GPH fuel burn could be done

IF the reduction gearing also was in line with the available prop diameter.
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Old 01-20-2016, 07:48 AM   #38
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sail boats use 2 blades a lot, but probably more for the ability to fold them when under sail.
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Old 01-20-2016, 07:57 AM   #39
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Actually...a couple of my sailing fiends took off their 2 bladed fixed props and went to the 3 bladed feathering type.


The feathering types are costly so they tend to show up on the higher end sailing vessels.


The improved performance was significant in all cases.
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Old 01-20-2016, 08:44 AM   #40
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If one has ample room in prop area for their sizable power boat; such as is predominant on TF - D, SD, or P: And, was going to change from a 3 or 4 blade prop to a 2 blade prop... In general, what would be the % change in diameter and could some of the apparently needed diameter increase be overcome by dwell/pitch factors as well as larger square inches per blade?

I wonder - Regarding 2 blades: Estimated improvements for nmpg fuel usage as well as the changed over boat's new handling capabilities (backing especially).

Isn't there a marine propeller expert/business owner on TF who could be brought into this thread? Maybe some member knows a prop expert who could be induced to join TF for this discussion... just might bring him some business! It would be good for all of us propeller-design-conjecturers to learn real numbers and experiences from a thread participant who previously-had or does-now actually live the prop-alteration game - hands on for years, that is.

Prop design is not rocket science - but far as I can tell - it's pretty darn close! LOL
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