Props - 3... 4... 5... - Blades

[Art]

From what I've always understood: 3 blade prop is quite efficient for screwing through the water. 4 blade a bit less so. 5 Blades even less, and so forth.

In old-school days before high tech balancing systems of today the more blades the easier it was to balance a prop.

With today'd improved prop balancing techniques/systems why do some boaters still use more than 3 blade props?

 

[Marin]

The fewer blades the more efficient the prop is. The more blades the smoother/less vibration will be generated when it turns.

Theoretically speaking the most efficient prop for moving water would have one blade, but it'd be a bitch to balance.

Also, the fewer blades the better the prop will "back." In other words, it will have less prop walk. Could be a good thing in a single engine boat, but not a good thing in a twin with counter-rotating props where lots of prop walk is what you want.

Some boats with powerful engines need more blades to make use of that power. And sometimes it's a matter of utilizing the power and minimizing the vibration. For example our friends with a 36' custom lobsterboat had the boat re-engined with a 420 hp Cat. The prop was originally a four-blade. But they were't getting the kind of performance from the boat they thought they should be getting so the shop suggested a five-bladed prop. This improved the performance to a noticeable degree.

So lots of variables. But the basic rule of fewer = efficiency, more = smoothness tends to hold true.

Our boat was built with a pair of three-bladed props with 24" diameter and 17" and 18" pitches. A previous owner replaced these for whatever reason with a pair of four-bladed props ostensibly to the same specs. Perhaps he wanted more maneuvering power (propwalk) from the props, I don't know.

When we thought we needed new props the prop shop we use in Seattle determined that in fact the props were physically fine but had been horribly set up prior to our buying the boat (so by somebody in the SFO Bay area).

On our shop's recommendation they pitched the blades 1" down because they said as a general rule a 4-bladed prop should have 1" less pitch than the same size three-bladed prop on the same boat. They also reduced the diameter of the 4-bladed props by an inch but that was for another reason.

 

[O C Diver]

All depends on what you want your boat to do. I've always preferred 4 bladed props for my charter boat and trawler.

Ted

 

[Gdavid]

Really complex issue to try and answer easily. Many factors involved but keep in mind that it is easier to get more surface area for a given diameter with more blades. Generally you balance drag versus "bite" so there is no single best solution it depends on the boats configuration and intended use. A prop that is spinning faster found on a cigarette boat will be more impacted by drag than a that of a harbor tug. The diameter prop you can use is limited by shaft angle, keel configuration and draft so you don't always have the option of a larger prop.

 

[Nomad Willy]

The reason to use 4 blade props is to increase blade loading to consume the power of the engine. Otherwise w our type of boat 3 blades are best.

 

[Ted]

Marin,
I've seen pictures of a light airplane (piper cub?) with 1 blade and
a short counter weight. Worked well according to the article.

Ted

 

[stubones99]

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?

 

[Marin]

Marin,
I've seen pictures of a light airplane (piper cub?) with 1 blade and
a short counter weight. Worked well according to the article.

Ted

Well, keep it under your hat or all the boaters will be wanting one, too.

 

[Marin]

If fewer blades is better, why do the new super efficient container ships use 5-6 blade props?

I don't think anyone is claiming fewer blades are better across the board. Fewer blades offer advantages in some areas like efficiency. However it's not a one-size-fits-all situation. I suspect the answer to your question may lie more in Eric's comment in his Post #5.

 

[Art]

1960's/70's had 4 blades on one of dad's boats; a single screw Perkins. Did OK cruising, not too good for backing. Other than that always used 3 blade props. The 3 blades we currently have on our Tolly twins run smooth and true. All my previous boats ran well with 3 blades too; and, each was good for backing. If 3 blade props can do all that's necessary regarding smoothness (no vibration) in boats that turn high RPM I could never understand why some boaters want 4 or even 5 blades on low rpm single diesels. Especially as the backing efficiency is reduced. Is it because of prop diameter needing to be too large?? Or... ???

 

[Marin]

I could never understand why some boaters want 4 or even 5 blades on low rpm single diesels. Especially as the backing efficiency is reduced. Is it because of prop diameter needing to be too large?? Or... ???

Possibly that or perhaps they encountered vibration problems with fewer blades. Lots of variables......

 

[Wxx3]

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?

Simple. People like simple answers to complex problems.

Clearly, it's a function of the boat, engine, and power and torque of the engine at the anticipated cruising speed.

The previous owner put a four bladed prop on Dauntless. He ran at 1800 rpm, I almost never do.

I may check out the three bladed prop i have.

 

[Ted]

Well, keep it under your hat or all the boaters will be wanting one, too.

sorry, it's a 1936 Taylor J-2

Ted

 

[eyschulman]

It is mainly a trade off between blade area and blade pitch. In some situations it may be better to do the job with less blades and more pitch in other situations increasing the # of blades and decreasing pitch gets the needed thrust usually with less potential for vibration. I have owned boats with 2-3-4- and 5 blades they all work the 5 blade with soft motor mounts and a aqua drive was the smoothest.

 

[markpierce]

My four-blade propeller is a good match for the boat and it's engine and transmission; boat reaching maximum speed a little more than 90 percent of maximum engine RPM.

 

[Nomad Willy]

Four and five blades sometimes are used to get a higher aspect ratio. See the 4 blade in the green bottom boat. Long narrow blades like on the green boat or the 5 blade lower pic are more efficient. Lower aspect ratio like the prop in pic #2 is less efficient. Most trawler blades are skewed (pic #3) and more efficient but they don't back as well.

But 4 blade props are usually used to handle the higher power of an engine that has too little propeller disc area to accommodate (prop clearance) a 3 blade wheel. Then a 4 blade is used. And perhaps a 4 blade w wider fatter larger blade area blades again to handle even more power.
There is an ideal range of dia/pitch ratios that should usually dictate how many blades to use.

Pics:
1. High aspect ratio four blade.
2. Low aspect ratio three blade.
3. Skewed prop.
4. Five blade high aspect ratio prop.

 

[psneeld]

Like ballast...it depends.


So far the biggest reason I have heard is loading and limited by diameter.


I have never heard that 4 bladed in backing were a big deal. The efficiency of fewer blades is usually noticed up the rpm/load range I thought...so if anything...the 4 and greater numbers may be more efficient. Smaller outboard boats I thought used 4 plus blades for "hole shots".....immediate max thrusting...so that would lead me to believe more thrust at lower rpm....not up the rpm chart.

 

[what_barnacles]

Smaller outboard boats I thought used 4 plus blades for "hole shots".....immediate max thrusting...so that would lead me to believe more thrust at lower rpm....not up the rpm chart.

Yah, and if high rpm running then a 3 blade needed to be used as a 4 blade would kill power at high rpms.

 

[FF]

"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.

 

[O C Diver]

If 3 blade props can do all that's necessary regarding smoothness (no vibration) in boats that turn high RPM I could never understand why some boaters want 4 or even 5 blades on low rpm single diesels. Especially as the backing efficiency is reduced. Is it because of prop diameter needing to be too large?? Or... ???

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.

Ted

 

[Art]

"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.

 

[Art]

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.

Ted

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 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...

 

[O C Diver]

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 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.

Ted

 

[Gdavid]

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.

 

[Art]

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.

 

[Nomad Willy]

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.

 

[Gdavid]

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.

 

[smitty477]

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

 

[kulas44]

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.

 

[Art]

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.