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Old 01-18-2016, 09:08 PM   #1
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Props - 3... 4... 5... - Blades

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?
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Old 01-18-2016, 09:20 PM   #2
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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.
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Old 01-18-2016, 09:26 PM   #3
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All depends on what you want your boat to do. I've always preferred 4 bladed props for my charter boat and trawler.

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Old 01-18-2016, 09:29 PM   #4
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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.
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Old 01-18-2016, 09:52 PM   #5
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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.
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Old 01-18-2016, 09:58 PM   #6
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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.

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Old 01-18-2016, 10:15 PM   #7
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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?
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Old 01-18-2016, 10:20 PM   #8
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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.
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Old 01-18-2016, 10:22 PM   #9
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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.
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Old 01-18-2016, 10:43 PM   #10
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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... ???
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Old 01-18-2016, 10:46 PM   #11
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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......
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Old 01-18-2016, 10:49 PM   #12
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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?
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.
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Old 01-18-2016, 10:52 PM   #13
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Well, keep it under your hat or all the boaters will be wanting one, too.
sorry, it's a 1936 Taylor J-2



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Old 01-18-2016, 11:01 PM   #14
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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.
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Old 01-18-2016, 11:16 PM   #15
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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.
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Old 01-18-2016, 11:46 PM   #16
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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.
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Old 01-19-2016, 06:22 AM   #17
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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.
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Old 01-19-2016, 06:49 AM   #18
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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.
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Old 01-19-2016, 07:27 AM   #19
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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.
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Old 01-19-2016, 07:54 AM   #20
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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.

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