Four-blade and Three-blade Propeller

Optimizing the performance of Boston Whaler boats
Acseatsri
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Four-blade and Three-blade Propeller

Postby Acseatsri » Fri Mar 22, 2019 5:51 pm

Are three-blade props really best for overall performance?

According to the article (linked below) the only thing a three-blade propeller offers over a four-blade propeller is more [boat speed].

Two years ago, going to a four-blade propeller from a three-blade propeller, I've found that the claims in the article about increased fuel mileage, lower planing speeds, and better holeshot are spot on. The boat speed decreased 2-MPH at maximum, but in EVERY other category the boat runs better, runs flatter, and has better fuel economy.


To me boat speed is the least important aspect of all the performance characteristics.

https://www.sportfishingmag.com/propell ... de-props-0

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Re: Four-blade and Three-blade Propeller

Postby jimh » Fri Mar 22, 2019 5:57 pm

I have tested a number of three-blade propellers and a couple four-blade propellers on my boat. I settled on the three-blade as the best all-round propeller. I also liked it because I already owned it, while the four-blade propellers were going to be a $600 expense.

While your results are in conflict with my results, they are just two results. I don't think you could establish a contradiction of the general precept that a three-blade propeller is a good all-round choice from our outcomes.

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Re: Four-blade and Three-blade Propeller

Postby Acassidy » Mon Mar 25, 2019 9:57 am

This is what I have found while trying out props. It really depends on your boat too. But in general:

Four- blade:
--will give you better [acceleration to plane from a standing start], lower planing speeds, hold better in low plane speeds, have less top end speed; have more stern lift, and more bow push down, which is a big plus;
--be better in rough water where finding a slower speed and maintaining it is good
--great on flats boats and shallow boats for shallow take off.
--no reverse, loss of high end speed

Three-blade:
--more bow lift
--more distinct going to plane and higher planning speed
--better high end speed and overall better feel while running in smooth water
--will seem to have more cup
--will seem to have more grip on the water while running;
--not be so good in rough water if you are trying to go slow and maintaining a lower speed between planing and displacement
--will make the boat really want to just run fast not push through waves.

Every boat, engine, and water condition are different. To test in your conditions is important.

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Re: Four-blade and Three-blade Propeller

Postby dtmackey » Wed Mar 27, 2019 10:15 pm

I have a 21 Mako that was a previous Florida Marine Patrol boat and Mako built them with bigger stringers and lots of extra glass adding lots of weight. After I restored the boat and mounted a 250-HP engine, I tested many three-bladed propellers. I was not happy with the performance, but once I tried Mercury REVOLUTION4 I was sold and bought one. For low speed and docking the REVOLUTION4 had a far better bite on the water. Handling felt better, and the REVOLUTION4 held a plane at lower RPM. The REVOLUTION4 was within 1-MPH of the best three-blade propeller tested, so for me they were very close; the gains of a four-blade were far more important than the 1-MPH loss on top end.

D-

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Re: Four-blade and Three-blade Propeller

Postby GoldenDaze » Thu Mar 28, 2019 9:26 pm

I also had very good success going from a 3-blade to a 4-blade propeller on my 160 Dauntless. In short, she accelerates much more quickly, gets on plane faster, and has a smoother ride in chop. It cost me 2 MPH that I've never missed. Details at http://continuouswave.com/ubb/Forum4/HTML/004868.html. All that said, comparing a single example of a 3-blade to a single example of a 4-blade is not exactly a rigorous study on the merits of each and can't necessarily be generalized to other propellers or boats.

-Bob
2003 160 Dauntless Golden Daze

Acseatsri
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Re: Four-blade and Three-blade Propeller

Postby Acseatsri » Fri Mar 29, 2019 6:13 pm

The reason I started this thread was because everyone always makes the claim that a three-blade gives the best all-around performance. I haven't noticed any loss of maneuverability in reverse as someone else claimed.

I believe a four-blade gives the best all-around performance in almost all situations, other than the slight loss of top speed.

I'll have another benchmark in a few more weeks when my friend tries a Powertech OFS4 prop in place of a Rebel prop on a 1987 22 Outrage.

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Re: Four-blade and Three-blade Propeller

Postby jimh » Fri Apr 05, 2019 9:37 am

While there is no end to the number of anecdotal reports based on unscientific testing that ascribe particular benefits, characteristics, and flaws for propellers for outboard engines, I prefer analysis with a bit more science behind it, such as available in the rather inexpensive text, PROPELLER HANDBOOK by Dave Gerr. Gerr is a naval architect. He writes about propellers in a very readable manner. You do not need to be a naval architect to read his book.

In PROPELLER HANDBOOK Gerr notes:

..three-bladed propellers have generally proven to be the best compromise between balance, blade area, and efficiency.


He remarks on propellers with more than three-blades:

EFFECT OF MULTIPLE BLADES

Four- or five-bladed propellers--and propellers with even more blades--are useful for two reasons. First, their extra blades create more total blade area with the same or less diameter. Accordingly, an installation that needed a 20-inch [diameter] three-bladed propeller but only had room for an 18-incher could obtain sufficient thrust from , say, a properly sized four-bladed propeller. The four-blader, however, would seldom be as efficient as the three-blader because the closer blades create additional turbulence, literally scrambling up each other's water flow.

Another reason to use more than three blades is to reduce vibration. If a propeller is in the habit of producing annoying, rhythmic thumping and humming, a propeller with more blades will go a long way toward curing the problem. Every time the blades of a propeller pass under the hull or strut [or in the shadow of a gear case or close to the anti-ventilation plate in an outboard engine--jimh], they cause a change in pressure that causes a push (or a suction). If the push is strong enough it generates a bang. Lots or rapid bangs equals vibration.


With regard to reducing vibration, I recall the famous Atlantic liner, NORMANDIE. When she was first put into service the hull suffered from vibration problems when underway at speed (which was over 30-nautical-miles-per-hour). This was a problem for the ship owners because the NORMANDIE was to be the pinnacle of luxury and comfort in trans-Atlantic travel. After about a year of service, the ship was refitted with four-blade propellers replacing the original three-bladed propellers (one of which had been lost, probably due to the vibration). The result was a reduction in vibration and much more comfort for the luxury-class passengers. (The NORMANDIE top speed was said to be 37-MPH, which is astonishing when you consider the ship was 980-feet long and we are talking about the 1930's. While you can find this sort of speed in modern ships, today it takes an aircraft carrier powered by nuclear energy.)

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Re: Four-blade and Three-blade Propeller

Postby jimh » Fri Apr 05, 2019 9:54 am

Regarding the ability of a boat to be put onto plane and kept on plane at low speeds, I don't think the propeller has the most influence. Generally the engine is the problem. To maintain a boat on plane at a low speed the engine must be able to develop the necessary power output at the low engine speed required for the low boat speed. It is common that the power curve of an engine is not particularly broad, and many engines cannot develop enough power to turn the propeller at a low boat speed when the boat is on a semi-plane. The result is the propeller load is greater than the availale power produced by the engine at that engine speed, engine loses speed, the boat loses speed, and the boat falls off plane.

If an engine has a very wide power band it will be able to produce sufficient power, even at low engine speeds, to propel the boat and maintain the boat speed on the semi- or plowing-plane attitude. I have done this extensively with my boat with my E-TEC engine. The E-TEC engine runs like a diesel engine, and if you set its speed it just stays at that speed. I have run many miles (and what seemed like many hours) into large head sea on a semi-plane with my boat, with its three-blade propeller, with the boat on a slow plane at a boat speed of about 10-MPH. The notion that this sort of outcome depends on having a four-blade propeller is not demonstrated in my experience.

I can see that in a particular instance with a boat that had an engine that could not deliver much power at low engine speeds, changing to a four-blade propeller might affect the ability to stay on plane at low speeds on the basis that the four-blade propeller might be much less efficient. That would mean that the propeller would have to be turned faster to produce the necessary thrust to propel the boat at the low speed, and that would allow the engine speed to increase enough to put the engine into a range of power that could sustain the boat on plane.

For more about the power curve of engines and the power curve of propellers, see my article in REFERENCE:
Propeller Power Curve
http://continuouswave.com/whaler/reference/propellerPowerCurve.html

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Acassidy
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Re: Four-blade and Three-blade Propeller

Postby Acassidy » Fri Apr 05, 2019 11:17 pm

Jim that is great information. I will order that book. It looks like an interesting read.
You are absolutely correct about this

I can see that in a particular instance with a boat that had an engine that could not deliver much power at low engine speeds, changing to a four-blade propeller might affect the ability to stay on plane at low speeds on the basis that the four-blade propeller might be much less efficient. That would mean that the propeller would have to be turned faster to produce the necessary thrust to propel the boat at the low speed, and that would allow the engine speed to increase enough to put the engine into a range of power that could sustain the boat on plane.


My experience is if you switch to a four-blade propeller from a three-blade you have to go down one or two inches of pitch to maintain the same maximum engine speed. Many propeller manufacturers offer four-blade propellers in even-numbered pitch dimensions, so that if you have 19-pitch three-blade you can go to a 18-pitch four-blade.

Lowering the pitches does give you higher RPM at a given speed which will put your motor into a engine speed range where it makes more. Plus you have more surface area of the propeller in the water reducing slip and maintaining lift. Of course, all of this is a trade-off from a three-blade propeller.

You hit the nail on the head on this one. One interesting thing that I just thought of is that racing boats with five-blade or more and offshore surface drives with five-blade or more clever type blade are always much smaller in diameter than you would think they would be.
Archie

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Re: Four-blade and Three-blade Propeller

Postby jimh » Sat Apr 06, 2019 10:25 am

Acassidy wrote:...[with a four-blade propeller with less pitch than a three-blade] you have more surface area of the propeller in the water reducing slip and maintaining lift.


I don't see any difference in how much of the propeller is in the water in the case of a three-blade compared to a four-blade. How much of the propeller remains in the water is a function of the engine mounting height, not the number of blades.

Regarding the blade surface area in a three-blade propeller compared to a four-blade, there is nothing intrinsic in a four-blade propeller that means its blade surface area will be larger than a three-blade. The notion that a propeller with four blades will have more total blade surface area than a three-blade is only true if the blades on each propeller are about the same size. A three-blade propeller could have larger blades than a four-blade propeller, and the total blade surface area could be similar in both. The comparison really depends on the particular propellers being compared.

The influence of the pitch reduction is a factor in letting the engine run at a higher engine speed during a slow plane, but generally it is only going to alter the engine speed by maybe 400-RPM at full throttle; at half-throttle the difference in engine speed due to pitch would be less. And if you used a three-blade propeller with the same reduced pitch, the engine would also run at a higher speed for a given boat speed.

The boat falls off plane because the engine decelerates. The boat does not fall off plane because the propeller slip is too high. If the propeller were the cause, the boat would never get on plane at the slow speed.

When trying to get to plane at a slow speed, the boat is usually put on plane at a higher speed, then the throttle is reduced. The boat speed slows. Then when the desired low speed is reached with the boat still on plane, the throttle is adjusted to keep the boat at that speed. What usually occurs is there is no setting of the throttle for the engine that will maintain the engine speed at the appropriate speed to keep the boat speed at the slow planing speed. That is because the engine cannot deliver the necessary power at that engine speed. If you set the throttle for the desired boat speed, the propeller load is too high, and the engine decelerates.

Usually the throttle on an engine is not a direct speed control. The throttle acts to increase fuel flow into the engine, causing the engine to produce more power. This causes the engine to accelerate in its engine speed. The acceleration continues until the power the load on the engine stops the acceleration. When the engine power and the load are in equilibrium, the engine stops accelerating and runs at a steady speed.

Modern engines controlled by sophisticated fuel injection systems tend to have a throttle action which has more direct control over engine speed because the fuel is directly controlled by the throttle position. This is opposed to older engines with carburetors.

With carburetors, as the engine speed increased the carburetors would meter more fuel into the engine as a function of the air flow being sucked into the engine through the carburetors. The carburetors just controlled the ratio of air and fuel, not the amount of fuel. The faster the engine ran, the more fuel it would suck in. When the engine stopped accelerating, then the engine fuel flow rate becomes constant, the engine speed becomes constant, and the power output and load are in equilibrium.

The problem in carburetor engines and low speed planing is that as the throttle is reduced to slow the engine speed, the throttle position reduces air flow, reducing fuel flow, reducing power output. But the required power to the propeller to turn the propeller at the speed necessary to maintain the slow plane is higher than the engine can produce at that engine speed. The boat driver ends up hunting all the time for the perfect throttle setting to keep the boat on plane. If the boat engine has a wide power band, there may be a throttle setting that lets the engine run just fast enough to make the needed power, and just slow enough to keep the boat on plane at the desired low boat speed.

In my experience with my own boat with two engines, both the same displacement, both the same horsepower, but one an older carburetor engine and the other a modern direct fuel-injection computer controlled engine has shown that with the same propeller the boat behaves remarkably differently at lower planing speeds. With the old engine I could never maintain the boat on a slow plane. It was either not planing or planing at 25-MPH. With the new engine I can get the boat onto a slow plane at 10-MPH and run like that for hours. The fuel consumption is high and the MPG is low in that type of planing because the engine has to make a lot of power to hold the boat on plane at the rather slow speed. That is because the engine is making a lot of power.

A boat on plane is really a boat that is running uphill, trying to climb up its own bow wave. At 10-MPH the bow wave is big and the boat is on a steep hill, trying to go up that hill. To go uphill you have to accelerate against gravity. That is why planing needs so much power compared to moving the boat in displacement mode. To move my boat at 5-MPH probably takes less than 10-HP. To move it at 10-MPH in a slow plowing plane probably takes 100-HP.

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Re: Four-blade and Three-blade Propeller

Postby jimh » Sat Apr 06, 2019 10:42 am

Gerr also talks about propeller pitch and how it is generally measured or stated. Gerr devotes several pages to his discussion of propeller pitch. I won't try to summarize his remarks because they are already very concise. The basic concept introduced is the face pitch [a term of art he explains in detail] or angle of the blade changes along the length of the blade. Gerr then remarks:

...measuring simple face pitch poses some problems. Since the blade angles vary all along the length of the blades, from the root to the tip, you would get a different pitch measurement depending on where you took the measurement. By convention, however, the face pitch is always measured at 70-percent of the radius out from the shaft center.

Propeller Handbook, Gerr, page 25.

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Re: Four-blade and Three-blade Propeller

Postby jimh » Sun Apr 07, 2019 8:01 am

Acseatsri wrote:I believe a four-blade gives the best all-around performance in almost all situations...


I don't think you have enough evidence to support this claim. If your theory were true, then everyone would be running four-blade propellers instead of three-blade propellers. In my casual observations, most outboard engine boats are using three-blade propellers. What explains that?

Boats that are seeking maximum speed generally use a two-blade propeller.

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Re: Four-blade and Three-blade Propeller

Postby kwik_wurk » Sun Apr 07, 2019 3:53 pm

jimh, The economics of 3 blade propellers prevail and hence population distribution. (As do the up sell opportunities for the dealer, etc.)

Are there any brands offering their outboard engine with 4 blade SS as the standard prop? (No it's almost always no prop, or a standard 3 blade aluminum, that gets traded in or kept as a spare.)

There are two common exceptions, High Thrust kicker (8-15 hp) often have and aluminum 4-blade, by design. And high performance racing engines (outboard), typically come with no prop... but end up with 5 blades for offshore, and 3 for inshore (pickle-forks)...

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Re: Four-blade and Three-blade Propeller

Postby dtmackey » Sun Apr 07, 2019 10:37 pm

I dug back though my 3 to 4 blade notes and found some information that I will offer. I can attest that Ken at Prop Gods nailed it with the suggestion of a single prop being spot on.

Boat is a 1990 Mako 21 with an Evinrude 250. Hull is a Florida Marine Patrol boat built to military and marine patrol specs and very heavy. I purchased and restored 13 years ago and it's a tank with 12' stringers and many extra layers of glass.

Image

I tested many props and was not happy with any of the 3 blades tested. They had high slip, poor bite and the boat didn't feel right. I tried plenty and do not have notes from each prop since I could tell right way if it was worth gathering speed and RPM numbers, but wasn't happy with any of them.

Notes from the SST2 are below:

14.75x19" 3 Bladed that was suggested as the ideal prop.

RPM SPEED SLIP
1500 6.5MPH 55%
2500 9MPH 63%
3000 11MPH 62%
4000 28MPH 28%
5000 37MPH 24%
5600 43MPH 21%

After working with Ken at Prop Gods, he recommended a Revolution4 prop and I couldn't be happier with the results. Boat holds a plane far better and slow speeds. Steering is lighter and boat feels prouder in the stern underway. Handling is far better around docking and the boat has amazing "bite" when backing or low speed handling.

My reading at WOT after testing the Revolution4 had reduced slip of 50% at WOT and low speed handling was greatly improved and prop bite was far better.

I know there are formulas that apply to a wide range of boats, but like anything, there's theory and application and the range of boats out there provide a wide range of application, so do not be afraid to search for that perfect combination.

D-

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Re: Four-blade and Three-blade Propeller

Postby El Rollo » Mon Apr 08, 2019 2:22 am

Great 'Real-World' Intel. Thanks for sharing.

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Re: Four-blade and Three-blade Propeller

Postby jimh » Mon Apr 08, 2019 9:01 am

Regarding "great real-world intel": I have "shared" detailed test results of my testing of over a dozen propellers including both three-blade and four-blade. I never found any four-blade propeller to produce any significant improvement in performance over the three-blade propellers. So again, with anecdotal reporting we have completely contradictory results.

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Re: Four-blade and Three-blade Propeller

Postby jimh » Mon Apr 08, 2019 9:26 am

Rather than continue to report various anecdotal boat test outcomes, I thought it would be useful to look at a series of boats over a range of boat size and range of engine size that have been tested with various propellers in a systematic manner to achieve the best all-round performance. I have chosen the performance tests conducted by Boston Whaler as a good source of information that meets that criterion.

Boston Whaler can be assumed to have access to the entire inventory of propellers produced by Brunswick under their Mercury brand, and can further be assumed to have tested many propellers and selected the propeller that gives the best overall performance with their boats. To conclude otherwise would be to suggest that Boston Whaler is improperly testing their boats and delivering them with propellers that provide less than the optimum overall performance. If that were true, we would reasonably expect to see many reports from owners of these boats attesting to how much better performance they were able to obtain from change to a four-blade propeller from a three-blade propeller. Yet I cannot recall a single report of that nature.

It can further be assumed that the cost difference between three-blade and four-blade propellers is not a factor, as these boats sell in the range of $50,000 to $300,00 (or more), and the choice of a propeller that might cost $100 more or less than another propeller could not reasonably be seen as a influence in the choice of the propeller. It is unreasonable to assume that there is greater benefit to Boston Whaler to be able to save $100 in propeller cost in a new $300,000 boat than to deliver the boat with the best all-round propeller possible.

On that basis I just browsed a few performance test results from the Boston Whaler website. I found these outcomes with regard to propeller choice:

210 MONTAUK: ENERTIA 15, 18
190 MONTAUK: ENERTIA 17, 19
170 MONTAUK: ENERTIA 18, 21

380 OUTRAGE: ENERTIA ECO 17 (twin), 18-19-18 (triple)
330 OUTRAGE: ENERTIA ECO 17, 19 (twin)
250 OUTRAGE: ENERTIA ENERTIA 19 (twin), 15 (single)

These results show that in every case Boston Whaler selected a three-blade propeller. Even in the case of the 250 OUTRAGE (a large and heavy boat) powered by a single large engine (a 350-HP VERADO), they chose a three-blade propeller.

There are two conclusions possible from this data:

  • Boston Whaler engineering has not been reading this thread and is completely ignorant of the notion put forth here that a four-blade propeller is the best all-round propeller for outboard engine boats, or
  • Boston Whaler engineering has found from their testing that a three-blade propeller, particularly in the ENERTIA and ENERTIA ECO series, is the best propeller for their boats for all-round performance.

It is also interesting to note that the ENERTIA and ENERTIA ECO propellers were developed by Mercury as their newest and best propeller designs. When introduced they were heralded as "miracle" propellers, giving significant improvement over all other designs in speed and fuel economy. It seems implausible that if a four-blade propeller is intrinsically the best design for best all-round performance in outboard engine boats, that Mercury would produce the ENERTIA and ENERTIA ECO as three-blade propellers. Surely if Mercury was wrong, some other propeller maker would be exploiting their mistake and offering four-blade propellers that outperformed the ENERTIA and ENERTIA ECO.

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Re: Four-blade and Three-blade Propeller

Postby jimh » Mon Apr 08, 2019 9:43 am

kwik_wurk wrote:The economics of 3 blade propellers prevail..


I cannot see an economic argument as valid. The cost difference between a three-blade and a four-blade propeller is not particularly significant when considered in the total cost of a boat. Even with 25-year-old Boston Whaler boats that are bargains, when those boats are re-powered with modern engines the total investment in the boat can easily be $30,000 or more. To suggest that all boaters ignore your postulate that a four-blade propeller is better than a three-blade propeller because there might be an incremental cost for the four-blade propeller of $100 makes no sense. To go boating typically means spending a $100 per day on fuel and other costs. In an activity whose on-going operational cost is as much or greater than a one-time cost to obtain a perpetual benefit of improved performance, the suggestion that economics causes three-blade propellers to be preferred in not reasonable. To justify your theory you also would have to demonstrate that most boaters are ignorant of economics and spend their money unwisely.

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Re: Four-blade and Three-blade Propeller

Postby jimh » Fri Apr 12, 2019 12:50 pm

Here is a presentation of propeller test data in a graphic plot that shows fueleconomy as a function of boat speed for four propellers. When I presented this information originally I obscured the details of the four propellers from readers. That was intentional, because I did not wish for readers to interpret the results according to any preconceived notions or ideas or theories about what propeller would be best based on the readers knowing which propeller brand, series, pitch, number of blades, and other dimensions were being tested. I just called the propellers A, B, C, and D:

Image
Fig.1. Plot of engine fuel economy in MPG as a function of boat speed for four tests of different propellers.

I can tell you that three of the propellers tested were three-blade propellers and only one was a four-blade propeller. If the theory that a four-blade propeller is the best propeller for all-round performance, identify the four-blade propeller in the test based on the data in the graph.

I will identify the four propeller in a follow-up post.

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Re: Four-blade and Three-blade Propeller

Postby Acseatsri » Sat Apr 13, 2019 8:02 am

You can post all the graphs and arguments you want, but the fact is that everyone here, with the exception of Jim, who has tested a four blade prop ended up with the 4 blade for the same stated reasons. In fact, Jim said that ,in the end, the additional cost of the 4 blade did not justify the purchase. But if you had no prop to begin with, would you have still made the same decision?
In the end, the boat just performs better in the rpm range that we run, while boat manufacturers base their selections on top speed, even though most of us rarely run more than 3/4 throttle.

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Re: Four-blade and Three-blade Propeller

Postby jimh » Sun Apr 14, 2019 8:13 am

There has been no four-blade propeller that I have tested that gave better all-round performance than the three-blade propellers I already own (three of them). I have not tested the ENERTIA three-blade propeller, but based on the overwhelming test data from Boston Whaler, they would be the next test I would try.

I don’t spend $600 just because a fishing magazine article says I should have a different propeller.

No one buys a $600 propeller and then posts about how it actually made no difference in performance—that is human nature.

For every anecdotal report you can find that insists the four-blade propeller was the best choice, I can find a similar anecdotal report that shows a three-blade propeller was the best choice.

To prove that the four-blade propeller is the best choice for outboard engines to achieve the best all-round performance will need a lot more evidence than one article in a fishing magazine and a few anecdotal reports.

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Re: Four-blade and Three-blade Propeller

Postby El Rollo » Sun Apr 14, 2019 6:53 pm

I wish we lived closer so I could offer up a ride in my 1988 15 foot Boston Whaler with both a 3 blade and a 4 blade propeller.
Heck, I'd even let someone drive it, and let them decide for their self afterwards.
As I stated before, my choice of 4-blade was to help 'band-aid' the additional weight I have in my 15, as well as the 'unfavorable' weight distribution.
The 3 blade ran faster, but the bow oscillation was uncontrollable at anything but very low speeds.
My 86 year old father who has boated his entire life even said, "Wow, what a big difference the 4-blade made", during prop testing.
Now I have never tested my fuel economy, but I have a feeling it went down with the 4 blade. There just seems to be more bite.
Also, I have not compared weights between both props, but I think it's safe to say the 4 blade is heavier.
There is one more 4-blade model that I plan to try.
When I finally decide on the best performer, I will send it off to to be balanced and blueprinted.

If I didn't have the additional weight / weight distribution scenario going on in my little 15, I'm sure I'd be running a 3 blade.
That being said, the 4-blade works best for my particular application.

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Re: Four-blade and Three-blade Propeller

Postby jimh » Mon Apr 22, 2019 9:47 am

Apparently the USCG has not read that article in the sport fishing magazine about propellers that has become the basis for the new standard being put forth here that a four-blade propeller will be the best propeller for all-round use, replacing the three-blade propeller:

Image
Fig. 1. Latest generation Small Response Boat, part of a huge acquisition by the USCG.
Note counter rotating engines with three-blade propellers.


The total acquisition of these boats by the USCG may run to as many as 500 hulls. Imagine all those boats with the "wrong" propellers. The horror!

dtmackey
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Re: Four-blade and Three-blade Propeller

Postby dtmackey » Mon Apr 22, 2019 9:53 pm

Maybe the iceberg was a conspiracy theory and it was the 4 bladed center prop that doomed the Titanic.

Image

On a more serious note, a 4 bladed prop is not ideal for every boat and functions better on certain applications. I would never even think of switching the bow thruster prop or main propulsion bronze prop on our tug to a 3 bladed. In fact, it's not even an option.

Image

Image

If you're ever in the Boston area, swing by and I'll gather the props I used in testing my Mako 21 and you can experience for yourself. A short heavy deep-v hull loves the extra blade surface area and I did see a drastic reduction in slip. Heck, if you come out, I'll even buy you lunch.

https://www.youtube.com/watch?v=Fybo_IBl2H0

D-

jimh
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Re: Four-blade and Three-blade Propeller

Postby jimh » Tue Apr 23, 2019 10:08 am

With very little difficulty one can find images of propellers in many unusual blade configurations. For example, see

http://i.imgur.com/80NGOTq.jpg

which shows a seven blade propeller.

In the case of ships there is usually a specific design criterion employed in choosing the propeller. For a submarine, the most important consideration may be to prevent blade cavitation. Propeller cavitation creates noise. Submarines generally prefer to remain as noiseless as possible in order to avoid detection. Propellers with multiple blades and unusual curving blade shapes are common and likely are an artifact of the desire to minimize blade cavitation.

In large ships a propeller design is probably aimed at highest possible efficiency in propulsion at a particular optimum speed. Massive ships burn massive amounts of fuel, so even a fractional improvement in propeller efficiency can result in very large savings in fuel costs over the decades of nearly continuous underway operation.

According to one reference which seems to be extremely comprehensive in regard to the TITANIC:

...at present, no known photos appear to exist showing Titanic’s propellers in place..


On that basic, I have some doubt that the image seen in a preceeding post is actually from the TITANIC. It was likely from the BRITANNIC, a sistership:

Cf.: https://www.encyclopedia-titanica.org/mystery-titanic-central-propeller.html

In any case, back in 1910 when the TITANIC was being built, the engineers did not have the benefit of the wisdom of the article in the sport fishing website that is mentioned in the initial post in this discussion and has become the basis for the theory that a four-blade propeller will be superior to a three-blade propeller. If they only knew about that postulate perhaps history would be different.

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Re: Four-blade and Three-blade Propeller

Postby jimh » Tue Apr 23, 2019 10:39 am

If two propellers that are different are tested in a rigorous manner on the same boat in matching conditions, there is a very good probability that the boat speeds and other elements of the boat's handling while underway will not be identical. Whenever faced with two sets of outcomes from two propeller tests, the possibility exists for one outcome to be preferred over the other.

Anecdotal reports that some boat with some engine with some three-blade propeller was improved in some way by changing to a four-blade propeller are certainly possible outcomes. But to establish that in all boats and all outboard engines a four-blade propeller will produce outcomes that are preferred to all outcomes with all three-blade propellers seems rather hard to establish, particularly in light of the many decades of use of three-blade propellers, the advice of naval architects, and the preponderance of propellers in use with outboard engine powered boats.

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Re: Four-blade and Three-blade Propeller

Postby jimh » Tue Apr 23, 2019 10:48 am

If the diameter of a propeller is constrained, as might occur when the propeller is mounted in a tunnel as often occurs with a bow thruster, the design of the propeller will reflect the limitation of that installation. In the case of a bow-thruster, the limitation on propeller diameter is quite severe, and as a result the choice of propellers for bow thrusters cannot be seen as a good example for guidance in choice of propellers in other applications where the limitation of propeller diameter is not so restrictive.

An argument that the design of a propeller for a bow thruster should be considered as indicative of the optimum design for all propellers would be very simple to dismiss on the basis of design being particular to a very limited diameter dimension.

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Re: Four-blade and Three-blade Propeller

Postby jimh » Tue Apr 23, 2019 11:04 am

Acseatsri wrote:You can post all the graphs and arguments you want, but the fact is that everyone here, with the exception of Jim, who has tested a four blade prop ended up with the 4 blade for the same stated reasons.


I think you have already forgotten my citation of the Boston Whaler boat tests, which all use three-blade propellers. Those tests are a rather big "exception" to your "fact."

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Re: Four-blade and Three-blade Propeller

Postby jimh » Tue Apr 23, 2019 3:20 pm

Again in reference to the OLYMPIC (a sister ship of the TITANIC)--and many thanks to DTMACKEY for bringing this ship into the discussion--and her three propeller which are probably what is seen in the photograph above: the photograph is misleading with regard to propeller diameter. In the image the three propellers look to be about the same size. The three-blade propellers are 23.5-feet diameter, and the center propeller is only 16.5 feet diameter. The four-blade is then 7-feet smaller in diameter. The three-blade propellers are farther from the camera, so they appear smaller than they actually are.

A further problem in trying to draw any inference from the propeller's nature as either three-blade or four-blade is the difference in the engines turning the propeller shafts. The two outboard shafts with large diameter three-blade propellers were turned by reciprocating engines at a shaft speed of 75-RPM and each producing 15,000-HP. The center shaft with smaller diameter four-blade propeller was turned by a turbine engine of 16,000-HP but with a shaft speed of 165-RPM.

As can be anticipated, with the very large difference in shaft speed, the center propeller pitch did not match the wing propellers' pitch. By some accounts, the three-blade wing propellers were pitched at 33-feet while the center four-blade was only 14.5-feet.

Some inferences that may be valid and can be drawn from this information:

  1. the location of the central propeller in an aperture in front of the rudder established a mechanical limit to propeller diameter
  2. if it took 23.5-feet diameter three-blade propellers to absorb 15,000-HP, then clearly a three-blade propeller of only 16.5-feet diameter driven by a 16,000-HP engine might not have enough blade area to be able to effectively handle the greater power of the central shaft engine.

As for how an inference can be drawn from the propellers on the OLYMPIC that will be applicable for outboard engines from 90 to 300-HP, I will leave that to greater minds--perhaps at sport fishing magazines--to figure out.

Yoshukai
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Re: Four-blade and Three-blade Propeller

Postby Yoshukai » Thu May 30, 2019 10:59 pm

I replaced a [unknown] three-blade propeller with a [unknown] four-blade prop on my [25-foot non-Boston Whaler boat with unknown propulsion engine]. In addition to other benefits [from use of a four-blade propeller] as cited above, the [25-foot non-Boston Whaler boat with unknown propulsion engine] is more responsive when in reverse--helpful for backing into a slip.

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Re: Four-blade and Three-blade Propeller

Postby jimh » Fri May 31, 2019 12:12 am

Among the more than a dozen three-blade and four-blade propellers I have tested on my REVENGE 22 W-T Whaler Drive with E-TEC 225-HP engine, I have never made any measurements of the difference in the boat speed or acceleration while operating with sternward propulsion. I would be interested to see the data from tests of three-blade and four-blade propellers while making sternward propulsion to better understand what changes occurred.

In all the published boat tests I have seen there is never any data about boat speed and acceleration while making sternway.

The only time I use sternward propulsion is when maneuvering around docks or moorings, and this constitutes a very tiny fraction of the time underway. I would be hesitant to base propeller selection on performance while making sternway.

As a general rule, propellers that have a lot of rake tend to be very bad for sternward propulsion, so if there is to be any validity to a comparison of propellers on the basis of number of blades, the propellers must have the same rake.