4 blade vs. 3 blade propellers

[colbysmith]

I know there are a few other threads on this topic, but since they are a decade old, thought I'd start a new one. I've been running a 4 blade prop on my CD-22. A 13.5 x 13 Solas. While doing some maintenance and winterizing my boat, I decided to put a 3 blade back on, a 14 x 13 Merc Prop I believe. Recently I started tracking my fuel useage, and seems like I am averaging about 3.3 mpg in the full range of speeds. What I'm wondering is has anyone else compared the 3 blade vs. 4 blade, and what was the difference in your fuel useage? It will be March before I get a chance to find out for myself about how the mileage works out on my boat. Previously, when I first changed to a 4 blade, I'm not sure I really noticed much difference in performance, but I was not tracking my mileage at that time. I just happen to have access to less expensive props with 4 blades, and have continued to run that. (Now, I own 2 4-bladed props, and 1 3-bladed. All are aluminum.) The one difference in my props, other than the number of blades, is the hub system. The Solas 4 blade uses a rubber Rubix hub system, whereas the Merc 3 blade uses their hard plastic Torque II system. Not sure how much difference the rubber vs. plastic makes. Colby

[chimoii]

Ok, I’ll give you a couple of rules of thumb based on years designing marine propulsion systems, all of which have caveats but all of which have data to back them up. Then I will wait for the storm to descend.

Three blade props are more efficient than four blade props.
Two blade props re more efficient than three.
Three blade props are smoother than two and four are smoother than three.

Smaller hubs mean less resistance.

Stainless props are usually thinner bladed than aluminum and therefore more efficient.

IPS drives are more efficient because the flow to the prop is cleaner.

All the above is dependent on getting the right prop specified in terms of pitch, diameter, cupping etc.

Going further, fixed prop sets are initially more efficient than outboards or stern drives BUT the latter are trimable meaning they can overcome their initial disadvantage by altering attack angle.

Water jets usually have a limit of approx. 70mph. Above that the amount of water pumped aboard and expelled produces an inefficient power/weight ratio.

Surface piercing propellers are the most efficient but can have initial acceleration issues depending on the prime mover. If articulated (Arneson drives) they can be extremely efficient and practical. I was in a Skater 36 with 2 x 1200 hp gas turbines that ran at 1.5 mpg doing 170 mph. It could turn on a dime.

What I can say in summary, every prop and system is a compromise. You don’t have an unlimited number of variations to play with and gas is probably the cheapest thing you put in your boat. Choose the prop that feels best.

[Spike]

Intresting info Chimoii. I have a question for you. Why is a surface piercing prop more efficient. I am not doubting you, I have seen enough hydroplane races and seen where the prop clears the water. Just seems to me that a prop spinning in the air is just a waste of energy. And dose the prop under go undo stresses slamming back into the denser water? Thank you

[colbysmith]

So in layman terms (lol), can I expect better, worse or about the same gas mileage by going from my 4 blade prop, back to the 3 blade prop? (13 1/2 x 13 /4 to 14 x 13 /3.) If I remember correctly, the 1/2" diameter difference allowed me to be within specs at WOT. Colby

[ssobol]

[chimoii]

[chimoii]

....
[/quote]

One bladed props are most efficient. (Don't laugh, it's been done on airplanes.)

I thought part of the performance improvement with SS props is because the material is harder and there is less blade flexing under load. This does have some drawbacks as far as the drive train is concerned if the prop hits something though. I stick to AL props for this reason. AL props are also easily dressed with a file if there is a ding or two.[/quote]

You are correct about fixing little dings in aluminum props. The counter argument is that a ss prop may not actually show a ding in the same situation. Replacing a ss prop is definitely more expensive. As for flexing, not a significant factor between the two. There are carbon fiber props designed to flex under load and change effective pitch. I'm not taking about the common variable pitch prop here. They are of a design unique to the vessel and drivetrain and certainly not in the usual pleasure craft realm.

[chimoii]

[drbridge]

We have used both a three blade and a four blade on our 22. I liked the four blade. It had noticeably favorable handling characteristic around the docks. Seemed to get a good bit more bite in reverse. It seems to run smoother than the 3 blade overall. In a mpg comparison we got about 3mpg with the four blade and around 3.25 with the three blade and that is while loaded heavily in both cases.

[chimoii]

[SeaSpray]

Bringing this back up since I am trying to decide on an new prop. I have a CD22 cruiser with the Johnson 4 stroke 90. It came with a 14 x 19 3 blade prop which has worked well but is now dinged and feels a little out of balance at low speed.

I would be interested to know what prop any others with the CD22 and the Johnson or Suzuki 90 run, and how they like it? Please list brand, diameter, pitch, and number of blades.

Thanks,
Steve

[thataway]

[Hunkydory]

I, like Bob, always carry aboard an extra set of props

From my latest research comparing 3 & 4 blade props due to soon replacing my twin 40 hp Honda’s with new twin Honda BF 60’s, I’ve come to the conclusion that on a CD 22 the 4 blade prop or props properly matched to WOT rpm is perhaps the better choice of the two. Yes a 3 blade is more efficient at higher speeds & will gain several mph on top end, but the CD22, which is designed for a slower cruise speed with most owners cruising on plane at between 13 & 20 mph, makes top end speed, where a 3 blade prop efficiency matters, not often realized. The 4 blade prop at displacement speeds, getting on plane & at slow cruising speeds may actually be more efficient due to it’s less prop slip provided by more blade contact with the water. The 4 blade should also run smoother with less rpm needed to achieve the same lower speeds than a 3 blade & make for better control of the boat while docking.

My conclusion so far comes only from research reading & not personal experience, as the only 4 blade props I’ve run on our CD22 were a cheap set made of composite material. When the new motors are installed, I will be starting fresh, first seeing which 3 blade props work best, as I should have to start, a set of 10, 12, 13 & 14 pitch props. From how these run at different boat weights & altitude, I will make the determination of what pitch 4 blade props I will try. At some point in the next 2 months to a year, I should have personal experience to see if my read research matches it.

My collection of 3 blade Honda props comes from spares, I had from the 40 hp Honda’s, which also work on the 60 hp Honda & a very appreciated combination prop gift & extremely low price on a set of 13 pitch 3 blade stainless from friend & fellow C-Brat, Lew of Catclaw.

Steve, if you do switch to a 4 blade prop, you need to be 1 to 2 pitches lower than your present 3 blade.

Jay

[SeaSpray]

I probably should have included more info in first post. My original prop is the aluminum 14 x 19 3 blade. A number of years ago I purchased a Pirahna brand prop system which is a composite 4 blade hub with changeable blades. A few Cbrats where using another brand of plastic/composite 4 blade props where you could replace a blade if one got dinged. Don't remember the brand but that brand did not have anything for my motor so I found the Pirahna and have 14 x 18 blades. After using it for awhile I was concerned that it let the motor rev too high at WOT. My orig 14 x 19 3 blade went to about 5200 rmp. the Pirahna goes to about 5700 rpm. I recently tried a Turning Point 4 blade 13.25 x 17 on the recommendation of the manufacturer. This prop runs up almost 6000 rpm.

My shop manual lists the WOT range as 4500 to 5500 but Turning Point and Sportcraft Marina are telling me the WOT is up to 6000. The next size that Turning Point has is 13 x 19 4 blade which I hope would get me closer to 5500 rpm at WOT. The 13.25 x 17 just feels like I have get rpms up much higher for the same speed compared to the original.

I don't want to over porp and cause excess wear on the motor. There are so many variables, 3 vs 4 blade, changes in diameter and changes in pitch. That is why I am wondering what others with the same boat motor combination are running.
Steve

[drbridge]

Here is something to think about when you go to a different prop. Our boat originally came with a four blade composite prop. The boat ran very well with it.
We took it for a freshwater bath at lake Crescent WA and about 3/4 around the lake at a 15 kt per hr cruise the motor started vibrating uncontrollably. I shut it of immediately and could see no apparent problems. I started it again and it vibrated again. Then I pulled the motor up to look at the prop. It was missing a blade! We did not hit anything and had not hit anything previously. The blade just broke off. I was so glad that it happened on the lake and not the strait of Juan De Fuca with 3 ft wind waves.
Anyways, No composite props on any of my boats anymore,