4 blade prop vs 3 blade prop

As I have been searching Yachtworld and other sites for my next dream boat I have seen that some boats have 4 blade props and others uas 3 blade props. What if any are the advantages of a 4 blade over a 3 blade props?

4 blades provide greater surface area for a given diameter of prop which generally reduces prop slip. The smaller diameter allows a lower (closer to horizontal) shaft angle as well as reduced draft. 4 blades have higher drag compared to a 2 or 3 blade prop and in some cases can produce a vibration behind full keels at certain speeds.

The shape of the hull will dictate the placement of the engine and prop shaft, this configuration tends to drive the selection of the best prop configuration for a given boat.

Three blade props are almost always better for trawlers.
The reason to use a four blade is or should be to give the prop sufficient blade area to work propperly w the power of the boat. It's almost always a blade area issue.
If your boat has the wrong gear ratio you may need a 4 blade and there are other unusual circumstances that may dictate the need for 4 blades.
Three blades are almost always more efficient and cheaper.

Basically there is no advantage to 4 blade props unless a more powerful engine has been installed in the boat. Usually it would take considerably more power to get benefit from a 4 blade prop.

Say it ain't so!!! My trawler has a five bladed prop.

The number of blades on the prop would be about number 99th on my list of things to check when buying a boat.

David

Dave,
I think the OP was just curious.
True not usually an issue. Unless the pitch to dia ratio is too far from ideal.

CC,
I tried a 5 blade but it was 1" more dia and was wayyyy too much prop. 2000rpm max. May have worked if it had been 1" less dia. The blades were skinny and long and I was liking that for possible efficiency increase. Still have the prop ... 19".

Does it matter? That was a technical choice when the boat was built and took in consideration the engine, the draft, the gear, etc.

One way to sum it up: If a 3b works well, gives good performance numbers, and does not have bad vibes, you are good. If not, you do the same with 4b or 5b and try to dial it in.

Prop selection is half engineering and half black magic.

Thanks for the information, I have noticed that the Bayliners mostly have 4 blades and just about all others have 3. Today I was looking at three different DeFever 44's two were 3 blade and one was a 4 blade. the all had Lehman 135's.

My four-blade propeller is a good fit for my engine and hull. I'd rely on those professionally knowledgeable to select the proper propeller fit.

In at least one case I know of, a cat owner had Michigan Wheel make a 4 blade unit to replace the original 3 blade that came standard on PDQ 34 catamarans with twin 75 or 100 HP Yanmars. According to the owners on the PDQ forum, the efficiency and vibration was improved markedly, especially the "singing" the three blade units were known for. At least one boat claimed to gain two knots on top end but all seemed ecstatic over the sound levels...some claimed 50 percent improvement. PDQ was known for their design prowess, but this was apparently something they missed. Rare case, but sometimes it works out.

My four-blade propeller sung. After two attempts, the propeller guy fixed it.

Not necessarily a simple answer....but simplistically.....

A 3 bladed will be slightly more efficient if your boat design will allow it's diameter...if not a 4 bladed might be better.

Plenty of similar boars that run one or the other and no one can tell which one is really better.

markpierce said:

My four-blade propeller sung. After two attempts, the propeller guy fixed it.

Click to expand...

Singing is a function of the leading edge of one or all of the blades being tapered too much, common by novice prop reconditioners. The leading edge needs to be slightly blunt. It pays to use a seasoned professional when it comes to prop work.

Ted

Singing on my 3b tells me my prop is nice and clean. No sing means I need to jump in and clean it.

I just had my 3 blade set of spares put on a couple of weeks ago, replacing the 4 blade set. Both sets are 20"x19", I gained 2 to 3 tenths of mph at 1000 to 1100 rpm. That is about the only difference I've seen. I'am going to take the 4 blade to a prop scan shop and reinstall at next years haul out.

RC,
What was your WOT rpm w both props? Does your 3 blade have very high blade area per blade? That's the only thing that would make them comparable IMO. Also IMO you should put the prop on that has the most rpm if it's less than 100 rpm over rated WOT rpm.

Five blade prop

Colony Cove,
Here's my 5 blade. Probably dos'nt look much like yours. the blades are very high aspect ratio .. long and narrow. I think it's a Michigan Star wheel.

Cool prop!! I've seen those on real old chris craft cruisers, others from back in the 50's?? I'd love to try one with modern power. Just have no idea how that skinny blade 5er would do..

Ski,
Did poorly on Willy. 2000rpm max. Not even smooth .. considerably more vibration than the three blade. Think it was sling's water off the skinny blade tips slamming into the bottom. A higher frequency vibe than would seem right. I maxed it out for pitch and got little improvement. If Willy was a trailer boat I'd cut her down till she got revs. But ....

Nomad Willy said:

RC,
What was your WOT rpm w both props? Does your 3 blade have very high blade area per blade? That's the only thing that would make them comparable IMO. Also IMO you should put the prop on that has the most rpm if it's less than 100 rpm over rated WOT rpm.

Click to expand...

I lost 100 rpm near as I can tell by the tachs. I was at the spec rpm of 3000+ with the 4 blade. The blade area are very similar per blade, the gain in speed may be attributed to current speed, but it was checked in the same area of the river and the level gauge reading was within 10" of the 4 blade run. You are correct on using the props the give the proper wot rpm and that is the 4 blade pair.

I don’t think anyone really answered the OP’s question, “Why do some boats have 4 blade props and others have 3?” The fewer blades the more efficient the prop. (Racing airplanes have been known to use one-blade props.) However, the prop also has to be able to handle the power from the engine. For a given diameter more blades are able to transmit more power to the water, albeit at a somewhat lower efficiency. Two blades would be the first choice. With a bigger engine, if there is insufficient room for a larger diameter prop then add more blades. Blade pitch, shape and position are important details best left to the experts.

Paul

Paul .... Spot on
Yes the only reason for a trawler to increase the # of blades is to provide the necessary blade area for the power.
Of course the speed of the shaft and the boat get involved too.

Proping

I have come to the hard learned conclusion that prop ing requires either a wizard or a witch, a voodoo consultant and likely all three an still it would not be right.

Unless there is a significant problem, substituting the boat builder's propeller is probably a fool's game.

What he said!