3 vs. 4 blade props

So now that the trawler is out of the water getting her topsides painted, should I swap out my 4 blade prop for a 3 blade? *The seller gave me 2 extra props that were newly reconditioned when I bought the boat. *If i were thinking, I would have had them throw on one of the 3 blade props when I moved the boat up to TN and tried it out for a few months and then see how it handled with 3 vs. 4 blade. *

So how has your boat handled different with a 3 blade vs. 4 blade prop? *Which do you prefer and why? *I can't remember the size of the different props- have it written down @ home.

I have heard that generally the fewer the blades the more economical it is to move your boat through the water.

on typical 8 knots or less boats a 3 blade properly tuned wheel should be your more frugal choice.

Which style prop typically has more "prop walk" when in reverse? I am not too terribly worried about fuel economy- 1.5 GPH or 2 GPH will hardly be noticed by me. prop walk however, is something I definitely notice!

Stop worrying about the difference in 3 vs 4 in reverse. Just like the lock through, it will be a piece of cake for you. You'll wonder why you even worried about it! Go Have fun on your new ship!

Jack- I hear you!! Not worrying about it- just that this will be the last time in a while I take her out of the water so if I was going to swap it out now would be the time. But I agree- time to start just enjoying the boat. Once we are done with the painting in 2 weeks I think I will be done with the big stuff for a while and have her to the point we can just enjoy her!! *You're definitely right about the locking through- not sure why I was even apprehensive! *Super easy. *Now....to get used to backing her into her slip instead pulling bow in. *

-- Edited by Woodsong on Wednesday 16th of February 2011 02:42:14 PM

Woodsong wrote:

Which style prop typically has more "prop walk" when in reverse?

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As was explained to us by the prop shop we use, the basic "rule" is that-- given the same basic diameter and pitch--- the fewer the blades, the more efficient the prop will be.** The more blades, the smoother the prop will be (in terms of vibration).* A prop with less blades will "back better" by which is meant less propwalk, than a prop with more blades.

At one point we were going to swtich our boat back to three-bladed props (which is the way it had been built in 1973).* This is when we learned the "rules" I listed above.* When we learned that the boat's current four-bladed props were not worn out as we had been told but were simply horribly set up by whoever did it in SFO, we elected to have the four-bladed props re-worked and balanced.* The difference was $350 per prop to tune them vs $2,000 per prop to replace them.

In the case of a twin engine boat, propwalk is a wonderful thing and the more of it you can get the better (in my opinion, anyway).* With a single-engine boat not so much maybe, although once you get the feel for your boat's propwalk it can be a very useful maneuvering tool.

*

"With a single-engine boat not so much maybe, although once you get the feel for your boat's propwalk it can be a very useful maneuvering tool."

Yes extremely useful. I'd be lost without it. It's the only way I can "steer" in reverse.

jleonard wrote:

"With a single-engine boat not so much maybe, although once you get the feel for your boat's propwalk it can be a very useful maneuvering tool."

Yes extremely useful. I'd be lost without it. It's the only way I can "steer" in reverse.

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Tony, don't know if it would make a difference in your boat, but a four blade prop on a full keel boat can set up some vibration at higher rpm.* Two of the blades are in line with he keel.* The 3 blade props have more percentage of blade pulling water, so can be a little more balanced.* Just an observation from experience.* I would stay with the 3 blade.

*

Tony, The PO probably had the prop that works best on the boat.

I think I would just run with it.

JohnP

Moonstruck wrote:jleonard wrote:
"With a single-engine boat not so much maybe, although once you get the feel for your boat's propwalk it can be a very useful maneuvering tool."

Yes extremely useful. I'd be lost without it. It's the only way I can "steer" in reverse.

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Tony, don't know if it would make a difference in your boat, but a four blade prop on a full keel boat can set up some vibration at higher rpm.* Two of the blades are in line with he keel.* The 3 blade props have more percentage of blade pulling water, so can be a little more balanced.* Just an observation from experience.* I would stay with the 3 blade.DonFrom my understanding, what you say is correct, depending on how close the blades are to the hull. When I propped up upon installing a larger engine, the boat manufacturer recommended a five bladed prop to reduce cavitation and subsequent vibration as I was goiing to be about 3" from the hull with the tips of the blades. He was right. I ran a four bladed prop when I first got the engine, because I was waiting for the new 5 blade to arrive, and it did ceate a little shutter against the hull. *The five blade is as smooth as running without a prop, and much faster (than without a prop). I do believe that I suffered a little speed loss, but not significant.

*

Woody

Larry at Accutech Prop is a great source of info. But if your current prop is not in bad shape, as already stated, enjoy the ride. At 8 knots it is not an issue. I have 4 blades, I trust the boat manufacturer to do right thing. Other than re - balancing once, I've seen*no issues.

The reason for having a four blade prop on a boat is to achieve a lower blade loading.
Most boats have a finite amount of room to swing a prop regarding diameter. One of the most important things to get correct or optimal is blade loading. So much hp per sq in of blade area. I think I could swing a 22" dia prop on Willy but I have an 18" wheel. I get a good blade loading w the 18" wheel w 40hp. More often than not power boats have less space (swinging room) for their propeller than Ideal. This is the basically the only time a 4 or 5 blade wheel should be used. And as Carey and Don pointed out the 3 blade wheel has a lower percentage of blade exposed to the turbulence coming off the keel. Another way to look at it is that a 4 blade has half the blade area in the keel turbulence whereas a 3 blade never has over 1/3 of it's blade area exposed to keel turbulence. For even smoother operation a more skewed blade w wider tips will deliver and the same prop will absorb more power without cavitation. Disk area is important too and highly related. If you imagine a circle scribed by the prop blade tips as they revolve all the area within is the disk area and it's relationship to the blade area is important too. Basically the only reason to go to a 4 blade is if your propeller is over loaded w power. You then need more blade area or more blades or both. If you need an undesirably high pitch to load the engine you need more blades or area. If you have way too little pitch (as in a 20" dia w 10" pitch) you may gain from going to a 19" w a 12 or 13" pitch. Basically a 4 blade on most of our trawlers is not ideal but you'd be hard pressed to notice the difference. Three blades are best.

Three blades are best.


Actually from an efficiency point of view the counter balanced ONE blade is "best" , but the diameter is huge.

Many sailboats use 2 blades with great efficiency , as they usually can carry the diameter .

6.75K on .75 GPH is what our 90/90 had done for decades , even with an antique MD3B Volvo , dual alternators and a large mechanical refrigeration compressor .

Yes it takes a custom (wide blade) prop (not the low drag Sailor from Michigan) to have efficient blade area and not cavitate.

The fewest blades the least amount of induced drag .

FF wrote:

Many sailboats use 2 blades with great efficiency , as they usually can carry the diameter .

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I have always believed the opposite.* The typical auxiliary sailboat has a small, two-bladed propeller.* When not using the engine under sail, the two-bladed propeller can be aligned with the keel to minimize drag.* We always checked/adjusted alignment when sailing and particularly when racing.* A three-bladed propeller would have been better for motoring, but a three-bladed (non-colllapsible) propeller would create a lot of drag while sailing.

*

"The typical auxiliary sailboat has a small, two-bladed propeller."

Some even use 1-1 gear boxes , so the pitch drag slows the boat less under sail.

That is because many are sold as "Cruiser racers" and only need to produce 5K to meet the racing rules.

Since it frequently is not possible to swing a larger prop , the 3 blade by default is the cruising prop.

AS "deadwood" is out of style (as is shallow draft) the newer fin keel spade rudder shows the entire prop as drag,

A folding prop is then required or its like towing a bucket , with the same diameter as the prop.

An advantage of four-bladed props on a twin is the increase in prop walk which make the boat maneuver that much faster and more responsively.

JohnP wrote:

Tony, The PO probably had the prop that works best on the boat.

I think I would just run with it.

JohnP

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No disrespect, but the PO of my boat didn't know squat.* He kept the boat in great condition, as he should have, but he personally didn't know what he was doing.* And some people get things done simply because of what they think is correct, even when it might not be.

I have corrected many things that the PO did so at this point I don't really trust his judgement.

Woody

*

I have a 5 blader???

And I am with Charles, a 4 blade prop has less vibration than a 3. *I would go with the 4.


-- Edited by Baker on Sunday 27th of February 2011 10:43:52 AM