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Old 02-01-2019, 12:56 PM   #1
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Question Prop Differences

Folks, Can someone educate me on the difference in operation between a 5 blade and 4 blade prop on the same model 38-40 foot boat, with same engine - 380HP QSB Cummins. Both Props are 24" NiBrAl. I am looking at two nearly identical boats and wonder why the mfg would have two different props as an option or why an owner would change from one to the other? Thoughts??

Thanks, Mark.
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Old 02-01-2019, 01:13 PM   #2
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More blades, smoother and quieter, but (usually) less efficiency due to blade surface drag.

Some boats can't put a large 3b prop due to diameter restrictions. Those are somewhat forced to go with more blades.

A bud with a planing hull swapped old three blades out with modern 4b DQX. Reports much quieter, but lost 2kts at top end.

Suspect in your case it was owner's choice.
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Old 02-01-2019, 06:34 PM   #3
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You need to find the pitch of each. The 5 blade should be less pitch to keep the engine load the same. More initial bite on 5 blade due to more blade surface area, but less top end due to less pitch.
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Old 02-04-2019, 10:27 PM   #4
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Are we talking about a trawler here?
Three blades are pretty standard trawler fare, less blades = more efficiency.
Five blade props are not some magic bullet that makes boats go fast.
They are pretty specialized, mostly used in situations where there’s lots of power available and not enough room to swing a recommended prop.
Read up on it in Dave Gerr’s ”Propeller Handbook”.
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Old 02-05-2019, 01:53 AM   #5
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Hi,

I have read a number of rewiews Nordic tug37/39 whith qsb 37 qsb5.9 engine rewiews and used as a standard blade 4 and sometimes 5-blade props.

In tests there is no difference between the achieved max speed or partial speed of the same RMP and the fuel consumed the same readings, maybe achieving a smoother or quieter sound / vibration, I don't know, I have 4 blades and all ok.

NBs
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Old 02-05-2019, 02:12 AM   #6
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Does anyone here operate their trawler with a big 2-bladed prop?
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Old 02-05-2019, 05:12 AM   #7
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You need to find the pitch of each. The 5 blade should be less pitch to keep the engine load the same. More initial bite on 5 blade due to more blade surface area, but less top end due to less pitch.
Pitch is function of speed and diameter and blade area a function of load. You can increase or decrease load by changing pitch but only narrowly.
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Old 02-05-2019, 05:25 AM   #8
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To kapnd. Yes I both identical late model trawlers. One owner has about 500 hours and the other about 100 hours. Full keel protected props. I will look at book and see if I can guess why someone with 500 hours would change props if that’s what happened.
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Old 02-05-2019, 05:39 AM   #9
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Probably changed to 5b due to noise issues at higher power settings.
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Old 02-05-2019, 07:17 AM   #10
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Our 90/90 has a large 2 blade prop that works fine.

It was a custom from Michigan as 2 blades are usually cast with fine blades , not the wider heavier blades usually found on a power boat style prop.

Took 9 months to have it special ordered!

With a prop lock the blade was indexed to no drag at 6K , and the start key is stowed on the lock release , just in case.

6.5K LRC is 3/4 GPH from an antique Volvo MD 3B, about 13 hp .

The engine is oversized as the large eutetic cold plates can add about 10HP to the load for 30 min with a warm box .

Frozen hard ice cream for 3 days , on the 4th the temp will start to climb from 5 F
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Old 02-05-2019, 09:21 PM   #11
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Pitch is function of speed and diameter and blade area a function of load. You can increase or decrease load by changing pitch but only narrowly.
Pitch is not a function of anything, it is a constant. And you can drastically change load based solely on pitch.

If a 24x24 prop is ideal factory spec for a particular trawler or motoryacht style boat, changing only pitch to 24x30 will most certainly overload it, keep the engine from achieving full rated rpms, and cause it to overheat while trying to achieve full rpms.
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Old 02-06-2019, 05:55 AM   #12
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Pitch is not a function of anything, it is a constant. And you can drastically change load based solely on pitch.

If a 24x24 prop is ideal factory spec for a particular trawler or motoryacht style boat, changing only pitch to 24x30 will most certainly overload it, keep the engine from achieving full rated rpms, and cause it to overheat while trying to achieve full rpms.
Read Dave Gerr's "Propeller Handbook" and then talk to me. To calculate pitch you need to have an idea what speed the boat is designed for and calculate the rate of advance and the rpm of the shaft, so yeah pitch is definitely a function of speed.
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Old 02-06-2019, 09:08 AM   #13
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Originally Posted by Mark P View Post
Folks, Can someone educate me on the difference in operation between a 5 blade and 4 blade prop on the same model 38-40 foot boat, with same engine - 380HP QSB Cummins. Both Props are 24" NiBrAl. I am looking at two nearly identical boats and wonder why the mfg would have two different props as an option or why an owner would change from one to the other? Thoughts??

Thanks, Mark.
I can't educate you 'cause I'm not that smart, but I can relay an amusing conversation I had with Giffy Full (American Hero) and a couple other very talented boatbuilders.

Fewer blades = greater efficiency. A giant one blade prop would be very efficient - but it would vibrate like the dickens and you'd probably not have enough room to swing a prop big enough.

So you'd go to a 2 blade prop. Now that would likely still not give you enough room for a giant 2 bladed prop but it would run a little smoother and have greater blade area.

Now for my application (it was a 30' lobster boat) he said the reasonable choice with the space I had was either 3 or 4 bladed. That could get me as large a prop as I could fit with good standoff between propeller tip and hull, good blade area, and either would work well at the speeds I targeted and my engine/reduction gear. My builder calculated diameter and pitch for a 3 blade saying a 4 blade would have two blades passing into the "shadow" of the keel at the same time and result in more vibration. They all agreed that two identical boats, one with 3 and one with 4 bladed props might produce a different vibration or might not.

They spent 20 minutes arguing over priorities (speed, fuel economy, efficiency, vibration) and ultimately agreed they stopped talking about physics and started talking about prop-magic halfway through the conversation.

Then Giffy pointed out "what you really want is 5 blades...that's smoooooothhh as a kitten!" So I went 3 blade and didn't look back.

Sorry, not really that helpful. I just like typing the word "Giffy".
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Old 02-06-2019, 11:38 AM   #14
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Our 90/90 has a large 2 blade prop that works fine.

So FF, do you experience "two blades passing into the "shadow" of the keel at the same time and result in more vibration"? This is a common statement that I read, especially for boats with blunt deadwood that is not tapered properly.
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Old 02-06-2019, 08:03 PM   #15
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Read Dave Gerr's "Propeller Handbook" and then talk to me. To calculate pitch you need to have an idea what speed the boat is designed for and calculate the rate of advance and the rpm of the shaft, so yeah pitch is definitely a function of speed.
No, it's still a constant. Inches through the water per rotation if moving through a solid. Choose a pitch that works, choose one that doesn't, it doesn't change based on boat speed.
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Old 02-06-2019, 09:34 PM   #16
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A 24X24” “square” prop is not good at all for a trawler.

A prop with the pitch being about .75 X the dia is far better. Square (or nearly so is for planing boats or thereabout.

Re the 4-5 blade it all has to do w the aspect ratio and blade loading. With one more blade you could go to a 5blade and less pitch .. say 20-21”.

About the 5 blade it’s probably much more money.
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Old 02-07-2019, 12:40 AM   #17
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Does anyone here operate their trawler with a big 2-bladed prop?
How many here have an "actual" trawler or trawler based derivative?

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Old 02-07-2019, 04:49 AM   #18
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How many here have an "actual" trawler or trawler based derivative?

I used to, it had a 38x33 four blade.Name:  Iroquois2.jpg
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Old 02-07-2019, 07:01 AM   #19
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"So FF, do you experience "two blades passing into the "shadow" of the keel at the same time and result in more vibration"? This is a common statement that I read, especially for boats with blunt deadwood that is not tapered properly."


NO, as when the boat was built the area for the prop to swing was created large, so there is no vibration or pulsing .


Will try to get a photo.
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Old 02-11-2019, 06:57 PM   #20
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A good prop shop (as least one here in the NW does) has a computer program into which you load the numbers and out comes a recommendation, usually followed by some minor prop pitch tweaks. Result: Problems solved, questions answered. I'm sure if you are remote you can do it by phone or email if they are going to do the tweaking.
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