Prop Differences

[Mark P]

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.

 

[Ski in NC]

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.

 

[Commander Nomad]

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.

 

[kapnd]

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”.

 

[North Baltic sea]

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

 

[Mako]

Does anyone here operate their trawler with a big 2-bladed prop?

 

[Fish53]

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.

 

[Mark P]

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.

 

[Ski in NC]

Probably changed to 5b due to noise issues at higher power settings.

 

[FF]

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

 

[Commander Nomad]

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.

 

[Fish53]

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.

 

[Bkay]

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".

 

[Mako]

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.

 

[Commander Nomad]

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.

 

[Nomad Willy]

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.

 

[Simi 60]

Does anyone here operate their trawler with a big 2-bladed prop?

How many here have an "actual" trawler or trawler based derivative?

 

[Fish53]

How many here have an "actual" trawler or trawler based derivative?

I used to, it had a 38x33 four blade.

 

[FF]

"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.

 

[chartervet]

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.

 

[tpbrady]

Eric,

I have 28" diameter 28" pitch 4 blade prop. What is the problem with that type of set up?

Tom

 

[Nomad Willy]

tpbrady,
Too much spilling of the water on the pushing side of the blades spills over the blade tips and wasting energy throwing water radially rather than aft making thrust. You probably have a very noisy boat right above the prop/s.
You’d benifit from an increase in dia and reduction in pitch.
Or an increase in blade area and a reduction in pitch.
Ask someone with more knowledge than me and they will probably agree.

Almost strange you should have that settup as most trawlers that are way off on their pitch/dia. ratio have too much dia and too little pitch.

 

[markpierce]

Four blades works well for the FD Coot. All the efficiency and responsiveness needed to move close to hull speed.

 

[FF]

For many boats a "square prop" (where pitch and diameter are equal) is considered ideal.

 

[Bkay]

tpbrady,
You’d benifit from an increase in dia and reduction in pitch.

I have great respect for this commenter's experience, but I'm not sure I agree with this statement. I believe you need to start with the largest diameter you can fit with a minimum of 10% standoff between propeller tip and hull - 15% is even better. If you don't have that standoff, then increasing diameter is not helpful.

Assuming you have the largest diameter propeller you can fit - then you need to look at other variables if there is some change you need to make. Frankly, I agree with the comment above that your square wheel may be an ideal compromise.

If you can run your engine up to WOT with a full load, then I'd have to ask why you think you need any changes to your setup. Is there some problem you are trying to solve or just curious?

I stand by to be corrected by those wiser than me.

 

[Fish53]

I have great respect for this commenter's experience, but I'm not sure I agree with this statement. I believe you need to start with the largest diameter you can fit with a minimum of 10% standoff between propeller tip and hull - 15% is even better. If you don't have that standoff, then increasing diameter is not helpful.

Assuming you have the largest diameter propeller you can fit - then you need to look at other variables if there is some change you need to make. Frankly, I agree with the comment above that your square wheel may be an ideal compromise.

If you can run your engine up to WOT with a full load, then I'd have to ask why you think you need any changes to your setup. Is there some problem you are trying to solve or just curious?

I stand by to be corrected by those wiser than me.

You don't fit the propeller to the boat you fit the propeller to the reduction, engine horsepower and RPM for diameter and the pitch to the calculated speed of the vessel with the fitted horsepower and reduction. Once these basic calculations produce a diameter and pitch it's time to fit the propeller to the boat. If the calculated diameter doesn't fit with a standard three blade then reducing diameter by adding blades is called for, pitch can be adjusted slightly for the vessels intended use. Blade surface also needs to be appropriate for the installation. There's nothing special about a square wheel but there's also nothing wrong with one. It is correct for lower speed vessels that the larger a diameter the more efficient the propeller will be which is also true with blades, less blades is more efficient for common production propellers.

 

[ben2go]

Four and five blade designs make good fans.

 

[Bkay]

Fish53 - I concede you are technically correct. However, in my limited experience, prop diameter has always been limiting factor. So the optimal diameter was interesting, but irrelevant. This means we have always run the calculations including a required fixed diameter and found the best outcome given that constraint.

This quote below is from the Boat Diesel prop calculator - note the comment about fixed diameter being an input. I would love to be in a situation where I had no prop diameter constraints and simply use the diameter and pitch the calculator produced, but that hasn't happened for me yet.

"Determine the correct diameter and pitch of a propeller for any given vessel and engine.
Input vessel, engine and transmission details and the program will calculate: Maximum achievable speed, estimated fuel consumption, recommended propeller diameter and pitch, a propeller pitch based on a required fixed diameter, plus it will suggest alternative propeller sizes using different gear ratios to provide better propeller efficiency."

 

[Ski in NC]

There is a sweet spot with prop diameter that depends on all the relevant boat parameters: Length, weight, hull shape, engine hp, engine rpm, gear ratio, desired speed, etc, etc.

Too small a diameter, and to some extent too little blade area and the "slip" will be too high. Slip is defined as theoretical speed (pitch and prop rpm) compared to actual attained speed.

All props have some slip since they are churning in a fluid.

Too much diameter and the skin friction begins to eat shaft hp. This friction can result in a loss of speed greater than the gain from reduced slip. At some point a big dia prop just won't fit, or blades too close to hull (noise).

So somewhere between small diameter and large diameter is the right diameter.

Once diameter is selected, you pick pitch. Pitch is much like a gear ratio, you pick that to get engine rpm to the desired spot. Most boats that is to get a little above rated rpm at full power.

Oh, with all that said, there is a good bit of black magic involved!!

 

[Bkay]

Oh, with all that said, there is a good bit of black magic involved!!

Truer words have not been spoken. This is an interesting discussion that I have learned from - I appreciate everyone's expertise.