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Old 11-08-2018, 01:59 PM   #1
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Experience upgrading from a 4 blade to 5 blade prop?

I'd be interested in hearing from anyone who has upgraded from a 4 blade to a 5 blade prop.


What improvements did you notice?


Was it worth while?


If you were starting from scratch, would you get a 4 blade or 5 blade?


Thanks
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Old 11-08-2018, 08:06 PM   #2
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I always assumed a law of diminishing returns - 2 blade to 3 blade to 4 blade+ - and that the sweet spot for non-electric trawlers was 3 or 4 blade.

I class electric differently due to the ability have usable torque at any rpm from 1 upwards, and trawlers differently due to speed.

There seems to be many trawlers who cruise at 6-8kn but have a possible top speed of 14+, and would seem to benefit from two entirely different props to maximise both options. Since that may be changing pitch and number of blades, I have no idea how one could do it !
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Old 11-08-2018, 10:18 PM   #3
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What is wrong with the original prop? I have a friend with a 40 ft CHB who spent a lot of $$$$$$ on 4-bladed props to replace the 3-bladed. He was very disappointed. Fuel efficiency dropped, speed dropped etc. He took them off this fall and put the 3-bladed props back on.

The factory should have done some test to determine the best prop.
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Old 11-08-2018, 11:31 PM   #4
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I see a lot of 5 blade props on go fast sportfishing yachts with too much horsepower and not enough room for a large diameter wheel.
Likely not the best choice for a typical trawler with relatively low power and plenty of room for a large prop.
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Old 11-08-2018, 11:42 PM   #5
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Look at the prop on a nuke sub. Bunch of blades. Was (may still be) a big secret. Gotta think there is a reason behind the multiple blades.
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Old 11-09-2018, 12:12 AM   #6
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Quote:
Originally Posted by wwestman View Post
Look at the prop on a nuke sub. Bunch of blades. Was (may still be) a big secret. Gotta think there is a reason behind the multiple blades.
Maybe underwater noise...

L
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Old 11-09-2018, 12:35 AM   #7
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I vote you go up to a SSN or SSBN and tell them they're a trawler
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Old 11-09-2018, 12:42 AM   #8
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Economy is rarely the issue in the USN. Sub prop blades are shaped to give the least possible noise. They are also inside a nozzle in newer subs. The prop is always covered when out of the water. The last one I saw had blades shaped very different from conventional blades.
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Old 11-09-2018, 12:43 AM   #9
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Prop blades

Props selection are both scientific calculations & some artistic sense combined. It can be a very complicated thing to work out comparisons on as so many variables. There is not enough room here on the forum to go very deep in to this subject, as there are so many variables. Maybe one of our TF friends here can do a short synopsis for us, but I know from aircraft props, it is not simple.

A lot depends on what you want to change ?


What is your goal you want to achieve with this proposed change ?


How much room do you have on the vessel Hull & how much HP do you have on tap & what speeds do you want to run at.


If there is not room to increase the diameter & the pitch would have cavitation occur at the RPM range of operation, then add blades to transfer the HP to the water to push your boat.

Pitch & diameter, if there is room, can both be changed & still stay with the same number of prop blades & the size of the blades can be varied as well for surface area of the blades & how much over lap they have, etc.

To give you a simple answer -- All things being equal - and they seldom ever actually are - 3 blade prop is more efficient than a 4 blade - but the 4 blade is quieter.

Again, All things being equal - and they seldom ever actually are - If your towing or drag boat racing, a 4 blade accelerates out of the hole better than a three blade, but you would have a faster top speed with the three blade.

The surface area of your prop & the pitch or bite of the blades along with the propeller diameter all contribute to your drive forward, so does the number of blades, but all also contribute to drag as well. Different pitch is more efficient at different shaft speeds and can have more or less cavitation at other loads or speeds. Vessel hydrodynamic & hull shape & depth in the water all create drag of various levels & displacement effects the load and hull shaping drag that is resistance you need HP to over come, to give you the speed or efficiency you want when you input HP to the prop to over come the drag to move the vessel through the water, but different law of physics occur between displacement speeds & plaining speeds.

If you put to much prop on the shaft & then don't have the HP, then the engine can't pull it.

Nuclear subs have more excessive HP, and they must NOT have cavitation of the prop when they feed lots of HP in to the prop to drive the vessel. Other wise it makes noise. Sub's are war machines & have special specifications that must be met for things we don't necessary need.

We do not have the HP that they have. so some of the assumptions you might make are not really valid.

If you want to go to a 5 blade prop, you better have the extra HP to drive it or you will have a big disappointment.


Look at the Nordhavn 56 motor sailor, it has a variable pitch prop & you adjust that just like I do on my aircraft to have better efficiency at different speeds & loads.

Good Luck & let us know what you find out.

Thanks.

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Old 11-09-2018, 02:18 AM   #10
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Hi


One aspect of optimizing propeller is material that is stronger than conventional this is called stainless duplex steel, the blades can be made thinner and powerful. stainless steel duplex steel Propeller is quieter, has better grip in the water, increased speed and lower fuel consumption.


Their reputation has found some US companies to use these performance-capable propellers.


Propellers manufactured from stainless Duplex steel


They have a reference of about 40 'SD which benefited from their optimized propeller at max speed of 2-3 knots (in Finland, no way to exaggerate marketing, cultural difference).


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Old 11-09-2018, 07:30 AM   #11
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"Gotta think there is a reason behind the multiple blades." (submarine)

With almost unlimited power, spinning up the prop too quickly can cause cavitation.

Cavitation is NOISE , just what even a cheap torpedo can track.

For most trawlers there is not the room to swing a 2 blade prop , and 2 blase units with wide power boat style blades are special order.

Took 6 months for Michigan to make a 2 blade for our 90/90.

3 blades usually has enough blade area for most trawler work 30-60 HP , 2-4 GPH does just fine.

The few times a 4 blade is required is usually for a poor laminate above the prop , and the sole in the soft after section bounces , or a re-power with an out sized motor to burn fuel quicker and toss a bigger wake.

5 blades is usually for the sport fish boats that cruise on the pin.

There are many on line sources for prop guidance
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Old 11-09-2018, 10:02 AM   #12
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A few years ago we were in final negotiations on an N55. The owner had gone from a 4 to 5 bladed prop. The compiled data and comparisons showed little material NMPG improvements. Another goal was vibration dampening at hull speed. We never got to a sea trial so cannot comment on that aspect.

Prop testing is expensive and involves much more than number of blades. I would guess Nordhavn has treasure trove of information on subject. Maybe not so much on twins though. The usual suspects can be queried if not already done so, such as Michigan, Accutech and maybe Delta Shipyard in Seattle.
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Old 11-09-2018, 11:16 AM   #13
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Hi,

Hard to believe that a D Hull amusing boat could benefit from a 4/5 inch screws. SD or planing heavy boat can benefit from getting less cavitation over D speed. When you have too much money, it's nice to buy new toys.

NBs
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Old 11-09-2018, 11:39 AM   #14
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This is not my area of expertise. Many years ago I had a very interesting conversation with the engineers at Bell Helicopters. We were discussing why the the 212 heli was 2 bladed and the 412 was 4 bladed, very similar helicopters but different rotor configurations.

The engineers said the most efficient system would have been a single blade with a counter weight. Problem was noise (the blade tip would be traveling faster than the speed of sound) and no one would fly in anything as strange looking as a single bladed helicopter.

The moral of the story is don’t forget how cool sometimes trumps common sense.

helicopters suffer from something called blade stall, so much of why they choose more or fewer blades dose not translate well to boats.
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Old 11-09-2018, 05:43 PM   #15
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All your questions can be answered here.

Its a really good addition to your boat library.

Fewer blades is more efficient. For our trawler-style boats more blades is a compromise to solve a problem, such as insufficient clearance to get blade area needed.
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Old 11-09-2018, 06:41 PM   #16
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Old 11-09-2018, 06:58 PM   #17
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twistedtree,
IMO you don’t “upgrade” your prop by increasing the number of blades. Other than materials and general quality of machining or casting the “best” prop has the best relationship of pitch, diameter and blade area. Number of blades (excluding special applications) has nothing to do w quality. More blades will make it a bit more expensive though. For every application there’s a range or ratio of pitch/diameter that makes a prop ideal. Too much diameter and you’ve got too much blade area. Too much pitch and there’s too much transfer of water from the pressure side of the prop to the back side. A loss of pressure differential. Many trawlers suffer from too much diameter and blade area. It’s a balance act to get it right. And once you get the dia/pitch right those numbers will indicate how many blades. So one dosn’t choose the number of blades. The variables will dictate the number of blades and for most trawlers a three blade prop is best.

I put a five blade on my Willard and it was way too much diameter and probably blade area. Only pulled 2000rpm. I coulda made it work by cutting the blades down until I pulled 3000rpm but that may have taken several to many haul outs and I wasn't willing to pay for it.
By the way the Prop is a old Michigan "Star" wheel.
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Old 11-09-2018, 08:46 PM   #18
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The most efficient prop would be a single blade, which has appeal but you can imagine how that would work. If you have the room, 3 bladed is best for a trawler.
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Old 11-09-2018, 09:04 PM   #19
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Originally Posted by tiltrider1 View Post
This is not my area of expertise. Many years ago I had a very interesting conversation with the engineers at Bell Helicopters. We were discussing why the the 212 heli was 2 bladed and the 412 was 4 bladed, very similar helicopters but different rotor configurations.

The engineers said the most efficient system would have been a single blade with a counter weight. Problem was noise (the blade tip would be traveling faster than the speed of sound) and no one would fly in anything as strange looking as a single bladed helicopter.

The moral of the story is don’t forget how cool sometimes trumps common sense.

helicopters suffer from something called blade stall, so much of why they choose more or fewer blades dose not translate well to boats.
Noise is one tradeoff, the other is forward speed. A multiblade (smaller diameter) rotor on the same airframe can be pushed to higher forward speed before encountering tip mach effects, which are not just noise, but also standing shock waves on the blades...which leads to controllability problems just like Yeager encountered on the X-1. This cropped up on the 222 during certification flight test at extreme cold temperatures. The speed of sound drops with temperature and we unexpectedly ran into control reversals at -45*F. Of course, the shock wave from the rotor going transonic was also waking up the dead and scattering livestock for miles around.
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Old 11-09-2018, 09:32 PM   #20
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No risk of going trans-sonic on a trawler. And I may have chosen my words poorly when I said "upgrade".


This is a new build, and I'm exploring the trade-offs between a 4 and 5 blade prop. Diameter will be 46". Engine is 400 hp @ 1800 rpm, and gear is 3.43 reduction so max prop rpm will be 525 rpm. Those are the fixed parameters. A 5 blade will presumably be a lower pitch than a 4 blade, but I think that depends somewhat on the DAR or each prop.


My original questions was looking for experience from people who have switched from 4 to 5 blade (or the other way around) on their boat, and what their experience was. Theory says 5 blades will be less efficient, but by how much? Yet 5 blades should be smoother, but by how much? It's subjective, but that's what I'm looking for. Other info is of course welcome, but I wanted to clarify the original query.


I recently got Gerr's book and will give a read through.
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