5 bladed props, performance enhancer?

[Buccaneer]

I see that "they" put them on Knit Wits,so I was wonderin about the little ol' 48MY I have... 425hp 6v92ta motors...

Thoughts, oh wise (well, opinionated ) ones?

 

[jim rosenthal]

I think they are supposed to be smoother; however, I think a lot depends on the prop design. You might get better efficiency with a more modern design, depending on what you already have. ZF and Veems wheels are supposed to be very good. You might talk to Frank and Jimmy's or General Propulsion, both are in FL, and see what they think. Also Austral props are supposed to be very good, and the design and manufacture better than Michigan Wheel.

 

[Trojan]

The best way to state it is 5 blades work well on a tug boat. The less amount of blades the more efficient your boat will be. In general if you can get the correct size, largest diameter you can swing and the least pitch and .05 cup. You will have the most efficient prop you can get. Most people settle for less. The NEW machined 3 blade props with large giant blades, big diameters and pitch less than square are said to work the best. They will be MY next investment. I would never put 5 bladed props on my boat, never. They are a power prop for operation at low RPMs. Lots of thrust at low RPM but very low thrust at higher rpms. To much turbulence around the blades at the higher rpms.

BILL

 

[luckydave215]

The reason to add more blades is to increase blade AREA for a given prop diameter, reducing per area loading, and therefore combatting cavitation. This is a great thing in a high power application, (go fast sport fisher or tug boat, for instance) where prop tunnels or other reasons limit blade diameter.
Does that description fit your boat?

 

[Walter P]

If memory serves me I seem to remember that some years ago when Hatteras started using very high reduction gear ratios, that part of the package was very large slow turning 5 (or more) blades. Something like 3 or 4 to 1 reduction. That would seem to jibe with 5 blades being probably an advantage on boats that are so equipped, but on most of ours with 1-1/2 or 2 to 1 reduction I think 4 blades or even 3 are better suited. I feel pretty certain that Hargraves firm did some pretty extensive calculations to determine a fairly close prop match. All that we can hope for is to have them as perfectly balanced and each blade being exactly like their siblings as well as proper loads on our boats. As such, tweeking up or down a little is probably the best we can do.

I'm certainly not an expert and I will read and keep an open mind on other opinions - who knows maybe the silver bullet does exist and one of you will find it.

 

[Boss Lady]

The only real advances have been with Highly Skewed Propellers and Carbon Fiber construction. This technology has been in use with the military for some time. There is one company which has made this available to the public. They have a US distributor which has jacked the prices to rediculous levels so not many have sold here, it doesn't help that the US company representing them is a bunch of bufoons. They have focused their attention on the highly profitable megayacht crowd, not realizing the real market for them is probably 1000 times bigger, which is the 35+ boats. I am working on an overseas contact to get a set directly from the manufacturer.

 

[JLR]

I have 5 bladed props on my 43 SX with a 3:1 gear reduction. It takes quite a bite when put in gear and makes the boat a snap to dock in close quarters. I am also doing about 6.5 kts. when just in gear so that must be kept in mind as well.

 

[Trojan]

JLR what kind of WOT speed do you obtain. I think I might be able to get a set of 3:1 gears for My 43DC. I just wonder if it would be worth the change.
Do you think you save any fuel with those gears and are your engines Ns or TIs?

BILL

 

[JLR]

The boat is new to me so take that into consideration in my answers. My engines are 692 TA's rated at 550 hp or so. She will top out at about 29kts. (2300) and cruise at about 25 kts (1950). The props are Rollo (sp?) and I was told, but have not confirmed, that they cost about 10K each. I never had any other props on so I can't tell you whether I save any fuel or not. At my cruise rpm, I burn about 38 gallons an hour (total).

 

[Trojan]

I don't think I have enough HP to do that. Very interesting though. Thanks.
BILL

 

[REBrueckner]

Also keep in mind even a nice improvement in efficiency should pay for itself in operational savings over some reasonable time period. That's unlikekly unless you run your boat a lot more hours than most of us.

Say you get a 5% improvement in efficiency with new fangled props. At 10,000 gal of annual fuel use thats about 500 gallons or maybe $1,500 dollars. So the budget available for two new props, let alone reduction gears, is limited if you expect any payback in, say, three to five years.

This subject is one for technical expertise...from a marine designer/engineer/consultant who has access to experimental as well as theoretical data. In the meantime, maybe those who haven't had their props set up to computer accuracy standards, like PROPSCAN and others, can gain a few percent improvement.

Meantime, it's fun to see perspectives on this subject, but I'll continue to run at moderate cruising speed instead.

 

[thoward]

Luckydave, did you get those props on yet?

 

[luckydave215]

They haven't arrived yet. Should be soon........
I'm not foolish enough to think I'll ever pay for new props with fuel savings. My existing props have been reworked so many times I can shave with them, so really I'm investing in smoothness (and maybe a couple more knots cruise, but that's secondary)

 

[Trojan]

For every blade and blade edge that you rotate, it will require added hp and torque. The boats that gain by using 5 blades or more are the boats that have an excess of HP and torque. They also lack the room for a large diameter wheel. Tugs and work boats and such. If you have over size engines, Lots of HP that you are not using and you CAN NOT turn a bigger wheel. Then you add more blades. The only time you can gain efficiency by adding extra blades is if you have extra HP You can't put to use with the size of prop that you are turning now...........period. No exceptions. You turn the largest wheel you can turn, with 3 of the larger blades with the least amount of pitch and you will have the most efficiency that you could have. That is why the Veam wheels are doing so well. They have a larger blade size than any other props made today. All the Michigan Nibral wheel are now being made over seas.


BILL

 

[TedZ]

Propeller design is some science, some intuition, some witchcraft. Other than this statement, there is no such thing as an absolute in propeller design, period, no exceptions.

Five bladed designs are the rage and seem to offer advantages, sometimes, to many classes of boats.

"Veam" by the way is Veem Engineering Group

http://www.veem.com.au/props_profile.php

Ted

 

[Boss Lady]

there is a large amount of wasted energy in the form of water being pushed to the sides instead of to the rear in the form of thrust. A lot of tugs use Kort nozzle to increase thrust, this shroud keeps the water on the blade so it can only go in one direction. These shrouds are very effective at low speeds, but the shroud itself generates enough drag at higher speeds to wipe out any increases. The early jet drives were built on the premise of generating propulsive thrust by reducing the output nozzle (like your garden hose nozzel) orifice. There has been some progress on axial flow dynamics and the center hub drag and turbulance is the major hurdle, they are looking at driving a ring type impeller from the outside, but still have problems with the seals. I have always felt that propellers need some form of wing tip kind of like used on jet wings, They keep air from spilling of the end of the wing and increase fuel efficiencies almost 10%. The 2 bladed prop is still the winner of the science contest, but every propeller has its' trade offs, so we see multibladed designs. The real answer is a variable pitch prop, but no one has come up with a practical design. (cost effective) or so it seems.

 

[SKYCHENEY]

Yeah, the one bladed prop is the most effecient. But for some reason they seem to have trouble balancing that.

 

[Boss Lady]

I think as far as 5 bladed designs go they could offer more thrust in the same diameter wheel, run smoother and offer the perception of better performance over range we typically operate our boats. This will give the perception of better performance, isn't perception the real reality, the science guys will tell us which is better theoretically, but what they usually leave out is that there is not a nickle's worth of difference in the real world. I would prefer a propeller that gives me the best performance where I need it. I always stress that if I could get 10% better fuel usage at cruise speed, I would trade it for bragging rights about top speed. A fixed blade prop is optimum at one rpm and one loading factor. It is off the bubble everywhere else, so I think you should design for where you operate the boat the most. We operate at cruise, the go fast boys are at max speed. Take your pick, but you can't have both. The europeans have had variable pitch props for some time, they are working on some for smaller boats like ours, I have no idea what the cost is but they are complicated systems.

 

[Trojan]

Until you come up with something better there is nothing better. There are more than just Veem making the new wheels. There were not the capabilities to manufacture this type of wheel until recently. It all started in the sand molds. I find no hocus pocus in the engineering of props. It remains the same. Only its get refined more and more as we learn to manufacture things better. The government has been trying for years. If we had a very pitch props on our diesels most people would be cooking there engines in no time at all. Only the Gasser's would survive. The cupping of the blades is to try and hold the water on the blades as long as possible to enhance the thrust. Just think, bigger blades with cupped edges. What a wheel. I will have the new design wheels real soon I hope. Most people don't understand the reasoning behind the WOT RPM thing. Let alone the theory and load tuning involved behind the very pitch and the diesel engine loading. If you could integrate the tach, pyros. pitch and speed all at once then you could make some real gains. Why doesnt one of you guys develop a nice small ,strong and cheap very pitch for our boats. We have all the other things. Look what Evenrude did for the new E-TEC engine, f-I 2 stroke. The same old design just refined. What a motor.

BILL

 

[yachtsmanbill]

Yeah, the one bladed prop is the most effecient. But for some reason they seem to have trouble balancing that.

The germans developed a single blade aircraft prop in the 30's -- it had one long blade and a balance weight much like a crank shaft. I think they encountered problems with harmonics tho... ws