Pitch/Diameter Ratio for Props

It's the ratio of pitch to diameter that counts and dictates if you've got too many or too few blades. If MV Content had a 30" X 19" prop better performance would probably be had w the 28X23 props. So as Eamonn says the prop calculation proved correct on the Content. A fourth blade would have required a big reduction in pitch to load the four blades correctly. That would mean the resulting prop would be about 28 X 18 .. just a guess. Huge difference in the pitch/dia ratio. Not pushing much water but eating up much power swinging that fourth blade around. If you keep decreasing pitch and adding blades eventially you'll wind up w a flat disk. Could be the same dia but probably will be more dia. All the power of the engine/s will be consumed turning the blades through the water and no thrust created. And of course if you go the other extreme w too little dia and too much pitch eventially you'll get 90 degrees to a feathered prop. Blades aligned w the prop shaft. With a certian blade size and dia all the power will be required to reach rated rpm and absolutely no thrust will be generated.

So either extreme will consume all the power and produce no thrust. So for every blade dia and area and blade design and speed through the water there is a perfect or better put optimal pitch and dia. The number of blades is almost meaningless but if you can't get enough dia to be in the optimal zone w 3 blades you then need four. So usually the factor that dictates 3 or 4 blades is swinging room for the propeller .. the maximum dia that is practical. There is a small range or zone that either 3 or 4 blades will produce the same efficiency. Then it matters not if you have three or four blades. The four blade just costs more money. So the basic rule of thumb is "three is best unless you need four".

And this "hole shot" talk has no place on TF. On outboards four blades are used to get blade area. Just like trawlers but in the case of OB's much more limited space is available.

I am obviously missing something here, or are you having a discussion with yourself?
It is an interesting one none the less!
Bruce

Bruce B,
On the "girls Bottom" thread we got off on 3 blade vs 4 blade props. So I tried to limit the hyjack. Prolly a little late though and my pc skills are limited.

I figured it was something like that.
Now I'll have to go and take a look!
I do find "girls bottoms" interesting...I just haven't paid any attention to the thread yet.
Bruce

Eric...slight topic swerve....I never see Cort nozzles on FD pleasure craft even though they are widely used on fish and work boats to improve efficiency at displacement speeds (?)

Ken E. said:

Eric...slight topic swerve....I never see Cort nozzles on FD pleasure craft even though they are widely used on fish and work boats to improve efficiency at displacement speeds (?)

Click to expand...

Thanks for bringing this up, Ken...

First time I've paid much attention to ducted props; i.e. Kort nozzel propellers.

https://upload.wikimedia.org/wikipedia/commons/thumb/e/e4/Bel_vis_346.jpg/300px-Bel_vis_346.jpg

Wikipedia: "A ducted propeller, also known as a Kort nozzle, is a propeller fitted with a non-rotating nozzle. It is used to improve the efficiency of the propeller and is especially used on heavily loaded propellers or propellers with limited diameter. It was developed by Luigi Stipa (1931) and Ludwig Kort (1934). The Kort nozzle is a shrouded propeller assembly for marine propulsion. The hydrodynamic design of the shroud, which is shaped like a foil, offers advantages for certain conditions over bare propellers.

Advantages are increased efficiency at lower speeds (<10 knots), better course stability and less vulnerability to debris. Downsides are reduced efficiency at higher speeds (>10 knots), course stability when sailing astern, and increase of cavitation. Ducted propellers are also used to replace rudders."

Thanks Art.
I'm no expert on shrouded props but I do know the clearence prop tip to shroud needs to be really small. So small that I kinda think the benifits are too hard to get. Same with airplanes. I suspect that the commercial appications are there mostly to keep nets and other stuff like lines and cables away from the prop. The efficiency is there ... just too hard to harness.

But like I said I don't know much about this. But Art I thought the Kort Nozzle was a steerable prop and shroud and a shrouded prop was with fixed prop and a rudder. But Wikapedia dosn't agree.

Bruce B,
Sorry no real girl bottoms. They are high on interest but low on interesting.

The subject of the hyjack on the "bottoms" thread was the value of using a 4 blade prop on a rec trawler. There seems to be a notion that 4 blade props are better than 3. Not so IMO and I've stated why I think so.
MV Content, a 58' Garden designed trawler is refered to in the thread about that boat.

Thanks for starting this thread Eric!


Being a novice in a marine prop's separated-blade design for the prop as a total piece to best cut through water; yet considerably experienced for vertical-draft designs to get air quickly moving axially-symmetric upward inside containment channels (e.g. specially designed chimney-twists)... due to individualized molecular air contents' heat expansions, channeled perimeter bounce-offs, and developed pressure drops.


Seems to me that a specialized-design "cork screw" of a thin yet solid twisted material, fitted up inside a specialized-design cavern on hull bottom, with support bearings on ends, would provide extreme efficiency for "cutting/screwing" through water. Properly designed there would be little to no cavitation chance and vastly improved per turn forward movement with each revolution of the "cork screw". Don't know if this would only be best for slow moving boats [< 10 knots] or could be further modified to assist fast boats. But I do know that to a large effect this premise of screw-propulsion holds merit.

Smith's original 1836 patent for a screw propeller of two full turns. He would later revise the patent, reducing the length to one turn. Rats. Only missed it by 181 years...So close...

Further...https://en.wikipedia.org/wiki/SS_Archimedes

Art,
The very high velocity water over the inside of the nozzle should produce lots of drag so the increased efficiency needs to overcome that increased drag before any increase in efficiency can take place. That's why I'm sure all the nozzles seem to be very short.

I also suspect that the use of a nozzle may be a substitute for larger dia propellers. Get the thrust w a smaller dia device. Torque reduction too. The engine and huge prop on tugs must have huge torque. The torque could be a problem the nozzle may largely correct. Another thing that occured to me is that greater pitch could effectively used w a nozzle as spanwise tip flow would be greatly reduced. Lots of variables.

MV Content, a 58' Garden designed trawler is refered to in the thread about that boat.[/QUOTE]


Tiny, respectful correction, Willy..... Bill Garden would have been 12 at the time, if he designed Content (1930)

She was built at Fellows & Stewart in California, and I can't remember who designed her... I'll check.

RT Firefly said:

Smith's original 1836 patent for a screw propeller of two full turns. He would later revise the patent, reducing the length to one turn. Rats. Only missed it by 181 years...So close...

Further...https://en.wikipedia.org/wiki/SS_Archimedes

Click to expand...

Rudimentary My dear RT!

That drawing of course has the most basic of design premise to which I speak. My design has substantial differences. That said: I'm not going to put any time/effort to bring my plan to fruition... too busy otherwise. Just saying that I believe there is/are a way to improve thrust/inch per foot of screw movement through water, other than the generally well known and utilized propeller.

Greetings,
Mr. A. "... other than the generally well known and utilized propeller.". Gasp...Heresy!

Nomad Willy said:

Art,
The very high velocity water over the inside of the nozzle should produce lots of drag so the increased efficiency needs to overcome that increased drag before any increase in efficiency can take place. That's why I'm sure all the nozzles seem to be very short.

I also suspect that the use of a nozzle may be a substitute for larger dia propellers. Get the thrust w a smaller dia device. Torque reduction too. The engine and huge prop on tugs must have huge torque. The torque could be a problem the nozzle may largely correct. Another thing that occured to me is that greater pitch could effectively used w a nozzle as spanwise tip flow would be greatly reduced. Lots of variables.

Click to expand...

Correct Eric... "Lots of variables." There are design-ways to reduce water-surface drag and increase thrust efficiency. I and my engineers hade been successful accomplishing similar reductions/increases with slow and high-speed airflows. I believe water-flow can be made to likewise somewhat follow suit.

RT Firefly said:

Greetings,
Mr. A. "... other than the generally well known and utilized propeller.". Gasp...Heresy!

Click to expand...

Standard blade props always work better than anything else will... on a flat world!

Art,
At least the original prop was probably chosen by a NA that knows lots more than we do. And the number of blades is included in the choice. So anybody taking off the three blade prop on their boat and putting on a four blade is saying they know more about it than the NA that designed the boat. That's kind-of a pissy thing to say and the boat may have a bigger more powerful engine requiring a 4 blade. Knowing you'll need a different prop means one should go to a good prop man and take specs of all the original parts and what's changed. No guarantee but the prop man will almost certainly make a better choice than we the boat owner.

Art what's a flat blade prop? Non cupped? I personally don't think cupped props belong on trawlers.

Nomad Willy said:

Art,
At least the original prop was probably chosen by a NA that knows lots more than we do. And the number of blades is included in the choice. So anybody taking off the three blade prop on their boat and putting on a four blade is saying they know more about it than the NA that designed the boat. That's kind-of a pissy thing to say and the boat may have a bigger more powerful engine requiring a 4 blade. Knowing you'll need a different prop means one should go to a good prop man and take specs of all the original parts and what's changed. No guarantee but the prop man will almost certainly make a better choice than we the boat owner.

Art what's a flat blade prop? Non cupped? I personally don't think cupped props belong on trawlers.

Click to expand...

Eric - As I've mentioned before... I know little about prop design intricacies, leaving that stuff up to experienced propeller engineers. I do understand the general means of prop blade designs for "cutting/screwing" through water. What I mentioned regarding corkscrew twisted flat sheet as a water mover rests on my considerable knowledge of cyclonic vertical airflows and how we efficiently create/enable them. It's easy for my mind's eye to transpose successfully improved airflow design apparatus into potentials for improved water flow design apparatus. Water-movement as a liquid is considerably similar yet somewhat definably different than air-movement as a gas.

BTW: We still have exact same prop design that came with our Tollycraft from the factory. Are they the actual original props??? - I'm not sure having owned the 40 yr. old boat since 2008. But I do know they are in really good condition and work fine with our engines, hull and loads.

Kort nozzles are generally used to improve thrust at low hull speeds, for example on tugs etc

Stone Beach .. yes that's where we see them.

Just a thought ....
Ever seen a modern windmill w any other than three blades?

Two blade three blade six blade. the difference may be in total blade area and vibration or smoothness the essential thing is sizing to the aperture and getting the proper load characteristics. You add more blades if you need area to load right in a restricted aperture or want smoother you still must match the same proper load and that depends on area/pitch. The theoretic efficiency differences between the # of blades is rarely a big concern. The difference in cost between a two-three and five blade to buy and maintain could be significant for those on a budget. I have owned two-three-four and five blade props they all propelled the boats and I was rarely aware of the # of blades so long as the system was properly set up.