2 to 3 blade ???

[Don Bodemann]

Hello Kevin,following Jim's question and perhaps shedding some light. If one were to look at the owner review section and read the review of the Cherubini 27's and 30's, you would see that many owners have found improved performence with aftermarket 3 blade props. (I believe the stock 2 blade prop was not "loading" the engines sufficiently). I suspect the benefit with the 3 blade is not so much in the number of blades but the amount of prop surface area with a given diameter and pitch. Correct me if I'm wrong but a 2 blade prop of larger diameter with the same total area as the 3 blade would yield "similar" performence. Assuming of course we did not alter the pitch or shape of the blade. If we were to use a much denser medium than water to run our props through, we would get a much better feel for why props are sometimes referred to as "screws". This is where pitch comes in. A greater pitch will at least try to "screw" itself through the water faster at a given RPM. Enough of my rambling...let's hear from the expert!Don

 

[Kevin L. Woody]

2 or 3 blade

Dear Don:EXPERT!!! I feel a set up here. A larger two blade in both diameter and blade area will eventually provide similar characteristics to the three blade. The problem we run into is that most installations we get involved in do not allow for additional diameter to be used at the prop. With this we need then to look at the vessel we are specifying a propeller for. If the displacement ratio is very low in most cases the 38% (apx) efficiency rate of the two blade works fine. When equipping a prop to a vessel, which is considered a heavy displacement ratio, we then have to rely on the 50+ percent efficiency rate of the three blade to push and then maintain the vessels speed. You bring up a great point of the denser the medium the closer to a true screw we find the propeller becomes. Jell-O is a great medium for this example. If a two blade is set to 12” of pitch, it will advance 12” per one revolution. A three blade set to 12” of pitch, it will also advance itself 12” per one revolution. Because the three blade maintains a third additional blade, as the medium gets closer to water in consistency, it will help to maintain this 12” advancement. Another problem we deal with is the limited HP we have to work with. As we add blades we add load to the motor. This is why you usually see a three blade recommendation either smaller in diameter or smaller in pitch when going from a two to a three blade. Hope this helps. Sincerely, Kevin L. Woody

 

[Alex]

A good reading on these topics..

..I just found on March 2000 issue of UK sailing magazine " Practical Boat Owner".It explain in very clear way various point of propellors -two or three and differences,pitch, slip and advance, how to choose the right one for certain boat lenght/displ/engine,what are the RPM for best result in speed , fuel efficiency ,etc., what are the engine hp needed for certain LWL and DISP boat , the actuall HP you get to the propeler ( amazingly much lower at most rpm than the max advise one on the engine..),etc.The example shown is for a Yanmar 2GM engine but the formulas and theory are valid for most.One of the best and clearest articles on this issue I read in recent years.