Posts: 3511
Location: Elk River, Minnesota | Hi Everyone,
I am going to agree with Merc here on this one....because if "all things being equal"(I take that to mean both props have the same diameter and pitch) except for the number of blades, the 3 blade prop would spin faster since there is less prop to spin, and thus less surface area cutting through the water. Merc is correct in his statement.
I am also going to agree with Guest based upon the two props mentioned...here's why...for the two props listed (Merc high 5 compared to the tempest) we have different props designed for different applications. Their respective diameters are as much as 1-5/8" different....and that is a huge difference. In order to keep RPM's up on 4-blade and 5-blade props, the diameter is reduced significantly so that the motor can run in it's recommended RPM range. The high-5, being designed primarily for handling and water sports, and applications that require a large amount of blade surface (thrust). None of those props has a diameter near 14". It must be this way or the motors could not turn them without undo stress on the engine and components. The smaller diameter also allows the motor to "jump" up on plane quickly, which is what is needed for ski's and tubes, etc.
The tempest, on the other hand is a 3 blade prop designed for more speed. It has a diameter of 14-5/8" and only goes up to a 23 pitch (as per US boat supply website..there may be more...just not listed there, but does not matter since the two props named here are within those limits) The tempest is going to have a bunch of cupping on the trailing edge which will allow the motor to be higher off the transom.
Now...the fun begins. For every 1/4" of diameter there will be roughly a 100 rpm difference in the motor. Lower diameter, up in RPM (not a hard and fast rule, but generally the norm for most applications) and vice-versa. So..if we look just at diameter alone for a second, the high 5, has a 13-1/4" diameter, the tempest 14-5/8"...So...an increase of roughly 550 RPM's just due to a reduced diameter. Also, by increasing the number of blades on the prop (again everything else being held equal here...pitch and diameter) you would see a drop in RPM that would be quite significant...up to a point....there will be a limiting factor here and due to some weight of the prop, there is some rotational "momentum" taking place with more blades... This is probably what Guest is getting at as for his reasoning for needing a higher pitched high 5 prop to stay within RPM limits.
I have not run either of these props mentioned, so I would be "out of line" trying to make a guess as to how they would compare against each other... Generally, any time you make a switch to a new prop some testing "should" take place to see if the prop is going to do what your application requires of it. So what guest mentions could very possibly be true. It certainly would not surprise me if things worked out that way. To be honest, I think there might be a bit too many variables here to make any good call on how the two compare. If you can, get out there and test them...enjoy running the boat if you have open water.
Steve |
