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Posts: 726
Location: Eau Claire, WI | all thing being equal, if you reduce the number of blades on your prop will the w.o.t. rpms increase or decrease? For example, go from at 21P High 5 to a 21P Tempest?
Thanks,
Jono
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Posts: 2323
Location: Stevens Point, WI | You should run higher rpm's with a 3 blade Tempest. A way to think of it is, with a 5 blade high five, that prop provides more lift and better holeshot. Also, with a high five, it will decrease the amount of throttle it takes to put the boat on plane because you have 5 blades. A tempest is built for speed and achieving your maximum speed. | |
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| Wrong. All things being equal, you would most likely go up to a 23 pitch high 5, if you use a 21 pitch Tempest. Stay with a 21, and you'll likely over rev, if you were at max RPM's. | |
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| I forgot to mention, you need to look at blade diameter, too. A REV 4 would probably have the same pitch, because it is a larger diameter prop. | |
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Posts: 32959
Location: Rhinelander, Wisconsin | My application of the Rev 4 on my last two rigs indicated exactly that, guest is correct. | |
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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 | |
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Posts: 726
Location: Eau Claire, WI | Thanks to all of you and I agree VMS Steve....too many variables! I was trying to simplify the question but I think I complicated matters by naming particular models. Turns out though that was a good thing because I had not considered the effect of diameter. Really all I wanted to know is again all things equal reducing the number of prop blades will do what to the rpms? I figured they would go up and looks like that has been confirmed.
I am trying to educate myself on props and it's really interesting. Seems there is as much voodoo as science.
My recently purchased rig currently has a 17P high 5 on a 150 opti and I'm getting 38 gps @ 5500 rpm w/ two guys and full fuel tanks. I think I can go faster than that so my thought was trying a 21P or 19P Tempest. The high 5 is great for holeshot but not great for high end. I don't have to go screaming fast but a 692 should go faster than 38. I was hoping to get close to 50 but just my guess. Other 690 series owners have told me they get 50 to low 50s with their 175s.
I appreciate all the help,
Jon
Jono
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| 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...
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.
last year i ran both the Tempest and High 5 props that the original post asked about (200hp Opti on my Tuffy 1890).
the results were about what you'd expect them to be...the Tempest was incredibly fast and powerful, whereas the High 5 had more lift leading to increased handling but slightly less top end speed (though still quite fast). the Tempest put you back in your seat with a hole-shot that launched the boat forward whereas the High 5 lifted the boat up and out with a smoother transition to moving forward.
the High 5 ran higher rpms than the Tempest.
as VMS notes, the High 5 has a smaller 13 1/4" diameter in order to be able to turn enough rpms and the 3 blades on the Tempest are just plain big at 14 5/8". it's kind of hard to compare "all things equal" since they're not made that way.
if you're like me, Jono, and don't really know the specific rpm changes to expect for each change in diameter or pitch, and if you've got access to a variety of props at your dealer, try testing the impact of either a change in pitch, change in blade numbers, or change in diameter in increments, ie., don't change everything at once, but rather make small changes with each test so you know what it is exactly that's giving you the performance change. watch your rpm gauge, and take good notes on the change each prop gives you and how you like it's performance.
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Posts: 3511
Location: Elk River, Minnesota | Jono,
No doubt that rig should be very near 50mph. As Lambeau stated, if you can find a good prop shop that will allow you to demo different props, by all means do so. If you find a tempest that gets you close to your recommended top RPM, then you are very close to having your rig optimized....from there, you can move the motor up a hole...possibly to the top hole and get those RPMs to the max, and gain on the top end. There is a compromise, there, though...hole shot may not be as strong, but it will be decent. I believe the tempest series of props have vent holes in the hub too, which will allow some exhaust to exit along the blades during hole shot...this lets the motor get into a higher RPM Range and jumps the boat up on plane quickly...
Steve | |
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Posts: 1096
Location: Hayward, WI | Jono, something does not add up with your numbers at 5500rpm with 17 pitch only doing 38 you are at %20 slip, which is high, I would expect 6-12%. Make sure the prop is in good condition and verify the tach is reading properly. | |
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prop blades/rpms