Prop
09-12-2008, 10:30 PM
#1
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Prop
I have a k&b 7.5cc engine and i have several different props. They are all 2 blade props but 2 of them are aluminum and 2 of them are brass. Is there a certain purpose in which brass can give it better top end but slower bottom end or the other way around? How about aluminum. How about size... does bigger props make a difference??
09-12-2008, 10:46 PM
#2
RE: Prop
Technically, they're beryllium copper and the others are either aluminum or stainless steel.
OK, there's a big depends on these. One manufacturer, Prather makes some in both aluminum and stainless. Octura props are most likely beryllium copper.
There should be numbers on the prop's hubs. On Octura's, you'll probably find an X then a 3 digit number. The first digit (maybe a 4) is the pitch that is 1.4 (using the 4) times the diameter of the prop in millimeters which is the second two numbers.
Let's say that you have an X-450 sitting there which is a good possibility. Take the 50 and multiply that by 1.4 to find the pitch in millimeters.
If you have en X-646/3 you multiply 1.6 times 46 to get the pitch. The "/3" just shows that it's a 3-blade prop.
Octura props have a progressive pitch which you can think of as a cupped hand when you're swimming. Prather's have a constant pitch so it's the same all the way through.
If the props have the exact same numbers and haven't been reworked there should be no difference between which type of metal that they were formed from so that should make no difference in speed.
Yes, prop size does make a difference as the larger a prop is, the more pitch that it will have also.
OK, there's a big depends on these. One manufacturer, Prather makes some in both aluminum and stainless. Octura props are most likely beryllium copper.
There should be numbers on the prop's hubs. On Octura's, you'll probably find an X then a 3 digit number. The first digit (maybe a 4) is the pitch that is 1.4 (using the 4) times the diameter of the prop in millimeters which is the second two numbers.
Let's say that you have an X-450 sitting there which is a good possibility. Take the 50 and multiply that by 1.4 to find the pitch in millimeters.
If you have en X-646/3 you multiply 1.6 times 46 to get the pitch. The "/3" just shows that it's a 3-blade prop.
Octura props have a progressive pitch which you can think of as a cupped hand when you're swimming. Prather's have a constant pitch so it's the same all the way through.
If the props have the exact same numbers and haven't been reworked there should be no difference between which type of metal that they were formed from so that should make no difference in speed.
Yes, prop size does make a difference as the larger a prop is, the more pitch that it will have also.
09-12-2008, 10:56 PM
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RE: Prop
the 2 brass prop that you have are not brass, we call them for "BC" (Beryllium, Copper and some if not most also have Titanium).
these 2 BC props are better than the aluminum props, the BC are harder, therefore no flexing of the blades meaning no performance loss.
the next best thing are "SS" props or Stainless Steel.
I'm guessing that what you have are Prather (brand) it should be written on the hub of the prop (where the nut would sit once tightened)you will also find the size of the prop at the same place.
Prop range For a K&B 7.5 would be a 225, a 230 and a 235.
I would not go any smaller or any bigger. too small will cause the engine to rev past its RPM limits and will damage the engine (no gain in performance).
Too big of a prop will create too much load on the engine which may cause over heating (working too hard) and loss of RPM's (no gain in performance but actually a loss of performance).
There are other good brand of props out there like Octura, ABC etc.... but I do not know which one would be correct, actually the only one that comes to mind from Octura is the X447 2 blades, not the 3 blade.
Dan.
Ron beat me to it.
these 2 BC props are better than the aluminum props, the BC are harder, therefore no flexing of the blades meaning no performance loss.
the next best thing are "SS" props or Stainless Steel.
I'm guessing that what you have are Prather (brand) it should be written on the hub of the prop (where the nut would sit once tightened)you will also find the size of the prop at the same place.
Prop range For a K&B 7.5 would be a 225, a 230 and a 235.
I would not go any smaller or any bigger. too small will cause the engine to rev past its RPM limits and will damage the engine (no gain in performance).
Too big of a prop will create too much load on the engine which may cause over heating (working too hard) and loss of RPM's (no gain in performance but actually a loss of performance).
There are other good brand of props out there like Octura, ABC etc.... but I do not know which one would be correct, actually the only one that comes to mind from Octura is the X447 2 blades, not the 3 blade.
Dan.
Ron beat me to it.
