Moderated Discussion Areas
ContinuousWave: Whaler Performance
posted 09-01-2001 08:39 AM ET (US)
I have reviewed the reference section, but was only able to find a passing reference to blade cupping. Could someone please describe the effects of cupping on performance, and how cupping could affect RPM - for example, does a 21" cupped prop have a lower RPM of a 21" non-cupped? It seems that it might because of the extra "bite", but then again, if I knew, I would be answering this question, not asking it....
posted 09-01-2001 09:48 AM ET (US)
"A cupped prop can be run higher to the surface, as it holds the water better and slips less than an uncupped version. A cupped prop is going to run less RPM when wide open as it is pushing more water. It is like running a higher pitched prop. If a boat is running a 21 inch uncupped prop, a 19 inch cupped prop will usually turn about the same RPM, but will have better power out of the hole, and might even run a little faster as the engine or drive can probably be trimmed up higher."
MidWest Prop Service
posted 09-01-2001 09:51 AM ET (US)
As the reference section says, cupping is added to most propeller blades to improve the propeller's bite on the water and decrease slippage. The effect of cupping makes the prop perform like a higher pitch propeller. So a cupped propeller should yield a lower WOT RPM than a non-cupped propeller of the same designated pitch.
posted 09-04-2001 04:24 PM ET (US)
Had my SS prop "double-cupped" and it dropped me about 200 rpms. I was hitting 6100 and had major blowout on turns. Double cup brought me down to 5900 and was an animal out of the hole.
posted 09-05-2001 06:16 AM ET (US)
Here are some simplified deffinitions of propeller terms: "Pitch" is the base angle of blade-to-hub expressed in inches of "pitch" which is the theoretical distance a prop would "screw" through the water assumming no slippage; "Cupping" is the twisting of the blades which produces a "progressive pitch" (increased pitch outward to blade tip)... ; "double cupping" is the turning down (or back) of the trailing edges of blades which further accellerates water aft. (this concentrates the vector and produces more directed thrust) ... There are other propeller aspects which influence how a particular prop performs, among which are: material and finish; blade design/shape (speed, work, general purpose etc); hub design (bazooka, flared, vented, exhaust-over-hub, etc); number of blades; diameter; etc,, etc... A planing-boat propeller acts as a "non-encased axial-flow pump" and accellerates water aft in a concentrated stream (jet) and action-reaction is what propells the boat forward. You can see this jet surface in mid-wake! All the propeller characteristics mentioned above affect the speed of this jet! Do I ramble on or what... will stop now! Whew, last word... testing with a tach is the only way to tell if you have the correct prop...in any cast, that's what I do.. Happy Whalin'.. Clark.. The Old Man and the Sea
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