In the lower rpm range, while traveling in displacement mode, (I do a
lot of that) will a 4 blade "push" more water at a lower rpm? That is to
say will a 4 blade 21"x18" stainless perform as well as a 3 blade
21"x21" or would I simply be overloading the motor?

Capt. Frank

Trond Solem wrote:
"Tony Thomas" wrote in message
news:eJDob.72698$HS4.632907@attbi_s01...

The most economical cruise is going to be at the lowest rpms and

maintaining

plane. A 4 blade will allow for a lower rpm than the 3 blade at minimal
plane speed. It will also give you better throttle response and quicker
planing.



Nope! Not 100% correct.
The most economical cruise is going to be near the most effective RPM of the
motor. It is going to be a bit higher than the max torque RPM, but still a
bit under max HP RPM.
To find what RPM gives the most economical cruise, you just have to do som
testing.
As speed increases, the drag increases. This will offset the economy RPM. A
4 blade prop has more drag due to one more blade to push through the water,
so the 3 blade may be better for economy. This again depends on propeller
design. A good 3 blade SS prop is probably best for economy. A good 4 blade
prop gives better accelration and less vibration, but lower top speed.

Trond