First of all, normally a 4 blade prop will have a smaller diameter than the
3 blade to compensate for the extra blade surface.
You want to go with the same pitch prop. The 18 will be way too small a
prop and will travel less and be less efficient than the 21. Stay with same
pitch which is inches of travel forward per revolution (in theory not
compensating for slip). A 4 blade will have less slip at lower rpms than a
3 blade which is why it will be more efficient at cruise.

--
Tony
my boats and autos - http://t.thomas.home.mchsi.com


"Capt. Frank Hopkins" wrote in message
.net...
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