"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

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

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

Ok, Tony,

That makes sense. Is there a formula for calculating a prop's thrust?
rpm vs hp? Given 21" pitch and the current 3 blade a 21" diameter
(stainless steel)

21?R²(0.66)=19.0250?R²(0.80) it doesn't work out exactly.

If the formula doesn't display in the newsgroup, I will try a different
venue.

Thanks,

Capt Frank



Tony Thomas wrote:
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.

Only formula I am aware of is the speed formula.

rpm x pitch
------------------ = theoritical speed x efficiency = actual speed
1056 x gear ratio

1056 is a conversion factor to get from inches of pitch and rev per minute
into miles per hour.

gear ratio will be 1.5 or 1.86 or 2.0 depending on your motor usually.

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


"Capt. Frank Hopkins" wrote in message
nk.net...
Ok, Tony,

That makes sense. Is there a formula for calculating a prop's thrust?
rpm vs hp? Given 21" pitch and the current 3 blade a 21" diameter
(stainless steel)

21?R²(0.66)=19.0250?R²(0.80) it doesn't work out exactly.

If the formula doesn't display in the newsgroup, I will try a different
venue.

Thanks,

Capt Frank



Tony Thomas wrote:
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.

Thanks a lot Tony.

I will see what else I can find on the subject and post it.

CF

Tony Thomas wrote:
Only formula I am aware of is the speed formula.

rpm x pitch
------------------ = theoritical speed x efficiency = actual speed
1056 x gear ratio

1056 is a conversion factor to get from inches of pitch and rev per minute
into miles per hour.

gear ratio will be 1.5 or 1.86 or 2.0 depending on your motor usually.

On Sun, 02 Nov 2003 05:16:31 GMT, "Capt. Frank Hopkins"
wrote:

Ok, Tony,

That makes sense. Is there a formula for calculating a prop's thrust?
rpm vs hp? Given 21" pitch and the current 3 blade a 21" diameter
(stainless steel)

21?R²(0.66)=19.0250?R²(0.80) it doesn't work out exactly.

If the formula doesn't display in the newsgroup, I will try a different
venue.
========================================

Capt Frank,

Recommend that you get a copy of Dave Gerr's excellent book:

"Propeller Handbook"

http://tinyurl.com/te13

I bought a copy earlier this year after pondering many of the same
questions and issues. Excellent reading and very thorough coverage
of a complex subject.

To make a long story short, Gerr says that prop slip is basically a
function of blade area. Three blades are more efficient if you can
swing a big enough prop to get the required area. Many boats however
have insufficient prop aperature space for a large prop without
compromising blade tip clearance between prop and hull. It turns out
that insufficient tip clearance leads to vibration because of
asymetric blade loading. For those boats with insufficient clearance,
a 4 bladed prop offers a way to increase blade area without increasing
the prop diameter (and decerasing clearance).

Hope that helps but I still recommend the book for the complete story,
and many, many, design equations and charts.

Thanks Wayne,
I will seek out the book.

Capt. Frank

Wayne.B wrote:

On Sun, 02 Nov 2003 05:16:31 GMT, "Capt. Frank Hopkins"
wrote:


Ok, Tony,

That makes sense. Is there a formula for calculating a prop's thrust?
rpm vs hp? Given 21" pitch and the current 3 blade a 21" diameter
(stainless steel)

21?R²(0.66)=19.0250?R²(0.80) it doesn't work out exactly.

If the formula doesn't display in the newsgroup, I will try a different
venue.

========================================

Capt Frank,

Recommend that you get a copy of Dave Gerr's excellent book:

"Propeller Handbook"

http://tinyurl.com/te13

I bought a copy earlier this year after pondering many of the same
questions and issues. Excellent reading and very thorough coverage
of a complex subject.

To make a long story short, Gerr says that prop slip is basically a
function of blade area. Three blades are more efficient if you can
swing a big enough prop to get the required area. Many boats however
have insufficient prop aperature space for a large prop without
compromising blade tip clearance between prop and hull. It turns out
that insufficient tip clearance leads to vibration because of
asymetric blade loading. For those boats with insufficient clearance,
a 4 bladed prop offers a way to increase blade area without increasing
the prop diameter (and decerasing clearance).

Hope that helps but I still recommend the book for the complete story,
and many, many, design equations and charts.

I have updated my web page about safety equipment & abandon ship info. I
would appreciate any comments.

http://www.home.earthlink.net/~aartworks/solas.html

Capt. Frank