3 or 4 blade props?
 

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.

3 or 4 blade props?
 

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.

Winterizing your safety equipment saves $$$
 

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

Winterizing your safety equipment saves $$$
 

Is there really an effective shark repellent these days? I remember reading
that none of them were worth a flip. Also, the inflatable life jackets are
now approved by the USCG, but have to be worn to count.

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

Winterizing your safety equipment saves $$$
 

I remember Batman had shark repellant on his utility belt, it seemed to work
pretty good.

"Keith" wrote in message
...
Is there really an effective shark repellent these days? I remember
reading
that none of them were worth a flip. Also, the inflatable life jackets are
now approved by the USCG, but have to be worn to count.

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

3 or 4 blade props?
 

Tony,

I don't disagree with the response as far as the qualities of 4 blades but he
was asking for the most economical cruise. I can't imagine a 4 blade being
more
economical than a 3. Common sense tells me there would be less drag with one
less blade.

Barry

Hey Barry

I am basically am looking to be able to plain at a slower speed than I am
currently achieving. My boat doesnt like to hold a plain at under 25mph, and
when it gets rough, I'd like to go slower than bouncing from one wave to
another. And yes I am also looking to maximize my range.

Thank you all, I appreciate your inputs...

Rick

Winterizing your safety equipment saves $$$
 

Yeah, but Batman's shark was stuffed.
:)
CF

Paul wrote:
I remember Batman had shark repellant on his utility belt, it seemed to work
pretty good.

"Keith" wrote in message
...

Is there really an effective shark repellent these days? I remember

reading

that none of them were worth a flip. Also, the inflatable life jackets are
now approved by the USCG, but have to be worn to count.

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

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

3 or 4 blade props?
 

Hey Barry

I am basically am looking to be able to plain at a slower speed than I am
currently achieving. My boat doesnt like to hold a plain at under 25mph,
and
when it gets rough, I'd like to go slower than bouncing from one wave to
another. And yes I am also looking to maximize my range.

Thank you all, I appreciate your inputs...

Rick

You need trim tabs, not a prop change.
http://www.bennetttrimtabs.com/

3 or 4 blade props?
 

Hey Barry

I am basically am looking to be able to plain at a slower speed than I am
currently achieving. My boat doesnt like to hold a plain at under 25mph, and
when it gets rough, I'd like to go slower than bouncing from one wave to
another. And yes I am also looking to maximize my range.

Thank you all, I appreciate your inputs...

Rick








Rick,

Sounds like trim tabs would do more for you than changing props. Their is
probably no better way to achieve slower planing speed than good trim tabs. Of
course, that certainly won't be the most economical speed. Most new outboards
are most economical in the 3500 to 4000 rpm range.

At certain rpm ranges, I imagine a 4 blade could be more economical than a 3
blade but I'd still be real surprised if you took an outboard motor on a
typical v-hull boat and tried just to maximize the most mileage per gallon
possible that could be gotten out of that engine, a 3 blade prop would be the
winner.

What do you think, Tony?

Barry

3 or 4 blade props?
 

In article , F330 GT
writes
Rick,

Sounds like trim tabs would do more for you than changing props. Their is
probably no better way to achieve slower planing speed than good trim tabs. Of
course, that certainly won't be the most economical speed. Most new outboards
are most economical in the 3500 to 4000 rpm range.

Surely you mean most efficient, not most economical. There is no way an
outboard running at 3500rpm gets less gph than one running at 1000rpm.
Equally, there is no way it gets better mpg than one running at 1000rpm.
Only, m/g/h would it be more economical. I can run all day on a river at
1000rpm (6mph) and only use half a tank. Run on a lake at 25-30mph and
3500rpm and I get through a whole tank in a few hours!
--
Nick