On Thu, 18 Oct 2007 21:49:59 -0700, druid wrote:

One problem with most of the prop-calc programs I used is they don't take
into consideration the HORSEPOWER of the engine. Same prop for an 8hp as a
20hp? I don't think so!

That is correct. More horsepower generally means more torque, and
more torque gives you the ability spin a prop with more pitch, all
other things being equal.

On Fri, 19 Oct 2007 17:37:17 +0700, Bruce in Bangkok
wrote:

the H.P. and the gearbox reduction
give the diameter of the propeller

Not really. Diameter of the prop is limited by tip clearance of the
blades. Generally speaking you should use the biggest diameter that
will still give 10 to 15% clearance. Too little clearance will cause
vibration as the prop thrust interacts with the hull and/or aperature.

There is a way of backing into the prop numbers based on the estimated
hull speed and slippage factor assuming that the engine will deliver
the required power to maintain RPMs.

Required horsepower is a function of displacement and drag. If the
prop pitch is too great, or the available torque is too low, the
engine will not rev high enough to deliver its rated power.

On Fri, 19 Oct 2007 07:18:08 -0700, druid wrote:

They do
this apparently because the ANGLE of their blade corresponds to a 6" pitch
(they curve the blade to get the extra thrust).

That is called "cupping" in prop language. Cupping will increase the
effective pitch which is fine if your engine has enough torque to
maintain RPMs. If not you haven't accomplished anything. There is no
free lunch with props.

On Fri, 19 Oct 2007 11:01:33 -0400, Red wrote:

....
118 Engine Torque ft/lb
....
172 Propeller Torque ft/lb


Red


This is a nit-pick, but everyone has his hobby-horse.

A measure of work is raising some weight through a particular height.
If you time how long this takes, you have an early way of specifying
power - James Watts's way, in fact.
That's feet times pounds all divided by time .
For example ft.lbs /min

A measure of torque is how much force a crank can apply at a given
distance from the shaft's axis of rotation.
This does not signify power, or work but it's sometimes good to
know.
The units for torque can be force in pounds TIMES distance from the
center in feet. This time, the force is at right angles to the
distance from the rotation axis. So it can be given as ft.lbs
also, but this makes it look like work, just one step away from power.
But it isn't.

So some people who like to keep the difference between work
and torque in mind, call the one unit ft.lbs
and the other unit lb. ft.

To summarize: there is no such unit as ft/lbs.
/ means divide by....
There, I said it! :-)

Brian W

On Thu, 18 Oct 2007 11:22:19 -0700, druid
wrote:

Hi,

My new-to-me boat (Crown 28) has a Volvo 10hp MD6A, and it's way
overpropped with a Campbell Sailor 11x6 3-blade (gets 1500 WOT instead
of 2400). The Campbell Sailor dealer says his calcs recommend a 12x8
(ie much MORE pitch). Now, my experience with CS props is you take the
calculated pitch and subtract 2", but I donno...

Engine IS very "tired" but it pushes the boat at 5+ knots with the
over-pitched prop, so it can't be TOO bad.

Those with MD6A's - what prop do you have on it?

druid
http://www.bcboatnet.org

[based on 2 minutes back of the envelope and positively NO
experience with a MD6A]....
Borrow a 10 X 5 and look for 6+ kts at 2200 revs out of
your ol' tired engine.
:-)
Brian W

On Oct 19, 8:35 pm, Brian Whatcott wrote:
On Thu, 18 Oct 2007 11:22:19 -0700, druid
wrote:

Hi,

My new-to-me boat (Crown 28) has a Volvo 10hp MD6A, and it's way
overpropped with a Campbell Sailor 11x6 3-blade (gets 1500 WOT instead
of 2400). The Campbell Sailor dealer says his calcs recommend a 12x8
(ie much MORE pitch). Now, my experience with CS props is you take the
calculated pitch and subtract 2", but I donno...

Engine IS very "tired" but it pushes the boat at 5+ knots with the
over-pitched prop, so it can't be TOO bad.

Those with MD6A's - what prop do you have on it?

druid
http://www.bcboatnet.org

[based on 2 minutes back of the envelope and positively NO
experience with a MD6A]....
Borrow a 10 X 5 and look for 6+ kts at 2200 revs out of
your ol' tired engine.
:-)
Brian W

Interesting discussion - here's an "update":

First, I realized I have a spare prop: a 14x8(? have to look...) 2-
blade non CS (BTW, DON'T tell CS that their patented curve is simply
"cupping" ). I can maybe repitch it or just leave it as is - in any
case, I can put it on with a quick-lift rather than lifting the boat
out, leaving it on the hard while I remove the CS, get it repitched
and put it back on.

Second, I've decided to pull the head and injectors and try to squeeze
a bit more life out of the engine. Already got them off, getting them
to the rebuild shops on Mon, but 'er back together next weekend, and
see if I can get a few more RPMs. If not, I can still slap on the 2-
blade...

druid
http://www.bcboatnet.org

On Sun, 18 Nov 2007 19:46:02 -0800 (PST), druid
wrote:

Interesting discussion - here's an "update":

First, I realized I have a spare prop: a 14x8(? have to look...) 2-
blade non CS (BTW, DON'T tell CS that their patented curve is simply
"cupping" ). I can maybe repitch it or just leave it as is - in any
case, I can put it on with a quick-lift rather than lifting the boat
out, leaving it on the hard while I remove the CS, get it repitched
and put it back on.

Second, I've decided to pull the head and injectors and try to squeeze
a bit more life out of the engine. Already got them off, getting them
to the rebuild shops on Mon, but 'er back together next weekend, and
see if I can get a few more RPMs. If not, I can still slap on the 2-
blade...

druid
http://www.bcboatnet.org
=======================

Let us know how it works out. The real world has a way of confounding
the theoretical at times.

On Nov 22, 7:36 am, Wayne.B wrote:
On Sun, 18 Nov 2007 19:46:02 -0800 (PST), druid
wrote:



Interesting discussion - here's an "update":

First, I realized I have a spare prop: a 14x8(? have to look...) 2-
blade non CS (BTW, DON'T tell CS that their patented curve is simply
"cupping" ). I can maybe repitch it or just leave it as is - in any
case, I can put it on with a quick-lift rather than lifting the boat
out, leaving it on the hard while I remove the CS, get it repitched
and put it back on.

Second, I've decided to pull the head and injectors and try to squeeze
a bit more life out of the engine. Already got them off, getting them
to the rebuild shops on Mon, but 'er back together next weekend, and
see if I can get a few more RPMs. If not, I can still slap on the 2-
blade...

druid
http://www.bcboatnet.org

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

Let us know how it works out. The real world has a way of confounding
the theoretical at times.

Well, I FINALLY got the engine back together and running! Rebuilding
the head and injectors only cost $650, but lots of grunting and
general blue-air. Took her out for a bit, but there was quite a bit of
tide and gusty winds, so it was a bit hard to see any changes. I think
I got maybe an extra 1/2 knot (didn't have GPS and was too busy to
check RPM on the hand tach).

One thing that DID happen is now I get the "classic" grey/black smoke
when I push up the throttle past where the RPMs get any higher,
telling me too much fuel for the engine speed. DEFINITELY a sign of
over-pitched prop! So: gotta pull her in the spring to check/replace a
thru-hull, so I'll reduce the pitch an inch, maybe 1.5 inches....

druid
http://www.bcboatnet.org

On 2007-12-17 17:06:03 -0500, druid said:

One thing that DID happen is now I get the "classic" grey/black smoke
when I push up the throttle past where the RPMs get any higher, telling
me too much fuel for the engine speed. DEFINITELY a sign of
over-pitched prop! So: gotta pull her in the spring to check/replace a
thru-hull, so I'll reduce the pitch an inch, maybe 1.5 inches....

Not necessarily. We are properly pitched, but have a little extra
throttle to throw in which blows pretty black. Even when we were
under-pitched our first season, that happened once we achieved the
engine's max RPMs and the governor cut in.

Measure the revs before you change the prop.

--
Jere Lull
Tanzer 28 #4 out of Tolchester, MD
Xan's pages: http://web.mac.com/jerelull/iWeb/Xan/
Our BVI trips & tips: http://homepage.mac.com/jerelull/BVI/