[Public-List] Propellers

Mon Nov 14 12:32:01 PST 2011

The existing prop is already correctly tuned to the Atomic 4 and the A30
(with a clean bottom as John Riley says). Theoretically the prop can
generate the thrust to move the boat at its design speed at the RPMS at
which the engine is at its maximum efficiency and can deliver the torque
required. Adding more blade area (at the same diameter) means you get
the design speed thrust at lower RPMS, but you are asking your engine to
generate that torque at the lower RPMS. It may be unable or inefficient.

More blades or a different pitch might work better for special cases of
extra drag,  a sick engine,  or counter current,  all of which change
the parameters.  

Mike
Rinn Duin #272

-----Original Message-----
From: public-list-bounces at lists.alberg30.org
[mailto:public-list-bounces at lists.alberg30.org] On Behalf Of Bart Knapp
Sent: Monday, November 14, 2011 3:10 PM
To: 'Alberg 30 Public List -- open to all'
Subject: Re: [Public-List] Propellers

This is a resend - too many email accounts.

Not usually, most of our (Sea Scout) boats are lucky to do 5, but this
year
we had schedule to keep, so we were constantly pushing, not the best of
circumstances.  Next year we are hoping to have a more relaxed cruise
like
we did in 2010 - sans hurricane Alex with a longer time to sail and
possibly
head east to Pensacola or vicinity instead of down the ditch.  That said
most of the youth do not get the opportunity to actually spend much if
anytime in the ditch, so this year they got the chance to experience
that
and get a handle on the protocols of the ditch. This last year we didn't
get
out into the Gulf like we did in 2010, so next year we are wanting to
change
that too.

With a three bladed prop, wouldn't you get more thrust for a lower RPM
and
therefore be more efficient?  Since racing the Alberg is not in the
picture,
I don't think a little more drag would make that big of difference.
Also,
we have a number of certified Scuba divers who love the chance to get
down
and do some in the water maintenance, especially in July and August. In
fact, I think the kid's goal is to have somewhere around 90% of the
youth to
be certified Scuba divers and we have one consultant who is a certified
instructor.

Bart

-----Original Message-----
From: public-list-bounces at lists.alberg30.org
[mailto:public-list-bounces at lists.alberg30.org] On Behalf Of Meinhold,
Michael J.
Sent: Monday, November 14, 2011 1:36 PM
To: Alberg 30 Public List -- open to all
Subject: Re: [Public-List] Propellers

I kind of feel like a three blade prop is just one more blade to scrape,
and another blade to drag through the water when sailing. If the Atomic
4 is healthy you would have to have some pretty strong currents to make
no headway with the originally designed prop.

Do the Sea Scouts always book it down the ICW at more than 7+ knots? 

I do our annual Fall Cruise in the company of some pretty big boats, and
I just get to spend more time on the bay than they do. I usually leave
anchorages earlier and arrive later but have more fun (at least I think
so!)

Mike 
Rinn Duin #272 (A4 and 2 blades)

-----Original Message-----
From: public-list-bounces at lists.alberg30.org
[mailto:public-list-bounces at lists.alberg30.org] On Behalf Of Bart Knapp
Sent: Monday, November 14, 2011 2:14 PM
To: Alberg 30 Public List
Subject: [Public-List] Propellers

In the refit of our A30, it occurred to me that now would be as good a
time
as any to upgrade the propeller.  This past summer I was on an Allmand
31
with a two bladed prop and in company with an Erickson 35 and Shannon
39.
We were on the ICW from Galveston to Corpus and initially had the wind
directly on the bow.  The Allmand had quite a task keeping up with the
other
two.  I know that just by size of the boat, there would be a problem,
but
with both the Erickson and Shannon throttling back, the Allmand had all
it
could do to keep within about two miles of the others - that was under
full
sail (after the wind had shifted) and the motor running.  The owner of
the
Erickson said that by changing from a two bladed to three bladed prop,
the
Allmand would have picked another knot or so.  The Allmand was pushing
to
get 5 knots over ground with motor only and was just tipping 7 - 7.5
motor
sailing with a favorable wind direction and about a 30-35 degree heel.

Even though the A30 is currently land locked on a lake and effectively
dry
docked because of the drought, if we get it to Galveston Bay, we will
then
be able to get out to the Gulf of Mexico and the ICW.  I am looking
toward
sailing on the bay and take an occasional trip on the ICW.  Those trips
may
be in the company of other boats including the Erickson and Shannon, but
some others that are close to the same size as the A30 and going on up
in
size.  Most of these cruises would be on conjunction with Sea Scout
cruises
as was this summer's.  I came away from the cruise this summer with the
feeling that the Alberg would not be a good boat for the type of use we
put
boats to on the summer cruise, but it is a case of a bird in the hand -
an
A35 would be better, but..

Anyway, if I do change the prop out, what would be the size and pitch I
would need to be looking for.  Currently it would be married up to an
A4.  

Any thoughts or ideas would be helpful.

Bart Knapp

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