Re: [Amel] shaft driven alternator

Eric Lindholm <etlindholm@...>

Dave I agree. Anyone who has pushed a pinwheel through the air knows that it has more drag spinning than if you stop it and push. That is why I said I would post the site for "amusement" for some. I am not to concerned about how much drag there is on my prop. 3 to 6 seconds per mile, or ten times that is little to worry about. In fact I am going to see if I can put on a larger diameter prop. next time I haul out to give me better performance under power against wind or waves, but that is another story. Eric 

- On Mon, 5/4/09, Dave Benjamin <dave_benjamin@...> wrote:

From: Dave Benjamin <dave_benjamin@...>
Subject: Re: [Amel] shaft driven alternator
To: amelyachtowners@...
Date: Monday, May 4, 2009, 2:36 PM


My professional background is in aviation and props in the water and props in the air have some similarities. Air and water are both fluids albeit water is a much denser medium. Also the laws of physics come into play. Anotherwords the prop is not freewheeling since the generator places a load on it. You cannot extract energy from the water in the form of  a prop generator without creating friction. People can believe what they want to believe. The laws of physics and hydrodynamics will still apply. The only way to really reduce the prop drag penalty is with a feathering or folding prop which is precisely why the racing rules give credits for a fixed prop. The handicappers know the fixed prop will slow a boat down and give a 3 second or 6 second per mile credit for it.

--- On Mon, 5/4/09, Eric Lindholm <etlindholm@sbcgloba> wrote:

From: Eric Lindholm <etlindholm@sbcgloba>
Subject: Re: [Amel] shaft driven alternator
To: amelyachtowners@ yahoogroups. com
Date: Monday, May 4, 2009, 2:12 PM

David,  I actually read a study where the results showed that there was less drag with the prop freewheeling, no alternator, like you said, rather than locked. I also agree that this goes against everything I have ever heard. Even after reading the article, I have a very difficult time believing it. If I can find it I will post it for your info, or at least amusement. It was a study comparing all of the props available for sailboats, and there efficiency in forward, reverse, and under sail. It compared 2 blade, 3 blade, maxi prop, etc. Eric maramu 105

--- On Mon, 5/4/09, David Mackintosh <dlm48@...> wrote:

From: David Mackintosh <dlm48@...>

Subject: Re: [Amel] shaft driven alternator

To: amelyachtowners@ yahoogroups. com

Date: Monday, May 4, 2009, 3:41 AM

YES and NO there is theory and there is practice.... ...on my O461 the boat

IS faster by up to a knot (unless you are at hull speed with plenty of wind)

if you let the prop free wheel and i know this is contrary to scientific

theory...i guess the prop is uneducated. I have discovered that this is the

case on quite a few boats i have sailed too - so this is not a one off


Driving a shaft generator would i 'think' cause more drag - as you dont get

nothing for nothing - and the energy to drive that alternator has to come

from somewhere. However if you have an excess of sail power you 'might' not

see a decrease in boat speed which after all is all that matters. I am

ignoring wear and noise issues (for the pedantic).



2009/5/3 Dave Benjamin <dave_benjamin@>

Ettore and Silvana,
You are defying the laws of physics and hydrodynamics if you really think
the shaft alternator does not slow the boat down. In fact the turning
propeller acts almost like a disc. It generated more resistance than it
would if it was stationary. I would use one if it was already installed but
I would not go out of my way to install a speed brake like that. There are
some solutions in the form of a unit that is deployed from the transom. One
of my neighbors used one to and from Hawaii and it generated electricity
quite well.
--- On Sun, 5/3/09, Arca di Noè <arcadinoesailing@< arcadinoesailing % 40hotmail. com>>
From: Arca di Noè <arcadinoesailing@< arcadinoesailing % 40hotmail. com>
Subject: Re: [Amel] shaft driven alternator
To: amelyachtowners@ yahoogroups. com <amelyachtowners% 40yahoogroups. com>
Date: Sunday, May 3, 2009, 9:43 AM
Hi David and Hazel,
We use the shaft driven alternator in our Mango with much satisfaction.
No reduction the speed on sailing.
It will give 7/9 amp when running at 7/8 knots.
It don't works down 4 knots.
Very very important: if you have Hurth gearbox or similar, is imperative to
run the motor half our every 12 ours, to lubricate the gearbox because r
shaft running isn't enaugh to lubricate the interior of the gearbox: if the
motor is not on, is easy to break the interior disks of the gearbox.
Now our Mango is independent for the electriciy of Marina: 2 solar panel
and 1 wind generator is perfectly.
With 15/20 wind knots We have cooking the pizza in the electric four at
Ettore & Silvana of Arca di Noè
----- Original Message -----
From: "David and Hazel Worthington" <spritoaffine@ uk>
To: <amelyachtowners@ yahoogroups. com>
Sent: Friday, May 01, 2009 1:43 PM
Subject: [Amel] shaft driven alternator
Hi All
Due to the charges being imposed by marina operators now for electricity
in the Med I have been considering various options to allow free battery
Our rattly Max Prop needs rebuilding and I was wondering about replacing
it with a standard fixed prop in order to use the propshaft alternator
am concerned about loss of speed and increase in noise when sailing, does
anyone still use their prop driven alternator or has everyone turned to
solar and wind power instead. I understand we would need a 19 inch prop
but am not sure whether the 13 mentioned is inches or degrees of pitch.
Our aft cabin is too noisy to sleep in on passage due to
bearing noise even after replacing the cutless bearing, is this normal?
Also would we go backwards straighter with the Max Prop or a fixed prop?
we don't have a bow thruster
Your opinions would be appreciated
David and Hazel Sharki 148
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