PPM- propellor efficiency
I agree that a VPP is a luxury on a passagemaker. Unless your lifestyle is
to live aboard and cruise very long distances, I don't believe you will ever
recoup, in fuel savings, the cost of a VPP. OTOH, if you cruise very long
distances with a conventional setup you will likely run at the most
economical/practical speed 99% of the time and therefore will not need a VPP. We find
that operating at a reasonable speed results in the best economy despite where
we are on the BSFC curve. And, that curve is very flat now, at normal
continuous speed on newer turbo engines with EFI.
BTW, it's interesting that Nordhavn has a VPP on their new 56' motorsailer.
I have to wonder why they chose to do this when most of us blow-boaters use a
simple feathering Martec or Max-prop, which works just fine whether sailing
or motorsailing. Nordhavn touts their VPPs benefit compared to a fixed prop,
but not to a feathering one. Curious.
Those interested in one man's experience with a VPP may want to see:
Regards,
John
"Seahorse"
Regardless of what engine you have, ringing the optimum mpg from it is of
the
utmost importance. That or carry WAY more fuel on a passage.A vpp can get all
your engine can produce, power wise, at any rpm, reducing the amount of fuel
required to make your passage, and gives you the ability to increase your
ballast, which increases your chances of actually making the passage.
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After reading the link that John provided I agree with the writers
observation on the need vs. cost of a Controllable pitch prop. My
experience with it on the 48' duck is similar. Save the money and you
can buy an additional 30 to 40K of fuel!! @ $3/gal that is
conservatively 10,000 gals and at 3 gals/hr translate into over 3000
hrs of cruising @ 7 kts and you can see the results of the savings!
God Bless - Ross 10&2
On Mon, Nov 24, 2008 at 7:18 AM, Truelove39@aol.com wrote:
I agree that a VPP is a luxury on a passagemaker. Unless your lifestyle is
to live aboard and cruise very long distances, I don't believe you will ever
recoup, in fuel savings, the cost of a VPP. OTOH, if you cruise very long
distances with a conventional setup you will likely run at the most
economical/practical speed 99% of the time and therefore will not need a VPP. We find
that operating at a reasonable speed results in the best economy despite where
we are on the BSFC curve. And, that curve is very flat now, at normal
continuous speed on newer turbo engines with EFI.
BTW, it's interesting that Nordhavn has a VPP on their new 56' motorsailer.
I have to wonder why they chose to do this when most of us blow-boaters use a
simple feathering Martec or Max-prop, which works just fine whether sailing
or motorsailing. Nordhavn touts their VPPs benefit compared to a fixed prop,
but not to a feathering one. Curious.
Those interested in one man's experience with a VPP may want to see:
Regards,
John
"Seahorse"
Regardless of what engine you have, ringing the optimum mpg from it is of
the
utmost importance. That or carry WAY more fuel on a passage.A vpp can get all
your engine can produce, power wise, at any rpm, reducing the amount of fuel
required to make your passage, and gives you the ability to increase your
ballast, which increases your chances of actually making the passage.
**************One site has it all. Your email accounts, your social networks,
and the things you love. Try the new AOL.com
today!(http://pr.atwola.com/promoclk/100000075x1212962939x1200825291/aol?redir=http://www.aol.com/?optin=new-dp
%26icid=aolcom40vanity%26ncid=emlcntaolcom00000001)
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There seems to be lots of agreement that a controllable pitch propeller will
not pay for itself in fuel savings. Are we using for that comparison
mechanically controlled engines, $3 per gallon fuel and a Hundstat
propeller? If so, then an electronically controlled engine running at low
rpms on $6 per gallon fuel and spinning a less expensive Saab CPP may alter
the equation.
But I am attracted to a CPP for our "perfect passage maker" regardless of
the ultimate cost for the following reasons:
- the fun of trying to squeeze out a remarkable nmpg with a smallish,
electronically controlled, turbo-charged diesel engine running at the proper
temperature despite very low rpms. When installed in a long, narrow hull
designed to run best at VL 1.2, this drive train would seem to approach the
only "perfection" open to us. - the impact that combined system will have on range and, thus, tank size.
- the ability to have as much thrust in reverse as in forward to pull our
PMM off the reef without a tow. - the ability to "down shift" to a finer pitch when running uphill and thus
wring acceptable performance from that smallish engine in conditions that
would, otherwise, make it "too small."
BTW, I think a controllable pitch prop is different from a variable pitch
prop in that the VPP changes its configuration automatically but the CPP
pitch selection is done by the captain.
And finally, would someone please comment on the propeller efficiency charts
the engine companies produce that have tilted me towards a CPP? Is that
fixed prop really working at such great disadvantage just when the engine is
at its most efficient?
Regards,
Bob Frenier
Advantage Apparatus, LLC
Chelsea, VT
I agree that a VPP is a luxury on a passagemaker. Unless your lifestyle is
to live aboard and cruise very long distances, I don't believe you will
ever
recoup, in fuel savings, the cost of a VPP.
John of Seahorse wrote: I agree that a VPP is a luxury on a
passagemaker. Unless your lifestyle is to live aboard and cruise very
long distances, I don't believe you will ever recoup, in fuel savings,
the cost of a VPP.
REPLY
There is one kind of application where a CPP or VPP is useful. When
cruising in areas of rivers and canals with controlled speed zones.
Michael Kastrn is a supporter of CPP like the Hundested drives.
When cruising in restricted waterways, particularly if many locks must be
transited, a adjustable drive makes sense. It makes even more sense if the
propulsion engine is also driving power generation quipment like a
cruising generator. Even DC output alternators require a certain speed in
order to produce sufficient output to meet demand. With a CPP the engine
can be run at the right speed to keep the alternator happy while the CPP
is adjusted to produce the required hull speed. I agree this will shift
the engine away from the optimal BSFC point on the curve, but sometimes
operational requirements overrule optimum fuel economy.
Most of the Great Lakes boats use CPP for this reason. They require the
greater flexibility that CPP offers in order to safely transit the locks
of the St. Lawrence seaway. And depending on season and rainfall the river
current can vary from 1.5kn to as much as 4.5 knots.