:PPM-propeller efficiency
I hope we can get back to the very lively and interesting topic of the
mythical perfect passage maker. It sure perked up this list when it started.
No matter whether the PPM costs $300,000 or $3,000,000, the efficiency of
the drive train is a key element of range and range is a key element of any
PPM. That brings me to the efficiency of the propeller. Whenever I look at
engine manufacturers' charts, there appears to be an extremely low level of
propeller efficiency at the engine's lowest rpms in a propeller that has
been selected to be perfectly pitched and sized at maximum engine rpms. And
since we have discussed at length the fairly extreme difference in fuel burn
at low rpms vs. high rpms, aren't we faced with a dilemma? In other words,
just at the rpm which brings out the best efficiency in the engine, the
efficiency of the propeller stinks. Leaving aside for a moment whether
buying a controllable pitch propeller ("CPP") will save enough fuel to pay
for itself, won't a CPP increase propeller efficiency dramatically at those
low rpms where the engine sips fuel? And won't the CPP put a perfect load on
the engine at any rpm, so running slow doesn't gum things up?
Most of the discussion I've read about CPPs dismisses them because they are
"designed for boats with varying loads," like fishing trawlers; but that
doesn't answer the question of range posed above. Any of you experience guys
care to weigh in on this?
A note on steel: there seems to be pretty widespread agreement that modern
coatings on steel have handled the rust issue--assuming no standing water is
allowed by the hull design and there is modest maintenance. Having a lot of
detailed photos of the welding and coating processes plus a written report
from an engineer who inspected the steel and coating in the boat builder's
yard before it all got covered up might allay the fears of a subsequent
buyer. The interaction between steel and aluminum has apparently been
handled with modern techniques that bond the two metals together, so a steel
hull with aluminum wheel house is pretty common and straightforward.
I know zip about composites. Would its advocates prefer to be aground on
rocks in one of them versus a steel hull? I'm ready to learn.
Regards,
Bob Frenier
Advantage Apparatus, LLC
Chelsea, VT
Do trawler manufacturers select the prop for perfect match at full RPM?
I don't know a lot about this, but given trawlers are rarely run wide
open, I'd think they would be focused on getting the best engine-prop
efficiency at S/L of 1.2 or so?
Or does that have a huge downside in terms of reserve power or whatever?
John Marshall
N55-20 Serendipity
On Nov 23, 2008, at 8:40 AM, Bob Frenier wrote:
I hope we can get back to the very lively and interesting topic of the
mythical perfect passage maker. It sure perked up this list when it
started.
That brings me to the efficiency of the propeller. Whenever I look at
engine manufacturers' charts, there appears to be an extremely low
level of
propeller efficiency at the engine's lowest rpms in a propeller that
has
been selected to be perfectly pitched and sized at maximum engine
rpms. And
since we have discussed at length the fairly extreme difference in
fuel burn
at low rpms vs. high rpms, aren't we faced with a dilemma? In other
words,
just at the rpm which brings out the best efficiency in the engine,
the
efficiency of the propeller stinks. Leaving aside for a moment whether
buying a controllable pitch propeller ("CPP") will save enough fuel
to pay
for itself, won't a CPP increase propeller efficiency dramatically
at those
low rpms where the engine sips fuel? And won't the CPP put a perfect
load on
the engine at any rpm, so running slow doesn't gum things up?
----- Original Message ----
From: John Marshall johnamar1101@gmail.com
To:
Passagemaking Under Power List passagemaking-under-power@lists.samurai.com
Sent: Sunday, November 23, 2008 10:09:25 AM
Subject: Re: [PUP] :PPM-propeller
efficiency
Do trawler manufacturers select the prop for perfect match at
full RPM?
In the early days at Willard Marine ('60's, '70's, '80's) We
always over-propped the boats. The theory was that if you could reach hull
speed at a lower RPM, you would save wear and tear on the engine. But
conventional wisdom changed over the years as marine engines were made more
efficient. Today, most marine engine manufacturers recommend a running gear
(engine, reduction gear, and prop) that will allow maximum rated RPM at WOT.
Prior to cessation of production, Willard was propping their boats to achieve
max. RPM at WOT.
Patrick
Willard 40PH
ALOHA
La Paz, MX
Interesting change... but does that give us maximum efficiency (fuel
per mile) at S/L 1.2 or lower?
I have seen data that shows the latest diesel gensets have nearly the
same fuel efficiency at 50% power as they do at 100% (although
efficiency falls off fast below 50%).
Not sure if that relates to propulsion efficiency... gensets have to
run at constant RPM.
John
On Nov 23, 2008, at 9:26 AM, Patrick Gerety wrote:
----- Original Message ----
From: John Marshall johnamar1101@gmail.com
To:
Passagemaking Under Power List <passagemaking-under-power@lists.samurai.com
Sent: Sunday, November 23, 2008 10:09:25 AM
Subject: Re: [PUP] :PPM-propeller
efficiency
Do trawler manufacturers select the prop for perfect match at
full RPM?
In the early days at Willard Marine ('60's, '70's, '80's) We
always over-propped the boats. The theory was that if you could
reach hull
speed at a lower RPM, you would save wear and tear on the engine. But
conventional wisdom changed over the years as marine engines were
made more
efficient. Today, most marine engine manufacturers recommend a
running gear
(engine, reduction gear, and prop) that will allow maximum rated RPM
at WOT.
Prior to cessation of production, Willard was propping their boats
to achieve
max. RPM at WOT.
Patrick
Willard 40PH
ALOHA
La Paz, MX
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but does that give us maximum efficiency (fuel
per mile) at S/L 1.2 or
lower?
Theoretically yes. I get right at 5nmpg at a 1.2 S/L (no genset
operating or stabilizers of any kind) with my John Deere 4 cyl. turbo diesel.
If I slow down, I can do better. If I speed up, efficiency drops off
dramatically.
I have seen data that shows the latest diesel gensets have
nearly the
same fuel efficiency at 50% power as they do at 100% (although
efficiency falls off fast below 50%).
Apples and oranges.
Patrick
Willard
40PH
ALOHA
La Paz, MX
I recently repowered with a non-turbo Deere 80HP-the torque peak is at 1200-1400rpm-twisting a 34 inch prop at 500rpm-Overpropped since the engine "stalls" at 1750rpm-out of a 2400rpm redline-but operating at 1200-1400rpm fuel economy is sub 1gph my sl is 1.125 at 7.5 knots and the torque the twisting action of the engine is at is max 220ft pounds demiinishing to 170ish ft pounds over 2000rpm as the fuel consumption goes over 4gph.
Deere would not warrenty an engine so overpropped- but it works well/better when you match cruising speed to the torque peak of the engine'
Kevin Kearney/Jolie/Baltimore
--- On Sun, 11/23/08, Patrick Gerety alohaboat@yahoo.com wrote:
From: Patrick Gerety alohaboat@yahoo.com
Subject: Re: [PUP] :PPM-propeller efficiency
To: "Passagemaking Under Power List" passagemaking-under-power@lists.samurai.com
Date: Sunday, November 23, 2008, 1:35 PM
but does that give us maximum efficiency (fuel
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Interesting... that's phenomenal fuel efficiency.
Does that still give you enough excess power to deal with big seas?
Can the engine be run at long periods at 1750 rpm?
Another way to ask... what are the downsides to the overpropping?
John
On Nov 23, 2008, at 11:31 AM, Kevin Kearney wrote:
I recently repowered with a non-turbo Deere 80HP-the torque peak is
at 1200-1400rpm-twisting a 34 inch prop at 500rpm-Overpropped since
the engine "stalls" at 1750rpm-out of a 2400rpm redline-but
operating at 1200-1400rpm fuel economy is sub 1gph my sl is 1.125 at
7.5 knots and the torque the twisting action of the engine is at is
max 220ft pounds demiinishing to 170ish ft pounds over 2000rpm as
the fuel consumption goes over 4gph.
Deere would not warrenty an engine so overpropped- but it works well/
better when you match cruising speed to the torque peak of the engine'
Kevin Kearney/Jolie/Baltimore
--- On Sun, 11/23/08, Patrick Gerety alohaboat@yahoo.com wrote:
From: Patrick Gerety alohaboat@yahoo.com
Subject: Re: [PUP] :PPM-propeller efficiency
To: "Passagemaking Under Power List" <passagemaking-under-power@lists.samurai.com
Date: Sunday, November 23, 2008, 1:35 PM
but does that give us maximum efficiency (fuel
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Kevin,
What is your transmission gear ratio and what hull and waterline length do
you have?
Ron Rogers
We built our boat including a Controllable pitch prop and in steel
with the new coatings. On the Prop, it works well and I like it both
for ease of running and trolling but I doubt you will ever get a
return based on fuel savings over the average time most people own
their boats. a nice luxury. On the steel issue we sprayed with the new
sound and insulation materials now available and hard to tell what
their effectiveness will be over the long haul. also expensive but
certainly less than dealing with rust after few years. God Bless -
Ross 10&2
On Sun, Nov 23, 2008 at 3:33 PM, Ron Rogers rcrogers6@kennett.net wrote:
Kevin,
What is your transmission gear ratio and what hull and waterline length do
you have?
Ron Rogers
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