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<DIV>Hi Marc,</DIV>
<DIV> </DIV>
<DIV>I think the premise is that for a <EM>given power input</EM>, a larger
diameter prop is going to <EM>result</EM> in a lower RPM, not that there is an
inherent advantage in lower RPM within the scope of the question. Perhaps
not everyone was seeing it that way until you brought it up.</DIV>
<DIV> </DIV>
<DIV>As you alluded, it also begs the questions of what range of power input and
what range of RPM are we talking about. That is, what do the curves
on the graph look like throughout those ranges. I suspect the curve and
slope are going to look somewhat different at the upper and lower ranges than
they do in the mid-range. </DIV>
<DIV> </DIV>
<DIV>I really had not given enough thought to the difference a sleek hull vs a
higher drag hull interjects into prop characteristics and choice. And I
probably won't bother to get too deeply into that level of sophistication at
least until the vessel has a number of dives in its log. The last 5% of
perfection gets awfully expensive. However you're sure to hear from me
before I lay the money down on the main prop and motor.</DIV>
<DIV> </DIV>
<DIV>Best regards,</DIV>
<DIV>Jim</DIV>
<DIV> </DIV>
<DIV>
<DIV>In a message dated 9/5/2013 7:42:44 A.M. Central Daylight Time,
piolenc@archivale.com writes:</DIV>
<BLOCKQUOTE
style="PADDING-LEFT: 5px; MARGIN-LEFT: 5px; BORDER-LEFT: blue 2px solid"><FONT
style="BACKGROUND-COLOR: transparent" color=#000000 size=2 face=Arial>It's not
the low speed, but the large diameter of the propeller that <BR>gives better
STATIC thrust per horsepower. The advantage of a larger <BR>prop is less under
cruise conditions where the mass flow through the <BR>propeller is larger due
to forward motion and it has only the drag of <BR>the sub to overcome. In
fact, you really need to go through detailed <BR>calculation to make sure that
a larger propeller is really of any use in <BR>your application.<BR><BR>Low
rotation speed is NOT an advantage, though it may be dictated by the
<BR>propulsion machinery. Low speed, high torque propeller drive gives high
<BR>swirl loss in the wake, while higher speed and lower torque minimize
swirl.<BR><BR>Best,<BR>Marc de Piolenc<BR><BR>On 9/5/2013 8:20 PM, greg
cottrell wrote:<BR><BR>> I was standing in Frank Busby's kitchen one day
many years ago<BR>> discussing this same question. He had returned
from Finland on a<BR>> mission to carefully document two deep diving subs
being built there for<BR>> the Soviets (Finland agreed under threat
of economic sanctions to allow<BR>> the inspection). The two subs in
question had very large stern props<BR>> driven by hydraulics that turned
very slowly. They were later named Mir<BR>> 1 & 2. Frank compared the
concept to ice breaking ships that use huge<BR>> props that turn very
slowly. It's quite efficient because there is<BR>> little energy wasted on
turbulence and cavitation.<BR>> Also, in the realm of human powered subs,
the winners have all had large<BR>> diameter low aspect ratio props that
look for like aircraft props.<BR>> But there are trade offs- gear reduction
will use a little power. Belt<BR>> reduction a little less. There is more
drag on a big prop when it is not<BR>> turning (unless you feather
it).Fouling a big prop is more likely<BR>> unless you put it in a big
cage. But if the goal is lower shaft speed<BR>> then it can be a good way
to go.<BR>> Greg<BR>><BR>> *From:* hank pronk
<hanker_20032000@yahoo.ca><BR>> *To:* Personal Submersibles General
Discussion<BR>> <personal_submersibles@psubs.org><BR>> *Sent:*
Wednesday, September 4, 2013 6:58 PM<BR>> *Subject:* [PSUBS-MAILIST] prop
size<BR>><BR>><BR>> I have a two part question,<BR>> Is it
beneficial to increase my propeller size to increase efficiency.<BR>> I was
under the impression from my boating days that a large slow prop<BR>> is
more fuel efficient. It should increase battery range and slow
down<BR>> the shaft speed, helping the seal.<BR>> I have a much larger
prop left over from boating and I am thinking of<BR>> using it on
Gamma<BR>> The next question is,,, is there anyone here with the ability
to<BR>> calculate the new rpm for my given prop size and motor hp.<BR>>
Hank<BR>> *From:* Jon Wallace <jonw@psubs.org><BR>> *To:* Personal
Submersibles General Discussion<BR>>
<personal_submersibles@psubs.org><BR>> *Sent:* Wednesday, September
4, 2013 6:18:15 AM<BR>> *Subject:* [PSUBS-MAILIST] There's gold in
them-thar waters<BR>><BR>><BR>>
http://www.usatoday.com/story/news/nation/2013/09/03/florida-family-gold/2757357/<BR>>
_______________________________________________<BR>> Personal_Submersibles
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_______________________________________________<BR>> Personal_Submersibles
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mailing list<BR>> Personal_Submersibles@psubs.org<BR>>
http://www.psubs.org/mailman/listinfo.cgi/personal_submersibles<BR>><BR><BR>--
<BR>Archivale catalog: http://www.archivale.com/catalog<BR>Polymath weblog:
http://www.archivale.com/weblog<BR>Translations (ProZ profile):
http://www.proz.com/profile/639380<BR>Translations (BeWords profile):
http://www.bewords.com/Marc-dePiolenc<BR>Ducted fans:
http://massflow.archivale.com/<BR>_______________________________________________<BR>Personal_Submersibles
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