[PSUBS-MAILIST] prop size

[JimToddPsub at aol.com]

Hi Marc,
 
I think the premise is that for a given power input, a larger  diameter 
prop is going to result 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.
 
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.  
 
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.
 
Best regards,
Jim
 
 
In a message dated 9/5/2013 7:42:44 A.M. Central Daylight Time,  
piolenc at archivale.com writes:

It's not  the low speed, but the large diameter of the propeller that 
gives better  STATIC thrust per horsepower. The advantage of a larger 
prop is less under  cruise conditions where the mass flow through the 
propeller is larger due  to forward motion and it has only the drag of 
the sub to overcome. In  fact, you really need to go through detailed 
calculation to make sure that  a larger propeller is really of any use in 
your application.

Low  rotation speed is NOT an advantage, though it may be dictated by the  
propulsion machinery. Low speed, high torque propeller drive gives high  
swirl loss in the wake, while higher speed and lower torque minimize  swirl.

Best,
Marc de Piolenc

On 9/5/2013 8:20 PM, greg  cottrell wrote:

> I was standing in Frank Busby's kitchen one day  many years ago
> discussing this same question.  He had returned  from Finland on a
> mission to carefully document two deep diving subs  being built there for
> the Soviets  (Finland agreed under threat  of economic sanctions to allow
> the inspection). The two subs in  question had very large stern props
> driven by hydraulics that turned  very slowly. They were later named Mir
> 1 & 2. Frank compared the  concept to ice breaking ships that use huge
> props that turn very  slowly. It's quite efficient because there is
> little energy wasted on  turbulence and cavitation.
> Also, in the realm of human powered subs,  the winners have all had large
> diameter low aspect ratio props that  look for like aircraft props.
> But there are trade offs- gear reduction  will use a little power. Belt
> reduction a little less. There is more  drag on a big prop when it is not
> turning (unless you feather  it).Fouling a big prop is  more likely
> unless you put it in a big  cage. But if the goal is lower shaft speed
> then it can be a good way  to go.
> Greg
>
> *From:* hank pronk  <hanker_20032000 at yahoo.ca>
> *To:* Personal Submersibles General  Discussion
> <personal_submersibles at psubs.org>
> *Sent:*  Wednesday, September 4, 2013 6:58 PM
> *Subject:* [PSUBS-MAILIST] prop  size
>
>
> I have a two part question,
> Is it  beneficial to increase my propeller size to increase efficiency.
> I was  under the impression from my boating days that a large slow prop
> is  more fuel efficient.  It should increase battery range and slow  down
> the shaft speed, helping the seal.
> I have a much larger  prop left over from boating and I am thinking of
> using it on  Gamma
> The next question is,,, is there anyone here with the ability  to
> calculate the new rpm for my given prop size and motor hp.
>  Hank
> *From:* Jon Wallace <jonw at psubs.org>
> *To:* Personal  Submersibles General Discussion
>  <personal_submersibles at psubs.org>
> *Sent:* Wednesday, September  4, 2013 6:18:15 AM
> *Subject:* [PSUBS-MAILIST] There's gold in  them-thar waters
>
>
>  
http://www.usatoday.com/story/news/nation/2013/09/03/florida-family-gold/2757357/
>  _______________________________________________
> Personal_Submersibles  mailing list
> Personal_Submersibles at psubs.org  <mailto:Personal_Submersibles at psubs.org>
>  http://www.psubs.org/mailman/listinfo.cgi/personal_submersibles
>
>
>
>  _______________________________________________
> Personal_Submersibles  mailing list
> Personal_Submersibles at psubs.org  <mailto:Personal_Submersibles at psubs.org>
>  http://www.psubs.org/mailman/listinfo.cgi/personal_submersibles
>
>
>
>
>  _______________________________________________
> Personal_Submersibles  mailing list
> Personal_Submersibles at psubs.org
>  http://www.psubs.org/mailman/listinfo.cgi/personal_submersibles
>

--  
Archivale catalog: http://www.archivale.com/catalog
Polymath weblog:  http://www.archivale.com/weblog
Translations (ProZ profile):  http://www.proz.com/profile/639380
Translations (BeWords profile):  http://www.bewords.com/Marc-dePiolenc
Ducted fans:  http://massflow.archivale.com/
_______________________________________________
Personal_Submersibles  mailing  list
Personal_Submersibles at psubs.org
http://www.psubs.org/mailman/listinfo.cgi/personal_submersibles

-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://www.whoweb.com/pipermail/personal_submersibles/attachments/20130905/c6aa96dd/attachment.html>