[PSUBS-MAILIST] prop size

[Marc de Piolenc]

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/
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