Long Shot

Long Shot

Peter Nuebler
Peter Nuebler

July 1st, 2011, 8:43 pm #1

Hi,
This may or may not be the forum to answer my question but here it goes. After checking Google and various other sites it appears that the maximum revolutions of a ship's screw/propeller is 100 RPM, with as few blades as possible. Anything above that and the screw becomes inaffective. Looking at the HMS Hood site, the final drive of the ship was four three bladed proppellers shafts, turning at 210 RPM at full speed. 1930-1950 technology but...
Any replies would be most welcome,
Regards,
P.N.
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Anonymous
Anonymous

July 2nd, 2011, 2:14 pm #2

looks to me like the faster the screws turn, the faster the ship goes. 100 RPM seems a little slow to me for a cutoff rate. Honestly, look how small the props are compared to the bulk of the ship..I'd think they'd have to have some power to push the thing along!
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Joined: December 11th, 2007, 2:03 pm

July 4th, 2011, 8:48 pm #3

Hi,
This may or may not be the forum to answer my question but here it goes. After checking Google and various other sites it appears that the maximum revolutions of a ship's screw/propeller is 100 RPM, with as few blades as possible. Anything above that and the screw becomes inaffective. Looking at the HMS Hood site, the final drive of the ship was four three bladed proppellers shafts, turning at 210 RPM at full speed. 1930-1950 technology but...
Any replies would be most welcome,
Regards,
P.N.
Propeller design and size is the result of a series of complex calculations that involve efficiency coefficients, torque requirements, engine considerations, hull design, blade shape and many more pieces of information.

In general, a ships propeller can operate anywhere from 50rpm max to over 1000rpm, depending on the ship.

100RPM max MAY be true for one very specific set of circumstances, but is a poor generalization.

Do some more research to understand the complexities better.

MIT has a decent web page on the calculations and propeller designs.
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Peter Nuebler
Peter Nuebler

July 5th, 2011, 4:25 pm #4

Darren,
Of course you are correct and I should have been more specific with my question. I was looking for information on 1930's tramp steamers technology regarding engines and propulsion and came up with the numbers I posted.
Thank you again for getting back to me,
Regards,
P.N.
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Joined: January 25th, 2010, 4:16 pm

July 5th, 2011, 11:26 pm #5

Too many revs and the blades will slip more than push. Adding power actually slows the ship, but not many had a big surplus of hp anyway. Blade angle is a big factor. In tramp days, blades were not variable. Smaller factors for a given set of equipment are salinity, draught, cleanliness of ship underwater, etc.

Note Brunel's Great Britain had a many-bladed prop (8 or 9?) and was touted as ahead-of-its-time. Many had only 2 bkades back then.

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Joined: September 28th, 2005, 2:47 am

July 8th, 2011, 8:01 pm #6

Hi,
This may or may not be the forum to answer my question but here it goes. After checking Google and various other sites it appears that the maximum revolutions of a ship's screw/propeller is 100 RPM, with as few blades as possible. Anything above that and the screw becomes inaffective. Looking at the HMS Hood site, the final drive of the ship was four three bladed proppellers shafts, turning at 210 RPM at full speed. 1930-1950 technology but...
Any replies would be most welcome,
Regards,
P.N.
Negative. As I recall, max RPM on USS South Carolina (CGN-37) was 232 turns for an AIII bell. In practice, this was torque limited to about 217 rpm because of a concern with one of the main reduction gears, which still gave us somewhere around 32+ kts.

Andrew P, PBFHS
CGN-37 (1992-1996)
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Peter Nuebler
Peter Nuebler

July 9th, 2011, 4:11 pm #7

Too many revs and the blades will slip more than push. Adding power actually slows the ship, but not many had a big surplus of hp anyway. Blade angle is a big factor. In tramp days, blades were not variable. Smaller factors for a given set of equipment are salinity, draught, cleanliness of ship underwater, etc.

Note Brunel's Great Britain had a many-bladed prop (8 or 9?) and was touted as ahead-of-its-time. Many had only 2 bkades back then.
Hank,
Thank you for your response. I guess for a tramp, 90-100 rpm is enough to push the vessel along at about 12 knots. Anything faster would require more HP/machinery = more coal/oil = more cost. This, plus 1930's technology may be part of the answer.
Regards,
P.N.
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Hank
Hank

July 9th, 2011, 4:14 pm #8

Yes, it took a LOT more hp/coal to get a LITTLE more speed, which is why White Star, as policy, designed ships for luxury and not speed records like Cunard's Mauretania, etc.
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Peter Nuebler
Peter Nuebler

July 9th, 2011, 4:20 pm #9

Hank,
Exactly what I was thinking. Plus, being privatly owned vessels, someone has to write the check.
Cheers,
P.N.
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Peter Nuebler
Peter Nuebler

July 9th, 2011, 4:32 pm #10

Negative. As I recall, max RPM on USS South Carolina (CGN-37) was 232 turns for an AIII bell. In practice, this was torque limited to about 217 rpm because of a concern with one of the main reduction gears, which still gave us somewhere around 32+ kts.

Andrew P, PBFHS
CGN-37 (1992-1996)
Andrew,
Very impressive and a good looking ship too!
Regards,
P.N.
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