Shock testing

[RustyBattleship]

Bilgepump asked: The inspection of the 4=b;aded prop off the SOuth Dakota currently displayed at WNY was surprising. Do most props look this dog earred when dry dock takes place?

My assumption is that this is a prop that had reached te end of the line when the South Dakota was cut up and hadn't seen any rehab in years

Oh yes, but usually not quite that bad. Remember, bronze AGE HARDENS. Even if you don't use it. All the propellers we put on the New Jersey and Missouri were re-conditioned. That meant that all cracks were brazed up and all chips out of the edges inserted and brazed in place. Then the brazing was ground flush and polished.

Then came the fun part: RE-ANNEALING. That takes a pretty big oven. After that they are sent to the the shipyard for balancing before installation.

[RustyBattleship]

I bet they were beauts too Rusty - Question though, when I was in they were experimenting with variable pitch props on the DDG's. I'm curious if they ever made it to capital ships?

The Oliver Hazard Perry class Frigates, Spruance class Destroyers and Ticonderoga class Cruisers were designed for variable pitch propellers. The reason why is that they were to run off of Gas Turbines that run only in one direction.

We had a heck of a time with the first proposed variable pitch prop mounted on the steam propelled ship USS Barbey. Things were working great except when they went from full ahead to full reverse on the pitch. One bolt for one of the blades had a sub-surface flaw in it. Barbey came back home with no blades, a big gash in the hull and the special windowed trunk for the camera flooded.

Back to the drawing board. I write about that in Chapter 21 of my book.

As of today, however, I believe ALL new US Navy ships now run on gas turbines with variable pitch propellers. Though the Barbey casualty was a big disappointment, it led to more stringent inspections of all pieces that went into their structure and they have been running quite well ever since. The only time a VP propeller stopped running was on the USS Roberts. But that was because a free floating mine broke the propeller shaft.

[Pacfanweb]

One of the fastest ways to cure vibration problems in the main propulsion systems. In theory all are the same "somewhat", in reality they are much different given the dynamics of the design of the BB the powerplant, reductions, main engines and bracing.

It took the USN sometime to overcome the NC and Washington vibration problems being the first BB's built after the Treaty collapse. Once corrected, they refined it over and over for the following SD class and Iowas. As Rusty had mentioned before the Iowas and Essex class CV's shared characteristics and where therefore interchangeable on certain props. Its quite possible they could have also moved her main thrust bearings foreward or aft to help solve the problem or atleast dampen them.

From what I have read they never truelly eliminated the problem but did bring it down to an acceptable level.
SIAM?

I have read the same.

Chasing the solution for the vibration on the USS North Carolina is what led to her nickname, The Showboat. She steamed in and out of New York harbor so many times, New Yorkers gave her the name. And one book I've read states that the reason for so many trips in and out of the harbor was because of the vibration issue.

[Dreadnought]

A blurb from Navsource:

North Carolina (BB-55) underway at New York early in 1941. N.Y. & Phila pooled their resources in constructing the North Carolina class battleships. New York prepared all plans except for turret plans, rotating shell rings, and lower roller path weldment drawings, which were prepared in Phila. The propeller plans were all done at Philadelphia as were most of the plans for improved internal bracing to alleviate the vibration problems.
Although these ships were ordered on 24 June with 14-inch guns, the Roosevelt administration announced its decision to increase the main battery to 16-inch on 10 July. Detailed weight calculatuions, including changes for the new main battery, weren't completed by the Bureau of Construction & Repair until October 1937. These calculations showed a "readjustment of the longitudinal center of gravity was required in order that proper trim conditions could be assured throughout their probable range of service loadings and that the full loads of liquids (fuel oil or replacing sea water ballast) could be carried.
This readjustment of longitudinal center of gravity involved moving the major part of the contents of the vessel forward two frames (8.0 feet) relative to the outer hull. As plan work had already started, and since the change was absolutley required, the New York Yard was instructed by the Bureau..."to proceed with the development of plans on the above basis..." The SecNav on 15 January 1938 authorized a one month extension in the building period for each ship

[Dreadnought]

This report shows how they corrected the issues in the North Carolina class and the following South Dakota Class BB's by using flexible couplings, thrust bearing charateristics, shaft arrangements, a different skeg design and distance from the hull to reduce distortional flow to the props and trying different prop arrangements. Thus reducing vibration to a suitable level.

Its a very good study through MIT. Enjoy the read:
http://dome.mit.edu/bitstream/handle...pdf?sequence=1

It is IMO, the best answers found as to how they corrected the problems in the later classes following the North Carolina class. By the time they reached the Iowa class, it was a known science.

[SharkPilot]

I have a question related to the propellers installed on both Iowa and Essex class ships. The Iowas made considerably more power per shaft than the Essex ships. How could they use the same propellers? Did the Iowas just turn them at higher revolutions?

Thank you all for any answers. I enjoy reading all of this and have learned much from these forums.
Jim

[Dreadnought]

Hull design would be a huge consideration. The Essex class CV's (Antitam in this case) developed approximately 150,000 SHP on a displacement of approxiamtely 37,000 tons, Later hulls like Oriskany was 150,000 SHP on a slightly heavier displacement at full load and longer hull form with the hurrican bow so she was probably slightly heavier at full load displacement. The Iowas being much heavier at full load around 58,000 displaced and producing 212,000 SHP to push them as fast as the carriers at about 33 knots. I would not think that it was the props that made the difference if they were interchangeable but the reduction gears were probably different. At flat out full speed the Iowas would tick about 208 rpms maybe slightly less. The Essex class CV's i'm not sure of but having less SHP and displacement and sleeker hull for maybe less shaft rpms. I would think the reduction ratio is much lower (meaning less reduction on SHP) then what would be used upon the BB's.

Translation:

For the Iowa's, the steam turbines are connected to the propeller shafts via the Main Reduction Gear (Main Engine). There are two sets of pinion gears for both the Low pressure side and the High pressure side of the engine. Its final drive being the Main reduction gear or "Bull" gear itself to which the propeller shaft is attached.

For the Essex class carriers the "Engine" or reduction gear is more then likely either geared differently through the pinions (maybe a single reduction instead of a double on both the Low and High pressure sides, or maybe more or less teeth cut into those pinions) or perhaps different number of teeth cut on the Main reduction gear or "Bull" gear altogether.

*It comes down to the Essex class being 20,000 tons lighter and the Iowas being heavier. Being heavier, the Iowas are going to need more SHP (62,000 SHP more in this case then the Essex class) to keep up with the carriers at 30+ knots.

So in the end your factors are:

1) Hull form
2) Displacement
3) Shaft H.P.

*IMO, Reduction is more then likely a key factor here since the props are interchangeable so that cancels out their effect on either. Both must have the same shaft size. But, their powerplants themselves are no doubt different. The Essex class powerplant would have to be smaller since they generate 150,000 SHP (So thats 37,500 SHP over 4 props) as compared to the Iowas (53,000 SHP over 4 props) generating 212,000 SHP overall.

IMO, Reduction and gearing ratio is the key between their speed characteristics given their different SHP ratings, hull form and displacement.

There is no doubt an empirical equation that solves this question. The problem is finding it. All of this was done before the age of "calculators" so slide rulers were the rule of the day way back then. And fluid dynamics and other computer generating conditions could not be done on computers since they didnt exist like they do today.

Those eggheads were pretty smart no?

[Dreadnought]

An easy way of substantiating any of the above claims is to have a look at the Main Engine plate from an Essex class carrier. Mike, if your reading this can you tell us what the Engines aboard Hornet are rated for?