High-speed Montana class: propeller design?

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TheMarcksPlan
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High-speed Montana class: propeller design?

Post by TheMarcksPlan » 16 Feb 2021 05:50

Some of the studies for the Montana class design (BB-65-8/9) specified 320k HP for 33 knots. Of course this wasn't adopted as the final design and the ships were never built but the propulsion studies for the Montanas were far advanced. Source is Friedman, U.S. Battleships, chart on p.337 and surrounding discussion.

Does anyone know what propeller specifications would have been used on the fast Montana designs?

I ask because the 53k HP on each Iowa-class prop was barely manageable and arguably not manageable. Speed trials show excessive cavitation, as discussed more here: http://www.navweaps.com/index_tech/tech-104.php

To use more HP than Iowa's without hitting a brick wall cavitation limit, fast Montanas would have needed:
  • 1. More propellers at same size/RPM;
  • 2. Deeper-set propellers to use higher ambient water pressure; or
  • 3. A cavitation-reducing design.
(1) seems unlikely. Even if the extra 13ft of beam were used for a fifth propeller, it doesn't get you from 212k to 320k HP.

(2) is impossible given the Montanas had the same draft as Iowa.

(3) wasn't available until CFD and other modern techniques came about.

The design studies for fast Montanas seem totally foreclosed by the fundamentals of ship design in the 1940's. What am I missing?
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Carl Schwamberger
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Re: High-speed Montana class: propeller design?

Post by Carl Schwamberger » 20 Feb 2021 03:40

I failed to see anything about cavitation in the linked article. Did you link the item you intended?

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Re: High-speed Montana class: propeller design?

Post by TheMarcksPlan » 20 Feb 2021 05:04

Carl Schwamberger wrote:
20 Feb 2021 03:40
I failed to see anything about cavitation in the linked article. Did you link the item you intended?
My mistake, an earlier article in that series mentions Iowa's propeller cavitation if spun up to 70k HP per shaft. http://www.navweaps.com/index_tech/tech-029.php

That's still far short of the 82.5k HP/shaft for the fast Montana's designs at 4 shafts. Even at five shafts (were that possible on 13 feet more beam - Iowa's props were on average 17.6ft diameter), that's 64k HP/shaft.

As the originally-linked article notes, the previous article far overstates the Iowa's speed capabilities. I.e. it's likely that cavitation was already taking a big bite from effective HP as the ship exceeded 31 knots at the 55.6k tons average trial displacement.

As the more recent article notes, extrapolating BB62's speed and HP cubically from its 1985 trial max HP to its listed max HP of 225k HP would yield 31.95kn.

If we look at the HP/Speed curves for that trial, however, we can see that New Jersey was already far exceeding cubic escalation of HP with speed to get to 31kn.

Image

The curve only gets steeper as speed-length ratio increases; it's unlikely that Iowas could get much above 31kn at 225k HP.

There's other support for Iowa's hitting a cavitation wall:
Early on in the pursuit of speed, it became apparent that propeller cavitation (7) was the
single-most important consideration at speeds in excess of 30 knots. Propeller design, therefore,
took on new meaning and importance if speed was going to be sought after. When cavitation
occurs, the propeller efficiency falls off drastically and propeller damage could result. Model
tests proved ineffective in the prediction of high speed propeller cavitation (Raven, Feiler, and
Jesperson, 1947).
------------------------------------------------------

Sorry about the misrecalled link... Anyway, it seems impossible to have designed a fast Montana under the design's specifications even with 5 props.
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Re: High-speed Montana class: propeller design?

Post by Kingfish » 20 Feb 2021 19:41

Bear in mind that the Montana class underwent a lengthy design phase that was influenced by the Washington naval treaty and beyond. This led to numerous proposals, of which a faster but less well armored variant was considered. It may be that the 33kt speed was a theoretical target, and it was only after the Iowas underwent their speed trials that the cavitation issue arose.
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Re: High-speed Montana class: propeller design?

Post by TheMarcksPlan » 21 Feb 2021 08:31

Kingfish wrote:It may be that the 33kt speed was a theoretical target, and it was only after the Iowas underwent their speed trials that the cavitation issue arose.
Montanas had only ~17% more friction area than Iowas, therefore only ~17% more friction resistance at 33kn. As Montanas were longer, their share of wave resistance would have been smaller.

If propulsive efficiency were similar, we'd expect fast Montanas to require only <15% more HP or ~245k.

So actually it appears the USN anticipated a massive cavitation penalty - there's no other way, AFAICS, for 320K to pencil out.

...I guess what I don't see as possible/feasible is to run headlong into that cavitation dynamic, which gets weird and unpredictable at extreme ends. Instead of merely eating the penalty (in form of 75k extra HP requirement), propeller damage seems as likely.
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Re: High-speed Montana class: propeller design?

Post by Carl Schwamberger » 21 Feb 2021 17:12

TheMarcksPlan wrote:
21 Feb 2021 08:31
Kingfish wrote:It may be that the 33kt speed was a theoretical target, and it was only after the Iowas underwent their speed trials that the cavitation issue arose.
Montanas had only ~17% more friction area than Iowas, therefore only ~17% more friction resistance at 33kn. As Montanas were longer, their share of wave resistance would have been smaller.
Which specific design are you taking this from?

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Re: High-speed Montana class: propeller design?

Post by TheMarcksPlan » 22 Feb 2021 04:54

Carl Schwamberger wrote:
21 Feb 2021 17:12
TheMarcksPlan wrote:
21 Feb 2021 08:31
Kingfish wrote:It may be that the 33kt speed was a theoretical target, and it was only after the Iowas underwent their speed trials that the cavitation issue arose.
Montanas had only ~17% more friction area than Iowas, therefore only ~17% more friction resistance at 33kn. As Montanas were longer, their share of wave resistance would have been smaller.
Which specific design are you taking this from?
The "fast Montana" designs from the exploratory studies. From Friedman:

Image
https://medium.com/counterfactualww2
"The whole question of whether we win or lose the war depends on the Russians." - FDR, June 1942

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Re: High-speed Montana class: propeller design?

Post by Kingfish » 22 Feb 2021 10:55

Let's see:
-# of main guns
-# and caliber of secondary guns
-Dry tonnage
-Full tonnage
-Length
-Width
-?
-?
-?
-?
-HP
-Max speed
-? for the rest

Can you fill in the blanks, or post the preceding page
The gods do not deduct from a man's allotted span the hours spent in fishing.
~Babylonian Proverb

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Re: High-speed Montana class: propeller design?

Post by TheMarcksPlan » 23 Feb 2021 04:51

Kingfish wrote:
22 Feb 2021 10:55


Can you fill in the blanks, or post the preceding page
Image
https://medium.com/counterfactualww2
"The whole question of whether we win or lose the war depends on the Russians." - FDR, June 1942

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