Endurance of RN ships

[ar]

See note about this subject in "bulbous bows".

Beginning in the late 1920s the US Navy put very large amounts of money into warship machinry development to a far greater extent than did the RN.
Post World War One construction through World War Two produced bigger US types in terms of displacement per ship. Generally this was in order to carry more fuel to get the increased endurance needed for Pacific operations.
By the late 1930s the money put into development began to pay off.
An RN engineering paper of the late 1940s atated that US machinery was about 20% more effecient at cruising speeds than RN machinery, but a little LESS at speeds over 26/27 knots.
When one puts the two things together you find that US ships built from 1940 will give about 15 to 20% greater endurance over a 15 to 30 knot range.

[phil gollin]

Doesn't work like that - you can't "put the two things together"

[ar]

Doesn't work like that - you can't "put the two things together"

US ships gave over a greater percentage of ship displacement to fuel, couple that will a bigger ship and....
Forget to add: RN ships had a greater percentage of displacement given to hull structure, which also affected endurence when compared to US ships.

The Gearings came into being to give extra endurence, NO OTHER reason. THey were bigger ships with a bit added in the middle. for extra fuel. Bigger ship, more fuel as a percentage of displacement. A good example. Speed dropped a just a fraction but endurence rose measurably. Size matters.
How about that.

[chuck]

The Gearings came into being to give extra endurence, NO OTHER reason. .

That's not true. There are many reasons why turbines output should be geared down before driving the propellers. That most of those reasons indirectly bear upon endurance is secondary.

[phil gollin]

Chuck,

I noticed your little propeller picture - as a matter of amusement do you know what it was that started the RN on its path to skewed and then overlapping propeller blades ? I doubt if you will guess - but it has to do with cavitation.

.

[Foeth]

I doubt if you will guess - but it has to do with cavitation.

I don't think so, I think it has to do with pressure fluctuations on the hull and radiated propeller noise.

[Werner]

Simply put, propellers operate best at slow speeds and turbines operate best at very high speeds. When ships were powered by reciprocating expansion engines, the length of the connecting rod and the crank offset of the main crankshaft caused the motor to operate fairly near to optimal propeller speeds. With a turbine, especially very early direct drive ones, the propellers operated at very high, inefficient rpms.

The British solution was to adapt gears to change the ratio of turns of the turbine to turns of the prop. The USA was behind (in 1900-1915) in the ability to make these very large, complex, strong gears, and looked to an area where they had two of the world's foremost experts: electricity with Edison's GE and Tesla's Westinghouse. Electric motors did not need gears, ran as well in forward as reverse, and could be powered efficiently by shutting down some of the boilers, thus greatly increasing range and fuel economy,

[phil gollin]

I doubt if you will guess - but it has to do with cavitation.

I don't think so, I think it has to do with pressure fluctuations on the hull and radiated propeller noise.

Well, the designs originally arose through experiments in WW2 when tufts of wool were attached to model propellers to act as telltales during tests to try to reduce cavitation. It was noted that the propellers had slightly reduced cavitation damage, but that it returned when the woollen tufts were removed. Post war the RN developed that into skewed blades and then eventually into large overlapping blades (but again the Agouti trials fed into that as well). Amazing what you can do with wool.

[ar]

The Gearings came into being to give extra endurence, NO OTHER reason. .

That's not true. There are many reasons why turbines output should be geared down before driving the propellers. That most of those reasons indirectly bear upon endurance is secondary.

I believe that you may have been mistaken as to what I was referring to.
The Gearing class had exactly fourteen feet added amidships between the aft enging room and the forward fire room. About 95% of the additonally space from the top of the second platform deck down to the very bottom of the ship is given over to fuel. The fuel compartments extend to the centre line. This comes from looking at official plans.This extra amount of fuel gave extra endurance. Nothing to do with turbine output, or being geared down, or propellors.

[chuck]

I don't think so, I think it has to do with pressure fluctuations on the hull and radiated propeller noise.

Well, the designs originally arose through experiments in WW2 when tufts of wool were attached to model propellers to act as telltales during tests to try to reduce cavitation. It was noted that the propellers had slightly reduced cavitation damage, but that it returned when the woollen tufts were removed. Post war the RN developed that into skewed blades and then eventually into large overlapping blades (but again the Agouti trials fed into that as well). Amazing what you can do with wool.

Phil, I somewhere that one of the RN destroyers was fitted with a glass window immediately above the propellers to facilitate observing propeller cavitation in actual service. Would that be related to the wool experiment?

[chuck]

......

I believe that you may have been mistaken as to what I was referring to.
The Gearing class had exactly fourteen feet added amidships between the aft enging room and the forward fire room. About 95% of the additonally space from the top of the second platform deck down to the very bottom of the ship is given over to fuel. The fuel compartments extend to the centre line. This comes from looking at official plans.This extra amount of fuel gave extra endurance. Nothing to do with turbine output, or being geared down, or propellors.

Ahh, sorry. my mistake. I should have noticed the capital G in Gearing.

[chuck]

Simply put, propellers operate best at slow speeds and turbines operate best at very high speeds. .....

The British solution was to adapt gears to change the ratio of turns of the turbine to turns of the prop. The USA was behind (in 1900-1915) in the ability to make these very large, complex, strong gears, and looked to an area where they had two of the world's foremost experts: electricity with Edison's GE and Tesla's Westinghouse. Electric motors did not need gears, ran as well in forward as reverse, and could be powered efficiently by shutting down some of the boilers, thus greatly increasing range and fuel economy,

Different countries tried different solutions to gearing down turbine output. The Germans for example used fluid coupling to reduce prop shaft RPM during WWI. Eventually everyone came around to the British solution.

[Werner]

Different countries tried different solutions to gearing down turbine output. The Germans for example used fluid coupling to reduce prop shaft RPM during WWI. Eventually everyone came around to the British solution.

Yes, it was the most efficient and had the lowest energy loss. Kind of like a manual transmission in your car vs a fluid drive automatic transmission.

The US in the first decade of the 20th Century had just completed state-of-the-art electric railway locomotives to move trains into Manhattan from the regional railroad lines. This technology was quite impressive for the time, and some of the locomotives either still serve or did so until recently. The ease of maintenance, reliability in all weather conditions, performance and flexibility of this national story could not have eluded Naval officers and others who make long-range plans.

[Foeth]

Phil, I somewhere that one of the RN destroyers was fitted with a glass window immediately above the propellers to facilitate observing propeller cavitation in actual service. Would that be related to the wool experiment?

That is a standard observational technique for field observations (I have performed a few). The wool experiment seems to be rather inconsequential and antiquated, even for the time. I don't see exactly how the wool experiment could have yielded nay useful conclusions. If you want to observe the flow direction, it's easiest to apply a wet pain (linseed oil with a fluorescent, for example). This technique is used even today, for the flow around ship hulls, and only now (yes, only now) calculations are sufficiently reliable to simulate such a paint test. Not on propellers, that is still beyond bread-and-butter calculation work.

[phil gollin]

Phil, I somewhere that one of the RN destroyers was fitted with a glass window immediately above the propellers to facilitate observing propeller cavitation in actual service. Would that be related to the wool experiment?

That is a standard observational technique for field observations (I have performed a few). The wool experiment seems to be rather inconsequential and antiquated, even for the time. I don't see exaclt how the wool experiment could have yielded nay useful conclusions. If you want to observe the flow direction, it's easiest to apply a wet pain (linseed oil with a fluorescent, for example). This technique is used even today, for the flow around ship hulls, and only now (yes, only now) calculations are sufficiently reliable to simulate such a paint test. Not on propellers, that is still beyond bread-and-butter calculation work.

I did post an answer, but it seems to have disappeared.

You are not reading my first answer correctly.

The original woollen tufts experiment was a model test during WW2. Specifically they were working on trying to quieten the cavitation on escort and submarine propeller blades - with erosion also in the mix. The tufts were meant to make the flow off the tips of the blades visable. That led to the immediate post-war model tests of skewed propeller forms. At the same time "Nightshirt" experiments during WW2 led to "Agouti" type blades. The two types of blades were developed post war and in the fifties with "nightshirt" not being taken up but Agouti being taken up the two forms of skewed blade plus overlapping blade with, and without, Agouti meant that there was a whole host of propeller forms being tested for either anti-cavitation, or intrinsic low-noise characteristics, or usefulness with Agouti. HMS Savage was the main, but not only, test-bed for full-scale implementation.

[Foeth]

There really doesn't exist anything called "anti-cavitation", although there are tricks to control it. Tufts are indeed nice methods to visually the flow (be it somewhat crude) but I do not believe that skew was applied as a result of visualisation studies. Skew is applied to reduce radiated noise, and that is very difficult (that is, really impossible) to ascertain using tufts. Perhaps skew was one of the variations in the tests leading to reduced sound signatures and hence was applied later on. Also, skew is not used to mitigate erosion. A clear disadvantage of tuft tests is that you severly disrupt the boundary layer flow over the blades and may up drawing completely wrong conlusions. So, perhaps the studies did find a few useful items, but I doubt there was a strong relation to the tuft tests.

[phil gollin]

There really doesn't exist anything called "anti-cavitation", although there are tricks to control it. Tufts are indeed nice methods to visually the flow (be it somewhat crude) but I do not believe that skew was applied as a result of visualisation studies. Skew is applied to reduce radiated noise, and that is very difficult (that is, really impossible) to ascertain using tufts. Perhaps skew was one of the variations in the tests leading to reduced sound signatures and hence was applied later on. Also, skew is not used to mitigate erosion. A clear disadvantage of tuft tests is that you severly disrupt the boundary layer flow over the blades and may up drawing completely wrong conlusions. So, perhaps the studies did find a few useful items, but I doubt there was a strong relation to the tuft tests.

Well you will have to take that up with the RN scientists who cme up with skewed propeller designs because of their observations - maybe they wrote up something that didn't happen

.

[Foeth]

Or perhaps you misread something that did happen, or you read something by someone who didn't quite understand what he was writing.

[phil gollin]

No, as it was written by the RN people who actually designed the skewed blades I think I prefer to believe them rather than you.

.

[Foeth]

So would I, but so far the "best" information I have is from someone who doesn't really have a grasp on the subject.