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Author Topic: Screw depth and efficiency?  (Read 2512 times)

Norseman

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Screw depth and efficiency?
« on: February 21, 2013, 07:26:25 PM »

Hi Folks

Just an idle question. I was considering whether the depth of a propeller on a full size vessel had any bearing on its efficiency or reduction in cavitation? The deeper it is set the more pressure - so is the water denser - if so is that an assistance?

Now if the answer is yes then can we apply that to models or is the possible difference in screw depth just not significant enough. Only an idle question and possibly daft.

Off to Prague in a few hours and hopefully find a boat or two to photograph.  :}

Dave
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dave301bounty

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Re: Screw depth and efficiency?
« Reply #1 on: February 21, 2013, 07:45:47 PM »

Dave ive got a good little booklet on this subject ,would you like to borrow it ,its very interesting and really explains .Dave .
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Liverbudgie

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Re: Screw depth and efficiency?
« Reply #2 on: February 21, 2013, 08:09:09 PM »

 Depending on the design and purpose  the deeper the propeller is the more efficient it is; cavitation is a completely different thing, and is more to do with the rotational speed of the propeller. As you may imagine this subject is extremely complex subject, which has exercised the minds of countless designers in the past, the present and will no doubt continue to do so into the future.
One of the early White star liners, the Britannic of 1875, had a system were the after part of the propeller shaft could be lowered and or lifted to suit requirements; it was not a success though and she was rebuilt as a normal fixed shaft steamer. See: "Power of the Great Liners" by Denis Griffiths and published by Patrick Stephens Ltd in 1990 ISBN 1-85260-016-0
LB
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Norseman

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Re: Screw depth and efficiency?
« Reply #3 on: February 21, 2013, 08:40:05 PM »

Thanks for the replies and references. Yes it is certainly beyond me to understand it all but I have to tax the grey cells or I will start watching telly. So any more opinions / experiences are welcome.

Yes please Dave I'll certainly read that when I get back.  :-))

Dave
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boat captain

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Re: Screw depth and efficiency?
« Reply #4 on: February 21, 2013, 09:27:57 PM »

When we went on sea trials we started ballasting at the Mersey Bar and sailed up to the Clyde to the Isle of Arran measured mile.  On arrival at Arran the vessel would be fully ballasted.  The reason we used the Arran mile was the fact it is a very deep water mile.  To get the full efficency of the propeller it needs to be as deep in the water as possible and the greater the depth of water under the keel the better.
Joe  :-)) :-)) :-))
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NFMike

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Re: Screw depth and efficiency?
« Reply #5 on: February 21, 2013, 10:37:57 PM »

Just an idle question. I was considering whether the depth of a propeller on a full size vessel had any bearing on its efficiency or reduction in cavitation? The deeper it is set the more pressure - so is the water denser - if so is that an assistance?

Yes, deeper is better.
No, the water is not significantly denser. At any reasonable depth water is essentially incompressible though as the temperature may be lower that will increase the density a little.
It's the pressure that makes the situation better. This is from Wikipedia:
"As an impeller's (in a pump) or propeller's (as in the case of a ship or submarine) blades move through a fluid, low-pressure areas are formed as the fluid accelerates around and moves past the blades. The faster the blades move, the lower the pressure around it can become. As it reaches vapour pressure, the fluid vaporizes and forms small bubbles of gas. This is cavitation. When the bubbles collapse later, they typically cause very strong local shock waves in the fluid, which may be audible and may even damage the blades."

Basically the water boils at a lower temperature due to the low pressure. So increasing the overall water pressure (by being deeper) will increase the pressure at the low pressure points as well thus increasing the boiling point and reducing the bubbling.

Norseman

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Re: Screw depth and efficiency?
« Reply #6 on: February 21, 2013, 10:41:51 PM »

I actually understood that ... Do it again  :-))

Dave
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NFMike

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Re: Screw depth and efficiency?
« Reply #7 on: February 21, 2013, 10:48:45 PM »

Here you go:
How to pour the perfect pint:
E = - (0.62T2 + 39.2W2 + 62.4P2) + (21.8T + 184.4W + 395.4P + 94.5M - 90.25V) + 50(S + F + 6.4)

Norseman

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Re: Screw depth and efficiency?
« Reply #8 on: February 21, 2013, 11:26:30 PM »

Hmmn
Off to Prague in a few hours - I will find the perfect pint there O0
It will be dark and rich .... and be one of many  :}

Dave
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Shipmate60

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Re: Screw depth and efficiency?
« Reply #9 on: February 22, 2013, 07:29:36 AM »

Most of what we call cavitation is actually aeration, where air is sucked into the prop by the prop displacing the water too fast.
As a role of thumb most commercial ships (full size) run on a shaft speed of 250 rpm.
On models we start about 4,000 rpm.
So depth can be crucial for stopping aeration.

Bob
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BarryM

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Re: Screw depth and efficiency?
« Reply #10 on: February 22, 2013, 09:05:07 AM »

Aeration and cavitation may be confused with each other but they are not the same thing, although both will effect propeller efficiency. 
Propeller speed and pressure on the blade faces are intrinsically linked to cavitation as Oci's excellent explanation makes clear.
Aeration is that effect which is most obvious when a ship is in ballast with the propeller only partly submerged and the air is entrained in much the same way as a kitchen whisk operates in cream.
250 RPM may be a common prop speed for smaller, faster commercial vessels but much of the worlds deep sea cargo fleet cruises at far less. e.g in my seagoing career (all in single screw vessels) I never saw a tacho exceed 110 RPM.
Regards,
Barry M
 
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derekwarner

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Re: Screw depth and efficiency?
« Reply #11 on: February 22, 2013, 11:23:38 AM »

It is not un common to see a 100,000 tonne bulk carrier entering port in a totally de-ballasted condition with her single prop thrashing the water  <*<
As Bob notes... 250 RPM may be considered a medium speed diesel......however must agree with BarryM that the bulk [by tonnage] of the worlds deep sea cargo fleet vessel engines would be less than 150 RPM.......Derek
 
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NFMike

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Re: Screw depth and efficiency?
« Reply #12 on: February 22, 2013, 07:54:31 PM »

Bear in mind that a bulk carrier prop is likely to be 100 times larger (diam, circumf.) than a model prop. Eg, 5m (5000mm) as to 50mm. So for the same tip speed the rpm will be that much different, so in the example above 4000rpm on a model equates to just 40 on a full size prop.

Bryan Young

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Re: Screw depth and efficiency?
« Reply #13 on: February 23, 2013, 03:22:13 PM »

Does all this have much of an effect when we're talking about scale models? To my mind, as long as the prop stays submerged and not driven so fast that a rooster tail isn't formed then it's OK. Unless the "tail" is a requirement for a particular sort of boat.
For "scale" models, keep the speed down. Easy enough. If you want "fast", then get a different sort of boat. BY.
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NFMike

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Re: Screw depth and efficiency?
« Reply #14 on: February 23, 2013, 04:19:13 PM »

Does all this have much of an effect when we're talking about scale models?

Probably not, but the thread is in Full Scale Ships, so it's reasonable to discuss. Model props is just a bit OT, but I was answering a previous comment which actually ties to a recent discussion elsewhere about scaling.

gondolier88

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Re: Screw depth and efficiency?
« Reply #15 on: March 01, 2013, 10:03:06 PM »

The short answer to the question, is yes, at different depths propeller's efficiencies change.


However explaining the differences is a very long answer that involves water density, oxygen content, propeller blade form and RPM.


One interesting point is that in very shallow water propeller's efficiency drops off dramatically as turbulence from reflected wash from the sea bed interferes with the propeller flow- I would say rule of thumb would be a minimum of half the prop' diameter of water below and around the prop to guarantee no interference.


Greg 
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Norseman

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Re: Screw depth and efficiency?
« Reply #16 on: March 02, 2013, 06:25:46 PM »

Thanks for the input lads - it has been very interesting to think about full size vessels for a change.

One of my long term mysteries is just  what is inside a funnel? I never gave that a thought until the day I saw a rare photo of the Kalakala ferry with her funnel cap removed, and therein I could see equipment instead of the gaping hole my ignorance had always supposed existed.

Dave
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BarryM

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Re: Screw depth and efficiency?
« Reply #17 on: March 02, 2013, 08:33:48 PM »

Dave,
Early funnels were indeed little more than flue pipes and present day steam launches carry on the design. However, as ships evolved the funnel became a cylindrical object capable of carrying several uptakes and/or exhausts, header tanks etc. and, in some cases, were not used for passing boiler flue gases at all but installed for aethsetic reasons or to make the vessel look more formidable or fast.  In short, the modern funnel is often mostly empty space.
Regards,
Barry M
 
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grendel

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Re: Screw depth and efficiency?
« Reply #18 on: March 02, 2013, 08:59:41 PM »

whats inside a funnel? - well here is a drawing showing Lord warden's funnel.



Grendel
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gondolier88

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Re: Screw depth and efficiency?
« Reply #19 on: March 02, 2013, 09:20:29 PM »

Titanic's forward funnel had a menagerie in it to transport passenger's pets to America!


Traditional (proper) steam ship funnels were a decorative casing over the flue uptakes from the boilers, however towards the end of the steam era they could contain a myriad of uptakes, vents, exhausts and flues.


Greg
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Norseman

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Re: Screw depth and efficiency?
« Reply #20 on: March 03, 2013, 11:38:49 PM »

Titanic's forward funnel had a menagerie in it to transport passenger's pets to America!

Never in my wildest dreams ... The Kalakala was diesel so definitely no pooches in there.

Hey Grendel, thanks but I couldn't make out what was written.

Dave
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deadwood

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Re: Screw depth and efficiency?
« Reply #21 on: March 05, 2013, 05:18:56 PM »

Just an idle question. I was considering whether the depth of a propeller on a full size vessel had any bearing on its efficiency or reduction in cavitation? The deeper it is set the more pressure - so is the water denser - if so is that an assistance?

On full scale ships the main causes of cavitation are the propeller load and the inhomogeneity of the hull's wake field in the propeller plane while the propeller is working.

Examples of highly loaded propellers are those of e.g. tugs or fishing vessels with a large added resistance when they operate in towing.
Also highly loaded propellers are those of ships of great displacement and of high block coefficients (i.e. fullness of the hull, where 1 would be that of a cuboid with the edges of the main dimensions V / L X B X T) such as VLCCs or ULCCs (CB often >0.85, with high curvature gradients in the aft ship).
Or high-speed propellers where beyond certain speeds, since cavitation cannot be avoided anymore, so-called supercavitating propellers are often employed.

As was already quoted from a Wikipedia article, cavitation occurs where the pressure drops beneath the vapour pressure of the water so that the water literally begins to boil in small bubbles until these bubbles reach an area of higher pressure again when they suddenly implode.
Because these bubbles or sheets of cavitation not only disturb the fluid flow around the propeller blades they also cause an increase of resistance due to the virtual widening of the propeller sections' thickness and flow separation which both results in a notable drop of propeller efficiency.
When these sudden implosions of the bubbles occur close to the propeller's (or the hull's or other appendages' surfaces such as rudders) not rarely erosion at the affected surfaces happens which can end up in the whole destruction of the propeller.
Also nasty noise and vibrations can be caused by cavitation.

Though the inception of cavitation is often hard to predict one can easily imagine that on the propeller's suction side (aka the propeller's back because we look at the propeller from stern to bow when defining whether it is rotating clockwise or anti-clockwise),
where the blade sections have convex camber, and the pressure drops significantly because the fluid gets accelerated and lift (or thrust) is produced (Bernoulli's Law)
that there are for sure areas susceptible to cavitation.
But propeller face side cavitation is also quite common.

In opening I mentioned that another common cause is the inhomogeneity of the flow around the propeller blades.
A poorly designed hull shape (especially this of the aft ship) or a hull-propeller-arrangement being coerced into operating in off-design states can also lead to heavy cavitation.
Sometimes the susceptibility to cavitation can be mitigated by using ducted propellers such as in tugs and trawlers.

Because the effect of the propeller's submergence on cavitation was addressed, yes it does have a small effect at least for the propeller blades in the 6 o´clock position because the higher hydrostatic pressure with greater depth may kind of retard cavitation there.
But the effect compared to the mentioned main causes propeller load and propeller wake field are I would think more negligible.

But indirectly the "depth" of the propeller has a great effect indeed.
In general, the bigger the propeller's diameter, the bigger its efficiency. And with diameter inevitably comes submergence.

As also mentioned in another post, water is considered an incompressible fluid and thus its density should only matter as far as its accompanying temperature is concerned.
Of course, the colder the water the later does it begin to boil (at constant pressure).

Quote from: dave
Now if the answer is yes then can we apply that to models or is the possible difference in screw depth just not significant enough.

Yes, as you presumed, I too would consider the effect in model scale insignificant.
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boat captain

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Re: Screw depth and efficiency?
« Reply #22 on: March 05, 2013, 08:04:02 PM »

Alittle bit of information about stopping cavitation.
Agouti Air is a method of reducing propeller noise from cavitation. The system consists of drillings along the leading edge of the propeller, which are supplied with pressurized air.  On modern warships it is usually a bleed off the turbines suitably cooled and reduced in pressure The air pumped through these holes ventilate the vapour layer, similar to a surface piercing propeller, reducing noise.
 
Joe  :-)) :-)) :-))
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