Raumotters to the Rescue

Discussions about the history of the ship, technical details, etc.

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Herr Nilsson
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Re: Raumotters to the Rescue

Post by Herr Nilsson » Fri Apr 23, 2021 1:05 pm

Bugschutz.jpg
Bugschutz.jpg (27.38 KiB) Viewed 341 times
... The devices operate the worst when sailing in front of the sea, at low speed levels and at a flat setting. Then it can happen that the paravanes no longer operate and come to the surface. To get the paravanes under control again, you have to take a course towards the sea. Driving towards the sea hardly affects the devices, even in rough seas.
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Marc

"Thank God we blow up and sink more easily." (unknown officer from HMS Norfolk)

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Re: Raumotters to the Rescue

Post by wadinga » Sat Apr 24, 2021 12:46 pm

Hello Marc,

And a kite won't fly when the wind doesn't blow. Deployed from a small ship travelling down weather at slow speed, when the vessel may virtually stop occasionally under wave effect, such unreliable performance would not be surprising. But when the string stays taut, the kites flies predictably.

Does this interesting Gothic item say anything about the tow loads experienced or any need to compensate with steering if only one is deployed?

Hello Paul, I suggested looking at this excellent book earlier, https://archive.org/details/paravanead ... /mode/2up it will tell you many interesting things about the Paravane's development for mine protection in all sorts of ships and additionally its use as an anti-submarine device before depth charges were deployed. This fact-packed book, published in 1919 is downloadable as a pdf and is not only a great read, but great value as it is free. It also avoids irrelevancies about the use of plates deployed from booms to stabilise small vessels and minimise rolling. There are impressive statistics of the number of vessels from battleships down to the smallest warships and many commercial vessels protected from moored mines by paravanes.
that one set of forces (the paravanes) would probably be exerted considerably forward of the propeller, leaving a really inconvenient lever arm between the two.
Hello Bill, I particularly suggested that the paravanes would need to be towed from points on Bismarck's after port quarter. There is a towing fitting on the very stern from which one could be towed. The after anchor has a hawsepipe for another, and there are fairleads and bollards on the deck edge to act as further strongpoints for towing. Most of these points are aft even of the rudder stocks, so would have a similar or better lever length to the vessel pivot point as the rudder(s). Drawings in Brower show a paravane deployment boom on each side abreast the rangefinder on turret Caesar. It would be a reasonable seaman's task to deploy the paravanes there, and drop them back under control until their tow-wire was taut.

Finally, Marcelo. I am a bit confused about where the angle of attack is addressed in your calculation. Unlike an aircraft wing, I believe the two sides of the rudder are symmetrical so as to minimise drag and so do not create lift in the same way as an an aircraft wing. They may for instance even be flat and parallel to each other. They act more like a kite where a fairly crude resolution of forces occurs with water flow hitting the blade at an angle of incidence, and creates two vectors, one being rearward drag, and the other a force at an angle close to right angles to the blade, transmitted through the rudder stock which causes the vessel to turn. Ship's rudders typically turn no more than 35 degrees to vessel axis, as beyond this, I believe vortex creation destroys the smoothish flow of water past the rudder and it ceases to be so effective. This site http://marinacivil.com/index.php/artic ... ing-a-ship is very informative about rudder effects.

It may well be that a straight comparison between single rudder blade area and the total area (wing plus body) of eight hard-working paravanes would mean the latter would be overmatched, but I am going to continue to search for information on tow loads and any helm compensation necessary when a single paravane was towed from the stern.

Thanks for the interest from all parties.

All the best

wadinga
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Re: Raumotters to the Rescue

Post by marcelo_malara » Sat Apr 24, 2021 4:37 pm

Hi Wadinga.

It is a most common misconception that a symmetrical airfoil will not generate lift. In fact, most aircraft wings are symmetrical, ranging from high speed propeller driven planes to commercial jets, asymmetrical wings with flat lower surfaces are confined to the general aviation low speed planes.

The angle of attack is hidden in the Cl coefficient. The Cl varies with:

-the profile of the wing (thickness, thickness location in % of the chord)
-aspect ratio (=span^2 / area)
-angle of attack

As the first two are fixed for a determined wing (except when deploying flaps), the Cl is a straight line, Cl increasing with angle of attack, till the stall.

You can see the difference in Cl between a symmetrical and an asymmetrical wing. The symmetrical wing has a Cl of 0 at 0° angle of attack, because the flow has the same speed on both sides, whereas the asymmetrical will generate some lift in the condition, because the flow accelerates while traveling over the curved upper side.

Image

So, the rudder, the paravane´s wing, even a sailboat keel will generate lift by this mechanism at low angles of attack (say till about 20°). Past that value, the surface will stall, the flow will not remain close to the surface (we say the flow separates), loose velocity and cease to generate lift. In the case of the plane, the stall will make the plane come to the ground. In the case of the rudder such catastrophe would not occur, and the rudder will continue steering the ship due to other forces (the flow impinging on the rudder), but with a much higher resistance.

Even a kite is analyzed in this form.

Image

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Re: Raumotters to the Rescue

Post by wadinga » Sat Apr 24, 2021 7:52 pm

Hello Marcelo,
In fact, most aircraft wings are symmetrical, ranging from high speed propeller driven planes to commercial jets, asymmetrical wings with flat lower surfaces are confined to the general aviation low speed planes.
I go to lot of airshows and I enjoy seeing teams like the Blades flying the Extra 300 variants which fly upside down a lot and occasionally backwards.

So I might respectfully have to take issue with you there :D

https://www.grc.nasa.gov/WWW/K-12/airplane/shape.html

www.fzt.haw-hamburg.de/pers/Scholz/HOOU ... Design.pdf

Both show very few wing sections are symmetrical, and then very often only for aerobatic aircraft which have to fly upside down! Also very high speed transonic aircraft, but for lift at a wide range of speeds including commercial airliners like the Airbus family, variations on the asymmetric theme apply. Of course this is a simplification as the section varies along the length of the wing, to give different characteristics to avoid tip stall etc.

However thanks for the excellent kite drawing which I believe shows the situation of a paravane except it doesn't show the "tension" as a vector. It has to be there otherwise the kite falls down, and is surely a greater factor than gravity.

This NASA diagram I believe is a better rendition and says more about the vectors. https://www.grc.nasa.gov/WWW/K-12/airplane/kitefor.html

It shows every part of a kite, or a paravane including the body section and tail provides lift. The whole plan view of each paravane generates tow load so must be multiplied by eight to measure the challenge to the remaining rudder. Also looking down at the view of a deployed paravane there is no "weight" vector as in the kite diagram. Drag from the tow wire tends to pull the paravane towards the towing ship, but by keeping them on a short rein, less wire gets dragged sideways through the water, and this effect is minimised and maximum lift ensues.

All the best

wadinga
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Re: Raumotters to the Rescue

Post by marcelo_malara » Sat Apr 24, 2021 8:19 pm

Yes, may be I took it too far saying that most airfoils are symmetrical, I should have said most airfoils have curved up and down surfaces. As you said, the airfoil section is not the same along the span.

I don´t understand this:

"The whole plan view of each paravane generates tow load so must be multiplied by eight to measure the challenge to the remaining rudder."

What do you mean?

Regards

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Re: Raumotters to the Rescue

Post by Herr Nilsson » Sun Apr 25, 2021 1:04 pm

wadinga wrote:
Sat Apr 24, 2021 12:46 pm
Does this interesting Gothic item say anything about the tow loads experienced or any need to compensate with steering if only one is deployed?
No, the Bugschutzgerät (bow protection device) is designed for simultaneous deployment on both sides.
Regards

Marc

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Re: Raumotters to the Rescue

Post by wadinga » Sun Apr 25, 2021 2:30 pm

Hi Marc,

Yes I understand that once the paravane tow wires are attached at deck level they are then both taken down the wire or chain to the Bugschutzgerät point (retractable pole) and the deployed towing happens from there. Therefore they have to be deployed simultaneously. Is there any information about the deployment of the after paravanes? There appears to be a paravane deployment derrick both sides abreast Caesar turret, and four paravanes are stowed in the after part of the ship. It seems to me this is for extra protection (a second chance) if the bow paravanes fail to catch a moored mine cable.

As described in that very informative WWI book, destroyers of that era deployed and towed their paravanes from the stern, as it was thought their small draft would allow them to pass safely over moored mines, set at a depth below the surface to catch bigger prey. Given the narrow stern these were almost certainly deployed one at a time, and it would be interesting for my study whether steering compensation was necessary when only one was out.

Hi Marcelo. Sorry for my poor explanation. What I mean is that because the tow point on the raumotter is significantly offset to the centreline of the paravane body, water flow is at an angle to the paravane body, maybe 15-20 degrees. Therefore it too, In addition to the wing and its supporting strut, provides horizontal lift based on its cross sectional area. The whole paravane has this angle of attack to the water flow, and like your symmetric aerofoil, it is the angle of attack alone which provides lift. If all eight paravanes are deployed on the port quarter their cumulative effort is available to counteract the jammed rudder.

This is just a thought experiment, and as Bill (and you) suspect, maybe even a whole flotilla of Raumotters would not be sufficient to "balance" the ship and allow her to steer east-ish instead of west-ish.

All the best

wadinga
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Re: Raumotters to the Rescue

Post by marcelo_malara » Sun Apr 25, 2021 5:36 pm

I have this graphic of the British paravanes, from British Naval Weapons of World War Two Vol 1. It has a biplane cambered wing configuration, much alike a WWI fighter plane. Interestingly, the amount of pull of the tow seems to act in the link 4, against the spring 18, to set the angle of attack of the biplanes.



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Re: Raumotters to the Rescue

Post by wadinga » Sun Apr 25, 2021 7:59 pm

Hi Marcelo,

Nice piece of research. You have latched onto a very specialised paravane. The Two Speed Destroyer Sweep was a minesweeping system fitted to several classes of British fleet destroyers in the 1930s. They deployed these paravanes from special davits on their stern and also an additional central depressor to take the tow wires deep. This allowed them to carry out minesweeping ahead of the fleet at speeds up to 25 knots or as slow as 9 knots. It seems the variable geometry system you identified allowed for this speed range, and to get maximum lateral spread.

I have a book on the Kelly class destroyers which gives details in that the tow wire was 2" diameter and there was a 30hp steam winch for each side. All this took up a lot of space on the fantail, limiting depth charge capability. I figure 2" wire gives a safe working load of 32 tons but obviously there a big safety factor. Still not much confirmation of tow load, but if you need 30hp to haul it in. :think:

All the best

wadinga
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Re: Raumotters to the Rescue

Post by marcelo_malara » Mon Apr 26, 2021 4:26 pm

There is no way that the pull would be 30 tons. Let me do some calculations. The paravane is subjected to two forces , lift and drag (plus weight and buoyancy, that have no part on this calculation), the resultant of them would give the angle of the tow. Should the resultant be 30.000 kg, the lift would be 21000 kg (far higher that the lift calculation already done), and the drag 21000 kg too. To overcome the 21000 kg drag, suppose at 20 kt (10 m/s), what would the power of the engine be?

That is the equivalent horsepower, the power needed to overcome a force at certain speed.

ehp = 21000 kg * 10 m/s = 210000 kgm/s = 2800 ehp (at 1 hp = 75 kgm/s)

But the power applied by the engine to a propeller (shaft horse power, shp) will not be totally converted to ehp, the relation ehp/shp is called the propulsive efficiency, and is around 0.5 for a mean ship. So for the 2800 ehp, the ship will need to have 5600 shp, for each paravane, 11200 shp for both, and still you need the power to overcome the other resistances of the ship, whereas most minesweepers do not have more than 5000 shp.


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Re: Raumotters to the Rescue

Post by wadinga » Tue Apr 27, 2021 12:28 am

Hello Marcelo,

The Safe Working Load of the TSDS tow wire is indeed hugely over-specified. However transient loads when the ship rolls/pitches and pulls harder on the wire and especially if the wire hits a moored mine wire can be very high.

We already have reliable information suggesting maybe average 4 tons at 20 knots from the WWI graph. If this were doubled by shock loads the wire still has a big safety factor.

The Kelly class details are specifically for the specialised TSDS equipment.

All the best

wadinga
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Re: Raumotters to the Rescue

Post by marcelo_malara » Tue Apr 27, 2021 12:44 am

Oh yes, the stress as you described would be higher than the smooth water towing. Other issue would be the towing cable friction against the mine´s mooring cable as it travels to the cutter.

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