Raumotters to the Rescue

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

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wadinga
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Raumotters to the Rescue

Post by wadinga » Mon Apr 05, 2021 2:34 pm

A workaround for the Steering Problem

Given the high level of interest and somewhat desperate enthusiasm in imagining ways Bismarck could have escaped her pursuers, here is an “Easter Egg” as a present for interested parties to consider. Please feel free to comment about its potential effectiveness.

Bismarck’s problem was that the jammed rudder(s) made her circle uncontrollably to port because they (it) created a thrust of water out to port at the stern which overcame the hydrodynamic tendency of the hull to steer in a straight line. It pushed the stern over to starboard, and thus the bow to port relative to a pivot point, somewhat back from the bows.

Steering a desired course to the SE and safety would be made additionally difficult since wind and weather would be from astern, and that always makes steering a stable course difficult, due to wave and wind action on the hull. For small vessels towing a drogue astern helps with course stability in these conditions, but a useful drogue for a 50,000 ton battleship and which also has to do something about the jammed rudder problem must surely be improbably huge.

It has been observed by Baron von Mullenheim-Rechberg that during her trials, Bismarck could not steer by her engines alone, and looking at a plan view from aft, it is clear her propulsion set-up was very compact laterally, despite her large beam. The limited distance between the outer shafts would limit their ability to influence hull orientation through differential thrust. However one should remember this trial was presumably conducted with rudders amidships, and thus acting as additional “deadwood “ reinforcing the hydrodynamic stability of the hull, and providing extra resistance to “steering by engines”.

What Bismarck needed was sideways thrust, at the stern, to act in direct opposition to the rudder thrust, and give the differential engine thrust a possibility of working, especially as there was no “deadwood” effect working against it.

In my opinion, based on what I have seen in seismic surveys, where hydrodynamic “kites” are used to tow lengthy streamers tens of metres off the track to side of the vessel, by creating tons of sideforce, Bismarck could have used her “Raumotters” to at least give a possibility of success.
Bismarck had a comprehensive outfit of paravanes (Raumotters) for mine wire deflection and hence her protection. If “paravane” is put in the search box on the forum summary page, descriptions of their standard use can be discovered. Some drawings/renditions of Bismarck show as many as eight stored on her upper deck, and these are created in port and starboard versions, but given the ship’s engineering capability they could surely all be converted for use towed from the port side.

I think given the paravane deployment derrick on the port quarter, and using fairleads at the stern, port quarter bollards and fairleads as separate tow points and with short tow wires, each paravane deployed might generate several tons of useful sideways force, counteracting the jammed rudder effect. Perhaps using up to eight paravanes would put the vessel into a situation where differential engine thrust would work and allow a gradual turn to and maintenance of the desired SE course, at a reasonable speed. Like the jammed rudder effect, the compensating “paravane flotilla” would generate sideforce proportional to speed.

Unlike “off the wall” ideas like welding hangar doors to the side of the ship, or even sailing astern for many miles in bad weather, paravane deployment was a standard technique for Bismarck’s crew and was surely feasible even in the poor weather conditions prevailing.

Would it have worked? One can only speculate. :cool:

All the best

wadinga
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Re: Raumotters to the Rescue

Post by marcelo_malara » Fri Apr 09, 2021 1:17 am

I don´t believe they would produce enough lift to counter a 50.000 t ship out of control.

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Re: Raumotters to the Rescue

Post by wadinga » Fri Apr 09, 2021 10:37 am

Hello Marcelo,

Glad to see someone wants to discuss my suggestion.

They don't have to fight 50,000 tons, they only have to counter a rudder's side thrust at 15 degrees. The ship is not really "out of control" it is responding to a jammed rudder and predictably circling as a result. The ship's 50,000 ton hull wants to steam in a straight line, it is the jammed rudder which is destabilising this. Counteract the rudder's effect and the ship is back in balance. If there is any chance at all of steering with differential engine thrust, and there is probably none, my suggestion gives it an opportunity.

I am currently trying to find actual loadings from paravane towing at various speeds, but their tow wires, according to the RN Admiralty Manual, were specified for 25 ton breaking strain. Why over specify unless very high loadings were to be expected? Hood's last maximum speed trial was conducted with paravanes out so they were operational at up to 28 knots.

If anybody has info on actual paravane tow loads I would love to see them.

All the best

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Re: Raumotters to the Rescue

Post by marcelo_malara » Fri Apr 09, 2021 2:36 pm

Physics is my second love! If you can measure the hydrodinamic surface of a paravane we can guesstimate the pull.

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Re: Raumotters to the Rescue

Post by marcelo_malara » Fri Apr 09, 2021 4:54 pm

I did my homework :D :D :D.

First let me review the formula of the aero/hydrodinamic lift of a surface traveling thru a fluid.

Lift (force in Newtons) = 0.5 * fluid density * surface area * V^2 * Cl

All are self explanatory except may be Cl, the coefficient of lift, a non dimensional parameter which depends on the profile of the wing, the angle of attack and the relation span/chord. For an approximation let it be 1.

The dimensions of the lifting surface of Bismarck´s paravanes, as per Brower´s, is 0.42 m x 1.6 m. Let assume a 20 kt speed (10 m/s).

So:

Lift = 0.5 * 1000 kg/m3 * (0.42 m * 1.6 m) * (10 m/s * 10 m/s) * 1

Lift = 33600 Newtons = 3360 kg

That would be the lift. Now for a proper calculation we should calculate the drag, quit difficult with the known data. Its importance resides that the relation lift/drag would give the angle of the cable attaching the paravane to the ship, and so you would know the component of the lift perpendicular to the centre line of the ship.

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Re: Raumotters to the Rescue

Post by wadinga » Fri Apr 09, 2021 7:03 pm

Hi Marcelo,

I might have found direct measured figures. If you can see this graph:

https://archives.imeche.org/archive/ins ... tyPhoto/0/

I believe it shows curves for paravane models (British) speed in knots on the bottom axis and tow load in C W Ts on the vertical. Hundredweights (cwt) are an archaic British weight measurement with 20 Cwt making one long ton Imperial. For the best performing model, it would appear 10 knots speed gives one ton (20 Cwt) of load rising to 4.25 tons at 20 knots.

You are doing a great job with your estimations, ( I am super-impressed :clap: :clap: :clap: ) but as we once described with torpedoes, there is a lifting body effect with the paravane fuselage and tail wing as well as the main wing as this device is dragged at an angle to waterflow, so effectively in a stalled condition. I estimate from diagrams in the Admiralty Manual the paravane flies at an angle of attack of maybe 15-20 degrees to water flow, and with the tow wire going out at 45 degrees to the paravane axis. The tow wire itself is being dragged through the water in a parabolic shape with the curve narrowing towards the paravane, creating a lot of drag, but for the proper mine deflecting job, lots of wire is a good thing clearing a wider, safer path.

I don't think the wing on a paravane has a conventional aerodynamic chord section, as like a kite it flies purely by angle of attack and the resistance of the towline and depends critically on the position of the towpoint. A centimetre forward or back adjustment of this can make a huge difference to the angle of attack and there is an optimum where it is slightly stalled but not excessively. :shock:

To try and counteract the Bismarck's rogue rudder, I consider a short tow where the resolved component going out at right angles to the ship's hull is maximised where the tow wire stays as clear of the water as much as possible would give the best result. However, this presumably means a lower overall tow load at the towing point, as no water drag on most of the tow rope. There are plenty of strongpoints on Bismarck's port quarter to use as towpoints for up to 8 paravanes.

The Admiralty manual says speed on deployment of the paravane should always be less than 8-12 knots and slower is preferable. The lifting derrick must have a working load of 2 tons. Once the paravane is deployed the lifting wire on the derrick does not handle the tow load, the dedicated tow wire does so.

How much side thrust the rudder(s) create is unknown, but I suppose it could be estimated and an educated guess made as to whether the Raumotter flotilla could get into the same ball park. Whether this would actually help Bismarck steer is probably unknowable.

I'll try doing some more homework myself, but I don't suppose I can match your efforts. :D

All the best

wadinga
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Re: Raumotters to the Rescue

Post by marcelo_malara » Fri Apr 09, 2021 7:38 pm

Thanks for your kind words.

So, if I understand the graphic well, for 20 kt the towing force would be around 80 cwt, 8000 pounds, around 3600 kg, so the approximation is close. As the lifting force varies with the square of the velocity, it is quiet logical that at half speed, 10 kt, the force would be 1/4, around 900 kg.

When I measured the total wing span, I purposely ignored the fact that the center part of the span is beneath the body itself and so does not generate lift. You can assume that the added lift replaces the body lift that I am not taking into account.

For the fact that the paravanes wings may not have airfoils profile, let me say that any surface exposed to a flow generates lift, even a flat plate. The difference would be two:

-the Cl coefficient is lower, so the flat plate would generate less lift force
-the angle of attack at which the Cl tops, and beyond which the surface stall, is much lower

Both things you can see in the graph.

Image

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Re: Raumotters to the Rescue

Post by marcelo_malara » Fri Apr 09, 2021 7:44 pm

Now remains the question if the force would be enough to correct Bismarck´s track. One may ask what happens if just ONE paravane is deployed in an undamaged ship, would the side force be appreciable?

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Re: Raumotters to the Rescue

Post by wadinga » Thu Apr 22, 2021 9:53 am

Hello Marcelo,

I guess a marine engineer :think: would have some knowledge of the stresses a Bismarck rudder was designed to create in use and withstand. This would give us an idea of how much force operating in opposition to that from the deflected water flow from the rudder would be required, and whether multiple paravanes could achieve it.

This informative book, spotted and brought forward by the most excellent Byron Angel

https://archive.org/details/paravaneadv ... 8/mode/2up

written in 1919, details the early history and use of the paravane, and it noticeable that Bismarck's units look very similar to the WWI variety, but given your point on aero/hydrofoil section maybe they had this improvement.

All the best

wadinga
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Re: Raumotters to the Rescue

Post by marcelo_malara » Thu Apr 22, 2021 4:42 pm

Hi Wadinga.

I have been thinking about this, but the very simple answer came to me this morning as I woke up :D :D :D .

Let as review again the lift formula:

Lift (force in Newtons) = 0.5 * fluid density * surface area * V^2 * Cl

Now, the rudder is many times bigger than the paravane´s wing. So, if both are inmersed in the same fluid, and advance at the same speed, the rudder has the power to generate much more lift than the paravane. In the formula, fluid density and V^2 would be the same, so the only way you can compensate for a smaller area would be a higher Cl, that I guess the paravane´s wing would not have.

So, if the rudders are centered I thing that a paravane can in fact turn a ship. But if one or two rudders are deflected, there is no way that the lift generated by the smaller area wing of the paravane would overcome the lift of the bigger aerea rudder.

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Re: Raumotters to the Rescue

Post by wadinga » Thu Apr 22, 2021 7:35 pm

Hi Marcelo,

OK I see and value your point, but what if only one rudder survived the torpedo blast and there are eight paravanes deployed, where do we stand?

Also, as I mentioned before I believe the body of the paravane, and its tail also provide lift because they are not proceeding through the water along the axis of the paravane, but are being towed at an angle of attack of c 15-20 degrees.

Depending on whether the rudder indicator recorded as showing 12 degrees accurately depicted the actual angle of the rudder blade, the paravanes may even have a greater angle of attack than the rudder.

Also the rudder is not in clean water flow but takes disturbed water flow from the hull. Unlike Hessen in A S-H's example there is no rudder taking direct thrusted water from a propeller and using it to enhance rudder effect. Paravanes are clear of the hull and taking smooth input water for maximum lift effect.

In a parallel, modern Formula One drivers are always complaining about "dirty air" and how their wings don't work so effectively if they are behind another car in turbulent air. (BTW Personally, I say chop the wings off and bring back real racing, based on chassis dynamics, guts and driving skill!)

All the best

wadinga
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Re: Raumotters to the Rescue

Post by Bill Jurens » Thu Apr 22, 2021 8:46 pm

Trying to synchronize propeller thrusts, rudder forces (which were probably uncertain and variable depending upon how much rudder was actually there, etc.), wind forces, sea forces, and paravane thrusts (which would seem to be difficult to regulate with precision) would probably be impossible, especially when one considers 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... And paravanes would only work at speed, whereas the residual rudder forces, whatever they were, would be predominant at lower speeds because the rudder is, essentially right in the propeller race.

I had the same thoughts previously expressed regarding rudder/paravane force areas. The rudder(s)(?) were pretty big, and the 'wings' on paravanes relatively small, so I'm not sure the paravanes could exert anything like similar forces.

So, overall, I don't think parvanes would represent a workable solution; certainly not over any prolonged time period.

Nice idea, though...

Bill

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Re: Raumotters to the Rescue

Post by marcelo_malara » Thu Apr 22, 2021 9:27 pm

From Brower´s, Bismarck rudders measured about 5.5 m x 3.8 m. At 20 kt the force would be:

Lift = 0.5 * 1000 kg/m3 * (3.8 m * 5.5 m) * (10 m/s * 10 m/s) * 1

Lift = 1045000 Newtons = 106600 kg

Again we are assuming a value of 1 to the unknown Cl. Now I can say that the forces needed for turning the ship are much higher than I expected. Taking only one rudder into account, about 30 paravanes would be needed to do the same work.

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Re: Raumotters to the Rescue

Post by paul.mercer » Fri Apr 23, 2021 9:05 am

Gentlemen.
Please forgive my total ignorance on this subject, your calculations are way above my understanding!
I looked up paravanes on Google which gave me some idea of what they were for and got the impression that when not used for military work they were designed to be a sort of stabilizer (or 'Flopper Stoppers' as some described them!) in bad weather or when trawling.
I also understand that they were used to cut mine cables and were used on specialized mine sweepers, but I do not understand their purpose on a battleship as surely a mine sweeper was often made of wood and was generally of fairly shallow draft so they could pass over moored mine with reasonable safety whereas a large metal ship like Bismarck with a deep draft would have a very good chance of hitting or setting off a magnetic one.
Earlier in this thread it was mentioned that Hood streamed paravanes at speeds up to 28 knots, but I'm afraid that with my lack of knowledge on this subject I'm puzzled as to why she did or why they would be carried on a large warship in the first place as they were designed to go out in the roughest of seas.

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Re: Raumotters to the Rescue

Post by pgollin » Fri Apr 23, 2021 12:07 pm

.

Most (all ?) WW2 battleships had paravanes and cutters which they could stream to help protect themselves. Each navy used their own favoured system of deploying them. Examples are the "original" trawler-like way of streaming from the fo'c'sle, but due to the large draught of most battleships means were employed to lower the paravane chains to the forefoot/front of the bow, either by a chain passing through an opening and being drawn through by rotating the chain through the hole, or, using a pole to take the chains down to keel level.

The drag was mostly in the bow arrangements. The RN intoduced an improved arrangement (called the Illustrious system that lessened the drag.

.

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