SA Minesweeper in DBN Harbour Damaged - Page 2





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  1. #21
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    Default Re: SA Minesweeper in DBN Harbour Damaged

    Modern mines are programmable. They can lie on the seabed for a long time and wake up when the right type of ship approaches or after a period of time. They then swim out to the target and detonate like a torpedo. They can be deployed by submarines.

    Submarines have been gathering the sounds of ships for decades and some individual vessels can be identified by name already. A modern mine can be programmed for the type of ship or even an individual ship. Combatting these is very different to the old type.

    Pic of a modern sea mine that can wake up and attack it's target.

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  3. #22
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    Default Re: SA Minesweeper in DBN Harbour Damaged

    SAS Durban M1499 Build by Camper & Nicholson Ltd , Grosport , launched 12 June 1957
    SAS Pretoria old HMS Dunkerton and Kaapstad old HMS Hazleton was the oldest of the 10 minesweeprs in SA Navy , built by Goole Shipbuilding 8 March 1954(Pta) and Cook Welton & Gemmel 6 Feb. 1954 (Kaapstad)

    I stiil have the original SAS Kaapstad sold Brass key holder for the main hatch into the living quarters.

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  5. #23
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    Default Re: SA Minesweeper in DBN Harbour Damaged

    Thank you Olyfboer, very interesting.

  6. #24
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    Default Re: SA Minesweeper in DBN Harbour Damaged

    Quote Originally Posted by Olyfboer View Post
    Hi Hunter. I'm not a minesweeper fundi but I do know about degaussing systems used to negate the magnetic fields of warships to prevent the detonation of electromagnetic mines. I was friends with the Chief Engineer of the Minesweeper SAS East London and was often invited for a day trip to sea.

    The minesweepers of the type in the thread were designed to combat the WW2 type of mine which was a contact mine. These are still used by countries like Iran and are a particular hazard in shallow (Littoral) waters.

    Other mines used for that era's minesweepers are sound activated mines which activate on the sound of a vessels engine noise and propellors and electromagnetic mines which activate on the electromagnetic field of the vessel above it. Some mines could have all three activation systems.

    The minesweepers in the thread were used to keep harbour entrances and channels free from mines. A ship sunk in a narrow channel would block all trade in that port. These minesweepers were built of non magnetic materials to negate any magnetic field generated. They had a shallow draught as they were so light, which made them bounce severely in rough seas. The shallow draught meant were less likely to touch a contact mine and also meant they were one of the few medium sized SA Navy vessels that could pass over the bar going through tThe Heads into Knysna. They also carried a noise maker that was towed behind them to detonate noise activated mines.

    Contact mines of the WW2 type are attached to weight by a chain and float near the surface. To sweep contact mines they towed two floats like a fishing trawler does to keep their net open. Instead of a net there are sweep wires attached between the floats that drag and hopefully snag the chain of the mine, either cutting the chain or contacting the mine and detonating it. Should the chain be cut the mine would surface and be destroyed with rifle or gunfire.

    If anyone is interested I will continue with another post about more modern mine warfare.

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    Pls do, highly interested.
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  8. #25
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    Default Re: SA Minesweeper in DBN Harbour Damaged

    Quote Originally Posted by hunter26 View Post
    Pls do, highly interested.
    Sure Hunter, will do then. Although I need to start with the basic first principles, so this may take a few posts. I will keep it simple and easy to understand though. Hopefully interesting too.

    The old WW1 and WW2 contact mines are still in use. The horns on those mines are what cause a detonation. They are made of lead so they bend easily if a ship touches them. Inside is a glass vial of acid. When this breaks due to the bending action, the acid is released onto two disimilar metal contacts which instantly become a battery. The power from this battery then sets off the detonator. This is why rifles are used to detonate a swept mine that floats - To bend the horns, not to penetrate the mine casing.

    There is still a German WW2 submarine laid minefield off the Agulhas Bank through which passage is restricted. In the 1980's one of these mines came up in a trawlers net. The SA Navy despatched specialised divers and demolition personnel who disposed of the mine using explosives after it was freed from the nets.

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    To be continued.
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  10. #26
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    Default Re: SA Minesweeper in DBN Harbour Damaged

    Brilliant, looking at the mine launch pad. What's stopping a mine going overboard from accidentally hitting the ship as it tumbles overboard ie hitting the boat as it falls overboard.
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  11. #27
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    Default Re: SA Minesweeper in DBN Harbour Damaged

    Quote Originally Posted by hunter26 View Post
    Brilliant, looking at the mine launch pad. What's stopping a mine going overboard from accidentally hitting the ship as it tumbles overboard ie hitting the boat as it falls overboard.
    The rails cause the cart toppling over the edge to be closer to the ship than the mine itself. But there are several safety devices to prevent the mine detonator being armed until it is deployed such as a timer, flood switch and pressure sensor or combination dependent on the origin of the mine.
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  13. #28
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    Default Re: SA Minesweeper in DBN Harbour Damaged

    Now for magnetic influence mines.

    All steel ships have a permanent magnetic field that develops during construction. The Earth has it's own permanent magnetic field that varies slightly over a long time. The reason survey maps have a date and annual magnetic declination marked on them. Magnetic influence mines are deployed in shallow water to lie on the bottom. They are usually cylindrical to allow launching from submarine torpedo tubes. They can be deployed by ships or aircraft as well.

    Once deployed they settle on the bottom and after a time switch on to sense the Earth's magnetic field. The time delay is usually settable by the layer to give time to be clear of the area when a vessel detonates the mine. The Earth's magnetic field will vary dependent on the location and the compass heading of the mine. The field detected is referred to as the baseline magnetic field for the mines sensors. Any rapid change in this field as caused by a steel ship sailing over the mine will trigger the detonator. This type of mine is difficult to detect as it lays on the bottom. Moving it causes the field baseline to change causing a detonation.

    The magnetic field of the Earth is known and is illustrated on this degaussing chart. Degaussing is performed on all steel warships. More about that in the next post.

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