Grounding with drive savers

Discussion in 'Boat Design' started by paulpoz, Mar 16, 2011.

  1. paulpoz
    Joined: Mar 2011
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    paulpoz Junior Member

    Hi Everyone,

    I'm rebuilding my 85 28' Trophy with twin 350 inboards. I put drive savers between the transmission and prop shaft hub. In the literature with the drive savers, it says that in helps control engine corrosion because it isolates the engines from an electrical bond with the water.

    My question is, is this a good idea? Also, if so, should I ground the fuel fill neck to a sea water ground or the engine ground. The fuel tank itself is engine ground, so the gauge fuel gauge works. I'm concerned with static as I fill the fuel tank, which is better, an engine ground, a sea water ground or have everything bound together. Obviously all the metal through hulls and running gear are bonded together to the hull zinc.

    I also have a tuna tower on the boat. I've read some articles that I should bond that to the sea in some sort of way to reduce or at least give a minor lightning strike a path overboard and not have it arc through the boat. Any thoughts on that one?

    Thank you in advance.
     
  2. BertKu
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    BertKu Senior Member

    Lightning is a funny thing, it does not strike in one massive discharge, but in very short bursts of up to 40 strikes. It also depends whether it is a discharge from cloud to earth, or from earth to cloud (which is seldom). If your boat is the highest potential for the strike while it finds its way to the earth, definitely you have a good chance that you will get it.

    There are electronic devices which creates a high potential at the top of the mast and thereby increases the resistance for the lightning strike, thus it finds a better shorter potential to discharge itself somewhere else. How well they work, I cannot tell you. It is all theoretical. Example, If your cloud has a charge of negative electrons of a few millions volts and your mast is positive charged, i.e. shortage of negative electrons, you will be the shortest way allow the negative charged electrons to equal its potential. If your top of the mast is negative charged, i.e. too many negative electrons, why should it try to put more negative electrons into your mast? It will discharge somewhere else.

    There are different opinions about grounding. I personnaly would keep the engine and electronics ground seperate, but connect it with a small or 100 Ohm resistor to the seawater ground. Just to avoid static build up. But lets see what other more qualified people has to say about this topic.
    Bert
     
  3. mark775

    mark775 Guest

    I'm not the qualified one you seek but all of my life I have been under the impression that lightening went from the ground up!
    As far as grounding the fills, the purpose is to prevent a static arc, isn't it? I'd run a strip to the tank to forestall this.
     
  4. CDK
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    CDK retired engineer

    The only thing resembling "ground" is a metal object in the water, so that's what you connect the engines, fuel tanks etc with to create a safe environment.
    A prop shaft doesn't qualify because the electrical contact passes bearings and gears, making the electrical path questionable. A rudder and zinc electrodes provide a suitable ground.

    Of course you must include the tuna tower and sea rail in the ground wiring, otherwise it may acquire quite a static charge whenever there is electricity in the air. But no matter what you do, you will not succeed in making it lighting proof.

    Like Mark775 and BertKu wrote, the discharge starts from the most negative point, whether or not there will be damage depends on the current carrying capabilities of your ground wiring. The currents involved in a direct hit exceed 20 000 Amps, so whatever cross section your cable has, it will not be enough.
     
  5. BertKu
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    BertKu Senior Member

    It may very well that you have in your part of the world more from earth to cloud equalizing charges. There are three equalizing charges. From Cloud to cloud. From Cloud to earth and from earth to cloud. If you take a high speed film camera, you can see the electron flow flowing from cloud to earth or visa versa. If a cloud has lost electrons in the outer ring, the core will be positiv. Thus there will be an equilizing flow from earth to the cloud to equalize this. If the cloud , because of the friction has collected an oversupply of negative electrons, you can expect that the electrons will flow from cloud to earth.
    As far as I can see where we are living here, most times it is from cloud to earth and cloud to cloud. It is debatable whether a small resistor between engine chassis/electronics/lights and the seawater ground is better or worse.

    One thing is for sure, the discharge from top of mast must be
    a) as sharp as possible point at the top
    b) as thick copper and straight from top to water earth as possible
    c) preferable have a faraday cage environment were you could have a chance to survive a direct hit.
    d) Your heart can withstand currents of up to 20 milliampere maximum. If you are not in a faraday cage enironment, the induced flow from head to feet by a lightning strike is sufficient to kick the bucket. Your body will be the secundair part of a transformer.

    If you step into your automobile, do you have a strap to earth? Maybe a chain to release static charge, but not a permanent strap to earth. It will be difficult to drive to be ancred to the earth. If you drive through a storm, before you step out of the car, you better make sure the chassis is discharged. If not, you get a nice kick when you get out of the car. What I try to illustrate is the fact that a autombile carbody is a faraday cage and you have a reasonable chance to get out of the car alive when a lightning strike hit a car. One maybe blinded. Thus the same applies to a type of Farady cage on a boat. The electrolityc behaviour of metal under water is a different story. Refer the very good thread from CDK on zinc.

    Bert
     
    Last edited: Mar 19, 2011
  6. paulpoz
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    paulpoz Junior Member

    Thank you all for your replies:

    It sounds like the way to go is keep the engines and fuel tank and the boat's electrical systems bonded together and then separately bond the outriggers, tuna tower, and the stainless gas fill all together along with the Rudders, thru hulls, hull zinc, and struts. The prop shafts are isolated via the drive savers and the rubber in the cutlass bearing. They have their own shaft zincs.

    I read on one's blog that the best way to help alievate static and attempt to discourage a strike or save yourself from a minor one, would be to to connect the tuna tower via a heavy guage copper cable to a beaten flat copper pipe attached to the hull in the shortest path possible. Something about the electricity likes sharp edges. It also said to avoid a grounding plate for this because the sinistered bronze can be blown apart.

    Please correct me if I'm wrong with regards to grounding the fuel deck fill, but I would think that it should be grounded to the water and not the engine to disapate a static charge since the fuel pump is grounded to the water. I wouldn't think that grounding it to the engine ground, if the engine grounds do not complete a circuit to the water, would eliminate static.

    All this because of a couple of drive savers! LOL Thank you everyone for your responses.
     
  7. gonzo
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    gonzo Senior Member

    As far as grounding to the water, Dynaplate is the best way. A 6X9" plate is the equivalent to 25 sq. ft. of copper plate. The ground should be a strap, preferably woven, and not a cable. High frequency currents travel through the surface of the conductor. A cable of the same gauge as a strap will have a much higher resistance to a lightning discharge.
     

  8. BertKu
    Joined: May 2009
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    Location: South Africa Little Brak River

    BertKu Senior Member

    I am inclined to agree with you. As said before, lightning surges are not just one discharge, but short burst of up to 40 , which hold all kinds of frequencies, due to the nature of the discharge. A woven strap is indeed the better methode.
    Bert
     
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