On my Sabre that we're renovating all the thru hull bonding connects to keel at the same point as the lightning bond. In another thread on this site & other publications say this is a bad thing to do. The bond wiring is in poor shape & needs to be replaced so....
In your opinion does it make sense to connect the bonding to the same point as the lightning system?
Lots to consider. Under ABYC TE-4 it is suggested that you have primary and secondary lightning bonds. The main down conductor goes straight to the keel, unless encapsulated where you'd use an immersed ground strip, and this wire is as as straight as possible.
I personally diverge from TE-4 in the sheer size of the main down conductor and prefer at least 2/0 wire. I often opt for 2GA - 4GA for chainplates
For the seacocks, on bonded boats, fuel tank, rudder post, stuffing box etc. etc. I prefer to run all bonding directly back to the ships single main earth ground point, usually the engine. This gets the bonding wires for seacocks off the keel bolts and out of "direct line" strike duty and makes them more of a "secondary" lightning bond. I only wire the chainplates and mast straight to the keel bolts.
Does it make sense not to bond the thru hulls if an isolator is installed between the AC/DC grounds?
All a GI does is prevent the current developed from connected metals in an electrolyte from flowing. This is called galvanic corrosion. It requires all the metals to be immersed into the same electrolyte and to also be hard connected via a mechanical wired connection. This is what you get on the green wire when you plug in at a marina. You connect your vessel to all the underwater metals on all the other boats. Aluminum, bronze, SS, Manganese bronze, zinc, yellow brass etc. etc. etc.... When you plug in at a marina you are connecting your underwater metals to all the other under water metals in the marina via the green wire.
All metals have a voltage when immersed into an electrolyte. The current/voltage created by connecting all these dissimilar metals via the green earth ground wire can be damaging despite rarely if ever being over 1 volt. GI's ONLY block voltages below about 1.2V which is where galvanic corrosion activity is occurring thus they only protect against this one type of corrosion issue.
This type of corrosion is but one small piece of the corrosion puzzle. A lot of people assume a GI is the be all end all and it only serves ONE VERY SMALL portion of marine corrosion, blocking galvanic currents. They usually won't do much if anything for electrolytic corrosion or stray DC voltage leaks.
For example a hot bilge pump wire in the bilge of a boat, I see this all the time, that is not bonded, may eat the nearest seacock to it. Bond that same boat and you've now spread this current amongst all the under water metals and the voltage/current is much less damaging when divided by a much larger mass. You will hopefully notice the rapid zinc erosion and address the issue. Without bonding you can simply and easily eat that seacock and glug, glug, glug down goes the boat.
This happened recently and was written about by Ed Sherman the education director of the ABYC. An installer installed some bronze under water lights on a big expensive power boat (don't even get me going on the sheer stupidity of under water lighting
Everything else on this boat was bonded but he did not bond the underwater bronze lights. The wire made contact with the bronze light fixture and ate it very rapidly until it simply fell out of the boat... The boat sunk in the slip. Had that light been bonded it would have been a much slower issue and the zinc erosion may have alerted the owner to a problem..
To bond or not to bond is not so simple and these are but two examples.....
I see a LOT MORE corrosion problems with moored boats than people would ever assume occur.
This DC corrosion occurred in less than 24 hours on a mooring. The owner changed the bilge pump switch and left a bare exposed connection because he used non heat shrink butt splices. This exposed 12V wire was in the skim coat of bilge water about 4" from this 12 week old seacock. It had recently been installed for a new bait well in the spring of that year.. The boat was not bonded thus the corrosion on this seacock was RAPID. This was the only seacock to suffer damage this bad. Another one about two feet away was also damaged but the closest one took the brunt.. If he had not caught this the boat would have sunk in another day or two.
People always assume marinas are the worst environment but the really bad corrosion issues, as in fast, aggressive metal eating corrosion (like above), is most often caused by your own boat and on-board DC leaks..
The only true "isolation" in a marina is to either unplug or use an isolation transformer.