Zefyros,
There should NOT be any potential at the mast!  My guess would be that you have a short in one of the wires to the radar or lights.  I'd suggest disconnecting one wire at a time until it disappears, and then you should have your culprit. The short could be in the bilge, as the wires enter the mast, inside the mast, or where the wires exit the mast.  That could be the reason you have more corrosion than normal.  Hope this helps.

PS-It would help everyone (and you!) if you could add a "signature" to your posts, to help identify your boat and you.  Replies can be more specific to your hull number, location, etc.  Click on the My Account link near the top of the home page, then Edit.  The Signature portion is near the bottom.  Don't forget to save your changes.  See some of the other posts in the Forum as examples.

I don't have my book in front of me, but per Nigel Calder's book, as I recall it is only significant and worrisome if the leak is greater than a few milliamps, i.e. it's not just about the voltage.  You also have to take the flow (amps) into consideration. 

Yianni, you seemed to indicate that you were planning to remove the mast, or have you already removed the mast??? With the mast removed troubleshooting the electricals will be much easier.

About the copper ground strap...in southern Cal that's not a bad thing to have. I suggest that if it's an easy job to reinstall the ground strap it would be a worthwhile thing to do as a lightning protection step.

Here's my concern with Yianni's problem as originally stated. Yianni says there's a voltage potential from the mast to the keel bolts. But my keel bolts are set into a lead keel, and lead is not a good conductor of electrical current. As far as I am aware, my keel bolts are not grounded. So why there should be ANY potential measurable to keel bolts is a puzzle. I am guessing that salt water in the bilge is causing the slight mast potential when you measure from the mast to the keel bolts, however "dry" you say it might be. There may be, in fact there likely IS, salt water trapped in areas of the keel box where it is not visible. This moisture is normal.

But potential to the keel bolts is not the most important part of this puzzle. 

We need to restate the problem, and Yianni, we need your help to do so. (All measurements with shore power disconnected.)

• What voltage potential do you have from the mast to the engine/ship's ground?    

I disagree strongly with the statement that suggests there is "an acceptable leak level." There should be NO - none, zero, nil, nada - voltage on the mast as measured to either the keel bolts or, much more importantly, to the engine/ship DC ground. 

My theory is that your mast is not grounded to the engine/ship's ground.

I suggest you start troubleshooting by (a) making sure you are using a high quality voltmeter, (b) that it is on the proper DC scale, and begin (c) by confirming the presence of any voltage between the mast and engine/ship DC ground. If there is any voltage, then, with your battery switch OFF, using a high quality volt/ohm meter (Fluke meter or equivalent) measure the resistance between the mast and ship's DC ground. This should be a very, very low number...say, one or two ohms...that is, a solid heavy wire connection between mast and DC ground.

If there is any potential between the mast and ship's ground, and if you then confirm a higher than a couple of ohms resistance between the mast and ship's ground, then you MUST ground your mast to ship's ground. Failing to do so will result in lots of problems, including...very dim or nonfunctioning steaming light, dim or nonfunctioning anchor light, poor VHF communications, possibly poor radar performance (although there is an internal ground in the scanner cabling), poor TV reception, and perhaps even an increase in lightning risk, especially for persons below deck. 

A question for those who pull their masts annually: IS the C36 mast separately grounded?

I have never pulled my mast, but I think I know every wire going into it. I don't recall there being a ground wire that specifically ties the mast to ship's ground. On a simple installation, let's say a boat with only a steaming light, a deck light and an anchor light  - and a VHF antenna - there are ground wires associated with the switched voltage run to each light. And the coax going to the VHF antenna has a ground shield. But I do not recall a ground wire going to ship's ground.

Anyone?

Thanks, Gary. That's what I thought.

Still, there awaits an explanation as to why there is a voltage measured on the mast. I maintain nonetheless that there should be NO voltage measured on the mast to EITHER the keel bolts OR the engine/ship's DC ground. 

Many years ago I was helping a neighbor troubleshoot the tail lights on his Lotus sports car. [Side note: The reason that the British drink their beer warm??? Because their refrigerators are made by Lucas.] A situation similar to Yianni's C36 was occurring, and I found the stray voltage to be coming through the filament of a taillight light bulb to the car's frame. The problem was the absence of a good ground connection on the non-voltage side of a light bulb.

This is my present theory about Yianni's problem.

This might be a stab in the dark. I am wondering if he is leaking thru his SSB antenna if rig mounted. Or if something is grounded to a chain plate with a ground leak which would be going thru the rigging to the mast.

Randy, usually an SSB antenna - on the backstay typically - would be insulated from the rigging at both upper and lower ends. The SSB transmitter's coupler would then tie into the insulated backstay, but the ground side of the coupler would go to a copper plate somewhere on the hull. Therefore, if Yianni's boat still has an SSB antenna I somewhat doubt that would be the cause of a voltage leak. 

I'm still betting on one of the lights (or other mast-mounted accessories - TV antenna? Nah.) to have a broken ground wire. One of my first actions would be, with shore power disconnected, to observe the voltage as described, and then while watching the voltmeter to progressively switch off each circuit breaker. If the voltage is still there with all CBs switched off, then begin removing fuses one by one. If the voltage is STILL there after all fuses have been removed, then I suppose to report back here to discuss a possible next step.