Keel Blistering

[Sans Souci]

I have a 2003 Hunter 356. During my first haul out, the keel had blistering. The thought was it had a bad barrier coat. Hunter paid for part of the repair. The second haul out the keel was again found to be blistered. The company that did the first repair assumed they had done a bad job and repaired it again. On my third haul out (recently) the keel was again found to be blister on both sides from about 1/3 of the way down to the bottom. This time an electrician was present and suspected an electrical problem. We put the boat back in the water to test the vessel's in water voltage potential. A problem was discovered that is too detailed for this post. After determining that that the connection of the lightning protection (which consists of bonding of the mast step, shroud chain plates and aft arch to the keel to the DC negative) was the cause of the keel coating failure a call was placed to Hunter Marine to confirm these systems were intended to be isolated as per factory specs. In the mean time, the problem was traced to be between the lightning protection system and the DC negative system to the VHF shield coming in contact with with the antennal bracket (at the top of the mast) attached to the mast. When the Hunter engineer called back, he confirmed that this is an improper situation and added that a the engineering specs do call for isolation to and between the two systems. Most VHF antennas come with mounting brackets that require special treatment to isolate them fully from the mast. The contact person at Hunter was unaware of any special recommendations to the mast manufacture or dealers to perform such changes or special treatments to the brackets in question. After relaunching the vessel the VHF antenna bracket was isolated from the mast and was tested once again with satisfactory results.

Keel repairs resulting from this recurring problem have cost several thousands of dollars. It was only with luck that a fully qualified electrician was around that saw a pattern that indicated an electrical cause and was able to track it down and fix it.

My question to anyone owning a Hunter 356 or perhaps a newer 36 - have you had similar problems or am I the only one.

 

[Paul356]

We're hauling our 2002 356 in the next week or so. We haul every year and I haven't noticed anything in past years, but I will check again and report if I see anything. Are you salt water?

 

[Don S/V ILLusion]

If I understand correctly, this makes no sense as any antenna shield will inherently be at the same (voltage) potential as the DC ground system. How an antenna bracket (which is physically and electrically interconnected to the shield and therefore the ground is an impossibility regardless of how it is mounted. If the keel problem is electrically related, it would almost certainly be the result of some other issue.

How did you measure Dc voltage in the ground system to the water and what are the readings you obtained?

 

[Ron Roman]

I have an '04 356 that I bought new and I haven't noticed any problems. But I will be taking a closer look Monday when she's being pulled.

 

[chuckwayne]

A problem was discovered that is too detailed for this post. After determining that that the connection of the lightning protection (which consists of bonding of the mast step, shroud chain plates and aft arch to the keel to the DC negative) was the cause of the keel coating failure a call was placed to Hunter Marine to confirm these systems were intended to be isolated as per factory specs. In the mean time, the problem was traced to be between the lightning protection system and the DC negative system to the VHF shield coming in contact with with the antennal bracket (at the top of the mast) attached to the mast. When the Hunter engineer called back, he confirmed that this is an improper situation and added that a the engineering specs do call for isolation to and between the two systems. Most VHF antennas come with mounting brackets that require special treatment to isolate them fully from the mast. The contact person at Hunter was unaware of any special recommendations to the mast manufacture or dealers to perform such changes or special treatments to the brackets in question. After relaunching the vessel the VHF antenna bracket was isolated from the mast and was tested once again with satisfactory results.

Please take the time to give us the details of what you found so we can evaluate what you're saying. Please describe/add photos of the "blistering" you observed on the keel. It's hard to picture this happening since they should all be at the same potential. A ground loop caused by multiple point connections among the grounds would still need something else to induce a current in the various ground paths

 

[Don S/V ILLusion]

Please take the time to give us the details of what you found so we can evaluate what you're saying. Please describe/add photos of the "blistering" you observed on the keel. It's hard to picture this happening since they should all be at the same potential. A ground loop caused by multiple point connections among the grounds would still need something else to induce a current in the various ground paths

Even multiple grd loops will cause a voltage potential; in fact, this is one of the most insidious causes of electrolysis. Without knowing more details as you said, this is one of those too vague to answer questions.

 

[Sans Souci]

If I understand correctly, this makes no sense as any antenna shield will inherently be at the same (voltage) potential as the DC ground system. How an antenna bracket (which is physically and electrically interconnected to the shield and therefore the ground is an impossibility regardless of how it is mounted. If the keel problem is electrically related, it would almost certainly be the result of some other issue.

How did you measure Dc voltage in the ground system to the water and what are the readings you obtained?

Here is a the report I recieved from Shurtz Marine Interprises of Olympia Wa regarding this problem. In answer to one of the other replies, the boat is stored in salt water. Here is the report:

Upon haul out of vessel for routine maintenance it was found that the keel coatings where showing blistering under the keel coating. Upon pressure was 60% of the lead keel was exposed. The condition of the lead surface was bright with no white powdering or corrosion evident with the mechanical prep from the last prep of surface still evident i.e. sanding scratches. This indicated that the keel was suffering form cathodic disbondment from over zinc protection or stray current.

It was decided that the best approach was to re-launch the vessel so as to test the vessels in water voltage potential. Test were conducted for stray AC/DC leaks and none were found. Testing the voltage potential of the keel, using a sliver chloride3 half cell, found the keel to read – 850MVDD. Testing of the DC negative cable (engine black) found identical readings. Performed disassemble of each of the four keel bonding wires and checked the keel contactors, had no change with the removal of both amidships wires for the shrouds and chain plates. Continuity was confirmed between the wires and the DC negative using OHM meter. Tested current flow from the wires in question to the now isolated keel and found readings to be 40MADC +/- 5. Confirmed the lack of stray current condition by isoloating the engine/shaft (zincs) from the DC negative and powering up the AC/DC loads. Upon examination of the systems it was determined that the connection of the lightning protection (which consists of bonding of the mast step, shroud chain plates and aft arch to the keel) to the DC negative was the cause of the keel coating failure.

This is the second time the keel coating have failed, upon he first failure the keel was striped to bare lead, coated with Interlux 200E, faired with epoxy filler, over coated with 5 coats of 200E barrier coat and then painted with modified epoxy bottom paint. Again, the condition of the keel surface at the points of failure indicate cathode disbandment, and not a chemical or surface preparation failure.

We recommend to the owner of the vessel, (Name), that the keel, mast and arch be isolated from the DC system. Hunter Yachts was contacted to confirm these systems were intended to be isolated as per the factory specs. Hunter Yachts indicated that they were unsure and needed to contact the engineering department to confirm. IN the meantime, the point of contact was traced out to be between the lightning protection system and the DC negative system to the VHF shild coming in contact with the antenna bracket and the mount bracket attached to the mast. Upon next contact with Hunter Yachts they were able to confirm that this is an improper situation, als indicated the engineering specs do call for isolation to and between the two systems. It was pointed out to Hunter Yachts that if isolation is called for then it is likely this is not the only vessel with this particular system sep up that is not compliant with the engineered specification.

Most VHF antennas come with mounting brackets which require special treatment to isolate them fully from the mast. The contact person at Hunter Yachts was unaware of an special recommendations to the mast manufacturer or dealers to perform such changes or special treatments to the brackets in question.

Following the reapplication of the keel coatings and launch of the vessel, the voltage potential of the DC negative system was tested and found to be reading 950-MVDC with one prop and shaft zinc. We isolated the VHF bracket from the antenna and check for any continuity from the lightning protection system to the DC negative and found none, which is now appropriate.

I do have good pictures of the damage, but do not know how to attach them. If you can provide instructions, I would be happy to include pictures.

 

[Sans Souci]

Even multiple grd loops will cause a voltage potential; in fact, this is one of the most insidious causes of electrolysis. Without knowing more details as you said, this is one of those too vague to answer questions.

Since there were so many typing errors in my last response, I am again attaching the electricians report from Shurtz Marine Enterprises. Perhaps this will make more sense:

Upon haul out of vessel for routine maintenance it was found that the keel coatings where showing blistering under the keel coating. Upon pressure wash 60% of the lead keel was exposed. The condition of the lead surface was bright with no white powdering or corrosion evident with the mechanical prep from the last prep of surface still evident i.e. sanding scratches. This indicated that the keel was suffering form cathodic disbondment from over zinc protection or stray current.

It was decided that the best approach was to re-launch the vessel so as to test the vessels in water voltage potential. Tests were conducted for stray AC/DC leaks and none were found. Testing the voltage potential of the keel, using a sliver chloride half cell, found the keel to read – 850MVDC. Testing of the DC negative cable (engine black) found identical readings. Performed disassemble of each of the four keel bonding wires and checked the keel contactors, had no change with the removal of both amidships wires for the shrouds and chain plates. Continuity was confirmed between the wires and the DC negative using OHM meter. Tested current flow from the wires in question to the now isolated keel and found readings to be 40MADC +/- 5. Confirmed the lack of stray current condition by isolating the engine/shaft (zincs) from the DC negative and powering up the AC/DC loads. Upon examination of the systems it was determined that the connection of the lightning protection (which consists of bonding of the mast step, shroud chain plates and aft arch to the keel) to the DC negative was the cause of the keel coating failure.

This is the second time the keel coatings have failed, upon the first failure the keel was striped to bare lead, coated with Interlux 2000E, faired with epoxy filler, over coated with 5 coats of 2000E barrier coat and then painted with modified epoxy bottom paint. Again, the condition of the keel surface at the points of failure indicate cathode disbandment, and not a chemical or surface preparation failure.

We recommend to the owner of the vessel, (Name), that the keel, mast and arch be isolated from the DC system. Hunter Yachts was contacted to confirm these systems were intended to be isolated as per the factory specs. Hunter Yachts indicated that they were unsure and needed to contact the engineering department to confirm. In the meantime, the point of contact was traced out to be between the lightning protection system and the DC negative system to the VHF shield coming in contact with the antenna bracket and the mount bracket attached to the mast. Upon next contact with Hunter Yachts they were able to confirm that this is an improper situation, also indicated the engineering specs do call for isolation to and between the two systems. It was pointed out to Hunter Yachts that if isolation is called for then it is likely this is not the only vessel with this particular system set up that is not compliant with the engineered specification.

Most VHF antennas come with mounting brackets which require special treatment to isolate them fully from the mast. The contact person at Hunter Yachts was unaware of any special recommendations to the mast manufacturer or dealers to perform such changes or special treatments to the brackets in question.

Following the reapplication of the keel coatings and launch of the vessel, the voltage potential of the DC negative system was tested and found to be reading 950-MVDC with one prop and shaft zinc. We isolated the VHF bracket from the antenna and checked for any continuity from the lightning protection system to the DC negative and found none, which is now appropriate.