View Full Version : Bonding

12-07-2008, 12:10 AM
I have a 30' 1956 African Mahogany planked sloop that has been bonded for the last 20 years. I just finished a partial re-planking and a complete re-fastening. My surveyor was out to check my work the other week and he was concerned that bonding everything together could distribute any possible problems. He suggested removing the bonding and to just have the 3 zinc's on my prop shaft. Currently I have a 3"x 6" zinc at mid ship that is insulated from the wood. All of the thru hulls are wired together and attach to this zinc plate.

The boat has simple electrical needs. I have a starting battery and a house deep cell that run the lights, GPS, etc. I'm getting ready to install a continuously running low draw exhaust vent for a composting head. I will probably install a small solar panel for this demand. The boat is moored in a small mooring area of about 20 boats in Casco Bay, southern Maine.

So do you recommend keeping the bonding or eliminating the zinc plate & removing the wires. Or something different.



Jay Greer
12-07-2008, 02:00 PM
Your surveyor is correct. Bonding often distributes iononic exchange potential in such a manner as to cause wood surrounding some components to become "zinc sick"; which is the cooking of the wood surrounding a metal fitting that is receiving too many Zn ions . Evidence of this phenominom can often be seen as zinc crystals or fuzz that have formed on the fitting itself. The surrounding wood will take on a swollen soft fuzzy state as well as the alkali cooks the wood. With the boat in the water, reading the potential of, unbonded, fittings in milliamps will give a clue as to whether or not they have potential for electrolytic damage. A good marine electrition can perform this test for you. Or you can do it your self with a good multi meter. I consider 1.5 to 2 ma to be within the safe area. While others may not agree, I have found it to be safe in my own practice.

Bob Cleek
12-07-2008, 02:33 PM
Ditto to Jay and your surveyor. Lose the bonding throughout and lose the 3x6 zinc. That's WAY too much zinc for a boat that size. Bonding was a fad some decades back. Big mistake. Don't go there.

12-07-2008, 05:11 PM
Hmm, 20 boats, Casco Bay... One of the islands: Peaks, Cushing? I agree, the bonding does more harm than good. Possibly why you had to replank and refasten. :(


12-07-2008, 10:32 PM
Thanks for your replies. I keep the boat off of Cousins in Casco Bay. You all confimed my plans. I will get rid of the wires and dispose of the zinc plate.

24 years ago I had a boatyard in midcoast maine replace keelbolts and the keelson. They also took it upon themselves to bond the boat. The boat was also partially re-fastened at the same time. The 24 year old fastenings were in pretty good shape. The original fastenings were pretty sad. After all they were 52 years old. The planks on my starboard side were in excellent shape. The port side was way worse, which I replaced this summer. My only thought is the port side was stored in the sun in it's early years. I bought the boat when it was 25 years old, 27 years later I think I have either replaced of fixed everything. Hopefully this is my last huge project.


Gary E
12-08-2008, 08:35 AM
Bonding Systems The purpose of a bonding system is to equalize the electric potential of dissimilar underwater metals by tying them all together with wire or copper straps. The benefits of a bonding system are wide ranging but little perceived. One is that it serves to dissipate stray current leaks. 12 volts of current focused on a small piece of metal will result in rapid destruction. But that same 12 volts spread over a much larger surfaces, causes less damage in proportion to the size of the water exposed surfaces of the metal. Bonding systems can reduce the corrosion potential of metals inside and on the bottom of the boat. Boat which have all the hardware bonded, such as the railings, will suffer much less corrosion.
As mentioned above, bonding systems are not maintenance free. The wire connections corrode too, and need to be reestablished periodically. This is done by cutting off the old terminal or connection, and then establishing a new one. It's as easy as standing on your head in the bilge ;-)


12-08-2008, 08:48 AM
Why not put a flexible coupling on your propellor shaft while you're at it and then you'll have isolated your whole electrical system from the water? No need for any zinc blocks at all if you do that (I think) and you may, like me, find that you'll have less vibration from the engine as well.

12-08-2008, 09:05 AM
I do have a flexible coupling on the shaft.

12-08-2008, 09:10 AM
Then check with someone wiser than me (not hard, just ask anyone in the street) and see whether you really need to worry about any sacrificial anodes at all. I'll be surprised if you do. I think you'll find you already have everything isolated except the shaft and propellor. Do you have any other metal hanging in the water?

12-08-2008, 09:38 AM
I do not hang anything in the water. I think I will keep putting zinc on the shaft but leave it at that. Dave

Don Z.
12-08-2008, 09:48 AM
Then check with someone wiser than me (not hard, just ask anyone in the street) and see whether you really need to worry about any sacrificial anodes at all. I'll be surprised if you do. I think you'll find you already have everything isolated except the shaft and propellor. Do you have any other metal hanging in the water?

It's possible I just don't get it, after all, I seem to be an expert at letting the smoke out of things electronic, but...

Isolated or not, is it not possible that stray current in the water will pull the zinc out of a bronze prop?

12-08-2008, 11:03 AM
Is it important to have zinc on the shaft so the stainless is not degraded?

Jay Greer
12-08-2008, 01:25 PM
Even though the primary concern and reason for placing a Perry Nut and or zinc collar on bronze props and stainless shafts is the protection of the prop itself from
electrolysis I have seen, unprotected, stainless shafts that were grossly pitted.

12-08-2008, 01:32 PM
It's easy enough to put the zinc on the shaft and the Perry Nut (I never knew what that was called) so I will continue to add the zinc. My shaft is about 24 years old, has no pitting and in good shape. Thanks, Dave

Peter Eikenberry
12-08-2008, 07:48 PM
Bonding is a good thing on FRP or metal boats, but not on wooden boats. As has been previously said it contributes to the deterioration of the wood around the fasteners and dezincification of the fasteners themselves.

However, don't confuse bonding with proper grounding of electrical equipment. Your electrical system needs a good grounding system to prtotect you and the system.

12-08-2008, 08:35 PM
Hmm, p'raps I'd better put some zinc on our propellor shaft too! I replaced the shaft fairly recently as I needed a longer shaft when I relocated the engine. the old shaft had no pitting but I guess it's better to be safe. Thanks! Rick

12-08-2008, 09:17 PM
So I'm clear that bonding a wooden hull is a bad idea, especially since I just repalced about 2,500 bronze screws.

I do have a 6" dynaplate. I have an inboard diesel with a starting and a house battery. I have a DC isolator and a circuit breaker panel going to every electrical device. The electrical needs are pretty simple. VHS, GPS, Bilge pumps & lights. Grounding is going to the battery. I'm not sure anything is going to the dynaplate, maybe the VHS. It's been so long since I re-wired that I forget about the VHS. And it's 10 degrees outside so I'm not going to look...How should I be using the Dynaplate? I carefully wirebrush the Dynaplate but I wonder if there is a way to clean the pores?

A related topic is lightning. My spars are Sitka Spruce. No lightning rod with the hope that lightning will be more attracted to others. Comments?

Thanks in advance for your insight,