My brother has a 28' fiberglass hull (Duffy Hull) lobster boat with a single screw and gas engine that he keeps on a mooring right on the ocean in Kittery Point Maine. The boat is approximately 150' from the docks where there are about 6 transient slips with power available that were put in about 3 years ago. The boat, new to him two years ago, is a typical lobster boat that does not have any means to electrically plug into power. The boat is kept in the water from April through October. This year he replaced the transom zinc plate, and I had him add a 4" round zinc to his rudder. He does not have any room to add a zinc to the SS shaft. This year he installed a brand new, Michigan 19" NiBrad wheel. When he pulled the boat in October, the edges of the brand new wheel were eaten and the blades had pitting, the transom zinc was approximately 50% gone, but the zinc on the rudder looked almost new. Is this a grounding issue or can the power from the new docks effect his boat approximately 150' away? There are several boats closer to the docks than his, but he did not talk to any of the owners. Thanks for the help in advance.
Zinc & Prop Eaten Up
- Thread starter tim f
- Start date
- Joined
- Jul 27, 2001
- RO Number
- 5508
- Messages
- 1,243
I doubt the boats 150 feet away have anything to do with his problems,
No room on the shaft for a zinc? That sounds like a problem.
Some boats have all of the underwater metals connected ( bonded) via a wire ( typically green), some boats go the opposite route and keep everything isolated. There are pros and cons to each approach. It sounds like this boat took the isolated approach, whether by design or failure of a bonding wire. End result is that the transom plate and rudders are not tied to the shaft and prop, so those two anodes didn’t provide any protection to the prop and shaft assembly. therefore the prop sacrificed itself to the shaft.
You need something connected to the prop and shaft, that has the higher potential than the prop and shaft ( or is it lower, I can never remember?).
Easiest fix is to get an anode on the shaft - if one won’t fit between the hull and the prop, you can probably find one that goes after the prop nut. Or dialing that, you need the shaft to be connected to an anode - whether it’s the rudders, or the plate on the transom Start looking around the bilge for a green wire that connects the rudders, shaft and transom plate together. if it was there, it’s probably broken.
No room on the shaft for a zinc? That sounds like a problem.
Some boats have all of the underwater metals connected ( bonded) via a wire ( typically green), some boats go the opposite route and keep everything isolated. There are pros and cons to each approach. It sounds like this boat took the isolated approach, whether by design or failure of a bonding wire. End result is that the transom plate and rudders are not tied to the shaft and prop, so those two anodes didn’t provide any protection to the prop and shaft assembly. therefore the prop sacrificed itself to the shaft.
You need something connected to the prop and shaft, that has the higher potential than the prop and shaft ( or is it lower, I can never remember?).
Easiest fix is to get an anode on the shaft - if one won’t fit between the hull and the prop, you can probably find one that goes after the prop nut. Or dialing that, you need the shaft to be connected to an anode - whether it’s the rudders, or the plate on the transom Start looking around the bilge for a green wire that connects the rudders, shaft and transom plate together. if it was there, it’s probably broken.
Here's one possibility to get the I/B propshaft bonded to the rest of the system so as to negate or minimize any voltage potential between U/W metal parts.
https://www.sbmar.com/articles/how-to-properly-bond-your-propeller-shaft-with-a-shaft-wiper/
https://www.passagemaker.com/technical/shaft-brushes-who-needs-them
https://www.sbmar.com/articles/how-to-properly-bond-your-propeller-shaft-with-a-shaft-wiper/
https://www.passagemaker.com/technical/shaft-brushes-who-needs-them
Thanks guys, I will take a look in the bilge and the website Sandy posted. There is absolutely no room on the shaft for a zinc. The shaft comes out of his keel and immediately the prop attaches. Duffy Hulls are great lobster boats, but I guess they didn't think, or care, about electrolysis. Thanks for the help.
- Joined
- Jul 27, 2001
- RO Number
- 5508
- Messages
- 1,243
Look for a prop nut zinc. Not as elaborate as the brush inside the hull, but much more commonly used. Start here. https://www.boatzincs.com/propeller-nut-zinc-anodes.html
I have to admit, my brother doesn't always look for things (electrolysis) like I do on my boat. The boat is two years old to him. Last year he thought the prop was just old and chewed up from the previous owner so he bought the new one. When he saw what happened to the new one, that's when he got me involved. There is no new electrical components or solar panels. I do not know what he has/had for a bonding system (I need to look at this).
The prop is connected to shaft, and the shaft is connected to the engine, so if I make sure there is bonding between the engine and the metals that have zincs, won't this fix the problem, like alk mentioned? If this won't work, I guess we will need to look at the prop nut zinc, or brushes and shaft wiper as sandy mentioned, after I check out his existing bonding system, if this boat has one. Thanks again for the help.
The prop is connected to shaft, and the shaft is connected to the engine, so if I make sure there is bonding between the engine and the metals that have zincs, won't this fix the problem, like alk mentioned? If this won't work, I guess we will need to look at the prop nut zinc, or brushes and shaft wiper as sandy mentioned, after I check out his existing bonding system, if this boat has one. Thanks again for the help.
The flange that connects the shaft to the transmission is not a reliable electrical connection. Shaft brushes are used to assure shaft to bonding system connections. Zinc prop nuts do help but most people believer everything should be reliably connected. How are his engine and cooler zincs doing?