Prop shaft too short for ZINC

[Matthewpdavis]

Looking at a 42ft motor yacht that had its shaft replaced. Looks like the yard made it too short. Does anyone have any input on how a zinc could be added to safeguard the prop…that is obviously being eaten by galvanic corrosion. OR does it need a new shaft?

 

[Gdavid]

Mine is a similar configuration and requires a zinc collar of the shorter style, but that picture looks more extreme than mine. If I were shopping, I would question this setup and note my concern when placing an offer, which would reflect the price to replace the shaft. If I already owned the boat, I would look into repositioning the shaft coupler, add a spacer at the coupler, or try and cheat the motor mounts further aft and re-align the motor.

 

[Matthewpdavis]

Thanks for reply and input. Still in the discovery phase and deciding what I’m willing to take on. It went under survey a few weeks ago and this was the big issue that came back. Potential buyers bailed and the sellers have since dropped the price 25k.

 

[mvweebles]

Called a rudder post zinc. I have similar configuration. I believe these zincs require 1.25" of shaft for a 1.5" diameter shaft.

Alternative would be a shaft brush connected to your bonding system on the inside, behind the transmission. Can be a bit difficult to source. But not impossible.

Peter

 

[Matthewpdavis]

Thanks for those suggestions. I’ll dig into those potential solutions.

 

[Jeff F]

Prop nut anode is another choice, assuming there is clearance aft of the prop.

 

[mahal]

When replacing prop shafts, some replace with a slightly shorter one to allow room for a flexible coupling. If this vessel doesn't have one, it may have had it in the past and for some reason was removed. Or there was a plan to install one but hasn't happened yet.

 

[GoldenDawn]

On my previous vessel, I had the same problem. I was able to shave the standard zinc collar on a lathe to leave enough room (sadly, I have lost the photos). You should leave enough room for movement of the shaft.

 

[Comodave]

Several things you might try,

Grounding brush on the shaft
Spacer in between the prop shaft coupler to move the shaft aft
Doesn’t look like there is enough threads for a prop nut anode
Cutting down an anode to fit but bear in mind there has to be enough room for cooling water to get to the cutlass bearing

 

[High Wire]

Yes, a Globe Drive Saver will give you an extra inch. But if you use that space you better carry a spare in case you break it.

 

[psneeld]

Nice thing about sticking a zinc collar in there with enough room for water to get out of the cutlass bearing is rope and debris can't wind themselves so tight in there they damage your bearing.

 

[wfleenor]

A prop nut anode is what I use. Work fine.

https://www.boatzincs.com/prop_nuts.html

 

[Bmarler]

Prop nut anodes replace the second nut on the shaft, so there should be plenty of room for it.
Mounting a precision spacer would be my pick though. There’s an outfit that makes them as replacements for drive savers, should be just right.

 

[luna]

It is generally accepted that the dimension from the bearing to the prop hub (a cantilever) should not exceed about 1x the shaft diameter. The shorter, the better, within reason. Things can go bad quickly should this dimension be 3x shaft diameter.
The yard/shaft supplier that replaced the shaft seems to have followed this school of thought.

Adding a spacer introduces another failure point or 2, results in a less robust system and seems a waste $. All for an anode location? Why?

Use a prop nut anode or shaft brush if you can keep the connection, suitably connected.

 

[PhilPB]

Not sure if this adds value to the conversation, but this is a picture of mine.

 

[psneeld]

From Boatzincs.com

C-9 2" 3-1/2" 1-3/8" 1.8 lbs 3/16"

The thickness of a 2" shaft collar zinc is 1 3/8" .... so putting a 1 3/8" shaft in a 2-4 inch space is hardly an issue.

Other sizes it works out to be around the same. Here's a 1" shaft.

C-3 1" 2-1/2" 1-1/4" 1.1 lbs 3/16"

So fitting a 1 1/4" zinc in a 1"-2" space on a 1" shaft is no big deal either as I believe the rule of thumb, especially on slower turning engines is 1X-2X the shaft diameter.

 

[ronobrien]

Just wondering if a contact brush in the engine room that is tied to the bonding system would work?

 

[Brooksie]

https://boatzincs.com/prop_nut_specs.html


https://www.boatzincs.com/prop_nuts.html


I'm happy with this on my boat

 

[Bmarler]

Just wondering if a contact brush in the engine room that is tied to the bonding system would work?

Yes. But it needs to be tested regularly with an ohm meter to be sure it’s making proper contact.

 

[overgill]

Shaft wiper

I have the same issue with my 32 Island Gypsy, even less space than your photo.
Seeing that the PO had not addressed this in any way one of the first things I had to do was replace the 4 year old Prop, that had suffered sever electrolyses.

As previousely recommended above, the vender I purchased the new prop from set me up with a prop zinc. The original set up was not very expensive consisting of a special nut spacer and the zinc. Once that is set up correctly all that is required from there is to replace the zinc annually. (not a big deal)

In addition to that,,, Per discussion on this forum a fellow boater from Australia suggested that I also install what he called a "shaft wiper". Took me a wile to find one but it turned out to be a simple kit consisting of a piece of SS about a foot long ( the dimensions of a 12inch scale). On one end is a piece of what looks like a beryllium bushing cut in half, secured with a screw and a solder less connector. Anyway this is secured to a stringer with the bushing end contacting the shaft and a wire going the the engine block.

I installed this setup in my boat 10 years ago, my prop looks brand new. The PO had the boat for 9 years and per the paperwork left on board he replaced the prop twice.

Hope this helps good luck.

 

[Endeavor GB 32]

https://www.suremarineservice.com/E...DHjlwPTjr_tv_tkTtVGPC3l0nP7vC4yUaAkyhEALw_wcB


I have had these on a number of boats vs. zincs. Much cheaper in the long run. Good luck!
Regards,
Scott

 

[AKDoug]

Another Prop Nut Anode guy here, the wear on my hull mounted zincs almost completely disappeared after mounting the prop nut anode. Additionally when it starts to wear away it's super easy to replace while the boat is in the water, one screw!