This isn't supposed to happen

[Frenchy]

The flange under the bolt apparently eroded away, allowing the zinc to dig a hole in the face of the rudder. Has
anyone seen this before? How can I stop it from happening again? Thanks - Jean

[Jim Walsh]

Jean,
If you want to exclude (or severely diminish) a section of the zinc from eroding just dab the area with nail polish. Nail polish is handy because it has an applicator and I get an old half used bottle from my daughter.
I use this method to limit erosion in the area of the mounting bolts on my tapered shaft zinc. Here are a couple photos I took a few years ago to illustrate what I do and the effect it has.

[Frenchy]

All right, Jim - I'll give it a try. I don't have any nail polish handy, but maybe some epoxy paint might do the trick. -
Jean

[sgbernd]

It looks to me like you (or previous owner or your boat yard) are using a standard clamshell shaft zinc which is intended to wrap around a smooth shaft but the prop is fitted for a "C" size prop nut attached with the hex screw at the back.

(http://zincwarehouse.com/prop-nuts-c-nu ... inc-c.html)

I am not sure how your arrangement is supposed to work but I can guess what they or your were trying to do.

The proper prop nut, attached with the single hex screw at the stern, is notorious for working loose and the zinc digging into the rudder as happened to you. Likely the previous owner or boat yard was trying to prevent it and switched to the clamshell type shaft zinc which is what you took off. I would suspect the damage was from the previous (and correct) prop nut rather than your current (incorrect) shaft zinc.

BTW, I never had any luck with nail polish but sometimes was able to keep them on using both loc-tite and compression loc-washers. Unfortunately, when you change the zinc in the water, the loc-tite doesn't set so well, with the result you have shown. Sometimes zincs would stay the entire year, sometimes work loose after a few hours of motoring. Sometimes the zinc would just quietly disappear and you would notice it missing when you clean the hull, sometimes it worked loose and you felt it impeding the rudder. Then you had to shut down the engine, jump overboard with your hex-wrench and tighten it back down before it could damage the gel coat.

Hope this helps.

Steve Bernd

[Frenchy]

Hi Steve, - On the clamshell type zinc, I've never had one on. There's no room for it, as there's only about a
3/4" space between the front of the prop and the rear of the prop housing.
The screw doesn't look like it backed out. It screwed in with difficulty, as it had a blue loctite type coating on it.
I really think that somehow the flange under the bolt got eaten away before it should have. Maybe the anode
wasn't making good contact with the propeller assembly anywhere but at the flange? Could that make a difference?
I don't know. I'm definitely going to pay more attention to this area in the future. Here's a pic of a new anode next to the anode assembly. Apologies for being a bit out of focus. Regards - Jean

[sgbernd]

Yes, you have the correct zinc. Your guess as to being eaten away beneath the screw sounds like the most reasonable. If the case, the recommendation of nail polish beneath the screw might actually work as it will slow the erosion there. I ended up diving on it several times over the season to check for tightness but still found it gone occasionally.

Ultimately, my problem was solved when I repowered and went to a small clamshell type on the shaft.

Good luck.

Steve

[Frenchy]

Update - I painted the underside of the screw and the screw counterbore in the zinc this season (as per Jim Walsh's instructions) and
voila - my zinc didn't fall off. To appease Poseidon this season, I've offered a pair of sunglasses and a ball point pen. - Jean

[Jim Walsh]

Excellent. Glad it worked for you.
I know the paint trick seems like an oversimplification but it works great. I've been doing it for years and I've never had a zinc disappear, or rattle because it had loosened, again.