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Go Back   SailNet Community > On Board > Gear & Maintenance > Engines > Diesel
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Old 05-16-2013
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Props for Saildrive, Stainless vs. Bronze

Looking to replace the prop on my Volvo Saildrive from the factory 2-blade folding to a 3-blade feathering for better performance under power. There are various choices in bronze and one in stainless. The current factory folding unit is bronze, it has an aluminum spline in a large rubber bushing. This contacts with a stainless bearing washer which is in contact with the bronze body. Submerged in salt water you in effect have a battery. One problem I am experiencing is that it is consuming prop zincs at a rate of 2 sets per season. There isn't any other elctrical issues leading to the galvanic issue. One problem is Volvo prop zincs are smallish. Anyway, I have heard 2 schools of thought on this, one is that by replacing the bronze unit with the all stainless one you are eliminating the battery effect created by all the dissimilar metals in contact and submerged in salt water and that's a good thing. It has also been proposed against stainless is that since bronze is a less noble metal then stainless it is acting to help protect the drive (and hence why the prop zincs are being eaten) and that by putting on an all stainless prop the drive itself will now be more susceptible. With the current bronze prop the drive zincs show little sign of errosion.

So this is where the confusion lies, it's over 3 grand to install the new prop, that's a good piece of change, to spend that and have it create a problem in the saildrive is not a very good idea as a drive problem can get very expensive real quick. I also don't want to be adding what could in effect be a $3K extension of the zincs. As many know trying to get any info out of Volvo on something like this is very difficult. So I'm looking for some insight.

Before I get any responses about stray current at the dock or shore power, the boat is on a mooring over 200 yards from the dock and is literally never connected to shore power. So these are not issues.

Thanks for any feedback.
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Old 05-16-2013
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Re: Props for Saildrive, Stainless vs. Bronze

SoC, eliminating the number of types of metals you have hanging off the boat has to be a good thing, otherwise you get one metal protecting another which is protectiing another and so forth. So, in your poistion I'd suggest a stainless prop - but you'll still need the anode to protect the drive.

Your post doesn't say where on the planet you are, but I can strongly recommend Autostream (Seahawk) if you're after a new prop. I've never had an issue with mine (in both forward and reverse it acts like a fixed-blader), each prop they sell is QC'd by real people and they are extremely helpful to deal with: Seahawk: Autostream Self Feathering Propellers for Sailing Yachts & Sailboats

In any case, their FAQ's might be helpful to you: Seahawk: Common Questions (FAQ)
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Re: Props for Saildrive, Stainless vs. Bronze

Hartley,

Thanks for the reply. The Autostream is obviuosly the all stainless prop I refer to. I was completely in agreement with the simplify the metal combo submerged in salt water theory initally. The issue is that it appears Volvo is using the prop as a second line of defense against galvanic action in the whole engine/drive system. Replacing a prop hub every 4-5 years as the spline corrodes away is cheap relative to a drive leg. I replaced the hub on my prop 2 years ago and the spline insert is already showing a good amount of pitting. My theory is they use decay of the spline insert as a 2nd line of defense for those who don't replace zincs twice a year. I sail a Tartan 3700 in New Jersey and even a short haul to replace the zincs is fairly expensive. Most divers don't know when the Volvo zincs are bad, so if you use one you just have them replace it regardless.

But that brings up another issue about factory specs for replacement and how a diver can't possibly meet those requirements as there are locktite and torque specs for installing the zincs. My engine and drive were under warrenty until this season so all work had to be certifiable if you needed to file a claim.

I had a discussion with the folks at Tartan today, they promised to look into it and get back to me. Anyhow, I'm now thinking along the lines of a bronze prop, but one that you can replace the spline. Don't know if that even exists, but I will do some research.

Back a few years ago I had a C&C 35 mk111 and installed an Autostream 3 blade feathering prop on it and I was extremely happy with the performance enhancement it provided. That's one reason i was leaning that way initially now.

Like I said previously, the tought that the thing may become what's in effect a $3000.00 zinc enhancement is what I have issue with. Evaluating the performance benefits against replacement costs every few years has to be considered in making a change.

SoC
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Old 05-17-2013
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Re: Props for Saildrive, Stainless vs. Bronze

Righto.. but don't forget that the Autostream props (at least) have a ruddy-great chunk of zinc anode hanging off the end of them by design that, in your scenario, will likely protect the prop, drive leg and drive anode in one go.. And said anode is very easy for a diver to check and change in the water. It's one nut - done up tight.



The only downside I see to using a bronze prop in your application is the chances of the prop being damaged if you forget to change the zincs.. but perhaps that's better than the drive leg?
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