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Thread: Corrosion Protection for an OB Prop

  1. #1
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    Corrosion Protection for an OB Prop

    My Yamaha OB is always in the water, and the lower unit, including the prop, encounters a significant corrosion problem. Yes, I do have a sacrificial plate above the prop and the Ariel is used in fresh water. The prop is an aluminim alloy.

    The problem of the disappearing propellor (and lower unit) is commonly recognized (The Yamaha prop is about $100, so, being cheap, I don't like to replace it annually).

    I have painted the prop with TRI-Lux II which seems to work well as protection. Tempo did not work. But the prop has to be repainted each year, with a significant buildup. My motor mechanic has told me that the rough surface of the paint on the propellor is responsible for loss of thrust I have been experiencing.

    The question is, what have others found best to protect the prop? Or is sanding it every year and refinishing it the only or the best solution?

  2. #2
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    A Plastic prop?

  3. #3
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    Do you know if they make such a thing, or is that just an idea (although a good one). My recollection is that the plastic was not rigit enough for a larger horsepower motor (They do have them for the 2 HP for example). Has something new come on the market?

  4. #4
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    Sounds like a 'hot slip' or a wiring problem on the boat.
    1- Get a heavy cable and attach a bunch of zincs to one end , the other to your ground . Hang in the water at the slip. Or go to West Marine and buy a 'Guppy Zinc' .
    2- Pull the motor out the well , thats what I do . $100 buys a lot of BenGay ointment.
    4- Is that the right prop , pitch wise ? Cavitation can cause your prop to erode too , and too much pitch will cause cavitation .

  5. #5
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    Exclamation

    Just re-read your post;
    If your motor is elec start , I'm betting on wiring problem .
    I left a motor in salt water for the same time and only got marine growth , no corrosion .

    Are you slipped anywhere close to some big powerboats , 1 or 2 slips away, thay could have a leaking electrical system.

    I had a home made 'slip meter' to measure stray current in marinas. Basic VOM with special leads and 3' long probes . Works great in salt water.
    Found an old Egg Harbour with enough stray current to shock a diver !

  6. #6
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    Thanks Mike for your comments. I can not pull the 85 lb motor out of the well. Further, I have concluded (be it right or wrong) that to try to remount a motor in a heavy sea is very dangerous. I don't know how others do it safely. And as for pulling the motor out when leaving the boat, that is a big hassle, and I don't have a couple young turks standing nearby to help.

    Yes the pitch is correct for a heavy load - minimum pitch, pitched for almost no forward motion at all. Actually, the pitch issue and cavitation, I think, would only arise with a low pitched prop used on a high speed/light boat. With a high pitch prop on a low speed hull, the prop would not cavitate, but, instead, the motor would not get up to speed and there would be no power. The engine appears to reach the right rpm.

    The problem I don''t think is wiring (I do not hook up to the AC when in port) The entire electrical system is tied into a large copper grounding plate in the water near the keel. The zinc anode on the outboard just above the prop should discharge any currents relevant to the motor (and yes, it is eaten away). As for powerboats nearby in the marina, yes there are tons of the floating kitchens. However, I can't really control other people's wiring. Further, I visit various marinas during the summer.

    Keep in mind that this problem is not just related to my set-up but is related to all outboards. The problem is discussed in the Interlux brochure, and outboard motor instruction manuals. It appears to be universal. Everyone says that if the motor is left in the water, the lower unit and the prop must be protected by some sort of supplementary finish. It may not be electrolysis but simply corrosion of the aluminum. It may be that the movement of the prop, or the motor through the water creates a static charge that eats up the prop. I don't know.

  7. #7
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    That is why we switched to the 55-lb 6hp , even my wife can pull it at the dock .
    Sounds like stray current getting to you , the zinc guppy works well , ground it to the motor . Most outboards don't have enough protection zinc wise .
    If you have enough zinc , it goes before the aluminum , if the aluminum is going then you need more zinc or change it several times a year. 50% deterioration of the zinc and it is time to change it .
    My neighbor left his Nissan 8hp in salt water for 1 year and had no loss of aluminum and very little zinc loss , of course he was on a mooring about 200yds from the nearest electric . That shoud have ate it up .

  8. #8
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    Think Mike is correct. There you are in fresh water (maybe not sweet), not the best electrolyte, so it can't be everyday galvanic corrosion, right? So it's the hot marina syndrome or your 'bonding' system has the motor in a loop. A loop that has a fault in it.

    Looked real close at 338's 8/4 (2001) Yamaha and was astonished to find two sets of small zincing wires (what's the word: depotentializing?) in totally vulnerable positions in or near the water. Both connect moving to stationary parts at the base of the clamp and at the bottom 'hinge' of the leg. These wires could easily experience a little accident and break a circuit, if circuit is the correct term.

    There is an exposed zinc on the motor at the clamp, and a larger square shaped zinc hidden under the cavitation plate right over the prop. You can't see this zinc (if you don't lift the OB) unless your boat is out of the water - or you are in the water. Wouldn't assume anything until you actually touched these places.

    Heard that lanolin is an excellent corrosion inhibiter on props. Maybe you can smear it on your prop in the water or the whole unit in the water, warm it up first, the lanolin - or the prop. Not kidding about sheep grease, look it up!

    You can over-zinc, over-power the anode effect of the zinc - ie have too many or have them too large, so they won't work properly. Was wondering from your description of your grounding plate if it could be creating a problem in a similar way? You know, something out of balance between the g. plate and the ob always in the water. Does your grounding plate show signs of acting as the cathode?

    Maybe our Yamahas are primarily set up for salt water. The zinc (which is about 3/8' of an inch above the tips of the spinning prop blade on 338's) may not work as well in non-salt water. So you are not getting the anode/cathode reaction as intended. Wouldn't you suppose the problem is occuring when the boat is inactive, sitting in the marina?

    The prop itself could be at fault - have impurities in the alloy or experiencing galvanic corrosion in the grain of the material. Maybe a bronze prop is the way to go. Painting the prop itself with zinc, or keeping it painted with 2-part urethane, maybe willie's 5200 method would work. Kidding aside, you could get the prop plated with a more inert metal like tin.

    But lanolin is SO comforting natural, much less expensive, and it's good for your hands, too
    Last edited by ebb; 02-27-2005 at 11:52 AM.

  9. #9
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    Theis wrote: "I can not pull the 85 lb motor out of the well." Have you considered Gene Robert's solution -- the Garhaur davit? Installation directions in the manual.

  10. #10
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    Several points:

    EBB, remembering that the motor is pulled once a year, and that I go on extended cruises, I question whether the lanolin will hold for the season, particularly on the lead edges of the prop, where the primary corrosive effect occurs. But it also occurs on the lower unit. (which is no problem with regard to painting using paint. It is the painting that is a pain in the propellor).

    I have just replaced the zinc anode on the cavitation plate because the old one was approaching the end of its life. As for the wire, I will check continuity between the zinc plate and the electrical ground, although the problem occured the first year (2000), before I started painting the lower unit and the prop. Now the only problem is keeping the propellor painted without creating prop drag.

    As for the type/brand/etc. of the prop, I am limited to the Yamaha reversing prop, which does not cavitate in reverse, as conventional props do. I would assume the metal is a mixture/compound of metallic elements, principally aluminum, but have no control over that. So, out goes a specialized/bronze prop - although I will enquire further about that and a plastic prop from Yamaha. I am not confident that a stainless prop will make any difference, other than lighten my wallet a lot more than buying several aluminum props.

    As for the copper plate, I have checked that, and there does appear to be any corrosion. Only oxidation. However, when I had the corrosion problem (2000, before painting the prop), I had not installed the copper plate. The copper plate was installed in 2001 0r 2002. At the time of the motor installation, the electrical contact with water was both a) through the motor, and b) through the water intake valve. But that valve does not appear to be affected.

    However, It may be noteworthy that, before I rehabed Solsken, the water intake covering plate had been totally eaten away over the prior 16 years of use with the prior Evinrude and McCullough motors). I don't recall there being a prop/lower unit corrosion problem, however.

    As for the Garhouer unit, it really is a pain pulling the motor, taking off the wires, removing the exhaust tube, disconnecting the water barrier between the motor well housing and the lower unit, etc. Once a year is enough. The motor has to be in the well when the boat is in use, and I have no place to put it, other than in the well, when not in use. As for the lighter unit, I require the full 8hp. 6 hp is not enough - the reduced thrust (equal to a 6 hp) is what has raised this whole issue. In my case the motor is not only for getting into the slip, but for driving into tough seas and head winds (motor sailing), and getting to a destination faster when there is no wind.

    So, back to plan A (Interlux paint), and plan B (plastic) with the possible addition of a coating of lanollin and see if it works for a full season. Any other ideas? but I will check the continuity of the wire, and the continuity between the battery ground and the zinc anode on the cavitation plate.

  11. #11
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    Plan C,
    A rubber bag that you pull over the lower unit and secure around the stern with bungies. Dump a little environmentally friend hydrogen peroxide down the well befor departing and bam! you've solved the problem!

  12. #12
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    Curiosity does not always bring forth the expected. Thanks to all the help, I now find there is a problem that I not only didn't realize I had, but one that I didn't even know could happen.

    The battery ground and the lower unit have a solid electrical
    continuity. However, the sacrificial anode, physically bolted to the cavitation plate, is not electrically connected. From the battery ground to the anode is an open circuit.

    When I replaced the anode, I got continuity. I would never have thought that a solid connection between aluminum and zinc could become non-conducting.

    So, for all you that leave your motors in the water, you now have another winter check up item. Take the anode off, clean it, and make sure it is electrically connected to the lower unit, not just physically connected.

    But back to the original issue, one thing I forgot to mention is that I do have to put anti-fouling paint on the prop anyway to make sure that it doesn't grow fuzzies - which will really slow me down and reduce drive.

    As for plan C, Ed, without getting in the water each time I take out and put away the boat (twice per sail), I have no way to put a bag around the lower unit so that the bag remains always above the water line. In a way, the paint becomes a permanent bag. Keep in mind that once I get Solsken in the water, I want to maximize the time available on the water and minimize the time getting the boat ready to go and putting it away. I am not part of the racing crowd.

  13. #13
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    Theis,
    Have been one to point out the expediency of keeping the horse
    harnessed to the cart at all times when in the barn.

    That is a great tip.
    Now it's obvious that the french fitted zinc hiding under the cavitation plate has to be carefully connected electrically to be a working anode.
    And I have written that into my Yamaha Manual. Thanks.

    Would be wonderful if that alone solved your terrible corrosion problem!
    Last edited by ebb; 02-28-2005 at 09:03 AM.

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