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Old 21-11-2006, 13:12   #31
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My question would be

How would someone apply STP under water???? Since the prop is wet I wouldn't think a petroleum product would stick to anything wet.
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Old 21-11-2006, 15:41   #32
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Oh, boy, here we go again

Quote:
Originally Posted by Alan Wheeler
For a hull that the only metal in the water is the shaft and prop, a single round anode will suffice. .
Not necessarily. How much anodic protection is required is often dependant on the environment the boat lives in, if it is plugged into shore power or not, is there an electrical problem aboard the boat or any of it's neighbors etc., etc.
It's probably safe to start with a single zinc on the shaft, but don't let the boat go too long without checking it. It is very common to need 2 shaft zincs.

Quote:
Originally Posted by Alan Wheeler
It can be placed anywhere ont he shaft, but close to the bearing is best as the shaft has the greatest support there and vibration will remain a minimum. Before or after is determined by room to fit the anode.
Agreed, but be sure to place it far enough from the strut to allow water to enter the strut bearing. My personal feeling is that putting a zinc directly in front of the prop may impede a clean flow of water over the prop.

Quote:
Originally Posted by Alan Wheeler
Now for the strut. Any metals under the water line MUST be protected. So all metals MUST be bonded together to ensure they are all at the same potential and a common anode must be fitted somewhere.
In theory this is probably true. But in the real world, most struts do not need anodic protection. They just don't usually have a corrosion problem because they are not in electrical contact with any other part of the boat. In fact, I have seen struts break where they were drilled to accept a zinc because the strut was weakened at that point. I tell my customers that the only time to add a zinc to the strut is if there is visible evidence of corrosion occuring.

Quote:
Originally Posted by Alan Wheeler
And to finish, it is important to get the right size of Anode. Usually a good rule of thumb is an anode that will bet mostly eatin away in approx 12months. So if your boat has been without an anode for longer than 13mths, you had better get it out of the water real quick, or you may not have a prop left. Inspect the prop for pinkish bloches all over it. This is a tell tale sign of de-zincification and if that is really bad, it can mean a new propellor.
It is always a good idea to put a set of eyes on your running gear at least a few times a year, if for no other reason than to check your zincs. This does not require a haulout. Hire a diver for a fraction of the cost. Of course, I am biased in that direction.
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Old 21-11-2006, 15:45   #33
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Quote:
Originally Posted by delmarrey
How would someone apply STP under water???? Since the prop is wet I wouldn't think a petroleum product would stick to anything wet.
I suspect you're right. Not to mention that introducing petroleum products to the marine environment is not a very good idea and is, in fact, probably illegal.
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Old 21-11-2006, 16:52   #34
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Quote:
Oh, boy, here we go again
Fstbttms, I am not following. Have you a problem with my post???
I don't read anything different in what I have said as to what you have stated, apart from maybe I could have expanded some details, but I did not think that was warranted on my part. Yet you seem to be taking some issue with each of my points. If you wish to expand on details, then good on you and I look forward to reading your facts. But don't go patronizing me.
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Old 21-11-2006, 16:55   #35
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No you can not put STP on under water, if you try it leaves an oil slick. If you try out of the water and then put the coated part underwater, it produces an oil slick. Then allt hat aside, it doens't work. Nor does any grease, lanacote or any other soft coating. It all eventually comes off and the time it takes depends on how long you run the prop. You can coat in Lanocote if you are wintering over and the prop is not going to be turned, but it comes off quick once you start her up.
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Old 21-11-2006, 17:04   #36
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Quote:
Originally Posted by Alan Wheeler
Fstbttms, I am not following. Have you a problem with my post???
No, no, I was just joking. Seems lately that you and I have had differing opinions on a range of subjects lately, that's all. I was not commenting on the validity of your post and I certainly did not mean to diminish what you had to say.
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Old 21-11-2006, 18:05   #37
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All right you guys. I didn't mean for my question to start a fight.

Here's some more information. My boat is fiberglass. I'll probably have some bronze throughhulls and an internal bonding system. The strut has no other link to electricity but I could bond it too internally via one of the bronze bolts holding it in place.

Thanks for the info about the anode being too close to the cutless bearing to prevent water flow. Very good point I hadn't thought of.

Thanks for all the other information too. I'm going to move the round anode up forward a bit on the shaft and add a smaller flatter one between cutless and prop.

The boat has been on the hard next to my house for years. No visible corrosion on the shaft since I just reinstalled it last week and won't put it back in the water for at least another few months. : )

It really is good to bounce questions off you folks. I get a great deal of good information back.

Really happy day today!! I lifted the engine into the boat (with the help of a very large machine) after the boat having been engineless since I've owned it '89. I've got threads about what engine and why in other areas.

Kind Regards, JohnL
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Old 21-11-2006, 18:24   #38
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Quote:
Originally Posted by SkiprJohn
Thanks for the info about the anode being too close to the cutless bearing to prevent water flow. Very good point I hadn't thought of.

Thanks for all the other information too. I'm going to move the round anode up forward a bit on the shaft and add a smaller flatter one between cutless and prop.
John,
There shouldn't be enough room between the strut and prop to put an anode. And, you don't want one there anyway!!!

The distance between the two is suppose to be the same as the diameter of the prop shaft, plus or minus a tad. Any more causes viberation and wear. Any less impeads the flow of water thru the cutlass bearing.
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Old 21-11-2006, 18:25   #39
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Quote:
Originally Posted by SkiprJohn
I'm going to move the round anode up forward a bit on the shaft and add a smaller flatter one between cutless and prop.
You'll be better served simply using two streamlined collar zincs on the shaft forward of the strut, rather than one foward and one aft. Plus, the one you intend to put after the strut sounds like a limited clearance collar. Not that it won't work there, but it will likely be taller than a streamlined collar and definitely will affect flow over the prop.

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Old 21-11-2006, 18:33   #40
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Quote:
Originally Posted by delmarrey
John,
There shouldn't be enough room between the strut and prop to put an anode. And, you don't want one there anyway!!!
There are many boats designed with very little shaft exposed between the prop and the cutless. This is a common feature on full keeled boats. I can also name half a dozen Beneteau and Jeanneau models that also have maybe a 1/2" of exposed shaft. In some of these instances, the only place there is room for a zinc is in that very small length of exposed shaft. That is what the limited clearance collar was designed for.
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Old 21-11-2006, 19:44   #41
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Quote:
Originally Posted by fstbttms
There are many boats designed with very little shaft exposed between the prop and the cutless. This is a common feature on full keeled boats. I can also name half a dozen Beneteau and Jeanneau models that also have maybe a 1/2" of exposed shaft. In some of these instances, the only place there is room for a zinc is in that very small length of exposed shaft. That is what the limited clearance collar was designed for.
Full keeled boats pump water thru their cutlass bearings unless there are vent holes for the water to enter.
By putting a zink behind the bearing, wouldn't it restrict the flow just as it would in front like shoving a banana in your exhaust pipe.

And on a fixed prop wouldn't it serve just as well to put a zink-nut on the end of the shaft?

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Old 21-11-2006, 19:54   #42
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Quote:
Originally Posted by delmarrey
Full keeled boats pump water thru their cutlass bearings unless there are vent holes for the water to enter.
Yes, but the point is there is limited space to install a zinc.

Quote:
Originally Posted by delmarrey
By putting a zink behind the bearing, wouldn't it restrict the flow just as it would in front like shoving a banana in your exhaust pipe.
As previously mentioned, you must be sure to leave space behind (or in front) of the cutless for just this reason.

Quote:
Originally Posted by delmarrey
And on a fixed prop wouldn't it serve just as well to put a zink-nut on the end of the shaft?

These "cap" type zincs can be used in many of these appilications, but I advise my clients to use shaft zincs if possible. The cap zincs are far more prone to spinning off the prop prematurely. I replace the machine screw they come with with a hex-head bolt. That way I can get a wrench on it and really torque it down. I don't like 'em much. But sometimes there is no other way to zinc the boat.
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Old 21-11-2006, 19:56   #43
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Fstbttms, Well actually I don't think our opinions differ at all. At least not on this particular subject. What you have stated is valid and what I have stated is valid. So is Del. But as you should know, this subject can be complex with many opinions , often none of those opinions being wrong as such. Just different schooling.
I have no objection to you commenting on my post itself, I welcome that. What got me was the "here we go again" comment. I didn't think that was justified.

Back to the subject, OK John, the fibreglass part helps. There are two trains of thought. Keep all metals bonded, and the Keep all metals isolated. Both arguments are valid, both have holes in the theory. I won't go further here now, but do a search as we have had some rather fantastic discussions on this subject in the past.

If you really want to know exactly how much anode you need, the best is to place it int he water and use a Galvanic meter to measure the current. This will ensure the correct protection. You don't want too much and you don't want too little. The issue (as I see it) is that shaft anodes are rather small. Most boats need something much bigger. This is determined by how much metal and what type is in the water. And many of the other points that Fstbttms stated as well. It may well be that a single or double shaft anode is all that is needed. But you have two ways of finding out. Either test for current flowwhich is (IMO) the best method or maintain a watch on the anode for "work". You need it to
"work", you don't want it over protecting. It must last as well however, you don't want to replace it every three months now do you.
By the way, In an extreme case, it has also been known that too much anode has caused paint and FG to bubble.
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Old 21-11-2006, 19:56   #44
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See now, the way I figure it a REAL dummy would have said "I don't understand why these zincs don't work, I bought four last year and I stowed them in the galley cabinet and they all still look GREAT what's the problem?" <G>

A zinc is supposed to fit tightly on the shaft, and I've found that in order to get them on snugly, I need to hold a maul on one side, then slap the other side with a heavy hammer to actually force them together tightly around the shaft. After that, the screws just secure them together, no great effort needed to tighten them down.

The shaft should be clean bare metal under the zinc, and you shouldn't paint over it. Being a bit compulsive, I still do use threadlock or paint on the actual screws, to make sure they don't work loose.
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Old 21-11-2006, 20:03   #45
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Quote:
Originally Posted by Alan Wheeler
I didn't think that was justified.
Alan, I really meant nothing more than "Here we go with differing opinions again" meaning I was about contradict you again. I absolutely was not commenting on the quality of your post. I hope you understand that.
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