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Go Back   SailNet Community > On Board > Gear & Maintenance > Martec Prop Cotter Pins
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Thread: Martec Prop Cotter Pins Reply to Thread
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Topic Review (Newest First)
04-06-2009 10:57 PM
rperret gotcha...thanks.

Quote:
Originally Posted by brak View Post
Shaft is big, and it is also towards the boat from the prop. So the electricity flow is from shaft to prop to water. This prevents much corrosion occuring at shaft. Pins are small and they are between prop and water, so since stainless is less noble than bronze, pins will act as sacrificial anodes and may dissolve relatively quickly.
04-04-2009 10:48 PM
brak [QUOTE=Fstbttms;470918]1.- The size of the shaft relative to the pins and its position relative to the prop and boat do not affect whether or not, or to what extent, electrolytic corrosion will occur.

2.-

The first part of this sentence (besides being completely irrelevant) has absolutely no relation to the second part.

Quote:
3.- The pins are made of monel because monel is galvanically closer to bronze than stainless is (as you correctly pointed out). However, this is to prevent galvanic corrosion, not electrolytic corrosion.
The two are intimately related, in particular when we are talking about prop shaft and anything attached to it. Practically all diesel engines are electrical ground for their alternators and as such, shaft/prop etc act as a ground path to the water (unless specifically disconnected through a coupler).

Boat ground is higher in potential than water and it does create current. Galvanic and electrolytic corrosion have essentially same mechanism - difference in potential created in one case by external source of electric potential and in another by different pontential between dissimilar metals in electrolyte.
Generally speaking simple difference of potential between stainless and bronze is not sufficient to create significant corrosion, but the ground current from the boat helps this process quite a bit.
04-04-2009 10:37 PM
Fstbttms
Quote:
Originally Posted by brak View Post
why not?
1.- The size of the shaft relative to the pins and its position relative to the prop and boat do not affect whether or not, or to what extent, electrolytic corrosion will occur.

2.-
Quote:
Originally Posted by brak View Post
Pins are small and they are between prop and water...
Quote:
Originally Posted by brak View Post
so since stainless is less noble than bronze, pins will act as sacrificial anodes and may dissolve relatively quickly.
The first part of this sentence (besides being completely irrelevant) has absolutely no relation to the second part.

3.- The pins are made of monel because monel is galvanically closer to bronze than stainless is (as you correctly pointed out). However, this is to prevent galvanic corrosion, not electrolytic corrosion.
04-04-2009 09:35 PM
brak
Quote:
Originally Posted by Fstbttms View Post
This doesn't even make sense.
why not?
04-04-2009 11:45 AM
Fstbttms
Quote:
Originally Posted by brak View Post
Shaft is big, and it is also towards the boat from the prop. So the electricity flow is from shaft to prop to water. This prevents much corrosion occuring at shaft. Pins are small and they are between prop and water, so since stainless is less noble than bronze, pins will act as sacrificial anodes and may dissolve relatively quickly.
This doesn't even make sense.
04-04-2009 07:46 AM
sailingdog IF the cotter pin acts as a sacrificial anode and corrodes away, like the zincs do, what is keeping your prop in place????
04-04-2009 01:24 AM
brak
Quote:
Originally Posted by rperret View Post
ah..so they did not advise me of that when I called them last year.

so help me understand if electrolysis is an issue - how come the stainless shaft does not appear to have any electrolysis issue?
Shaft is big, and it is also towards the boat from the prop. So the electricity flow is from shaft to prop to water. This prevents much corrosion occuring at shaft. Pins are small and they are between prop and water, so since stainless is less noble than bronze, pins will act as sacrificial anodes and may dissolve relatively quickly.
04-04-2009 01:17 AM
Fstbttms
Quote:
Originally Posted by rperret View Post
so help me understand if electrolysis is an issue - how come the stainless shaft does not appear to have any electrolysis issue?
Because the shaft is likely not made of stainless steel, but rather Monel, just like the cotter pins.
04-03-2009 06:31 PM
rperret yeah yeah - not the cost - need them quickly...

actually now found a local source....after an hour of web surfing...

Quote:
Originally Posted by cnc33voodoo View Post
dude, its $10.
04-03-2009 05:33 PM
cnc33voodoo dude, its $10.
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