[rperret]

I've got a martec folder on my S2. Last year I replaced the pins - getting new monel pins from martec (at a cost of $10...).

Question, do these props really need monel pins v. stainless? Anyone out there using stainless cotter pins on their martec? Monel pins are impossible to find and i don't really want to spend yet another $10 on new pins.

I haul every year so would put in new stainless pins annually.

Thx.

 

[sailingdog]

Ask yourself "What is the worst that can happen if that cotter pin breaks from crevice corrosion...or the cotter pin rots the prop" and then see what repairing that is going to cost you, and figure out how many years of cotter pins that repair would pay for...

That's why I have AAA Plus for my car and a TowBoat US membership for my boat.

 

[JimsCAL]

Martec STRONGLY advises against using stainless cotter pins to avoid electrolysis with the bronze prop. Bite the bullet and get them.

 

[rperret]

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?

Martec STRONGLY advises against using stainless cotter pins to avoid electrolysis with the bronze prop. Bite the bullet and get them.

 

[cnc33voodoo]

dude, its $10.

 

[rperret]

yeah yeah - not the cost - need them quickly...

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

dude, its $10.

 

[Fstbttms]

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.

 

[brak]

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.

 

[sailingdog]

IF the cotter pin acts as a sacrificial anode and corrodes away, like the zincs do, what is keeping your prop in place????

 

[Fstbttms]

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.

 

[brak]

This doesn't even make sense.

why not?

 

[Fstbttms]

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.-

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.

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.

 

[brak]

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.

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.

 

[rperret]

gotcha...thanks.

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.