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Old 24-10-2019, 16:28   #1
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Broken propeller

I hit some misfortune today, one of my propeller blades broke off and the other one cracked. I heard a loud bang, followed by massive vibrations (that was following a 4 hour motoring session). I am looking for a replacement but the whole experience made me think... what if this happens while at sea? Should I carry spare props? Is it more likely that it cracked due to age (35 year old boat) or I hit something? How would I hit something strong enough to crack a propeller without damaging the rudder? Very scary experience.
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Old 24-10-2019, 17:25   #2
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Re: Broken propeller

Have you kept up with the anodes? They get pretty weak with electrolysis eating them.
Was it a bit pinkish in colour? A spare prop is a good idea if off the beaten track.
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Old 24-10-2019, 18:18   #3
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Broken propeller

Definitely carry a spare prop. That way it can sit in the hold forever.
Usually its the guys with the complex props that need a spare, but for some reason it seems that Cats spit props, cause Ive met three on Lagoons that have.

For the record, I carry a spare, its the fixed prop that came off to fit my complex one, I assume that is where most spare props come from.
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Old 24-10-2019, 20:34   #4
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Re: Broken propeller

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Originally Posted by Compass790 View Post
Have you kept up with the anodes? They get pretty weak with electrolysis eating them.
Was it a bit pinkish in colour? A spare prop is a good idea if off the beaten track.
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Old 24-10-2019, 20:47   #5
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Re: Broken propeller

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Duct tape?
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Old 24-10-2019, 21:28   #6
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Re: Broken propeller

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I guess it wasn't lapped
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Old 24-10-2019, 22:09   #7
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Re: Broken propeller

Nah, they were running the fridge off it!

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Old 24-10-2019, 23:01   #8
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Re: Broken propeller

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Originally Posted by Compass790 View Post
Have you kept up with the anodes? They get pretty weak with electrolysis eating them.
Was it a bit pinkish in colour? A spare prop is a good idea if off the beaten track.
CORRECTION: I should have properly described the possible problem as galvanic corrosion, as being more likely than stray current corrosion.
Surprised my a***e didn't get kicked earlier but I sense a treacherous undercurrent in this thread that maybe occupying minds.
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Old 25-10-2019, 14:53   #9
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Re: Broken propeller

....a dastardly, cruel, destroying undercurrent!
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Old 25-10-2019, 15:13   #10
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Re: Broken propeller

For what is is worth, for a boat to get a Category "A" open ocean certification from the EU authorities, a spare prop used to be required equipment. I assume that is still true.

For a prop to come apart as the OP described it almost certainly was weakened by galvanic corrosion. Bronze is normally ductile enough that even a significant impact will rarely fracture it, but rather bend it. Bronze that has been de-zinced by galvanic or stray current corrosion is fragile and brittle. That only happens on a boat that has been neglected through apathy or ignorance.
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Old 25-10-2019, 16:45   #11
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Re: Broken propeller

It can also happen if you're in an electrically leaky area of a marina...over one weekend: happened to friends of ours just before the marina upgraded its wiring, and iirc, cost them about 10K for a new leg to their sail drive, and new prop.

However, back to Pizzaz's question. If he didn't strike rocks, then, I agree with billknny, about the galvanic corrosion.

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Old 26-10-2019, 15:51   #12
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Re: Broken propeller

I agree that it was galvanic corrosion. I think neglect or apathy are strong words... I do replace the zincs on time but Marina Del Rey is a very hot Marina and the propeller was old.

On a different note, I sourced a relatively unused Martec folding prop which I plan to install this weekend. I looked at a number of old Martec props, some still on the shafts and while I like the overall design, I came to the conclusion that once you put one on, given the special long bronze nut they use, it would be very hard to take off after a number of years. So, the question is is there something I can do to make it easier for the bronze nut to come off when the time comes to replace the prop again. Do I just put marine grease? Is there a better compound? Shall I try to source/make a stainless steel nut?

Thank you.
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Old 26-10-2019, 16:06   #13
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Broken propeller

When I had a Martek I used a bit lubriplate water pump grease. I had an adjustable wrench large enough to fit on the flat sides of the hub, and a 1/2 breaker bar with an adapter to 3/8 and a short extension to get in to the nuts 3/8 square socket.
No problems once I started doing that.
If it were today I would use a hint of tefgel.
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Old 26-10-2019, 18:19   #14
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Re: Broken propeller

Pizzazz, first time you use it, check how long it takes to stop the boat in reverse. Our old Martec was not a great boat stopper. The FOF 3 blade works much better in reverse.

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Old 26-10-2019, 20:08   #15
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Re: Broken propeller

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ISo, the question is is there something I can do to make it easier for the bronze nut to come off when the time comes to replace the prop again. Do I just put marine grease? Is there a better compound? Shall I try to source/make a stainless steel nut?
I hate to say it but all of your suggestions are very bad ideas. You absolutely should not lubricate a prop nut of any kind. And having removed and installed many Martec folders, I can tell you that removing the Martec barrel nut is not a problem provided you use the proper tools. And as for having a stainless nut fabricated, that is an extremely very bad idea. Monel is the only metal that should ever be used with Martec props. But again, there is no reason to replace the existing nut. Not even sure why you think it would be necessary.

BTW- Considering that MdR is the largest man-made marina on the planet, I think it's a bit of a stretch to make a generalization like it's "a very hot harbor." It almost certainly is not. In fact, the "hot harbor" is largely a wive's tale, IMHO.
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