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post #1 of 6 Old 10-11-2010 Thread Starter
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C&C 121 Prop

We just repropped our C&C 121, after the original prop broke. The short story is that our new prop, a 16"x13P 3-blade folding prop from FlexoFold, seems to be just right.

Some notes:

Volvo Penta no longer has the original 2-blade folding 16x13 prop that was on the boat when delivered new to me. The boat was very underpropped with that prop.
They no longer have the 17x14 2-blade folder they recommend for the MD2040 engine on the boat.
They also don't have the 17x12 fixed 3-blade they recommend for the engine.
I ordered the 17x14 2-blade fixed they recommend for the engine. But it doesn't fit - the 120S saildrive installation in the 121 hull won't accommodate the 17" prop diameter. This explains why the boat was shipped with a 16-inch prop.

So we found and bought the 16x13 3-blade FlexoFold. It's great. It's also expensive. It seems to drive the boat at hull speed at full throttle at 3500 RPM, a little bit slower at 3000, and at an acceptable cruising speed at 2500 RPM. If I can get around to it, I'll get some decent GPS-based speed numbers (I don't trust my speedo's very high numbers, and I've been exclusively in strong currents the last couple weeks). The Volvo MD2040 seems to be rated to 3600 RPM (although the literature is a little vague). At 3500 RPM, it sounds as if the engine's working awfully hard, and I wouldn't run it more than a short while at that speed; 3000 RPM feels and sounds much better, and I'd be comfortable running it for hours at that speed. If I were making a long crossing, I'd probably back off more.

Anyway, setting aside my satisfaction with the FlexoFold, my general point is that a 16x13 3-blade prop is much better than the inadequate 16x13 2-blade that originally came with the boat. I hope this is helpful.

Last edited by CarpenterSF; 10-13-2010 at 12:12 AM.
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post #2 of 6 Old 11-23-2010
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Thanks for the post-- I have hull #34 and are quite disapointed in the boats performance under power. I have looked into a new prop and am glad to hear thet the Flexofold seems to work. Any issues with installation? Does there seem to be any increased drag?
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post #3 of 6 Old 11-24-2010 Thread Starter
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Installation and other notes

Installation: The installation instructions are very good, very clear. The Flexofold comes in a bunch of pieces, and you install it piece by piece onto the shaft/saildrive. That is to say, you do NOT assemble the prop into one piece and then screw the entire assembly onto the saildrive, the way you might with a fixed prop. Out of the water, it's easy: 10 minutes. We did it out of the water, since the boat was already hauled. A diver who's changed props before should be able to do it underwater, but it would probably take a little longer, and would definitely be a good idea to have a helper reading instructions and handing parts and tools to the diver. You definitely want someone doing it who is careful about reading instructions, including the note to save the spacer ring from the prior prop. Though a zinc was included with the prop, I ordered a couple spares from the dealer (North Harbor Propeller, aka GetAProp.Com, very nice and helpful folks). The zincs were about $40 apiece, and various other notes on the net say that they go away pretty quickly, with various alternatives offered.
Drag: The prop folds into a pretty compact piece, and my diver says the prop seems to fold freely at the dock. The prop isn't spinning when under sail, which I take to indicate that it's folding (during our brief period with a fixed prop, we definitely had to shift into reverse to stop the prop spinning when under sail, so I would be able to detect if the prop were spinning). Now, Flex-o-Fold makes a racing prop, which creates less drag than than their regular folders. It also costs more. It's also a 2-blade, and would presumably require more pitch. I don't know when more pitch starts not to make sense. I cared enough about drag to pay for a good folding prop in the end, but not enough to care about the difference between the regular and racing folders, which I can't imagine results in a difference of even 1/4 second per mile. If I were racing in a very large, competitive one-design fleet, I might care about it. I certainly don't notice any performance issues under sail, but that may just be me.
Finally: I remain happy with the prop. At near full throttle, we are definitely going hull speed, with an enormous amount of water being pushed astern. Harbor maneuvering is great, with rapid starts and stops and reverses. None of that was true with the original prop.
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post #4 of 6 Old 11-25-2010
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Thanks for all the great info. I'm in the Northeast so the boat is now out of the water. I am strongly considering replacing my prop thanks to your great info!! Sounds like the install should be easy out of the water!
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post #5 of 6 Old 11-25-2010
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Quote:
Originally Posted by CarpenterSF View Post
A diver who's changed props before should be able to do it underwater, but it would probably take a little longer, and would definitely be a good idea to have a helper reading instructions and handing parts and tools to the diver. You definitely want someone doing it who is careful about reading instructions, including the note to save the spacer ring from the prior prop.
+1
An experienced diver should not need your presence to do the install however. Flex-O-Folds are quite easy to install, once you know what you are doing.

Quote:
Originally Posted by CarpenterSF View Post
Though a zinc was included with the prop, I ordered a couple spares from the dealer (North Harbor Propeller, aka GetAProp.Com, very nice and helpful folks). The zincs were about $40 apiece, and various other notes on the net say that they go away pretty quickly, with various alternatives offered.
Flex-O-Fold zincs are about 20 bucks here:

FX Flexofold Propeller Zinc Anode

These zincs are relatively large pieces of metal and in my experience, should last quite a long time. If you are getting less than 9 months or a year out of yours, I suggest that there is an electrical issue aboard your boat that needs looking into.

Last edited by Fstbttms; 11-25-2010 at 01:05 PM.
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post #6 of 6 Old 06-05-2011
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I launched the boat a couple of weeks ago with a new flexo-fold prop and what a difference!! The boat now will do 7.6 under power where before it would barely do 6.5 kts. It also has reverse! Thanks for the info!
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