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1979 C&C 30 Mk I - 2QM15
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Discussion Starter #1
This is a newbie question I'm sure but here goes...

At lot of the late 70's early 80's boats I'm looking at right now have the standard OEM fixed 2 blade props they came with from the builder. And since many of these boats are on the hard for viewing, it seems like it might be an opportune time to upgrade the prop before they get splashed.

Is there a standard formula that calculates the most efficient prop you can use with parameters like: Engine horse power, Engine max rpm, Shaft diameter, etc...

I'm also curious about things like:

When should you (or can you) use a 3 blade prop vs. a 2 blade?

How much of a difference in speed does a folding prop really make under sail?

If you need to... Assume a 1980 deep fin keel - tall rig Catalina 30 as a baseline platform for the sake of argument.
 

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When should you (or can you) use a 3 blade prop vs. a 2 blade?
Anytime you want 33% more drag under sail you can use a 3 blade, and a folder on a 30 year old cruiser is perfect for when you got way too much money burning a hole in your pocket.
 

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1979 C&C 30 Mk I - 2QM15
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172 Posts
Discussion Starter #3 (Edited)
Ok... So I understand why a 3 blade would be a bad idea, although I have seen them on out of the water cruisers and never really understood why some people use them. And I assumed that a folding prop (expense aside) would only be good for a few tenths of a knot at best, I just wanted some validation on that. And I'm aware of how most folding prop designs create added headaches when backing up.

But that still doesn't really address my original question, so maybe I should rephrase it, since I'm basing that on an assumption as well...

My assumption was that the OEM props that came on boats from the late 70's early 80's might not necessarily be the most efficient prop that could be used. (This may or may not be true).

But if it is true, I was wondering if there's a formula that can be used to determine the most efficient prop size and pitch for any given application based on the variables I mentioned in the first post?
 

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Like most things on boats a prop is a compromise, gotta be able to get the boat moving (first gear) without too much strain on components and still achieve a reasonable hullspeed without running the engine too fast (high gear). Many people like 3 bladed props 'cause they believe them to motor better, I'm sure engineers have some way to calc. a range of props that would be appropriate for a boat and engine, no propeller is 100% "efficient", they all have some "slip".
As long as the prop moves the boat at an appropriate speed why fix something that isn't broke ?
BTW OEM props can change, my boat was built in 1993 and came with a 2 blade, my friend has a 2006 model of the same boat and it came with a 3 blade. More people want 3 bladed props now than in the 90's.
 
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