Fixed vs. Folding Props: How Much Difference? - SailNet Community

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  #1  
Old 07-13-2007
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Fixed vs. Folding Props: How Much Difference?

The boat we're having surveyed mid-week has a folding prop on it. (The PO raced the boat.) He said he still had the fixed prop and he'd give it to us. I'm considering asking him if the fixed prop could go on while the boat's up in the sling for the haul-out survey, if the initial report from the surveyor doesn't contain any show-stoppers. We'd still have the sea trials to go, but I strongly suspect that if neither the stuctural survey or the haul-out survey stops the deal, the sea trials aren't likely to, either.

But before I ask so bold a question of the PO...

From a standpoint of sailing: How much difference in boat speed does a folding prop make? (The boat's a 30' Pearson P30.)

From a standpoint of motoring: What are the performance implications of a folding prop vs. a fixed prop? For example: When we finally get to cruising, there are times we'll have to go up-river against 2-4 kt. surface currents for 32-40 miles. What about in dockside maneuvering?

The point is: If the folding prop isn't going to hurt motoring and (under power) maneuvering "much," but will noticeably improve performance under sail: I'd prefer to keep it. But if the performance under sail wouldn't be all that much improved, and motoring performance and maneuvering will be significantly negatively impacted: Perhaps I ought to go ahead and ask.

TIA,
Jim
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Old 07-13-2007
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Keep the folding prop, your boat will perform much better as a sailboat than with the fixed prop. The lost of power is maybe 10%.

If you ever have to go against 2-4 kn current for 32-40 miles in a 30 foot sailboat, stay home. FWIW one way to duck the currents where the shore line permits, is get into shallow water along the shore where the current dies out...
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You want the folding prop. Not because it's so much faster, but because it folds under sail. A fixed prop is either constantly spinning and making a racket (if you're trying to sleep in the quarter birth) or you have to remember to lock it, in reverse.
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Old 07-14-2007
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Take both, use both, evaluate the difference for yourself. They're easy to change underwater, you have a spare, and who knows, maybe can sell it for $50 bucks at a swapmeet. Sancho Pancha said that if a man offers you a horse, run for the reins.
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I'd go for both... especially if you prefer to sail, rather than firing up the iron genny every time the boat speed drops below four knots... then the folding prop is well worth it..
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Hey Sd, it's 0213 hrs on the east coast...... GO TO BED!!!! Give us other guys a chance
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Quote:
Originally Posted by sailingfool
Keep the folding prop, your boat will perform much better as a sailboat than with the fixed prop. The lost of power is maybe 10%.
That seems to be the consensus.

Quote:
Originally Posted by sailingfool
If you ever have to go against 2-4 kn current for 32-40 miles in a 30 foot sailboat, stay home.
Here is a Landsat image of Lake St. Clair, the lake upon which we'll be docked. (Compliments of WikiPedia.)



LSC is 26 miles long at its longest, 24 miles wide at its widest. Much of it is not navigable by sailboats due to lack of depth. See those rivers to the NE and SW? Those are the St. Clair and Detroit Rivers (they're really straits). They're 40 and 32 miles long, respectively. You have to use them to get in and out of LSC. In a boat, anyway. On the water. So, it's either stay in LSC or deal with their currents. I'll deal .

Quote:
Originally Posted by sailingfool
FWIW one way to duck the currents where the shore line permits, is get into shallow water along the shore where the current dies out...
Thanks for the hint. Yes, I've heard of that before. That's why I wrote "2-4 kts," instead of "4 kts": It depends on where you are in the river. Same with the St. Clair River, tho on that one there is at least one spot with a 6 kt surface current. (I think I'll just avoid that .)

In fact: I'm told that, on the St. Clair River, if you stick to the sides, there's actually a "reverse current" in some parts.

Btw: See that bit of land sticking out near the upper left? We'll be docked very close to that.

Jim
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Okay, I'll just leave it w/the folding prop. Thanks for the feedback. As usual: Sailnet's community comes through.

I really cannot imagine what this journey would've been like w/o Sailnet. Certainly I would've been a helluva lot more clueless than I am .

Jim
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Out of curiosity, find out what kind of props you have. Some are better under power than others. A Pearson 30 is about as representative a '70s design as can be imagined (like a Catalina 30, but less common), so I have a good idea of your hull shape. Let's assume you have an Atomic 4 or a Yanmar 18 HP as a motor. The fixed prop will likely be a 12 x 6 or 7 Michigan Wheel two-blade, with about an inch of tip clearance. The folder will likely be a Gori or a Martec two-blade of about 11 x 8. This means you will need more revs to get the same speed with the folder, but you will have slightly more thrust and a finer control at low speeds. This is desirable in docking and maneuvering. Allow me to give you an example: With a fixed prop, my "dead slow" in forward was about 2.6 knots. Consequently, I learned to shift into neutral and coast into dock, depending on winds blowing me on or off. I could, however, make 5.6 knots in flat water at half-throttle. At full throttle I could make 6.5, but cavitation and the noise made it hardly worth it.

By contrast, my "dead slow" with a Gori folder was 1.5 knots, and my stopping and reversing power were increased by about 25%. I also suffered less from prop walk, which meant that I actually used the engine MORE with a folder, because I could maneuver with more throttle control when I had options under 2 knots of speed over ground. I would also reverse off the dock instead of simply pushing the boat backwards with the engine in neutral and then hopping on amidships and throwing the tiller over until my bow had cleared and I could push the shifter into forward.

As for sailing, I gained approximately 1/2 knot in speed under equivalent wind conditions. I actually had to mentally shift my sail changing "bands" in the sense that I would opt for a No. 2 sail if the wind was over 12 knots for a daysail whereas I would have waited for 14 knots with the old fixed prop. The reason? I had less resistance under the boat and its sailing characteristics became noticeably livelier (as did its acceleration) in light-to-medium airs.

My experiences with a folder contributed greatly to our decision to purchase a feathering VariProp for our ocean cruiser...the pluses greatly outweigh the few minuses in my view.

But don't throw out the fixed prop...if the folder is damaged, you've got a solution one hour in a TraveLift can effect.
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It really depends on the manufacturer of yor props.

Some folding props are really bad and crappy, and I'd rather have a good fixed than a bad folding.

Same the other way around.

So beware, just because it folds does not mean its good, same the other way around.

I vote for folding, off course, but a GOOD ONE, not a crappy one.

What brands are they?

edit...Val beat me to it
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