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· 1979 C&C 30 Mk I - 2QM15
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Discussion Starter · #1 ·
A racing trick that a lot of racer's without folding props use to reduce drag while sailing is to mark the prop shaft so that someone can jump down into the cockpit locker and set the prop to be vertical (in-line with the keel) to reduce drag. (This assumes that you have a 2-Blade prop).

What's something you can mark the metal shaft with that will be resistant to water, oil, etc..., but not something that will damage the metal?
 

· 1979 C&C 30 Mk I - 2QM15
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Discussion Starter · #7 ·
You might want to let it spin if you're not on a full keeler..

Prop Drag Test Video (LINK)
The only problem with that test is that it eliminates the keel and it's effect on the water turbulence in front of the prop. So it doesn't accurately simulate the conditions of drag for a sailboat.

Even without a full keel, a fin keel will still change the water flow in front of the prop. I've never seen a published test, but I know on a race boat, setting the prop has a measurable effect on boat speed. Not something a cruiser is going to notice, but in a race where tenths of a knot can determine the outcome, it makes a difference.
 

· 1979 C&C 30 Mk I - 2QM15
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Discussion Starter · #10 · (Edited)
No need to get defensive... I just pointed out the very obvious fact that the original test you cited drags a fixed prop through the water in no particular orientation vs. a free wheeling prop while completely leaving out the specific test condition that was in my original post of "vertically aligning" a "2-Bladed" prop with a leading keel and the overall effect on parasitic drag.

The second study you cited (even with the collective brainpower of MIT), also doesn't vertically align a 2-blade fixed prop with the keel. It just tests them in no particularly cited orientation in fixed vs. free wheeling mode.

And the last study you cited only graphed out the drag of 3-blade props that could in no way be aligned with the turbulence pattern of a straight vertical keel.

It's all good info, but it's just not relevant to what I stated in my original post, which was the common routine of racers with fixed 2-blade props to set them so that they're aligned with the keel. If you've got a study that cites that particular condition and shows a negligible, or negative affect on drag, I'd be more than happy to read it. Because I didn't devise or propose the theory, I just know that many racers follow it...
 

· 1979 C&C 30 Mk I - 2QM15
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Discussion Starter · #15 ·
I would SWAG that "MOST" racers today will skip the fixed prop and buy a feather/folding prop vs using a fixed prop in ANY position be it aligned or not with the keel.

Marty
I guess that really depends on whether or not dropping $2500-$3000 on a folding prop is a trivial thing for your wallet... But I know plenty of racers that use 2-Blade fixed props. I don't know for sure, but it might have something to do with One Design rules that don't allow you to change anything from the original design on the boat. J-Boats for example (I think) all come with fixed props.
 

· 1979 C&C 30 Mk I - 2QM15
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Discussion Starter · #16 ·
While sailing my Bristol 24 this weekend in light wind; I checked my 2 blade prop alignment and found the mark to be 90 degrees from verticle. I watched my knotmeter as I turned the blade to straight up and down. The speed increased 2/10s as soon as the blade was verticle. I moved it back to sideways, and the speed dropped 2/10s. I have a 2 blade, 12" diameter x 14P bronze prop. The prop is completely enclosed in a cutout behind the full keel.
This would be in line with what most of the J-105 racers believe the difference is in boat speed with an aligned set prop vs. one that's left to free wheel. (A couple tenths).
 

· 1979 C&C 30 Mk I - 2QM15
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Discussion Starter · #26 ·
On my old C-36 I had a two blade fixed Michigan wheel. The boat was faster with the prop freewheeling vs. locked in any position. I experimented with this for a full afternoon with some racing buddies who doubted this. In the end they agreed the boat was faster freewheeling sometimes by as much as almost a half knot in lighter winds when we were not already near hull speed.

...Every ounce of credible scientific data suggests that a freewheeling Michigan Wheel style prop causes more drag locked.
This was really what I was looking for... A test that involved actually aligning the blade with the keel. And if you were seeing a 1/2 knot difference in drag, then I have no reason not to believe your results.

Oh, and btw... you're right on the J boat Martech's, (my bad). But I know several J boat skippers that still set the prop shaft orientation even though it's a folding prop.
 
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