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Old 06-08-2014, 11:39   #1
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Bigger Diameter 2-Blade vs. 3-Blade

Bought a catamaran a couple of months ago, and the fixed props are driving me crazy. Not only the whining noise from them rotating (SD drives, has to be in neutral), but I also get significant vibrations especially when sailing between 6-8 knots changing speed in following seas.

So I plan to change to folding props and more or less decided on flexofold but for my question/thinking its independent of brand.

Folding (and feathering) props are kind of pricy (need two on a cat) and a 3-blade prop is 50% more expensive than an equivalent 2-blade. The benefit of the 3-blade is that it has more bite, gives a higher thrust, mostly from that it has more surface area. Better maneuverability.

Standard prop size and also what flexofold recommend is 17" dia. However, there is plenty of space between the propeller tip and hull (130mm, 5"), so from what I understand it could easily house a bigger diameter propeller.

So if I go with an 18" 2-blade prop, maybe I get the best of both worlds. Better maneuverability than a 17" 2-blade, and maybe not that far from an 17" 3-blade. Then there is the cost saving over a 3-blade

In addition, a 2-blade is more efficient when motoring and lower drag when sailing. Some extra bonus.

The boat is a Fountain Pajot, Orana. 44 foot catamaran, 9.8T displacement, 2x 40 Volvos on saildrives.

What do you think?
Am I on the right track?
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Old 06-08-2014, 11:47   #2
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Re: Bigger diameter 2-blade vs. 3-blade

Two 3 blade props are going to be very costly. In addition, you have haul out fees for the installation. The props must be pitched correctly by the manufacturer or you will not be able to obtain full RPM on your engines.

Have you considered a brake installed at the coupling? This would be a lot less expensive and eliminate your prop spinning while under sail.

Also, I would recheck with your SD manufacturer. Amel's use SDs and can use reverse gear as a shaft brake when underway.
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Old 06-08-2014, 12:49   #3
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Re: Bigger diameter 2-blade vs. 3-blade

Props spinning under sail reduce prop drag by a good 50% versus stationary.

Locking his props would be like dragging a bucket in comparison. The Flexofolds will reduce drag by 90% of a stationary prop.

I believe a call to Flexofold might be in order.
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Old 06-08-2014, 13:02   #4
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Re: Bigger diameter 2-blade vs. 3-blade

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Originally Posted by Seafarer24 View Post
Props spinning under sail reduce prop drag by a good 50% versus stationary.

Locking his props would be like dragging a bucket in comparison. The Flexofolds will reduce drag by 90% of a stationary prop.

I believe a call to Flexofold might be in order.
Forgot to say that we have an AutoProp 3 blade folding prop.

To clarify, I think the OP will still find prop spin an issue unless a way to lock the prop is installed.
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Old 06-08-2014, 13:02   #5
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Re: Bigger diameter 2-blade vs. 3-blade

What size shafts? You don't want to push the prop diameter too far without the right shafts. 2 blade props are not more efficient last I heard, less efficient..? But I don't think it matters unless you have minimal HP and thrust at this time. I like the reverse thrust of a feathering myself, but I guess some of the newer folders are good too. In the overall scheme of things, I'm not sure the amount of drag by a couple 2 blade props is that much compared with the drag of the whole boat. No way to brake the shafts?
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Old 06-08-2014, 13:33   #6
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Re: Bigger diameter 2-blade vs. 3-blade

[QUOTE=Seafarer24;1600586]Props spinning under sail reduce prop drag by a good 50% versus stationary



Are you sure about that? I only know for sure about airplanes so I would take a stationary prop over a prop wind milling anyday. Air and water are both fluids so why would there be a difference between the two?
It makes a dramatic difference in how far you can glide when you stop the engine and prop.
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Old 06-08-2014, 13:54   #7
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Re: Bigger diameter 2-blade vs. 3-blade

[QUOTE=Guy;1600608]
Quote:
Originally Posted by Seafarer24 View Post
Props spinning under sail reduce prop drag by a good 50% versus stationary







Are you sure about that? I only know for sure about airplanes so I would take a stationary prop over a prop wind milling anyday. Air and water are both fluids so why would there be a difference between the two?

It makes a dramatic difference in how far you can glide when you stop the engine and prop.

Yeah, we have sort of beat that to death already, I assume it has to do with the cord of a boat prop being so wide?
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Old 06-08-2014, 14:03   #8
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Re: Bigger diameter 2-blade vs. 3-blade

Thanks for the replies.

Given that I have a saildrives, I don't think there is a way to create a shaft brake. It's all integrated. Also, according to a sizable propeller test done by Yachting Montly in 2009, doing drag tests a locked propeller creates twice the drag to a free spinning one and accounts for 25% drag of a 32 foot boat at 5 knots. And I have two props.

Volvo specify that for their saildrives fixed prop must be free-spinning and a folding should be locked in reverse.

So I will change the props. Reason going with a folding vs. feathering is cost and lower maintenance. Flexofold is water greased, so the only maintenance needed is cleaning (I will not haul out every year).

From what I have read, the pros of 2- vs 3-blade props are:'

2-blade: Higher top speed/Lower fuel consumption, Less drag when sailing, Lower cost
3-blade: Better maneuverability, better motoring in strong headwind / sea

So my thinking is .... As I have so much space between the prop and hull, by going up in diameter on a 2-blade prop, I can maintain the benefits of the 2-blade and add some of the goodness of a comparable smaller diameter 3-blade.
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Old 06-08-2014, 15:28   #9
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Re: Bigger diameter 2-blade vs. 3-blade

Comments:

1. Our F-O-F three blade prop is now 10 years old and still in excellent, as new condition. It has performed exactly as advertised, and we are quite happy with it. However, the literature that came with it says to grease the gears that link the blades when the boat is slipped for painting, and we have done so. An easy job, too. The only problem has been shell growth in the gears causing some jamming. In our case, simply getting in the water and moving the blades by hand a couple of times has completely cured that issue. All in all, one of the most honestly advertised and well engineered products we've ever had on a boat.

2. Two blade props are indeed theoretically more efficient than three blade. Whether this is a noticeable difference is likely dependent upon many factors outside the prop itself, things like water flow in the area of the prop, speed (fast planing boat or displacement hull), other appendages nearby and so on. Not easy for a non naval architect/prop specialist to determine on paper.

3. I found the folks at F-O-F to be approachable, so will second the suggestion to contact them re the tradeoffs that are concerning you. I for one would appreciate your posting what they tell you if you do so.

4. And for mcerdos, no, prop spin will not be an issue with the F-O-F. You simply pop the tranny into reverse with the engine stopped, there is a "clunk" as the blades close, and all rotation stops instantly. Then if one wishes, the tranny can be returned to neutral. With the blades closed, there is no torque produced to induce spinning, and the will remain still.

Cheers,

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Old 06-08-2014, 20:32   #10
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Re: Bigger diameter 2-blade vs. 3-blade

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Originally Posted by Jim Cate View Post
Comments:


4. And for mcerdos, no, prop spin will not be an issue with the F-O-F. You simply pop the tranny into reverse with the engine stopped, there is a "clunk" as the blades close, and all rotation stops instantly. Then if one wishes, the tranny can be returned to neutral. With the blades closed, there is no torque produced to induce spinning, and the will remain still.
Jim, The op stated in an earlier post the SD instructions will not allow him to put it in gear. This is why I stated that the OP should check with the manufacturer as most SDs can be place in reverse to avoid prop spin. Just switching to a folding prop without the ability to stop the spin, even for a short time will not prevent prop spin. Hope this is clearer for you.
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Old 06-08-2014, 20:41   #11
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Re: Bigger diameter 2-blade vs. 3-blade

Quote:
Originally Posted by mcerdos View Post
Jim, The op stated in an earlier post the SD instructions will not allow him to put it in gear. This is why I stated that the OP should check with the manufacturer as most SDs can be place in reverse to avoid prop spin. Just switching to a folding prop without the ability to stop the spin, even for a short time will not prevent prop spin. Hope this is clearer for you.
Popping the SD into reverse for the purpose of stopping the shaft rotation MOMENTARILY should not hurt anything. Once folded, which takes but a fractional second, the trans can be returned to neutral, although with no torque it really shouldn't matter. I believe that the admonition to not use reverse is for fixed props where the continuous torque will cause the cone clutches to slowly slip and wear.

Checking with the OEM is always a good idea, if one can get hold of someone who actually knows something rather than parroting general info.

Cheers,

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Old 06-08-2014, 22:20   #12
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Re: Bigger diameter 2-blade vs. 3-blade

[QUOTE=Guy;1600608]
Quote:
Originally Posted by Seafarer24 View Post
Props spinning under sail reduce prop drag by a good 50% versus stationary



Are you sure about that? I only know for sure about airplanes so I would take a stationary prop over a prop wind milling anyday. Air and water are both fluids so why would there be a difference between the two?
It makes a dramatic difference in how far you can glide when you stop the engine and prop.
The difference is that an airplane prop is bolted to the crankshaft. Put your transmission in gear and sail fast enough that the prop is turning your engine over, and then you're talking about the same thing.
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Old 06-08-2014, 23:13   #13
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Re: Bigger diameter 2-blade vs. 3-blade

[QUOTE=Dustymc;1600934]
Quote:
Originally Posted by Guy View Post

The difference is that an airplane prop is bolted to the crankshaft. Put your transmission in gear and sail fast enough that the prop is turning your engine over, and then you're talking about the same thing.
Oh, I thought in the spinning disk area of a propeller, the entire diameter of the prop was drag, not the surface area of just the prop blades if they were stopped.. Unless the prop is driving the boat of course, then it does not matter.
So you are saying, with a 2:1 gearbox, the the prop can turn over the engine if my boat is doing 6 knots? I don't think so but that would be really nice. No more worries about dead batteries or a bum starter.
Thats probably not what you are saying.
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Old 06-08-2014, 23:57   #14
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Re: Bigger diameter 2-blade vs. 3-blade

[QUOTE=Guy;1600948]
Quote:
Originally Posted by Dustymc View Post

Oh, I thought in the spinning disk area of a propeller, the entire diameter of the prop was drag, not the surface area of just the prop blades if they were stopped.. Unless the prop is driving the boat of course, then it does not matter.
So you are saying, with a 2:1 gearbox, the the prop can turn over the engine if my boat is doing 6 knots? I don't think so but that would be really nice. No more worries about dead batteries or a bum starter.
Thats probably not what you are saying.
What I'm saying is that unless you have a disconnect on your airplane or a REALLY big prop on your boat, it's probably not a great comparison.

If you have a big enough prop on a small enough engine, then yea, it should spin...

When your airplane prop is spinning, it's making enough lift to turn your engine over. When your sailboat prop is spinning, it's making enough "lift" to turn bits of a small transmission.

On my boat I can gain a bit of speed by letting my prop spin, I think, maybe, sometimes.

I don't have enough math to really understand the drag effects of disk area, or the difference between pushing an 80" prop through air at 60MPH vs. a 16" prop through water at 6MPH, turning or not. My primary flight instructor did have a fetish for yanking on mixture knobs, and the difference between spinning and stalled "feels" more significant in an airplane. Part of that feeling may well be explained by the psychological effects of a silent airplane on a 20-hour pilot....
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Old 07-08-2014, 00:11   #15
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Re: Bigger diameter 2-blade vs. 3-blade

Guys, the subject of fixed vs spinning prop drag on SAILBOATS has been beaten to death, here on CF and in many other places, some of which actually have knowledgeable people involved. It seems to be well demonstrated that in the case of SAILBOATS the spinning prop causes less drag. The aircraft analogy is not valid, so forget about it.

A folding or feathering prop will cause far less drag than a fixed prop, spinning or not, and that too is well established. On most boats it is significant, and that's why lots of folks are willing to fork out the money for such props.

The OP is wondering about a specific question re relative efficiency and drag between a smaller three blade and a larger two blade. It is an interesting question, one which could affect the decisions of many boats with saildrives (where there is typically more tip clearance than in most shaft drive boats). Lets see if more applicable discussion comes up.

Thanks.

Jim
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