2 vs 3 blade prop
- Thread starter flyingcr
- Start date
I would guess that a 2 blade prop is adequate. You might find smoother operation with 3 blade but 2 blades are more efficient - better application of power. I assume that cost may be a consideration, so 2 blade is more economical, especially if considering a folding prop. I'd check with your nearest Volvo penta dealer for selection.
It depends; the three blade prop results in more drag when under sail but cuts down in prop walk and may deliver smoother operation at low speed. I'm personally one that wants that predictable prop walk to assist in docking.
It depends on what you mean by BEST, and if its a folder.
A three blade fixed will torture your light air sailing; a two blade fixed - a bit less so.
If you care at all about performance, get a folder.
I have a 2 blade folding Flexofold on my First 260 and love it. If you read this very comprehensive review there is little performance difference between the 2 and 3 bladers in most tests.
http://www.flexofold.com/upload_dir/docs/Test_YachtingMonthly_low.pdf
A three blade fixed will torture your light air sailing; a two blade fixed - a bit less so.
If you care at all about performance, get a folder.
I have a 2 blade folding Flexofold on my First 260 and love it. If you read this very comprehensive review there is little performance difference between the 2 and 3 bladers in most tests.
http://www.flexofold.com/upload_dir/docs/Test_YachtingMonthly_low.pdf
I read the flexfold "test" last winter. I viewed it kind of as propaganda. If your in the business of selling folding props, which are not cheap, how does one sell more stuff.
If a 3 bladed prop costs X & a 2 bladed prop is considerably less than X, what better way to sell more 2 bladed props.
What do we know?
A 3 bladed prop gives us more bite when motoring but more drag when sailing.
The folks that go the EP route want the biggest 3-4 bladed prop they can fit. Reason being slow turning big wheel with bite equals less amperage & draw on the propulsion battery bank. When sailing they allow the wheel to spin so there's no draw on the battery bank thus getting a defacto folding prop.
I realize I'm comparing apples to oranges but if 2 bladed props were efficient than most folks going the EP route would have a 2 bladed prop under their boats.
Make sense?
Bob
Propeller efficiency is not a static condition, so it is really not possible or necessary to split hairs over the efficiency of a 2-blade vs. 3 or 4 blade prop for a sailboat. General rules indicate that, except for high speeds (sailboats are definitely NOT high-speed), the most efficiency is provided by high-diameter, slow RPM, least blades. A single-bladed prop with higher diameter and slow RPM is theoretically 'more efficient' than equivalent 2 or 3 blade props. But nobody would choose that because the imbalance would drive you nuts! Also, the slower RPM is probably bad for our engines. Factors that also influence the selection is the clearance with the hull, which limits diameter. Obviously, if calculations are performed to determine the thrust between 2-blade and 3-blade props, the diameter limitations have to be considered. The clearance between the hull and propeller tip, if a factor, should be no less than 2", I believe.
Generally speaking, for a sailboat under 30', I can't see any reason to consider a 3-blade prop for an auxiliary engine. It's a sailboat. Why increase drag while sailing to accommodate possibly some smoother motoring operation? But, I would also take the advise from the engine manufacturer first.
I don't think there is any equivalency between a free-spinning 3 wheel prop vs. a folding prop, so I don't get the de-facto reference in that regard. Folding or feathering blades obviously reduce drag significantly compared to fixed-blades.
I found this article to be pretty helpful to explain in layman terms .... http://www.psychosnail.com/boatingarticles/boatpropellers
Generally speaking, for a sailboat under 30', I can't see any reason to consider a 3-blade prop for an auxiliary engine. It's a sailboat. Why increase drag while sailing to accommodate possibly some smoother motoring operation? But, I would also take the advise from the engine manufacturer first.
I don't think there is any equivalency between a free-spinning 3 wheel prop vs. a folding prop, so I don't get the de-facto reference in that regard. Folding or feathering blades obviously reduce drag significantly compared to fixed-blades.
I found this article to be pretty helpful to explain in layman terms .... http://www.psychosnail.com/boatingarticles/boatpropellers
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make sense? no.
First the 'propaganda' test was conducted by a well respected UK sailing journal, with reps from each of the prop manufactures there to install their prop and witness the test. They are the first to point out that its a test on one boat, but what else can you do?
AS far as I'm concerned there is no such thing as a 'best prop'. Listen to what people that have your boat and sail like you like. Get that. My boat model replaced the OP's boat. If he sails like me he'll like the flexofold. I'm betting.
No one said 2 blade was universally better. 3 blades often (but not always) gives better/faster start/stops. But as mentioned, fewer blades are often more efficient.
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He is planning electric propulsion so the prop will have almost no mechanical resistance if it is unpowered.
Unless racing, I've always opted for the most blades I can get. A 3 is better balanced and has less flex than a 2 blade prop. A 4 blade is even better. I once designed and had a 5 blade built to improve sailing performance and lessen vibration under power. 5 feather shaped blades worked very well.
I disagree about a 2 blade having more power unless we are just using different terms. A 3 or 4 will have much more thrust at slower speeds than a 2. This is much better for close quarters maneuvering; being able to kick the stern around, etc.
The more blades, the less wobble on the shaft and the less vibration under power.
As for the correct prop for one's boat, the prop manufacturers are the ones to contact about that. If you have a common production boat with a standard engine, then they should be able to tell you exactly what you should be turning, on the spot. It takes a bit more to figure out with a custom boat or engine installation.
I disagree about a 2 blade having more power unless we are just using different terms. A 3 or 4 will have much more thrust at slower speeds than a 2. This is much better for close quarters maneuvering; being able to kick the stern around, etc.
The more blades, the less wobble on the shaft and the less vibration under power.
As for the correct prop for one's boat, the prop manufacturers are the ones to contact about that. If you have a common production boat with a standard engine, then they should be able to tell you exactly what you should be turning, on the spot. It takes a bit more to figure out with a custom boat or engine installation.
Three bladed will give you better power assuming that is a properly sized three blade prop but it will also create more drag when sailing. As a racer I have a 2 blade folding prop so less speed when motoring. If I was a cruiser I would have a three blade fixed prop so I could maximize my engine's power.
There may be more power with a 3-blade, but that doesn't necessarily translate to higher speed. I don't think there is a difference between 2 & 3 blade with regard to speed. I suppose it might if you are pulling something, like a dinghy, with a lot of drag. I agree that there are advantages that a 3-blade has over a 2-blade. For a Beneteau 265, I still think that a 2 blade folding prop would be the clear winner for sailing and cruising. For a fixed blade on *this* sailboat, I think I would still use 2 blades, unless there is some other compelling reason to suffer the additional drag.
Not necessarily speed, but definitely thrust. It is just physics - more blade surface area moving water. I have a high-thrust prop for my little Tohatsu 9.8 - four blades instead of the original 3. The end result is more initial speed to get me up on plane with a full load... quickly.
My thoughts...If you are racing, day sailing, or care about light wind sailing AND you can't buy a folding prop go with the two blade prop.
If you are cruising and you spend some time under power then you want the three blade prop.
Everything on a boat is a compromise.
If you are cruising and you spend some time under power then you want the three blade prop.
Everything on a boat is a compromise.
Props are measured by their diameter and their rated inches through the water per revolution (24X18 for instance is 24" diameter and 18" of forward movement per rev), so the pitch will have much more to do with speed than the number of blades or diameter. Add to that the gear box ratio (very important info when seeking a new prop) and you could pull the Queen Mary with a pretty small boat very slowly, or get that same boat up on a plane differently propped and geared but at the same RPM.
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Well, Capta is accurately stating that boat speed is a function of RPM and pitch (and slip). Speed isn't directly related to the number of propeller blades or diameter. Of course there are several variables that affect overall performance, but a 3rd blade doesn't necessarily equate to higher speed over a 2-blade prop. Check out the charts from post #4. That pretty much says that the speed differential between 2 or 3 blades is negligible.
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Referring to Capta and Scott's comments about relationship of pitch to speed - the previous owner installed my Max-Prop 3-blade, it is a common Beneteau upgrade. He fiddled endlessly to find the best pitch adjustment of these adjustable pitch propellers finally settling on the one that just got him to the right engine speed while maximizing the pitch on this B411. It was not the pitch specified by the manufacturer.