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Old 27-06-2017, 18:25   #1
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How many blades

I have a 2015 leopard 40 with 29hp yanmar and sd 50 saildrives. Looking to put feathering propellers.how many blades 3, 4, or 5?
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Old 27-06-2017, 18:27   #2
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Re: How many blades

You forgot 2 blade, cheaper and more efficient.
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Old 27-06-2017, 18:54   #3
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Re: How many blades

I have read that the four-blade provides the least vibration -- a quieter and smoother ride while in the midst of a rare motoring event. And that it does so with no loss in efficiency. I don't have one, however, so I'm just repeating something I read elsewhere. For what that makes it worth ..
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Old 28-06-2017, 19:47   #4
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Re: How many blades

2-blade Flexofold, in my opinion, gives you the optimal ratio of cost, ease of maintenance, and performance. If you go with the folding props option in a new Yanmar saildrive, that's what they will be.

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Old 28-06-2017, 22:06   #5
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Re: How many blades

Thanks for the help. I have some time before I buy so I do some research
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Old 30-06-2017, 05:27   #6
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Re: How many blades

As to the "rare motoring event," you'll be luckier than most if that's true!

Regarding folding props, I just had a conversation with a cat dealer who recommended never getting folding props (we were discussing 40-48' sizes) because they make manouvering in tight quarters in marinas difficult. This was the first time I'd ever heard this.
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Old 30-06-2017, 05:59   #7
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Re: How many blades

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As to the "rare motoring event," you'll be luckier than most if that's true!

Regarding folding props, I just had a conversation with a cat dealer who recommended never getting folding props (we were discussing 40-48' sizes) because they make manouvering in tight quarters in marinas difficult. This was the first time I'd ever heard this.
Not so.

Catamarans with folding props work just fine.

Catamarans are so efficient with folding props, because when the wind is down you only cruise with one engine.

Catamarans are one of the most efficient boats on the planet for their size, because of this.

We have backed into so many tight places with a 44 foot catamaran with absolutely no backing up issues. We have Volvo Penta 3 blade folding props.
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Old 30-06-2017, 06:51   #8
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Re: How many blades

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Not so.

Catamarans with folding props work just fine.

Catamarans are so efficient with folding props, because when the wind is down you only cruise with one engine.

Catamarans are one of the most efficient boats on the planet for their size, because of this.

We have backed into so many tight places with a 44 foot catamaran with absolutely no backing up issues. We have Volvo Penta 3 blade folding props.
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Thanks Mark,

We're vacationing in Florida and went to see the Leopard 40 & 48 (45 wasn't there). I'd never heard that about folding props, so I'm glad I could post it to share.

The 48 is a beauty, but 5.1' draft is a bunch, esp in the Chesapeake Bay.
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Old 30-06-2017, 07:04   #9
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Re: How many blades

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Thanks Mark,

We're vacationing in Florida and went to see the Leopard 40 & 48 (45 wasn't there). I'd never heard that about folding props, so I'm glad I could post it to share.

The 48 is a beauty, but 5.1' draft is a bunch, esp in the Chesapeake Bay.
Leopards are nice catamarans for sure, but the DRAFT is a BIG issue for most cruisers doing the USA east coast, ICW and Bahamas.

It almost has as much draft as the Catalina 380 mono keel boat we used to own with 5.6' draft.
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Old 01-07-2017, 01:58   #10
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Re: How many blades

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Originally Posted by tugz View Post
I have a 2015 leopard 40 with 29hp yanmar and sd 50 saildrives. Looking to put feathering propellers.how many blades 3, 4, or 5?
3 blades are more than sufficient, 4 and 5 blades don't add anything in terms of speed, or maneuverability or anything else on a 40' boat, it might be different for large boats (say over 60'), where the inertia of the boat during mooring operations must be contrasted by large blades surface.
On the other side 4 and 5 blades are much heavier (bigger inertia switching from fwd to bkwd), so they could damage the S-Drive gearbox of your S-Drive , they have more drag than a 3 blades (folding ones remain in a closed flower position and feathering anyway offer more drag because of more blades in the water).
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Old 01-07-2017, 03:09   #11
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Re: How many blades

To begin with a propeller there's some constants in play.

To be as efficient as possible:
1# choose the biggest propeller diameter with adequate clearance around (generally 10 to 15% of the diameter)
2# maximum power and rpm for the prop. From there you can determine the blade area. Too small is prone to cavitation, too much lessens the effiency.

Now you can choose the number of blades. How many depends of multiple factors thou best usually with 2 or 3. More only if there's problem with point 2#

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Old 01-07-2017, 03:39   #12
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Re: How many blades

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To begin with a propeller there's some constants in play.

To be as efficient as possible:
1# choose the biggest propeller diameter with adequate clearance around (generally 10 to 15% of the diameter)
2# maximum power and rpm for the prop. From there you can determine the blade area. Too small is prone to cavitation, too much lessens the effiency.

Now you can choose the number of blades. How many depends of multiple factors thou best usually with 2 or 3. More only if there's problem with point 2#

BR Teddy
Hi Teddy,

I agree with most of your comments, increasing slightly the diam is somewhat helping for sailboat propellers which are relatively slow in rpm, anyway the propeller diameter (and pitch) are something more complicated than filling the space minus 15% of diameter (your point #1), so the largest diameter that you can fit is not a good parameter.
A too large diameter also leads to cavitation, because blade speed is higher at the external edges of the propeller and higher speed = higher risk of cavitation
If needed you can calculate your own prop diam here: Calculate your propeller

BR
Sergio
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Old 01-07-2017, 11:23   #13
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Re: How many blades

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Hi Teddy,

I agree with most of your comments, increasing slightly the diam is somewhat helping for sailboat propellers which are relatively slow in rpm, anyway the propeller diameter (and pitch) are something more complicated than filling the space minus 15% of diameter (your point #1), so the largest diameter that you can fit is not a good parameter.
A too large diameter also leads to cavitation, because blade speed is higher at the external edges of the propeller and higher speed = higher risk of cavitation
If needed you can calculate your own prop diam here: Calculate your propeller

BR
Sergio
Hi Sergio
You are right of course. Thou most sail boats don't have so much room that too big would become a problem.

I need a quote sometime next winter :-)

BR Teddy
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Old 01-07-2017, 12:22   #14
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Re: How many blades

Two years on, we could not be more happy with our two blade Flexifold propellers. Less efficient maneuverability? Bah! These props were a distinct improvement over the stock Volvo fixed two blades.

Fixed blade props have to be a comprise between providing fwd / reverse thrust when running and not creating too much drag when not in use. Folding props eliminate the second constraint and can be optimized for the first.

As for vibration, we have never noticed any.

Why not let the on line Flexifold configuration program, (or helpful human representative) provide guidance based on vessel weight, engine HP, gearbox ratio, prop clearance, etc... They seem to know their business.
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Old 01-07-2017, 12:37   #15
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Re: How many blades

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You forgot 2 blade, cheaper and more efficient.


For aircraft and I assume boats, the fewer the blades the more efficient and lighter. More blades are used when you can't get more blade area by going longer due to RPM tip speed limitations or ground clearance. More blades are usually smoother though. Fewer blades are usually cheaper to buy and maintain as there are fewer moving parts.
Lots of three blade props are sold for small airplanes cause they look cool, same I believe for boats.
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