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Old 07-08-2014, 04:03   #16
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Re: Bigger diameter 2-blade vs. 3-blade

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Originally Posted by mcerdos View Post
Two 3 blade props are going to be very costly. In addition, you have haul out fees for the installation.
There is almost never a reason to haul a boat to remove or install propellers, especially a boat with saildrives. This kind of work is easily done while the boat is in her slip by a knowledgable diver with the right tools.
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Old 07-08-2014, 05:12   #17
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Re: Bigger diameter 2-blade vs. 3-blade

Has the OP contacted Volvo and or FP, the manufacturer of the Orana? Many FP's leave the factory with folding props. There are many FP's in the charter fleet I work for that have folders. Volvo sells them. I don't know if they are making the props or just reselling another brand. As stated, no need to haul to replace them.


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Old 07-08-2014, 05:56   #18
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Re: Bigger diameter 2-blade vs. 3-blade

Interesting idea, keen to see what the experts think, having a folding prop myself. from experience a two bladed folder can work very well. I delivered a 45 foot motorsailer with a two bladed folding prop. I was very sceptical about it, but found it had tonnes of grunt in ahead and astern. We punched our way into all sorts of stuff as if she had a fixed 3 blader, infact I thought she was way better than most boats I have been on.

I finally got to see the prop. It was a big wide thing. the blades where like plates, almost round. The swept area was massive unlike most yachts props with narrow skinny blades.

Another boat had an oversize and overpitched 2 bladed folding prop to give more efficiency motor sailing in the light stuff. It was pretty crap at punching into anything.

he has since reduced the pitch and it works much better. got to find out if he also reduced the diameter.

A big benefit of the folders for your cat is improved steering control under sail. A big spinning fixed prop right ahead of the rudder really messes up the flow.
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Old 07-08-2014, 07:12   #19
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Re: Bigger diameter 2-blade vs. 3-blade

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Originally Posted by Snowpetrel View Post
.

A big benefit of the folders for your cat is improved steering control under sail. A big spinning fixed prop right ahead of the rudder really messes up the flow.

On the Orana, the rudder is in front of the prop.


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Old 07-08-2014, 10:17   #20
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Re: Bigger Diameter 2-Blade vs. 3-Blade

Thanks for the replies.

First, you inspired me to do the installation myself. It seems dead easy. Flexofold has a foolproof locking mechanism and even comes with waterproof thread locker pre-applied for underwater installation. Main challenge will probably to get the old ones off as Volvo's fixed props has no locking mechanism. Just force and thread locker I guess.

Btw, the boat has a 4' draft and I'm 6'7" tall. The props are 3' from the stern where the hulls are real narrow. I can reach the props and still have my mouth above water.

I found an site that had a formula to calculate the most efficient diameter for a boat propeller:

D = (632.7*SHP^0.2)/RPM^0.6
SHP = Shaft horse power
RPM = shaft RPM, ie after gearbox reduction.

If I plug in my number I get to 16,53". Flexofold is recommending me 17", 2- or 3-blade. I also found another site with a graph, and again I got to the same (16,5") diameter.

So maybe a chicken out trying an 18" 2-blade, even if I think above formula should have an input for how many blades the propeller will have. Looking at airplane sites, they often debate, Bigger diameter 2-blade or smaller diameter 3- or 4- blade.
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Old 07-08-2014, 11:00   #21
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Re: Bigger Diameter 2-Blade vs. 3-Blade

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Originally Posted by Sail IC View Post
Thanks for the replies.

First, you inspired me to do the installation myself. It seems dead easy. Flexofold has a foolproof locking mechanism and even comes with waterproof thread locker pre-applied for underwater installation. Main challenge will probably to get the old ones off as Volvo's fixed props has no locking mechanism. Just force and thread locker I guess.

Btw, the boat has a 4' draft and I'm 6'7" tall. The props are 3' from the stern where the hulls are real narrow. I can reach the props and still have my mouth above water.

I found an site that had a formula to calculate the most efficient diameter for a boat propeller:

D = (632.7*SHP^0.2)/RPM^0.6
SHP = Shaft horse power
RPM = shaft RPM, ie after gearbox reduction.

If I plug in my number I get to 16,53". Flexofold is recommending me 17", 2- or 3-blade. I also found another site with a graph, and again I got to the same (16,5") diameter.

So maybe a chicken out trying an 18" 2-blade, even if I think above formula should have an input for how many blades the propeller will have. Looking at airplane sites, they often debate, Bigger diameter 2-blade or smaller diameter 3- or 4- blade.
This is funny, seaplanes use the biggest prop you can have. The limit seems to be a tip speed at a bit over the speed of sound where things go wacko.
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