Flexofold size vs. fixed prop

[Davidasailor26]

We're considering switching from the 3 bladed fixed prop on our Beneteau 37 to a Flexofold 3 blade. I've reached out to our dealer and to Flexofold to find out what size they would recommend. The original fixed prop is 17" diameter x 12" pitch, 3 blades. Our dealer said they would outfit it with a 16x12 3 blade FOF. Flexofold themselves has recommended a 15x10. I'm curious why the Flexofold sizes, both from the dealer and especially from FOF are so much smaller than the OEM fixed size. Has anyone made this conversion and had similar size changes? If so, how has that worked out in terms of cruising speed under power?

Boat displacement is 14,400 lbs. Engine is Yanmar 3YM30. Currently we cruise at about 7.2 kts at 2775 RPMs.

 

[DougM]

Did you provide the dealer or Flex with the transmission gear ratio or are they perhaps taking a stab at that when they specified the prop?
If the original prop at 17 x12 is correct for the boat and you are satisfied with that, why would you not stay with the same diameter and pitch for the new prop?
In any case neither source is recommending a pitch change, but there is some difference in diameter, but an inch of clearance from the prop tips to the bottom of the hull.
There must be some logic driving either of the two recommendations.

 

[Davidasailor26]

Thanks Doug. I did provide FOF with the gear ratio (at least the one in the manual; I haven't gone back to the boat to verify what's stamped on the transmission). FOF did recommend changes to both diameter and pitch (10" pitch from FOF vs 12" fixed).

I agree there must be some logic behind the recommendations. I guess what I'm wondering is whether there's something about the geometry of the FOF that makes it more efficient at smaller sizes/pitches than a fixed prop - and if anyone has experiences to back that up.

 

[capta]

I would highly recommend that you do NOT put a folding prop on your boat. They use centrifugal force to open them and just a little fouling can inhibit proper unfolding. Feathering props like the Maxprop use GEARS to feather/unfeather them, and they function with almost any amount of fouling.

 

[Davidasailor26]

Capta, Thanks for the input. I was a bit concerned about this at first as well, but in the many threads I've read I can't find a significant number of folding prop users with that complaint. Also we're in an area of nearly fresh water that gets slime but basically no hard growth, so I wouldn't expect that to be a problem for us.

 

[BarryL]

Hey,
My boat (C&C 110) has a two bladed folding prop that is geared. I don't know who makes it but it's the OEM prop.

Barry

 

[whatfiero]

A folding prop is to improve sailing performance usually with slightly reduced motoring speed and delay in direction changes the smaller prop is usually so the folded prop will fit (folded) behind the rudder

 

[Davidasailor26]

A folding prop is to improve sailing performance usually with slightly reduced motoring speed and delay in direction changes the smaller prop is usually so the folded prop will fit (folded) behind the rudder

Good point about the rudder. I hadn't considered that it needs to fit there when folded, which could explain the smaller diameter if that's an issue. I'll have to measure and see if that could be the reason. Still doesn't explain the lower pitch though. In fact I'd expect that if the diameter were forced smaller then the pitch would need to be greater to compensate.

 

[Justin_NSA]

FWIW, my folding prop works great. I am on a big pond though. No significant fouling.

Does more pitch induce more prop walk?

 

[Sandy Stone]

Feathering props like the Maxprop use GEARS to feather/unfeather them, and they function with almost any amount of fouling.

Note that the flex-o-fold prop blades are also geared, so they open in unison. I installed a 2-blade on my previous boat, and it was the best improvement in sailing capability short of new sails. A maxprop was on my current boat when I bought it, and is even better, (much more thrust in reverse) but quite a bit pricier.

 

[Davidasailor26]

Note that the flex-o-fold prop blades are also geared, so they open in unison. I installed a 2-blade on my previous boat, and it was the best improvement in sailing capability short of new sails. A maxprop was on my current boat when I bought it, and is even better, (much more thrust in reverse) but quite a bit pricier.

Thanks Sandy Stone. I don't suppose you remember how the size of the FOF you installed compared to what it replaced, and how the resulting motoring compared? (I know the sailing speed would be significantly greater)

 

[DayDreamer41]

Don't know about sizing your prop when changing to a Flexofold our boat came with the FOF installed, I haven't had an issue with it as of yet, just wanted to mention that while you are at changing out the prop make sure you also install a line cutter while you are at it, you may never need it, but the day you do you will be happy you invested in one.

 

[LakeOntario270]

I went from a 3 blade fixed to a 2 blade FOF and my sailing speed AND motor speed went up.
I can now back up straight.

 

[JohnShannon]

So they are suggesting a smaller, finer pitched prop than what you have? That will really compromise motoring performance. My old boat had a Kiwi prop (feathers) and that worked well, initially it was set too fine, but can be adjusted by means of set screw and once coarsened up worked well, diameter was same as original.

 

[Gunni]

I would assume that FoF is recommending the 10 pitch so that they don't get you into a problem loading your engine with too much prop pitch. Can't see anyway that this will not reduce your prop power (unless you run your engine at a higher RPM). Really depends on what is most value to you, propulsion efficiency or sailing efficiency. A feathering prop pretty much gives you both (plus power reverse) despite what the people at FoF say. If you just value low wind sailing performance and passage making efficiency the FoF is a decent compromise. And yes, the Beneteau prop sits very close to the rudder for better effect so they are shrinking the blade radius to fit the slot.

 

[capta]

Don't know about sizing your prop when changing to a Flexofold our boat came with the FOF installed, I haven't had an issue with it as of yet, just wanted to mention that while you are at changing out the prop make sure you also install a line cutter while you are at it, you may never need it, but the day you do you will be happy you invested in one.

Boy, I second that motion. Since we've installed our Spurs I no longer worry about all the fish traps or the broken mooring buoy lines that are so common down here.
Come on try to grab this boat Mr. line. Arg....

 

[Sandy Stone]

I went from a 3-blade fixed to a 2-blade folder, can't remember the dimensions cause it was some years ago. I used the Flex-o-fold recommended size and it seemed to work OK under power, but you had to anticipate a little if you needed to back down. Despite what a lot of people say, I didn't notice that much difference in prop walk.

 

[Ken13559]

Why not write in to FOF again and ask for recalculation. I finds it discerning when the difference between FOF and her dealer is so wide.
When i bought my 2bladed FOF, I was very sceptical of FOF sizing but FOF stands by it. I wasn't happy as it had too much pitch. But i use it anyway. And along the way I serviced my engine and clean up the hull (though it was bad) and lo and behold FOF sizing was just about right!

 

[Davidasailor26]

Ken (and all), thanks very much for your advice. I have an email in to FOF and am awaiting their response.

 

[Davidasailor26]

Lesson learned - don't blindly trust the service manual for transmission gear ratios. I went to the boat and found the ratio stamped on the transmission is 1:2.62, different from the 1:2.21 listed in the manual. FOF took the new ratio and recommended a 16x12 3 blade to replace our existing 17x12 fixed 3 blade. Makes much more sense now. If we go forward with that I'll report back once we see the performance changes.