Prop Size

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May 31, 2007
758
Hunter 37 cutter Blind River
I need a new propeller for my H37C. Will be a three blader, either a feathering or a folder. Flex-i-fold recommends a 15X12 and is around $2000. However, I do have the opportunity to purchase a used 17" Max prop for around $1400. The Max's pitch can be adjusted. My concern is the larger diameter. I do have ten inches between the shaft and the bottom of the boat so clearance is reasonable. Question, then, is what are the consequences of going with a larger diameter assuming the pitch can be reduced enough.
 
Jun 2, 2004
5,802
Hunter 37-cutter, '79 41 23' 30"N 82 33' 20"W--------Huron, OH
We have seen it written here that a too large prop creates more turbulence than thrust. It is especially a problem in reverse I hear. But what is "too large"? Will you really have 1.5" of clearance? And is that enough? I remember Blaise commenting on that one time. Meanwhile I love my Flex-O-Fold and it is 15x12. Here's a picture of my original Flex-O-Fold, also 15". It is open so you can see the clearance.
 

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Ross

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Jun 15, 2004
14,693
Islander/Wayfairer 30 sail number 25 Perryville,Md.
The circle the prop make is abot 28 percent bigger for the 17 then for the 15. can you take the pitch down to about 9.3 and will it turn fast enough?
 
May 31, 2007
758
Hunter 37 cutter Blind River
The engine manufacturer is recommending a pitch of 10. I don't know if the MAX will adjust that far. Anyone out there know? Also, is there an advantage or disadvantage to having large diameter with small pitch as opposed to small diameter with large pitch?
 
Dec 2, 1999
15,184
Hunter Vision-36 Rio Vista, CA.
I would recommend that you let the mfg of the prop tell you what prop is correct for your boat.

We have had numerous posts about props over the past 10+ years. The biggest problems have typically been from new boat owners (including my own). The prop was over-pitched. Many times they have been under pitched so the boats are able to maintain power in rough/windy conditions.

Let the prop people determine (and be responsible) for the proper sizing. Typically you want to maintain 15% of the prop diameter for clearance. So a 15" prop would require about 2.25" of tip clearance.
 
May 31, 2007
758
Hunter 37 cutter Blind River
Thanks, Steve. It would appear then that a 15" prop will give me 2.5" clearance - more than the 15% minimum. A 17"inch will give 1.5" clearance, or 8%. So clearly the Max prop is not going to be a good investment for me. What is the reason for the 15% clearance. Is it due to wasted energy from turbulence near the bottom of the hull? Increased cavitation at the blades? Prop wash increase?
 
Dec 2, 1999
15,184
Hunter Vision-36 Rio Vista, CA.
It is what Ed was referring to in his post. When the prop tip is too close to the hull it causes cavitation (can be very noisey). When you are in reverse it increases prop walk.

You may also consider the Kiwi Prop (feathering) or a Campbell Sailor (fixed 3 blade). The Campbell provides better power and decreased drag under sail.
 
Jan 22, 2008
8,050
Beneteau 323 Annapolis MD
You would get a thumping against the hull as the blade swings close to it and directs a wave of water thereon/upon/against.
 
Dec 2, 2003
1,637
Hunter 376 Warsash, England --
My H376 came with a two blader 17" prop so it is certainly not too big for this size of boat. I now use 17.5" x 15" pitch on 3JH2E Yanmar 3,600 RPM 2.81 gearbox.

Look on the web for range of adjustment for MaxProp which IMHO are the best.
You will find a feathering prop has vastly superior brakes (not quite the same as going into reverse!)
See U Tube http://www.youtube.com/watch?v=B4xwJkUfU6o
Particularly the Flexofold test 4min 20sec from the start.
 
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