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  #1  
Old 02-03-2011
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Props 3 blade Vs 2 blade

Hello want some good advice here, prop shops all want to sell their product so I am asking sailors not prop dealers.
We currently have a 31' boat fairly heavy @12k Power is via a Westerbeke 4-91 (25hp) connected to a Hurth transmisson. We currently have a 2 blade 17" prop of no special make. The boat only sees about 4 knots @ 1500 rpm @2000 rpm not quite a knot gain.
I am told by friends a 3 blade prop will geatly improve both speed and backing ability.
My concern is am I trading better motoring for slower sailing due to more drag from an exta blade on the prop? I really feel the boat should be faster under power. (not that we are wanting to waterski)
Should we look at just a different pitch 2 blade? (unsure of current pitch)
The Hurth owners manual says to let the prop freespin under sail and not lock it in forward. It comments if I choose to lock it out to do so in reverse only.
I am not really in a position to afford a feathering or folding prop.
Thanks,
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Old 02-03-2011
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If you got a feathering prop, like the Kiwi prop, you would have better backing, lower drag and likely more power in forward as well, though that can vary depending on the pitch, diameter, and gearing of your engine. It would help if you said what boat you had, since the hull speed is highly dependent on the waterline of the boat, and without knowing which 31' boat, it is hard to say what your boat should be able to do. A boat with a 25' waterline should be able to get 6.7 knots at hull speed...and your boat is badly under-propped by the sounds of it, since you're only making 5 knots.
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Old 02-03-2011
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Campbell Sailor Prop 3 blade should be good. Its not pricey. You have to ask Norm Ross (West by North) for correct diameter & pitch. You have to send him details of your engine/boat/2blade performance.
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Old 02-03-2011
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My 26' C&C just had a 3 blade 'sailboat' prop installed, to replace a 'powerboat' prop which a PO installed. I asked about a 2 blade,(so I could 'lock' it inline with the keel), and was told the drag of the 3 blade was not enough to worry about? My trans is in netural when the sails are up, this is all the info I can offer. It will go to hull speed with the 8hp yanmar at almost full power. I hope this helps somewhat?
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On Adriana, Pearson Vanguard 33', I changed the two blade 13 x 15 for a three blade 13 x 13 in anticipation of a lot of windward motor-sailing going down island. This improved things somewhat, giving hull speed of 6.25k at 2400 rpm on a Yanmar 3GM30F. The two blade struggled to get hull speed at all.
You will lose some light airs sailing performance, no doubt.
The Vanguard has the prop in an aperture and it was a tight fit to get the three blade on and off. You need to check this before buying a new prop.
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Old 02-03-2011
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Quote:
Originally Posted by DulceSuerna View Post
.....My concern is am I trading better motoring for slower sailing due to more drag from an exta blade on the prop? I really feel the boat should be faster under power. (not that we are wanting to waterski)
Should we look at just a different pitch 2 blade? (unsure of current pitch)
The Hurth owners manual says to let the prop freespin under sail and not lock it in forward. It comments if I choose to lock it out to do so in reverse only.
I am not really in a position to afford a feathering or folding prop.
Thanks,
There are many threads on Sailnet about this subject. You might start with this one:
3 blade prop

First you should know and use the load RPM for your engine, that this the engine power at which you should be expecting whatever boat speed you expect. It is good for the health of the egine for it to be run a that power.

The power delivered by your prop is largely a factor of the blade area, as power demands go up for larger boats, they go to three blade props for more blade area as the diameter of bigger two blades wont fit under the hull.

Two blade and three blade propellers witht he same blade area will produce basically the same power, but the three blade costs more... Just adding a three blade without knowing how it fits your application would be a very bad idea, and wont fix any issues you have.
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Old 02-03-2011
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You might look at ebay or equal for a used folder or equal prop if you go with a 3 blade. A 3 blaed will give you more bite if motoring in currents, and not slow you down as much. If you can find out the prop diam, some like my maxprop, the pitch can be changed, so if you found say a 14" diam 2 blade on ebay for a good price, it functions etc, but the wrong pitch, it can be change in about an hour on a flat work surface or even on the shaft itself. I am not familiar with other folding/feathering props, as I have a max prop.

BUT, it does sound like the prop size or pitch is not correct for your application, and that would be the first thing to figure out, what prop/pitch you should have.

I can do 6.5-6.6 knots with a 13" max prop at max rpm or about 2800 or so with my 16hp twin cyl yanmar, I am about 7000 lbs total. 2000-2500 I am in the mid5-6knot range. I do not have a tach, so I may very well be doing the same speeds as you. BUT, if you top out at 2000 rpms with you motor, you DO need a new prop to deal with this rpm to get you to hull speed.

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Old 02-03-2011
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I'd agree that your current setup is probably not properly sized/pitched. We have had good success in the past with the 3 blade Campbell Sailor as a midpoint alternative to the costly folding/feathering option. A 3 blade prop will give you a much smoother ride, at some cost in drag, obviously, over the same 2 blade prop.

Our son's C36 with the 3 cyl 21 HP Universal does better than 6 knots at around 2200 - similar weight or heavier than your boat. A stiff headwind does slow them down, though.
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Old 02-03-2011
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My catalina 27 has a 2-blade prop, 30-HP Atomic-4 and with slack tide and a clean bottom she'll do 7.2 to 7.5. There are lots of myths about 2-blade props V/S 3-blade. There's also some common sense factors to consider as well.

If the boat has a relatively thick keep and the prop does not have a lot of clearance between the prop and keel, and prop and rudder, there is a turbulence issue to consider--particularly when backing. Under these circumstances the additional width of a 2-blade prop will provide about 20 to 30 percent more keel/rudder clearance, thereby providing more thrust against the water. A 3-blade prop, with its much smaller diameter doesn't have the needed clearance, therefore much of the prop thrust may be against the keel or rudder.

I've always locked my prop using the transmission while sailing to prevent it from turning. I've read that this is important because to transmission is not being lubricated with circulating oil when the engine is not running. That made sense to me! Additionally, there's no noise and less chance of wrapping up a crab pot line because the prop isn't spinning. Granted, you could hook a crab pot line, but it's not going to wrap around the prop shaft if the prop isn't spinning.

Good Luck,

Gary
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Old 02-03-2011
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A speed of 7.5k is remarkable performance for a displacement boat with a LWL of under 24'.
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