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  • 2 Post By Jaramaz
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  #1  
Old 08-12-2014
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2 vs 3 blade prop

Someone askd about 2 vs 3 blade props on the same boat. My 28' S2 came with the original yanmar 1gm engine with 6.5 hp, not the bigger 1 gm10 of today and it was just too small with a 2 blade and wouldn't get to hull spd in calm water.
I replaced it with a 3 blade for cruising and it made a big diff although I am sure it affected sailingperformance.
Later, I replaced the tiny 1 gm with a 13 hp 2gm and I then I had plenty of thrust even in rough water. So, I the replaced the 3 blade with a 2 blade PerfectPitch prop that is about 1.5" greater diameter and still get good thrust but better sailing performance.
Moral of the story, check PerfectPitch cuz they can prob improve yer performance with a 2 blade.
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Old 08-12-2014
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Re: 2 vs 3 blade prop

I will assume perfect pitch is a fixed prop? if so, any 2 blade fixed will ssail better than a 3 blade. A folding/feather 3 blade will sail better, and bit better than either of the fixed blade options. Generally speaking that is.

The larger diam 2 blade probably gave you the same prop face to grab water with motoring, along with the higher HP to propel you along.

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Old 08-12-2014
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Re: 2 vs 3 blade prop

Quote:
Moral of the story, check PerfectPitch cuz they can prob improve yer performance with a 2 blade
No, not at all. Moral is, see to that your prop is correctly dimensioned.

/J
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Old 08-12-2014
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Re: 2 vs 3 blade prop

I've had 3 props on my Pearson 28-2, so I feel like I can make a pretty fair comparison.

The first prop was a 2 blade Michigan Sailor. It was the stock prop on the boat. It worked pretty well, but had big flat blades that seemed inefficient. Diameter and Pitch were 16x10. I used this prop for only 3 or 4 months, so I don't have a strong opinion on it. I've kept it as an emergency spare.

The second prop was a 3 blade Campbell Sailor. I upgraded to this prop because the blade design is much improved, it has a foil shaped cross section that provides more bite, and a smaller blade that provides less drag. In fact my 3 blade CS prop had less blade area than my 2 blade MS prop. I went with a 3 blade prop because it was supposed to allow the motor to run more smoothly since the blades aren't coming in and out of the turbulence behind the keel at the same time. In retrospect I don't think that made any difference on my boat (a fin keel with tons of room between the keel and prop) and I should have purchased the cheaper (and less blade area) 2 blade prop. Diameter and pitch were 14x8. That sounds like it would be under pitched, but the blade profile gave that smaller diameter and pitch prop more bite than the original.

My newest prop is a 3 blade feathering prop (it is a Featherstream). I wanted better sailing performance, and this prop has made a huge difference in drag when sailing in light air. I considered 2 blade or 3 blade options, but Featherstream only makes 3 and 4 blade props so I bought the 3 blade one. This prop is 16" in diameter and has the equivalent of 9.5" of pitch.

The Campbell Sailor motored a little better than the Featherstream in forward. It seemed like I could go about half a knot faster without overdriving the motor. The Featherstream has less prop walk in reverse because the blades flip over and drive in a more efficient way. Both have a helpful amount of prop walk for steering in tight areas.

I don't think 2 blade vs 3 blade makes a difference on my boat, but I could see it making a big difference on a full keel boat where the prop is mounted in an aperture between the keel and rudder.

If I were buying another fixed prop it would be another Campbell Sailor in 2 blade. Since I love sailing and don't really like motoring I'm much happier with my feathering prop. My CS prop is now pushing around the boat of another Sailnet member.

To me it sounds like Frogwatch's original prop was incorrectly spec'd. Selecting prop pitch has an element of luck to it. One advantage of feathering props is that the pitch can be changed pretty easily when the boat is hauled out. The ideal pitch for a diesel engine has you moving near hull speed at about 75-85% of the engine's max RPM (where the engine is near the top of the torque curve). For my Yanmar 2GM20F that means I want the boat to move around 6 knots at about 2700-2900rpm. With the Featherstream it is more like 5.5 knots since the prop is not as efficient when motoring.
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Last edited by Alex W; 08-12-2014 at 11:36 AM.
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Old 08-12-2014
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Re: 2 vs 3 blade prop

Here is my "pitch" for PerfectPitch. The PerfectPitch are fixed blade massive plastic castings. When you order, you tell what boat and motor and they decide if you can handle a larger diameter. If not, they have a shaft extension that often allows one to use a larger diameter. One advantage of this large thick plastic prop is that it does not corrode although you should still use a shaft zinc to protect your strut and shaft.
In light chop with a smooth hull, my cruising speed is 5.9-6.2 kts (a 28' boat).
Folding props may be an option but in my home waters the fouling is so extreme that it is rare to see one functioning well.
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Old 08-12-2014
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Re: 2 vs 3 blade prop

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Originally Posted by Frogwatch View Post
Here is my "pitch" for PerfectPitch. The PerfectPitch are fixed blade massive plastic castings. When you order, you tell what boat and motor and they decide if you can handle a larger diameter. If not, they have a shaft extension that often allows one to use a larger diameter. One advantage of this large thick plastic prop is that it does not corrode although you should still use a shaft zinc to protect your strut and shaft.
In light chop with a smooth hull, my cruising speed is 5.9-6.2 kts (a 28' boat).
Folding props may be an option but in my home waters the fouling is so extreme that it is rare to see one functioning well.
Frog, you are just promoting PerfectPitch without any background knowledge of prop design.
How nice of them to decide for you! And to have some shaft extensions ...

And on top of this you should have a zinc to protect your shaft? Made in stainless?

A prop is an expensive item. When one buys one, some care should be spent on selecting a serious vendor/retailer. They will assist in dimensioning the prop (boat owner takes the decision, please). Without any mumbo-jumbo. No zinc to protect stainless. No extras to pay for.

/J
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Old 08-12-2014
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Re: 2 vs 3 blade prop

A. My prop strut is bronze so needs a zinc to protect it.
B. I went with the PerfectPitch prop because I had corrosion problems and the Plastic PerfectPitch worked well.
C. I made a direct comparison of their prop to my old 3 blade prop.
D. I easily reach hull speed in chop.
E. Experience always trumps theory.
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Old 08-12-2014
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Re: 2 vs 3 blade prop

Anyone posting numbers on propellers must include engine model or at the least it's horse power in their post or the post isn't as helpful as you think.
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