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Old 23-03-2009, 13:21   #1
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Two-Blade vs Three-Blade Prop

From what I understand each has it's advantages.

2 blade is cheaper, more efficient and less resistance when sailing, particularly folding props

3 blade has better "acceleration" (for want of the proper term) which would help in manouvering and maintaining speed in a chop. Possibley less vibration as well.

Anybody have experience with both on the same or similar boats?

Mike
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Old 23-03-2009, 14:25   #2
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we have a catalina 27 that we just put a new campbell sailer 3 blade prop on it replacing our 2 blade. our 2 blade was a fixed prop but the 3 blade was an incredible difference in acceleration and slow speed manuverability. under sail i havent noticed any real difference although if you read the reviews on this prop it is supposed to reduce drag more than a 2 blade fixed prop. when we pulled the boat our in nanaimo to have the prop changed the prop builder himself stopped by to oversee the job. im extremely impressed with the performance, quality and helpfullness of this prop and its maker.
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Old 23-03-2009, 16:15   #3
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Quote:
Originally Posted by mikereed100 View Post
2 blade less resistance when sailing, particularly folding props

3 blade has better "acceleration" (for want of the proper term) which would help in manouvering and maintaining speed in a chop. Possibley less vibration as well.
With my racing background all I wanted was a nice, sleek, folding, feathering, furling proplett that would'nt disturb the boat speed.....

Then I turned my engine on with its BIG FAT 3 BLADES chruning the water! Man, my boat moves! 6 kts at 2,000 rpm and into any sea theres still plenty of power where and when you need it.

Who gives a tinkers cuss about loosing 1/4 of a knot sailing when you can hit 8kts if you want?

When you are tired and pissed off and just want to get somewhere you have the option to do so.

Save the extra thousand dollars for a new comfy bed matteress (or beer!)


I love my fat 3 blades!

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Old 24-03-2009, 02:40   #4
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It also depends on the boat. My Ingrid was a fat old girl and had a 3 blade, a full keel and 26,000lbs displacement. You could have been dragging a 5 gallon bucket and not noticed a difference!
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Old 24-03-2009, 04:34   #5
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I have a fixed 2 blade which is about 18" and I have no problem with performance. When sailing it aligns with the keel and offers little resistance and seems to have plenty of power forward or reverse.. though I don't reverse very often, more like I use reverse to kick the stern in and slow the boat or use it to make very tight radius turns.

I can't see the justification for the expense and the maintenance for the supposed small improvement. I don't race either.
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Old 24-03-2009, 06:47   #6
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The differnce..

The difference, according to the PHRF handicap ratings, is about 3 seconds per nautical mile between a fixed two and three blade..

If that makes a big difference to you as a cruiser then keep your two blade. If three seconds +/- a mile is no big deal then go with a Campbell Sailor. It is the most efficient fixed three blade prop in terms of both pushing the boat forward and reduced drag when compared to say a Michigan MP sailor prop..

The most studied handicap ratings in sailing, which are constantly re-evaluated, are the PHRF handicaps.

PHRF AUXILIARY POWER:

Two bladed solid propeller on an exposed shaft +6 sec/mile

Three bladed solid propeller on an exposed shaft +9 sec/mile

Three bladed solid propeller in an aperture +3 sec/mile

Two bladed feathering propeller in an aperture -3 sec/mile

Three bladed feathering propeller in an aperture 0 sec/mile

Outboard fixed in well with two bladed solid propeller +6 sec/mile

Inboard with exposed shaft in place of outboard +3 to +9

The widest spread here is still only about 12 sec/mile. However the price of a feathering three blade vs. a fixed Campbell Sailor is an astronomical difference..

Again if you are worrying about the 3 sec/mile difference then stay two blade.

It should be interesting to note however that a Michigan MP two blade has nearly the identical drag at 5 knots as does a three blade Campbell Sailor.
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Old 24-03-2009, 07:47   #7
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As with many things, it depends on what you are using it for.

For cruising, my personal preference is for a 3 blade fixed prop. Quite a bit less expensive than a feathering prop, simpler, bulletproof. 3 blades gives you more bite in the water and potentially more thrust. You really do motor a lot when cruising.

I also actually like to have propwalk...especially if it is pronounced...because then it is predictable...and thus useful.

I switched from a 2 blade fixed to a Campbell Sailor prop, just before taking off. About the same drag, but more propulsion. This prop is very good when clean....and therein lies the rub.

Props foul and when they do, they lose their efficiency. The hydrodynamic shape of the Campbell Sailor prop makes this more significant. It's shape also makes it a big more of an issue to scrape barnacles off of.

I am content with it, however, if I had to do it all over again...I would get either a Michigan Wheel 3 blade fixed, either standard or their Sailor prop (which is just narrower blades, no difference in shape). I would also over-prop...would get the same diameter and pitch as the 2 blade, rather than going down in pitch or diameter with the addition of the 3rd blade.

And again...the above is JUST for cruising. Daysailing, racing etc... different needs.

Hope this helps

best

John
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Old 25-03-2009, 12:46   #8
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John - I have always wondered this, and since you mentioned it - I will ask you / others!

You mentioned going down in pitch, when going from a 2 blade to 3 blade. Is this a standard, or accepted "rule"? I would think that a boat would feel / perform somewhat more overpropped (especially in a headwind & chop) with a 3 blade vs a 2 blade, of the same pitch, as there is more blade to "work" through the water. Is this correct?

Don't want to steal the thread, but since I am having a couple of inches of pitch taken off my 3 blade fixed next week (as I am seriously overpropped now), I thought I would ask the question.

Prop calculators show my boat needing about a 14x8.6 or 15x7.5 prop. But does that matter if it is a 2 blade or 3 blade??
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Old 25-03-2009, 13:07   #9
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Hello

I will chime in for the sake of a quick reply, but I am sure others could give you more information on the subject.

Essentially, a diesel engine should operate under a certain load. Too little prop diameter and pitch, and the engine has less load than optimal and will also be turning the prop at a much higher rpm than it should for a given amount of thrust.

Too much prop diamter and pitch and the engine is operating under too great a load and turning the prop in lower rpm's than would be optimal.

I am given to think that there is a range in there that we can play with. If the engine is not overheating, not over revving, not putting out black smoke or white smoke....then seemingly, we are playing in the right range.

I personally want to be a little over pitched. It means I am operating at lower rpm's for a given speed and ...if needed...I have more bit in my prop.

Sailors who desire less drag and less prop walk want less prop diameter and lower pitch (or at least lower pitch). If they are day sailing or sailing in protected waters, they do not need more bite to the prop.

This is how I personally look at it. Other's may have a different view.

And those prop calculators...I used them ad nauseum, had the prop tech's at Campbell Sailor prop make the determination as to what size and pitch I should change to, when going from 2 to 3 blade. In the end...I wish I had asked an old friend who is a former fisherman (and old salt). He had the right answer...and I ended up having the prop re-pitched up 2 inches. And now , quite frankly, if I had to do it all over again....I would have just gone with the same diameter and pitch going from 2 blades to 3 and been over propped....probably perfectly for me. But that is me.

My old salt friend told me that the rule of thumb he would use is to go down in pitch 1 inch for a change from 2 to 3 blades and keep the same diameter. I probably should have done that to begin with. The prop calcs told me to go down 1 inch in diameter and 2 inches in pitch. That was OK for protected waters, but not for cruising...and only right when the prop was perfectly clean.

Again....this is just an opinion and I cannot say whether or not this would be good for you.

Hope this helps

best

John
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Old 27-03-2009, 05:46   #10
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Hi John,

Thanks very much for taking the time to reply. I tend to think you are correct now, in believing an old salt, over the prop calculators.

Since my diesel starts to smoke hard when I reach about 2000-2200 rpms, with no further increase in speed and I am already at 5 1/2 - 6kts (nearing) hull speed, I am pretty sure that I am overpropped. (bottom and prop clean). I can rev to 3600 in neutral.

I don't think I should be getting that much speed at such a low rpm - but since it has too much pitch, it gets there quick and then cannot rev / go any further.
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Old 27-03-2009, 07:23   #11
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Considerations On Choosing A Sailboat Propeller
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Old 27-03-2009, 13:31   #12
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Any suggestions?My boat is slightly technically underpowered,and overpropped.Not sure about going down to suggested size of 12x6 from present 3 blade 15x8 Campbell sailor.My boat cruises in flatwater at about six knots at 2000rpm.With present prop it will reach peak rpm at about 2800,@7.5 knots.Black soot particles appear in exhaust water ,but no smoke,above 2400rpm.It doesn't appear to be running hot or overloaded.Its more the transmission I'm worried about overheating.Engine is supposed to reach 3600rpm,which it does in neutral.I read here some like to be overpropped,but what is acceptable?If I reduce prop size I will lose more speed and headsea performance.Boat specs. read like this;LOA38ft.LWL30ft.,Beam 10ft.6in.,Beam at waterline about 10ft.4in.,Draft 6ft.6in.,Disp.18000lbs.,engine ;Universal M320B,Hurth Trans.,2.05 to 1,Aperture to full skeg rudder.Engine ,shaft and prop,has 269 hrs.,trans. has 5 hrs.Do I spend the $650. and hope for the best?Bigger motor is not an option.
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Old 27-03-2009, 18:21   #13
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I don't know the "numbers" for my prop aside from the fact that it is a fixed 2 blade 18" diameter. How does one measure the pitch?

I understand that pitch is the theoretical distance that the the prop moves forward in one complete revolution. I am not sure how to understand actual mechanics of this concept. I understand how the pitch of a screw relates to how far it "advances" in one revolution. You can set that by looking at the spacing of threads. Fine thread move less in one turn, coarse threads more. You can actually measure it in threads / inch.

How do you measure the pitch of a prop?

It would seem that regardless of the number of blades, props with the same pitch advance the prop through the water the same amount. More blades - more bite, but they would require more power to turn?

As you can see I need a good "Propellors for Dummies". Any suggestions?
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Old 27-03-2009, 22:59   #14
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While more critical to power boats this article is a good primer to understand your choices and trade-offs.

Propeller Selection - article

A single blade (paddle) provides the most efficient thrust per energy expended whereas the highly skewed props (4 and 5 blade) cut down dramatically on vibration but are geared more to higher rpm.

I have always felt that 2 blade props were more vulnerable than 3
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Old 19-05-2009, 09:09   #15
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2 bladed v 3 bladed prop on INGRID 38

I used a 2 bladed variable pitch Sabb prop for 22 years on my Ingrid 38. We had a Sabb 22hp diesel and could do a max 6 kts in flat calm!! We put 4,500hrs on the engine & prop. We made 6kts flat out in calm conditions but with a stiff headwind we were down to 3kts or less. Replaced last year with 3 bladed feathering Max Prop and new 64hp Westerbeke. Now we motor into 30 kts at 6kts and have a cruising speed of 6kts at 1600rpm. Close to 8 knots max.
Sailing performance has not been compromised one wit!!
Fuel consumption is about the same...0.6 to 0.75gph
My recommendation........go wid a three bladed max-prop & plenty of HP..........
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