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Go Back   SailNet Community > On Board > Gear & Maintenance > 2 blade or 3 blade propeller
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Thread: 2 blade or 3 blade propeller Reply to Thread
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Topic Review (Newest First)
10-11-2012 10:00 PM
HMoll
Re: 2 blade or 3 blade propeller

I'll tel you what's dragging: this thread! For cruising in heavy currents, on a small boat: 3 blade-folding. I went from feathering 2-blade maxprop to 3-blade folding flexofold. Gained a full knot motoring and maybe lost .2kts sailing, which is unmeasureable, 'cause I probably "lose" more than that just chillin' and enjoying the ride! Get 3 blades. No brainer.
10-11-2012 09:27 PM
sailingfool
Re: 2 blade or 3 blade propeller

Quote:
Originally Posted by Capesailor View Post
Hello,

I'm a new member. I'm buying a new Marshall Sanderling and am deciding on a two blade vs three blade prop for the 10 hp Nanni 2.10 diesel. Is there more drag with the 3 blade? Is the three blade quieter with less vibration? I am interested in getting max thrust in a tidal outlet to the Atlantic.

Capesailor
That's a small diesel, it certainly will work fine with a suitable size 2 blade. I would doubt they make a 3 blade to deliver so little thrust...
10-11-2012 05:11 PM
Gene T
Re: 2 blade or 3 blade propeller

Generally better to start a new thread than use one 8 years old. 3 will have more drag with lower vibration probably only felt on the tiller. Drag with a 3 blade is so significant most sailors will go with a folding or feathering 3 blade, and pay the difference.

Gene
10-11-2012 04:30 PM
Capesailor
Re: 2 blade or 3 blade propeller

Hello,

I'm a new member. I'm buying a new Marshall Sanderling and am deciding on a two blade vs three blade prop for the 10 hp Nanni 2.10 diesel. Is there more drag with the 3 blade? Is the three blade quieter with less vibration? I am interested in getting max thrust in a tidal outlet to the Atlantic.

Capesailor
12-29-2004 08:12 PM
dougyoung
2 blade or 3 blade propeller

I looked into this same switch. A local prop shop shop said to go with the same dia w/ 2" less pitch. Think about it, 150% of the thrust area of a 2 blade, reducing the pitch takes the strain off the engine at the same rpm. The big difference that was explanied to me was with a 3 blade, two blades were always thrusting in less distrubed water from the keel, with a 2 blade there is only one.
11-11-2004 12:17 PM
JohnDrake
2 blade or 3 blade propeller

OK, just got an email from Mich Wheel. Sent them all my specs and asked for a quote. They recommend a 3 blade fixed....17x11.

Interesting, Campbell also said 17x11 at first blush but then changed that to 16x9 when I sent them my trans specs later on.

Hmmmm......Jeff????? Jack??? Anyone?

I am going to write campbell back and ask them to recheck. I can see 16", not a big deal. But going from 12" pitch to 9"?

J
11-11-2004 12:05 PM
JohnDrake
2 blade or 3 blade propeller

Thanks Jack. I will have to look up Gerr''s books. I like his writing and if Nigel recommended them....all the more. Nigel is a great guy. He really likes Hood 38''s too, so...that colors my opinion.

I am trying to ''ground truth'' the prop specs recommendation from Campbell by using some net resources (though would really like real world examples still). I have written Michigan wheel to get their recommendation...they have a similar "sailor" prop. A 3 blade with thinner blades for less drag. They said about 10 days to get back to me. Whew.

If your plans include more Med sailing this coming summer, check out La Maddelena. One of my favorite places and the one I''d like to get back to most.

Best

John
s/v Invictus
Hood 38
11-11-2004 09:04 AM
WHOOSH
2 blade or 3 blade propeller

John, I''d definitely recommend the 3-blade prop. Given your near-term but also long-term cruising plans, I suspect you''ll be happier with it despite its add''l drag.

Have you worked Campbell''s pitch & diameter recs against either of Dave Gerr''s books? (Prop Book and also the Nature of Boats). I think a lot of the latter and have heard the former recommended by savvy folks (Calder is one), and while neither book will produce an iron-clad conclusion for you, it might prove useful to have an ''independent, theoretical but expert'' view to compare against everything else you hear. (The Nature of Boats will also keep you out of the bars at night; it''s a very thoughtful, helpful read, if you haven''t already consumed it).

Understand completely about the ''better choice'' being the one you can afford. After struggling with the Euro and the Pound for the last two years, we understand. At least your near-term cruising will be done in the U.S. Dollar''s neighborhood.

Jack
11-11-2004 06:34 AM
JohnDrake
2 blade or 3 blade propeller

Hi Jack and Jeff,

It is great to chat with both of you again. I appreciate your points, all good ones.

I too am surprised at the Campbell pitch and diam recommendation and so was wondering if anyone had made this specific change and had comparable numbers. Apparently though, Campbell says that they have a lot of experience with the engine, displacement and tranny in this range and likes these numbers best. Still, I would like to hear from anyone who has actually made this change.

I do think the campbell prop will have much less drag than a similar three blade and more than likely less drag than the existing 2 blade prop (it has broad blade surfaces). I am not that concerned with drag, it is a fact of life. I cannot afford and otherwise do not wish to have a folding or feathering prop. I want a fixed prop as it is simpler, cheaper and more reliable.

Jack, completely agree with your points about using prop walk (and do not have a bow thruster). I do use it and there are situations in which it is great. But...invariably, there are more situations in which I would like to have much less. If need be, we could use a spring line to accomplish much the same as the walk.

My cruising plans have changed a little bit due to family reasons. I have to stay closer to FLA and the east coast (so that I could hop a plane to FLA to take care of my folks if the need arises). So, I will be in FLA and the islands. Probably much of the caribbean and cruise up and down the east coast. Perhaps the Med later, that was my goal, but it will have to wait. Marina stays are likely and so backing into or out of a slip will be nicer with less walk.

The bottom line really is that my existing prop was dinged by the previous owner and is probably out of tune as well as chipped. It has been ok, but I have budgeted a change at this point. Since a Campbell or MW sailor prop is so cheap, why not go that route. And 2 vs 3....all the experts I talk with suggest 3 blades as they run smoother so less stress on the trans. These new "sailor" props should have less drag than my existing prop. I do not have any speed or performance issues with the boat so I will be satisfied. I know that a feathering prop would likely be better, but it is just not in the budget and as stated, I feel a fixed is more robust and reliable. But really, cost is the greater driver.

Appreciate your thoughts always.

Your friend

John
s/v Invictus
11-11-2004 05:00 AM
Jeff_H
2 blade or 3 blade propeller

A couple minor points, a three blade prop of equal thrust generally will have significantly greater drag than the two blade under sail. In most regions PHRF gives six seconds a mile for a three blade fixed prop vs a two blade fixed and I have talked to owners who convert back to two blades for racing because they claim that they can''t sail to their rating with thier three blade props. Obviously it depends on the boat and the props.

I am also surprised at the big change in pitch. Since you are not changing the transmission, I would expect that the pitch should be roughly the same for both propellors since pitch is theoretically the amount that the boat would advance with one turn of the propellor assuming zero slipage. The slippage should be fairly similar between the two props in flat water and so I would expect the pitch to be much closer.

Jeff
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