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Question:

A three (or 4) blade prop always has at least one blade sticking out, which creates drag.

Oops.  I meant to say they always have at least *two* blades sticking out. OTOH, having that one blade sticking out from the turbulence behind the keel means WAY less vibration. I never thought of that until I got my (3-blade) Campbell Sailor for Far Cove. "Prop-based" vibration now virtually nil. And I didn’t see a difference in sailing speed (although I don’t race).

I think I misread you here.  But I didn’t realize it until I checked out the website you provided and took a look at their FAQ.  Yes, I very much agree that a two blade prop is *much* more prone to vibration than a three or four blade prop, when it’s motoring.  And in fact, the vibration is made even worse on a full keel boat than a fin keel boat. But what I was talking about was when the boat is *sailing*.  Not motoring.  Maybe you knew that already… if so, I misunderstood… sorry. PhantMan again: By "some sailboats", what I meant was a full keel type boat where the prop actually tucks in closely behind the keel.  A Catalina 36 has a fin keel, IIRC, and the prop really has no place to hide.

Lloyd Sumpter again: Here’s a website on the Campbell Sailor: http://www.westbynorth.com

Thanks. Interesting stuff. As I said, I didn’t notice any difference in sailing speed when I went to the 3-blade.

Makes sense. When you switch from two, up to three blades (especially three *thin* blades) on a fin keel boat, you’re dragging only one extra blade through the water.  But when you make the same switch on a full keel boat (or any boat that can completely hide a two blade prop from the water flow), it’s like going from no prop at *all*, up to "towing a bucket". Anyway, I think your prop is a fine idea for your boat. Especially when you’re approaching a dock after a day of sailing. Sure beats wondering/worrying about whether your folding prop will unfold, or your feathering prop will unfeather  :-) Rick

Response:

Hi, I would like to change the propeller of my boat (18 D x 19 P) of 3 blades for one propeller of 2 blades. Which mathematical relationship should use to have the same acting, or better? Can anybody help me? Thanks

Response:

Don’t do it.  2 blade props were great speed props but have almost no hole shot and will cavitate like crazy. Based on the size of your prop, I would assume it is a large boat and a 2 blade prop will not work on that. Why are you even considering this if you want the same acting? Tony Thomas My speed boats at http://members.home.net/thomastl1

– Hide quoted text — Show quoted text – Hi, I would like to change the propeller of my boat (18 D x 19 P) of 3 blades for one propeller of 2 blades. Which mathematical relationship should use to have the same acting, or better? Can anybody help me? Thanks

Response:

Carvalho  wrote I would like to change the propeller of my boat (18 D x 19 P) of 3 blades for one propeller of 2 blades. Don’t do it.  2 blade props were great speed props but have almost no hole shot and will cavitate like crazy. Based on the size of your prop, I would assume it is a large boat and a 2 blade prop will not work on that. Why are you even considering this if you want the same acting?

I don’t speak for the original poster, but here’s my 2 cents. On some sailboats, it’s more efficient to sail with a 2 blade prop locked in place (up & down) behind the keel. A three (or 4) blade prop always has at least one blade sticking out, which creates drag. For sailing efficiency, sometimes you just have to sacrifice a little "hole shot"  ;-) Rick

Response:

- Hide quoted text — Show quoted text – On some sailboats, it’s more efficient to sail with a 2 blade prop locked in place (up & down) behind the keel. A three (or 4) blade prop always has at least one blade sticking out, which creates drag.   OTOH, having that one blade sticking out from the turbulence behind the keel means WAY less vibration. I never thought of that until I got my (3-blade) Campbell Sailor for Far Cove. "Prop-based" vibration now virtually nil. And I didn’t see a difference in sailing speed (although I don’t race). By "some sailboats", what I meant was a full keel type boat where the prop actually tucks in closely behind the keel.  A Catalina 36 has a fin keel, IIRC, and the prop really has no place to hide. Different situation for a prop on a very short shaft  (2"-3") sticking out from behind a full keel though.  The choice between a 2 or 3 blade prop can really matter (or at least it certainly did on mine). Interesting point about the prop vibration though. Most of the fin keeled boats I’ve sailed have two blade standard, folding, or feathering props. All of which are compromises in one area or another. And lately I’ve heard some new debate on whether it’s better to lock ‘em down, or let ‘em spin (speaking now of *non*-feathering/folding prop on a fin keeled boat). Sailing efficiency wise, I think it’s better to let ‘em spin, but the noise and vibration are unwelcome. Plus there’s the wear and tear on the running gear to consider. Apparently most racing sailors disagree with me though because alot of ‘em lock ‘em down. And any racer would *gladly* put up with any irritations and expense for a little more sailing efficiency. What’s different about a "Campbell Sailor" prop?  I don’t recall sailing a boat with one (that I know of), I’d be interested to find out more about them though. Rick

   Here’s a website on the Campbell Sailor: http://www.westbynorth.com It’s basically a highly efficient prop with a "patented" airfoil shape. Yes, I know about the feathered/folding/whatever props. As I said, I didn’t notice any difference in sailing speed when I went to the 3-blade. If you look at a 36ft boat, and then at the prop, you can see that it just doesn’t make that much difference. Racing, yes. But cruising (even "performance cruising"), no.    And as for Harry’s suggestion: they’re called "outboards" and they cost about $8000. And the prop comes out of the water when the boat hits big waves. Lloyd Sumpter "Far Cove" Catalina 36

Response:

On some sailboats, it’s more efficient to sail with a 2 blade prop locked in place (up & down) behind the keel. A three (or 4) blade prop always has at least one blade sticking out, which creates drag.   OTOH, having that one blade sticking out from the turbulence behind the keel means WAY less vibration. I never thought of that until I got my (3-blade) Campbell Sailor for Far Cove. "Prop-based" vibration now virtually nil. And I didn’t see a difference in sailing speed (although I don’t race).

By "some sailboats", what I meant was a full keel type boat where the prop actually tucks in closely behind the keel.  A Catalina 36 has a fin keel, IIRC, and the prop really has no place to hide. Different situation for a prop on a very short shaft  (2"-3") sticking out from behind a full keel though.  The choice between a 2 or 3 blade prop can really matter (or at least it certainly did on mine). Interesting point about the prop vibration though. Most of the fin keeled boats I’ve sailed have two blade standard, folding, or feathering props. All of which are compromises in one area or another. And lately I’ve heard some new debate on whether it’s better to lock ‘em down, or let ‘em spin (speaking now of *non*-feathering/folding prop on a fin keeled boat). Sailing efficiency wise, I think it’s better to let ‘em spin, but the noise and vibration are unwelcome. Plus there’s the wear and tear on the running gear to consider. Apparently most racing sailors disagree with me though because alot of ‘em lock ‘em down. And any racer would *gladly* put up with any irritations and expense for a little more sailing efficiency. What’s different about a "Campbell Sailor" prop?  I don’t recall sailing a boat with one (that I know of), I’d be interested to find out more about them though. Rick

Response:

- Hide quoted text — Show quoted text – Carvalho  wrote I would like to change the propeller of my boat (18 D x 19 P) of 3 blades for one propeller of 2 blades. Don’t do it.  2 blade props were great speed props but have almost no hole shot and will cavitate like crazy. Based on the size of your prop, I would assume it is a large boat and a 2 blade prop will not work on that. Why are you even considering this if you want the same acting? I don’t speak for the original poster, but here’s my 2 cents. On some sailboats, it’s more efficient to sail with a 2 blade prop locked in place (up & down) behind the keel. A three (or 4) blade prop always has at least one blade sticking out, which creates drag. For sailing efficiency, sometimes you just have to sacrifice a little "hole shot"  ;-)

   OTOH, having that one blade sticking out from the turbulence behind the keel means WAY less vibration. I never thought of that until I got my (3-blade) Campbell Sailor for Far Cove. "Prop-based" vibration now virtually nil. And I didn’t see a difference in sailing speed (although I don’t race). Lloyd Sumpter "Far Cove" Catalina 36

Response:

- Hide quoted text — Show quoted text – Carvalho  wrote I would like to change the propeller of my boat (18 D x 19 P) of 3 blades for one propeller of 2 blades. Don’t do it.  2 blade props were great speed props but have almost no hole shot and will cavitate like crazy. Based on the size of your prop, I would assume it is a large boat and a 2 blade prop will not work on that. Why are you even considering this if you want the same acting? I don’t speak for the original poster, but here’s my 2 cents. On some sailboats, it’s more efficient to sail with a 2 blade prop locked in place (up & down) behind the keel. A three (or 4) blade prop always has at least one blade sticking out, which creates drag. For sailing efficiency, sometimes you just have to sacrifice a little "hole shot"  ;-) Rick

Sounds like an expensive, overly complicated Rube Goldberg device that raises and lowers a heft prop into the waterstream would be the ideal answer… Only $25,000, helps you pick up a knot.

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