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  #1  
Old 11-03-2004
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2 blade or 3 blade propeller

I need help deciding between a 2 blade or 3 blade propeller
for a Catalina 36 with a wing keel. Planning to do little
or no racing and I''m non experienced in docking.
How practical is the 3 blade when docking? And is it worth
the extra drag when sailing?
Thank You
Danny
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  #2  
Old 11-05-2004
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2 blade or 3 blade propeller

I have a three-blade on my Hunter 335 and can really notice the performance difference. More prop-walk with the three during docking. I don’t really notice any negative sailing difference. Let''s face it, if I was in a hurry I''d by a motor boat.

Ian
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Old 11-05-2004
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2 blade or 3 blade propeller

When you say that you don''t notice the difference, have you sailed a 335 that had a two blade prop or sailed your boat with its three blade propeller near a boat with a two blade prop. The difference between using a two blade vs a three blade shows up primarily in low windspeeds when it might make the difference in whether you can sail or not and at high windspeeds when it can result in greater heeling and more weather helm, and of course less controllable speeds.

Jeff
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Old 11-08-2004
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2 blade or 3 blade propeller

Jeff,

We had a choice between a two blade and the three blade when we launched the boat for the first time after purchase and decided on the three blade right off the bat. We have not done comparisons between performance of the two vs. three directly on our boat. Compared to other boats we sail with we have absolutely no trouble holding our own. Keeping it in perspective, I''m an ex dinghy racer turned cruiser so - is my sailing ability overcoming the slight drag difference - I don''t know but it might? Most sailors we go out with have more of a set-it and forget it mentality than I do. I prefer to tweak sail trim more than most. Our marina is tight and our slip is a little awkward to get to. I really notice the performance of the prop when docking in heavier winds. The ability to “hit the breaks” when things get a little out of control is a real confidence booster. The weather helm issue you mention is one I have not experienced. The 335 is a fractional rig and it just simply works better with a reef in the main. I’m usually one of the first to reef and my wife prefers it if I keep the boat flat. (Like a dinghy) I hope this helps even if it’s not the science you were looking for.
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Old 11-08-2004
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2 blade or 3 blade propeller

I think that your post does help a lot. As an ex-dinghy racer I would think that you would be a bit more sensitive if the drag was noticably hurting your performance. With that in mind, the fact that from your perspective the three blade does not appear to noticably effect performance suggests that the impact of the three blade prop on the Hunter 335 is an obvious issue.

I also found your observation on reefing interesting. I find that I generally have to reef less with a fractional rig (at least on frac''s with backstay adjusters and decent deck hardware) but that when there comes a time when reefing is necessary, the reef produces a boat that has very nice handling characteristics, pretty much like a cutter sailing under mainsail and staysail only.

Thanks for your comments.
Jeff
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Old 11-10-2004
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2 blade or 3 blade propeller

I still don''t get what the coversation of 2 versus 3 is supposed to be about...

My understanding is that thrust is a factor of blade area, and the proper blade area is determined by engine HP and boat displacement. You can get your blade area two ways - a larger prop with only two blades, or a smaller prop with three blades, but both with the same blade area, or your engine will not run at cruising RPM. Regardless, the performance or the props will be basically the same...you only NEED a three blade when the proper sized two blade would not have adequate clearance around it. If you buy a three blade when a two blade would fit, you have simply wasted some money to no benefit...
Which is a common enuff syndrome in sailing...
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Old 11-11-2004
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2 blade or 3 blade propeller

It is not quite that simple. You can get greater blade area in a number of ways. On a two blade prop you can increase the width of the blades without increasing diameter (often called a ''power prop''). This adds some power but is relatively inefficient since it does not increase the leading edge of the prop but does increase the tip drag. In order to get the full benefit of increased blade area out of a two blade prop, the two blade propellor needs to have a larger diameter. Because of the longer lever arm of the larger diameter prop it requires greater torque to turn a two blade prop of equal area to a three blade at the same RPM. Since the determinant of horsepower in a rotating shaft is rpm x torque (times a constant I believe, but that I can''t recall) the greater torque will require more horsepower for a given thrust. That is the theory. The reality can be a little different because the greater reciprocating mass and lower efficiency of a three blade can offset this increase in HP a little.

Respectfully,
Jeff
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Old 11-11-2004
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2 blade or 3 blade propeller

I am swinging a 17"x12" 2 blade fixed, which I need to replace. It produces significant drag and prop walk (which can be used on occasion but I would rather have less).

I am looking into getting a new Campbell Sailor Prop, 3 blade fixed, mostly because of cost ($485) and this will be for cruising. This will reportedly give less drag, less prop walk, smoother action and more bite in forward and reverse.

The boat is 38ft LOA, 31ft LWL, 22,000lbs. Perkins 4-108 engine, Hurth V tranny doing 2.13:1.

Campbell is recommending a 16"x9" 3 blade fixed.

Does that sound in the ballpark?

My best to all.

John
s/v Invictus
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Old 11-11-2004
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2 blade or 3 blade propeller

John, I apologize for backing you up a bit in your considerations but I notice you are going to reprop and then use your boat (a Hood 38, right?) for cruising.

Where are you thinking of cruising? And to what extent will you be wanting to use marinas, refuel and otherwise maneuver in close confines? And finally, do you have a bow thruster?

I mention all this because I find, more and more, that prop walk is far more often our ally than our opponent when maneuvering, and that this growing awareness has resulted from the cruising areas we''ve frequented (less so in the Caribbean, moreso in some parts of the NE USA and Europe). And our boat is not unlike your own (42 x 33 x 21,000#). WHOOSH does have a bow thruster - it''s my wife, with a boat hook - but even so, we pretty much have to rely on what we can make the boat do via the engine, and by being cagey about wind direction. INVICTUS is also a fair amount of boat for the engine she carries and I believe, like us, lacks the deep fin around which a boat might otherwise pivot a bit more sharply.

I''m not sure any of this argues against a Campbell prop. Guess I just wanted to stir some add''l reflection of the benefits of prop walk given the limitations we otherwise probably share. <g>

Jack
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Old 11-11-2004
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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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