Three blade vs. two blade

Sep 25, 2008
464
Catalina 30 MKIII Varuna Boat Club
Hi all,
I'm in the process of finalizing a purchase on a 1997 Cat30 MKIII. My previous boat, a 1991 Cat30 MKII had a 3 blade prop and I noticed the new boat had a 2 blade prop. Both boats have Universal 25xp engines.

Other than a cost issue, why would Catalina put a 2 blade on a MKIII?

I salvaged the 3 blade from the old boat and am planning to put it on the new one considering the increase in performance suggested.

Any opinions?
 
Mar 1, 2012
2,182
1961 Rhodes Meridian 25 Texas coast
two blade has less drag when sailing.

I used to own a 35 foot Cross tri. I had a two blade prop on it, worked fine. The new owner switched to a 3 blade. Lost 1/2 knot sailing in light air, due to extra drag
 

Gunni

.
Mar 16, 2010
5,937
Beneteau 411 Oceanis Annapolis
Depends on your definition of performance I suppose. Guys that lean to sailing performance will put a 2 blade prop on and mark the shaft so that they can orient it with the axis of the keel. Less drag. If your prop needs to freewheel the 2 blade prop doesn’t spin as fiercely as the 3 blade.
 
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Feb 14, 2014
7,400
Hunter 430 Waveland, MS
On my boat the transmission is water cooled with the engine sea water pump. My 2 bladed prop spins freely when in neutral [per owners manual], especially on a 20° heel. The transmission gear oil still gets very warm.
If you are going to race, 2 blades.
Jim...
 

nat55

.
Feb 11, 2017
210
Gulfstar 1979 Gulfstar 37 BELFAST
My experience, our previous boat was a Islander 32 MKII, it had a 3 blade prop, got along just fine except that it was a pot warp magnet, took a bit to learn how to deal with that. When we bought the Gulfstar I noted that it had a 2 bladed folding Gori, I was a skeptic, not anymore, it drives that 20,000 boat great and backs like it is on rails, well most of the time, and when sailing I don't worry about catching line any more.

Two blades will move that 30' Catalina just fine.
 
Jul 12, 2011
1,165
Leopard 40 Jupiter, Florida
I've never owned a two-blade, but I've been told by the 'old guys at the dock' that even-numbered blades accentuate imbalance more, and vibrate more frequently. May be an old sailor's tale. What have you guys learned?

Also, to @JamesG161 's post ... if you're going to race (or care about sailing performance in lighter air), then spend the money on a feathering or folding prop. Feathering is the compromise between great sail performance and poor motoring performance of a folding, and the reverse of a fixed prop.
 

capta

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Jun 4, 2009
4,766
Pearson 530 Admiralty Bay, Bequia SVG
You can't just change the prop willy-nilly. A prop is suited to the specific boat in relation to its weight and hull shape, the horsepower and torque of the engine and the reduction gear on your transmission. There is also no guarantee that both shafts even have the same taper, something else you must consider.
Most props can be altered a few degrees in pitch, but of course, the diameter is fixed. Putting on the wrong prop can easily cost you the engine, never mind a loss of performance.
 

Ward H

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Nov 7, 2011
3,645
Catalina 30 Mk II Barnegat, NJ
I have no experience but when I was asked by my instructor buddy what prop I had, I have a 3 blade, he said "good, it will stop on a dime". He has had an 87 C30 for over 25 years.
 

capta

.
Jun 4, 2009
4,766
Pearson 530 Admiralty Bay, Bequia SVG
Yep!
If balance was the key, we would have 3 arms.:cowbell:
_____
Vibrations would come from an improper alignment, mainly.
Jim...
The more blades, the better the balance.
 
Feb 14, 2014
7,400
Hunter 430 Waveland, MS
The more blades, the better the balance.
Well, not so fast.
This is not a US Navy submarine silent drive propulsion.
My Propeller spins relatively slowly thus doesn't need balancing at all.

If more blades is better, then why not 10 bladed propellers?

The manufacturing cost would be excessive.

I would say that less "cavitation" from more blades, for sure.
Jim...
 
Jan 27, 2008
3,045
ODay 35 Beaufort, NC
More blades is more efficient. Current aircraft propeller technology is using 6 to 8 blades. The fan on a hi bypass turbine engine uses dozens of blades. Outboard motors use three or four blades, never seen two blades above a 2 to 4 HP outboard. Your propeller needs to be matched to your motor and boat. With engine running wide open it should be running at its' specified maximum RPM's. If the propeller is over pitched the motor will not get to full RPM's. So you need to see if the prop/motor/boat combination is a match.
 
May 20, 2016
3,014
Catalina 36 MK1 94 Everett, WA
, never seen two blades above a 2 to 4 HP outboard. .
. Look at any racing outboard where they have flying starts including (not an outboard) unlimited hydro’s with 2000+HP are all 2 bladed props. Very efficient at higher speeds not so much at slow speed.

Les
 

capta

.
Jun 4, 2009
4,766
Pearson 530 Admiralty Bay, Bequia SVG
Well, not so fast.
This is not a US Navy submarine silent drive propulsion.
My Propeller spins relatively slowly thus doesn't need balancing at all.

If more blades is better, then why not 10 bladed propellers?

The manufacturing cost would be excessive.

I would say that less "cavitation" from more blades, for sure.
Jim...
On a sailboat 4 is about the maximum for efficiency and lowest drag. Thinner blade surface (more feather shaped), less cup and thicker metal. I'm sure you can see how that is less likely to bend under load.
If your prop is not balanced, it may cost you big time in the end. It will bend the tailshaft with every revolution, which can even lead to a shaft failure. It will wear out your cutlass bearing much quicker and can lead to transmission failure.
I have mine checked for balance every haulout, even if I don't think I've hit anything.
 
Jan 7, 2011
4,727
Oday 322 East Chicago, IN
You can't just change the prop willy-nilly. A prop is suited to the specific boat in relation to its weight and hull shape, the horsepower and torque of the engine and the reduction gear on your transmission. There is also no guarantee that both shafts even have the same taper, something else you must consider.
Most props can be altered a few degrees in pitch, but of course, the diameter is fixed. Putting on the wrong prop can easily cost you the engine, never mind a loss of performance.
I believe the OP said he had a Mark II and now a Mark III, but with the same engine...so probably not an issue using the 3 blade from the Mark II on the Mark III.

Greg
 
Jun 6, 2006
6,990
currently boatless wishing Harrington Harbor North, MD
It is a SAIL BOAT. so the only time you need to actually get to be under power is on a no wind (waves flat?) Sunday when you need to get home or when you have a sail malfunction of some sort converting it into a motor boat and have to get to a safe harbor to fix it or to enter a marina. Consider that in determining the "design speed and RPM Sadly, the concept of "I'm on sail time" is rapidly becoming a distant memory for a lot of us.
you want to pitch the prop so at max continuous power (not WOT BTW) the boat is going hull speed. Variables include transmission ratio, prop slip factor (three blades have less than two blades BTW), max continuous power RPM, hull speed of the boat and of course those are used to compute the desired pitch (inches forward travel while at design speed per prop rotation). The inched forward travel at hull (design) speed is just:
(hull speed nautical miles/hour)*(6000 feet/nautical mile)*(12 inches/foot)*(1 hour/3600 seconds)*(prop slippage factor) = inches/sec @hull speed
The "per rotation" is just:
(max continuous engine RPM)*(transmission ratio)*(1 minute/60 seconds) = revolutions/second @ hull speed
and
Inches/second)/(revolutions/second) = inches / revolution aka the prop pitch
the only trick to this is the slippage factor. It is different for every RPM and wave condition, and wind condition and hull cleanness....... the prop manufacturer is a good starting point but you are basically guessing.
If drag is a concern then get a folding prop. Get one that has adjustable pitch so you can experiment if need be.
 
Sep 25, 2008
464
Catalina 30 MKIII Varuna Boat Club
Thank you all for your input. Lots of good information. Be advised that Catalina offers the 3 blade prop as an option. My MKII came with that option. It's a 13x10x1. Same shaft for either prop. According to Catalina, the props are interchangeable.
In response to use of the engine, I must travel a heavily used, very narrow channel for about a mile until I have the opportunity to sail without getting in everyone's way.
I prefer to stern into my slip and always wondered how a 2 blade would compare in reverse?
Ultimately, after all the years of sailing, experience has always been the best teacher. So, maybe I'll hold off on the optional 3 blade and try the 2 blade for at least this season and then make an educated conclusion.

Happy Sailing
 
Jun 8, 2004
2,841
Catalina 320 Dana Point
I did some trials using a '94 270 with 2 blade and a 2006 with 3 blade, when backing with the 2 blade at idle you could turn to starboard slightly. With the 3 blade you could not go to starboard at all, so a small increase in propwalk was really the only noticeable difference. And the 2 blade boat did have a slight advantage under sail.