2 blades vs 3 blades for motoring

[cmedsailor]

Any idea how much difference would have a 3 blade fixed propeller in comparison to a 2 blade fixed propeller during MOTORING?
I must admit it's probably a difficult question because the only way to know is to have used both propellers on the same boat.

 

[Eeyore]

Are the two propellers of the same diameter and pitch? If they are, then the two blade would be more efficient, giving the same amount of forward motion with only two thirds of the friction (drag). I think!! /forums/images/graemlins/confused.gif

 

[bendyone]

I swapped from 2 to 3 with a GM10 engine and found the 3 bladed was better, more grip and push when running into a sea.

 

[cmedsailor]

"Bendyone" is it possible to say how much better? For example how much increase of speed did you get for the same rpm?
Thanks

 

[PCUK]

I went from two blade folding to three blade folding and gained 2 knots for the same revs, from 4 knots to 6 knots.
Both props were the correct size and pitch for the engine.

 

[jeremyshaw]

No expert, but 3 blades makes more sense to me. The job of a prop is to provide thrust “so the more blades the better”, but that’s offset by the inefficiency of disturbed water, so three is probably the best compromise. Useful discussion on props here:

http://www.westbynorth.com/Choosing.aspx

Extract for those who want the nub of the argument here:
“Every keel has disturbed water coming off of it, be it a streamlined fin keel or a full keel. Water meeting on the trailing edge takes time for the flow to smooth out. Two-bladed propellers turning, go through the disturbed water at the same time. This causes a slippage, in turn inefficiencies because it is on/off/on/off etc.
A three blade propeller is the better way to go. You only have one blade moving through the disturbed water at any time. Two blades are still pushing the boat. We have more efficiency, more thrust, more control and less vibration. Depending on the propeller mind you it may cause slightly more drag.”
Also from here:

http://www.solarnavigator.net/propellers.htm

“How many blades should your prop have? Three blade propellers are the most common, but 4 and even 5 blade props are available. The immediate benefit to increasing your propeller's blade count is increased thrust and a smoother ride (Much like adding more cylinders to your engine). Theoretically, the more blades the lower the efficiency of the propeller. However, increased blade count generally means that each blade no longer has to deal with as much horsepower and consequently the blades can be made a bit thinner which improves their individual efficiency. For example, on a three blade prop running on a 300HP motor, each blade has to handle a 100 hp each. On a 4 blade prop on the same motor, each blade only has to handle 75 HP! This allows the designer to build a prop with a thinner blade without sacrificing stiffness or strength. Unfortunately, the overall efficiency of a multi-bladed propeller is reduced when the blades are forced to run through confused waters - increasing drag. Equally, drag rises to the square (approximately) of speed. Also, a narrow blade, just like a thinner blade passes through the water with less drag. Ideally, therefore, we want a thin section, narrow twin blade, revolving as slowly as possible. Which is just not practical for most boating applications.”

So will this be like an anchor fight then?

 

[cmedsailor]

Jeremy you have confused me now!! Does you answer refers to sailing boats or motor boats?

 

[jeremyshaw]

[ QUOTE ]
Jeremy you have confused me now!! Does you answer refers to sailing boats or motor boats?

[/ QUOTE ]
I was talking about sailing boats. The WestByNorth site is entirely about that - the other is more generic.

 

[ERICH]

Hi PCUK ,
could you give me more detail about the manufacturer , pitch , diameter & approx. cost of your two blade and three blade folding props ?
We currently have a Hunter 27OOD with a 1GM10 driving a two blade folding prop and could do with more oomph !

 

[boatmike]

Cmed, this is a fairly complex subject and needs a hydrodynamicist to give you a full answer. I am not an HD but as an engineer who has been involved with HDs in the design of power systems for small craft of all kinds I think you should look at it like this.

If you calculate the pitch of a prop X its RPM is should give you distance travelled in a minute. Easy to convert this to knots then. Unfortunately this assumes 100% efficiency and nothing is 100% efficient. The difference between theoretical distance travelled and what happens in practice is best referred to a "slippage".
A 3 blader has more area than a 2 blader and therefore gives you less slippage. Unfortunately it's not possible to be precise regarding how much because it requires calculations involving power, hull resistance, headwind, etc which are not uniform constants. The advantage of 2 bladers is that they present far less resistance (drag) when sailing, especially when orientated vertically in line with the keel. There are no other advantages though and if you don't care about that, then the 3 blader will give you more "shove" under all conditions due to reduced slippage. That's why Mobos invariably have 3 blades or more and you only normally see 2 bladers on sailboats.
How much is difficult to say but I would say that......

IF you are not achieving "hull speed" i.e. that speed at which resistance becomes very high, which is probably at a guess about 6 1/2 knots for your boat.
and
IF you have a severe reduction in speed when motoring into headwind relative to flat calm
and
IF you disregard it's disadvantages when sailing

.....then a good 3 blader might well give you an extra 2 knots. i.e. if you are only getting 4.5 knots into a headwind it might get you back to 6 1/2.
It might however also reduce your sailing speed from 6 1/2 knots to 5 1/2 or less though.

Frankly, having read your earlier posts I would tend to leave it be and be more inclined to use more throttle. The extra cost of fuel burnt will be less than the cost of a new prop I suspect and it certainly won't hurt your engine. In fact it may do it good!

I hope this helps put things in perspective....

 

[cmedsailor]

Boatmike thank you very much. Yes indeed your advice on earlier thread was to increase rpm. Actually I am looking forward "testing" this the next time I am faced with headwind.
To be honest, most of the times I was faced with headwind was also faced with waves and actually had to decrease speed to avoid too much "up and down motion". So, it's another question if I really need the extra speed.

 

[Marsupial]

My SO37 when made had a 2 blade, this I changed for a 3 blade - no contest, the three blade was far superior. Ask any MOBO guy and he will tell you that for motoring the more blades the smoother the ride. My current boat has 3 blade volvo folder, the best of both worlds.

 

[davidbains]

Get a quote for a Kiwi prop then you can have three blades, adjustable pitch, and feathering.
I went from a sailing 2 blade prop to a three blade Kiwi and there is a significant improvement in motoring power, and sailing speeds in light airs especially are improved. Reverse is amazing, it transforms manouvering, as long as your gearbox has a higher ratio in reverse.