Are 2 blades more efficient than 3?

[cmedsailor]

Please ignore for the moment the drag issues of the 3 blades propellers and their better bite against heavy seas.

Some people claim that 2-blades are more efficient whereas some others say they are not because of the water disturbance. So, what’s the story with having a 2-blades propeller? Assuming you have the CORRECT SIZE propeller will 2-blades perform better than 3-blades in calm to moderate conditions?

 

[savageseadog]

I think one of the benefits of 3 blade propellers is the reduced diameter for equivalent drive over a 2 blade. Other than that I can't really comment.

 

[Coaster]

From the Propeller Handbook by Dave Gerr (ISBN 978 0 07 138176 5 if you want to buy it), Chapter 4 "Blade Characteristics".

"How many blades? Surprisingly, the ideal is one. A single blade does not have the other blades disturbing the water flow ahead of it. Unfortunately, trying to get a single-bladed propeller to balance is like trying to clap with one hand. Having two blades is the logical answer. Both sailboats trying to reduce drag and high-speed motorboats frequently use two-bladed propellers. The problem with two-bladed propellers for most vessels is that such propellers require very large diameters to get the blade area for effective thrust. As a result, three-bladed propellers have generally proven to be the best compromise between balance, blade area and efficiency."

So the propeller choice depends on the boat in question and what you want to do with it. For a motor sailing, which we do a lot, a three-bladed Bruntons Autoprop was the answer.

 

[Nigel_Ward]

As already stated the "optimum" 2 blade prop will be slightly greater diameter and slightly more efficient than the optimum 3 blader to do the same job.

However, blade number choice is down to other factors e.g. vibration.

 

[cmedsailor]

Just to make it more interesting... I recently had the chance to make a 500 miles trip with a friend that involved a lot of motor-sailing. Two boats; my boat with a 16X11 2-blade fixed propeller and a friend's exactly same model and year boat with a 16X10 3-blade fixed propeller (the two boats were more or less loaded the same, same sails, both having same number and weight (!) of crew). For the whole trip and all kinds of sea state (from calm to 1 meter waves at the bow) the two boats seemed to maintain exactly the same speed. The only difference was that I had to use 100-200 rpm less probably due to having a very clean hull and propeller in comparison to the other boat (or due to the different pitch?).

But I must admint that depsite having the engine aligned and four new mounts I still get some vibrations whereas he doesn't. And I am sure I have to blame the propeller for this.

 

[sarabande]

My Lords Commissioners of the Admiralty are of a different opinion...


but I bet they don't have problems with pot buoys.

 

[lw395]

1) A fixed prop has no place on a SAILING boat.
2) Either 2 or 3 fixed blades should be perfectly balanced dynamically. This gets to be a bit more of problem with folding props as the moving parts wear.
3) Efficiency wise, a 3 blade prop can be further from the hull, because it is smaller for the same blade area, alternatively it can have more blade area for the same diameter. So a 3 blade prop tends to be more efficient. But also more efficient as a brake when sailing.

A clean hull is the most important thing, followed by having a prop of good design and correct pitch. Other things can affect 'identical' yachts too, the engines may be the same make and model, but they coke up their exhausts, the timing drifts a little etc etc, and power output can change markedly. Also the engine doing a few more revs may not have been using more power or fuel.

 

[Robin]

You have to transfer the horsepower used and when this is too much for the blade area you have a problem with slip. For motoring a 3 blader is much better.

We had a Liz 30 with a Bukh 10 25 years ago and were in company with another identical one trying to get out of St Peter Port with NE 6-7. The only difference between us was We had a 3 bladed fixed and the other a 2 bladed fixed, but we made headway under engine against the wind and sea (wind against tide and very rough) and our 2 bladed friends did not, their prop was cavitating.

If you want better performance under sail then go for a self pitching feathering prop like a Brunton Autoprop, that will give better results under power AND sail.

If you want a 2 bladed 17" dia bronze folder however I can sell you one!

 

[NormanS]

If the propeller is mounted astern of a sternpost or similar structure, the water flow to it is obstructed by the sternpost. This can cause vibration and lack of power with a two bladed prop. A three blade prop is still affected by the sternpost, but more evenly.

 

[Little Rascal]

It’s important to differentiate between ‘efficient’ and ‘optimum’. Efficiency becomes important with low power available but with oodles of hp the way to transfer it is with more blades - if you don’t want a bigger prop.

It’s one reason that the Spitfire Mk I had a two-bladed airscrew but by Mk22 it had 5 blades and more than twice the hp.

That’s also why big ships have multi-bladed props.

Optimum for your boat may not be the most ‘efficient’ under test conditions.

 

[NormanS]

It’s important to differentiate between ‘efficient’ and ‘optimum’. Efficiency becomes important with low power available but with oodles of hp the way to transfer it is with more blades - if you don’t want a bigger prop.

It’s one reason that the Spitfire Mk I had a two-bladed airscrew but by Mk22 it had 5 blades and more than twice the hp.

That’s also why big ships have multi-bladed props.

Optimum for your boat may not be the most ‘efficient’ under test conditions.[/QUOTE

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