Glen-L.com

Good Morning boaters,
Just a quick question here. What are the main differences between a 2 blade and a 3 blade prop? It would seem I would expect to get more speed from a 3 blade? Anyone?
Dan.

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Anywhere But Down
http://dansboathouse.blogspot.com/

The main difference between a 2 blade prop and a 3 blade prop is that the 3 blade prop has one extra blade.





I'm sorry, I just couldn't resist that. Please forgive me.

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Isn't it amazing!! The person that never has the fortitude to pursue his own dreams, will be the first to try and discourage you from pursuing yours.

The three blade catches more grass. THe extra blade I suspect causes more drag on a smaller torgue motor which will probably use more electric. Maybe this is the reason for the two blades on the smaller electric motors.

All kidding aside, I think Mike is correct. I believe that the 2 blade prop will be higher revving meaning it will use up the battery more slowly but the 3 blade prop will take a bigger bite. Better for a larger boat.

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Isn't it amazing!! The person that never has the fortitude to pursue his own dreams, will be the first to try and discourage you from pursuing yours.

All of the above are correct but what it rely boils down to is what your end result is. If you are only going to run any kind of motor at one speed then the propeller size and blade number are easy to calculate. But boaters seldom do that as we change speed for countless numbers of reasons and the load onthe motor changes accordingly. If you expect your motor to operate at various speeds then the propeller size and number of blades is always a trade off.

Ok then,
If I'm understanding all of you correctly the end result is this,

If I run a 3 blade I might have more torch until I hit the weeds. Then I will vary my speed accordingly and wish I had 2 blades. But then I realize that by slowing down I have more battery left. At which time It dawns on me that the only difference between the 2 & 3 blade prop is 1 blade.

Now I'm thinking what a guy could do by adding 1 more oar to a boat....

_________________
Anywhere But Down
http://dansboathouse.blogspot.com/

Propellers, whether they are on an electric, gas or diesel motor are probably one of the most misunderstood factors on the boat.

Here is some "general" information on props -

For a given pitch the prop that would give you the MOST SPEED (all else being equal - horsepower or thrust of the motor, weight of the boat etc) would be a SINGLE blade prop.

You won't find one because it would run so unbalanced that it would potentially cause damage to the engine.

For each blade you add to the equasion, you create more drag, but create more forward thrust (and make the motor work harder to turn it), but you also reduce vibration which smooths out the ride.

So a 2 blade will give you more speed than a 3 blade, and the 3 more than a 4, or a 5 etc, again when all else is equal.

On the flip side of that, for each rotation of the prop, you get more forward thrust with a 4 blade than with a 3 or a 2 - but again, it is more difficult for the motor to turn a 4 blade than a 3 or a 2.

Likewise, if you increase the PITCH or DIAMETER of prop you are using, it is more difficult for the motor to turn it. Unless you are "underpropped" to start with, running more pitch will do little to increase your top speed and could in fact result in engine damage.

If the motor is working harder, you burn more fuel (unless you work it so hard that it stalls).

So in the case of your "electric" - you will use more electricity (draining your battery quicker) to turn a 3 blade at the "same" rpm as a 2 blade, but you will get more thrust (more forward motion) at that specific rpm compared to the two blade.

General "rules of thumb" for props -

If you want the MOST SPEED possible you run a 2 blade

Best all round "average" performance - 3 blade

Best "out of the hole", "lower rpm plaining" and "smoothest running" - 4 blade or 5 blade

Each boat/motor combo needs to be fitted with a prop that is best suited to how the owner or operator will use the boat.

An electric trolling motor was never designed to produce speed (as it's primary reason for existance), but if you use it to poke around on weedless flats a 3 blade prop would work well. However, as has been noted here, if you are in a very weedy environment a 2 blade will catch less grass and tends to "self clear" a little easier - and when you want to go fast, hoist out the electric and fire up the gas outboard

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Graham

Yes, Plywood is "real" wood

A "professional" is someone who gets paid for their work - it doesn't necessarily mean they are good at it

In spite of all the kidding, this turned out to be a good thread. Galamb, that is an EXCELLENT explanation. I knew what it was but I didn't know how to explain it.

Thanx again.

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Isn't it amazing!! The person that never has the fortitude to pursue his own dreams, will be the first to try and discourage you from pursuing yours.

Galamb,
I agree with Dave. Very well said and now understood.
D.

_________________
Anywhere But Down
http://dansboathouse.blogspot.com/