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galamb
Post subject: Re: Trolling Motor 2 or 3 blade prop
PostPosted: Tue Jan 05, 2010 8:29 am
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Joined: Sat Jan 17, 2009 1:37 am
Posts: 715
Location: Inverary, Ontario - Cuddy Sport (modified)
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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