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Trolling Motor Prop


Question

Posted

I've seen a 3-bladed prop that will fit on my motor(38# Endura) and was wondering if it would be worth buying. The maker claims it will give you more speed, use less power, extend runtime, and just plain make life great.

I don't know much about prop design.

So,are 3 blades better than 2?

11 answers to this question

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  • 0
Posted

Never tried one but it sure sounds like it may work.

  • 0
Posted

From other things I've read, two blade is actually the best design for a prop (peformance wise). The problem is that at high RPM's you will get wobble and vibation.

Myself, I think it's just a marketing ploy. Unless you start getting up into some high RPM's I think 2 blade is fine.

The way I think about it, is that it's only logical that it would take more energy to spin 3 blades vs 2.

  • 0
Posted

On the two blade vs. three blade topic, my dad's got a Mercury 9.8hp (I think it's an '83) that has a two blade prop on it, and it works fine. I've never had any problems with wobble on it. I think it's the only outboard with a two blade prop that I've ever seen, though.

  • 0
Posted

A lot of the older, small outboards, had a 2 blade prop, again not high enough RPM's to notice and significant side effects.

  • 0
Posted

I'm not a physics major but I would think the amount of energy used will be more dependant on the amount of thrust generated (water moved over time) rather than the number of blades. Certainly there is an added friction component but I would think this is minimal. I would also be inclined to think that three blades would be more efficient than two blades and provide for more balanced load on the motor. This could increase motor efficiency as well. Finally, as the number of blades are increased, is the prop pitch reduced proportionally since there are 50% more blades moving the water? If so, the friction component becomes even less a factor.

Bob

  • 0
Posted

It seems to me that total surface area of the prop figures in to this somewhere.

  • 0
Posted

If you want to get scientific, here's what factors into the design of a prop:

+the number of blades B

+the axial velocity v of the flow (flight speed or boat speed)

+the diameter D of the propeller

+the selected distribution of airfoil lift and drag coefficients Cl and Cd along the radius

+the desired thrust T or the available shaft power P

+the density rho of the medium (air: ~1.22 kg/m³, water: ~1000 kg/m³)

The design procedure creates the blade geometry in terms of the chord distribution along the radius as well as the distribution of the blade angle. When you look at the resulting geometry, you might understand why highly efficient propellers for man powered aircraft, indoor free flight models or FAI-F1B rubber powered models look as they look like. The local chord length c depends mainly on the prescribed lift coefficient Cl - if you would like to have wider blades, you have to chose a smaller design lift coefficient (resp. angle of attack) and vice versa. It should be noted, that the design procedure does not work accurately for high thrust loadings as they occur under static conditions. If you receive nonsense values for the blade chord, the power loading of the propeller is probably too high. Check if the power coefficient Pc is less that 1.5, otherwise the theory is not fully applicable and may lead to errors.

The Number of Blades

The number of blades has a small effect on the efficiency only. Usually a propeller with more blades will perform slightly better, as it distributes its power and thrust more evenly in its wake. But for a given power or thrust, more blades also mean more narrow blades with reduced chord length, so practical limits have to be considered here. The chord length can be increased while decreasing the diameter to keep the power consumption constant, but a diameter reduction is usually a bad idea in terms of efficiency, as long as the tip mach number or tip cavitation is not an issue.

The Velocity

The velocity of the incoming fluid together with the velocity of rotation (r.p.m.) determines the pitch distribution of the propeller. Large pitch propellers may have a good efficiency in their design point, but may run into trouble when the have to operate at axial velocity. In this case, the blades tend to stall. usually the best overall propellers will have a pitch to diameter ratio in the order of 1.

The Diameter

The propeller diameter has a big impact on performance. Usually a larger propeller will have a higher efficiency, as it catches more incoming fluid and distributes its power and thrust on a larger fluid volume. The same effect can be shown for lifting surfaces, which results in sailplanes having large span but slender wings.

Lift and Drag Distributions

Instead of the lift and drag coefficients, it usually convenient to specify an airfoil with a prescribed polar and the design angle of attack at each radius. In JavaProp, definition sections are spread along the radius where airfoil and the design angle of attack can be prescribed. The distribution of Cl and Cd along the radius can be examined later by performing an analysis for the design point. For maximum performance, the airfoils must operate at maximum L/D. But if the propeller should also work reasonable well under off-design conditions, it is usually necessary to use a lower angle of attack for the design. Again, you can check the Cl and Cd distributions for off-design cases by performing several single off-design analysis for different settings of the flow velocity v. Stall should occur gently when the velocity is reduced. The analysis code will probably give unreliable results for very small velocities.

The Fluid Density

The density of the fluid has no influence on the efficiency of a propeller, but strongly affects its size and shape. As the forces and the power are directly proportional to the fluid density, a hydro-propeller will have much smaller dimensions than a propeller working in air. Also, lifting surface under water tend to develop cavitation when the local pressure of the flow field falls below the vapor pressure. Therefore it is not possible to use high lift coefficients in hydro-props, usually they have to stay below Cl = 0.5. The same is true for high speed tips of aircraft propellers, where not cavitation, but supersonic regions may occur if the pressure gets too low. Therefore the tip sections of propellers operating at Mach numbers above 0.7 should be designed to operate at small lift coefficients below 0.5 too. The analysis module of JavaProp uses the fluid density to calculate thrust and power during the multipoint analysis. The dimensionless coefficients Ct and Cp are not affected by a variation of density, but the values for thrust and power are. Thus a propeller engine combination will find different operating points depending on the fluid density. This makes a difference for aircraft propellers, where the performance of propeller and engine depends on the altitude.

If you really care about the source, e-mail me.

  • 0
Posted

You CANNOT swap props around on electric trolling motors like you can with gasoline outboards. By playing with pitch, diameter, blade flex strength, # of blades, etc... you may end up drawing more amps. This may burn wires, switches, or armatures. It certainly will void any electric motor warranty!

Do NOT do this!

  • 0
Posted

Now that's what we're talking about!

Great info.

Bob

Also, good advice regarding the motor amp draw. Make the motor work too hard and you just may overheat the windings.

Bob

  • 0
Posted

I field-staff for Motor Guide. I would be very leary of using any prop. on a trolling motor that is not manufactured by the same trolling motor company. On Motor Guides a 2 blade prop will give you more power than the 3 blade Machete or 3 blade Hydrilla Hacker. The new Hydrilla hacker is my new best friend, this baby has gone thru everything I've attempted to motor thru and not clogged up.

  • 0
Posted

Thanks for the replies everyone.

I kinda thought it was one of those "too good to be true" kind of things.

I think I'll just leave well enough alone.

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