Replies to This Discussion

Permalink Reply by Tom Hruby on May 30, 2012 at 1:55am

Hi Greg,

I checked out the web page, and wow, that's a nice looking prop.  However, I think it probably is overkill for an electric drive and probably that much more expensive than a regular prop. 

This prop was designed to optimize the power coming from an ICE (internal combustion engine) that has a completely different  power curve from an electric motor.  The efficiency for an electric motor is fairly linear for the entire rpm range of the motor so you do not loose power (or use extra battery power) if your rpms change.  This is not true for and ICE.  For example, many diesels have a "sweet spot" right around 1800 rpms where you get the maximum power per gallon of gas.  Since an ICE is only at most 50% efficient the power losses at higher and lower rpms can make a difference. 

An electric motor on the hand is usually around 90% - 95% efficient so it puts out all the power it gets from the batteries to the prop.  Furthermore the "sweet spot" for many electric motors (at least the ones I have looked at) is closer to 3000 rpm.  This means that it would take a lot of adjustments to the prop to make it work as designed, but you would get only a few percentage increase in the efficiency of the system.

 

I think a bigger issue for electric propulsion is sizing the propeller correctly.  Since the motor is quite efficient it is the efficiency of the prop that will determine the overall efficiency of your system and your biggest power loss.  For sailboats and workboats traveling close to hull speed the general rule is "the bigger and slower the better."  Big propellers moving as slow speed lose the least amount of power to "slippage" in the water.  Today ICE's run at high rpms and this requires smalll propellers unless you have big gear reductions. This means you have a lot slippage and power loss.  Today one compensates for that with more horsepower.  in the "good old days" the Acadia gas motor that was used to power St. Pierre dories was only 1.5-3 horsepower.  This was all that was needed because the motor ran at 400 rpm and turned a 20" prop directly for a 26ft boat.  Very little slippage so the all the power went into pushing the dory to hull speed.

You can run an electric motor quite efficiently at 1000 rpms.  All it will require bigger cables to the motor because as the rpm's go down the current has to go up for any given power level. This doesn't mean it uses more power from the batteries, just that electric motors run lower voltages at low rpms and require higher currents from the speed controller. So a variable pitch prop is not really needed for an electric system.

Tom

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Permalink Reply by Bill Choice on June 1, 2012 at 2:55am

I have two three-blade 22" MaxProps on my 55' electric cat.  I tested them against fixed 19" props recomended by the propulsion system company and believe I only lost around 5% in performance.  The great plus is the ablility to feather the props for sailing if desired.  My cat is high performance and I like to see it perform.  Draggin fixed props all of the time, although providing regeneration, slows the boat and creates drag noise which I don't like.

 

MaxProp now has a newer prop design than mine that I believe will provide better performance when motoring, but retains the option to feather and go fast!

 

Bill

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Permalink Reply by Bill Choice on June 26, 2012 at 2:35am

Tom:

 

Regarding your above response, in your opinion would I benefit in motoring performance by increasing the dimension of my props?  I have room to increase them by 2 inches I think.  Would this potentially overheat the electric motors?  I would like to find a way to increase performance in both max motoring speed and in torque when motoring in heavy weather situations.  And if a prop becomes closer to the hull, is there danger in damaging bottom paint or causing cavitation?

 

My cat will cruise at 12 - 15 knots under sail and at that speed dragging fixed props makes a lot of noise I don't like to hear.

 

Thanks.....

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Permalink Reply by Greg Martin on June 22, 2012 at 5:36pm

Thanks for the replies. I think there might be two benefits to these types of propellers. One, it would allow you to adjust the pitch, especially if you were experimenting and wanted to get just the right compromise for thrust and regeneration. Second, it would allow you to feather the props for sailors concerned with drag. I would be interested in having this just to experiment, as I am especially interested in getting my system to regenerate. How much drag would you have to tolerate in order to maximize regeneration? 

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Permalink Reply by Tom Hruby on June 26, 2012 at 2:05am

Greg,

Your question about drag is open ended.  It all depends on how much power you wish the prop to produce.  Think about it in terms of reverse power needs.  Your boat needs a certain number of horsepower (kilowatts) to move it forward.  In reverse if you want the motor to generate power you need to decide how many watts(or kilowatts) you want to generate.  As you configure the motor and the prop to provide more power for generation your drag will increase.  You need to decide how much regeneration you want, and therefore how much drag you will accept.  The power curve for electric motors is pretty linear, so there is not a very big "sweet spot" where you get the best regeneration rate for a given drag.

Also, if you go to regeneration you will need some kind of charger control that controls the voltage to the batteries. The volts coming from the motor will be variable and at slow speeds will be below the charging levels needed for the battery pack.

 

Tom

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