Prop/Speed Question

[CraigP]

Hey guys, what do you all think is more efficient for lower current draw?
Run a large prop slower or a small prop faster? Question asked in reference to just typical setups...

[martin]

Personally I think a lower kv motor with a larger diameter prop, the larger prop should be more efficient as well.

[srislash]

The theory is the larger prop, but... I find that corners suffer on a twin cat with larger props. This may differ with the mixer involved. With rpm come the spikes in amps so it depends on the application.

[fweasel]

When the word efficiency is used to describe a prop, does that refer to the amount of slip in the water?

[NativePaul]

There are 2 parts to prop efficiency.

One is blade slip, we are not driving a screw through a solid lump of wood, screwing through water as we do there will always be some slip, so the speed you travel will be less than the prop's pitch times the RPM, the less slip you have the more efficient the prop is. For a given design and weight of boat a larger diameter prop will slip less, and thus be more efficient.

The other aspect to efficiency is that it takes power to spin a prop through the water, whether it is doing work or not. Obviously the higher pitch a given diameter prop has the more power it takes to spin it at a given RPM, but now imagine a 2 blade prop with no pitch at all, it is not doing any work, but it does still take power to spin it, and the bigger the diameter, the more power it takes, just to spin it before any work is done, so bigger props are less efficient.

As you can see, these factors are in opposition to each other so like almost everything in boating what is beat for you will be a compromise, I think that with higher power levels (for a given design and weight of boat) the slip percentage will be more important, and for lower power levels the power consumed just to spin a prop is more important.

In the Naviga racing I do we are strictly limited in power via long run times and lower battery limits, and yet the majority of us run higher RPM than the majority of you in the states do, but with smaller props and throttle control to bring the average currents way down.

[fweasel]

Great info, thanks Paul

[CraigP]

Thanks Paul, that helps me tremendously!

[Speed3]

To turn a smaller prop that can plane the boat efficiently at a higher speed is more efficient in terms of energy consumption and in speed.

Example I ran a impulse on 5s turning an octura m545 with a 1800kv aquacraft motor. The boat does 44mph on gps.

Now the same boat, same motor with an octura x642 on 6s pulls similar amps but it does 49mph in this case.

So from this example I think it is evident that spinning a smaller prop faster can give more speed with similar runtimes. Or similar speed but greater runtime using the smaller prop.

[Moonie]

A bigger prop has to work harder per revolution than a small prop does. Lower kv motors will need to work harder per revolution than a higher kv motor going at the same speed. I use to believe in the low KV big prop theory now I’ve found it to work better with more KV same voltage and small prop.

[CraigP]

Yes Moonie, but the big question is does the drop in motor efficiency going with a higher KV offset the torque requirement to spin a larger prop. Lower KV motors have a better eff. So a big consideration is the KV between the two test cases. From my experience, I’ve seen between 5-6% difference in eff between a 1050kv and a 2200kv motor. So thinking about that, are there other thoughts?

[TRUCKPULL]

What I have found over the years is;
Every hull when setup, Has a preferred prop diameter to run at it's best.

Now all you have got to figure out is what is the correct motor size, KV, batteries, prop pitch and blade area, and over all weight and setup to make that hull run it's best.

Larry

[CraigP]

Absolutely true! What I’ve experienced so far as a general rule, that as the boat weight goes up, a lower KV motor and bigger prop seem to work better. There’s some good ideas and thoughts here, it’s been a good discussion!

[Moonie]

Yes Moonie, but the big question is does the drop in motor efficiency going with a higher KV offset the torque requirement to spin a larger prop. Lower KV motors have a better eff. So a big consideration is the KV between the two test cases. From my experience, I’ve seen between 5-6% difference in eff between a 1050kv and a 2200kv motor. So thinking about that, are there other thoughts?

I don't think about things to much before I go rite out and try it to find out the results. I've try before and I believe smaller props are better over large ones. Better is more efficient in this convention.

[Moonie]

I think of it as the rpms go up as the weight goes down and vise Versa.

[CraigP]

I think of it as the rpms go up as the weight goes down and vise Versa.

I think so too... Seems the bigger, heavier boat can handle the additional prop torque over a smaller, lighter boat. I do try to select motor, kv, voltage and prop as part of planning a build. I like thinking about that stuff and see how close I come to optimum out of the box...