Electric Conversion – Huge Prop vs Rear Reduction

[TitoSoto]

I'm putting all the plans together for my 38' 48v electric sailboat conversion project.

Wouldn't it be better to skip the gear reduction on the electric motor altogether and instead get a huge prop with a lower pitch?

In other words, instead of compensating for RPM's via the motor, do it via the prop?

[paulajayne]

High RPM on the prop will cause cavitation.

[TitoSoto]

Whoops I totally meant a smaller (high rpm) prop.

[stefano_ita]

get a lot of reduction ratio, a quite big diameter and quite long pitch propeller running at low rpm (500-600 rpm, best efficiency)

[motion30]

does the electric motor reverse or will you need a transmission with a reverse gear?

[stefano_ita]

Originally Posted by motion30

does the electric motor reverse or will you need a transmission with a reverse gear?

they can be reversed any time...just a cheap 2 gear for the reduction and it's done

add solar panels and battery...a small emergency gen and you are done

[TitoSoto]

Originally Posted by stefano_ita

they can be reversed any time...just a cheap 2 gear for the reduction and it's done

add solar panels and battery...a small emergency gen and you are done

So you would recommend doing the reduction via a gear VS a smaller prop? Both, I think, would probably produce the same outcome but a gear would add more complexity and cost...

[Stumble]

Part of the issue in adding a larger prop is most boats come with a maxed out prop size size anyway. It would take a pretty serious modification to the boat to allow a much larger prop than what is already being spun.

[GuidoY]

Just 2 cents, and worth as much.

Larger prop w/2:1 may end up being same as smaller prop direct. But the addition of a reduction may give you more options as to your exact power output when configured with various prop sizes.

I think prop size is limited to how close the edge of the blades come to the hull, ideally that space is a minimum 15% of prop diameter.

But basically your looking to require less torque and lower RPMs from the motor to produce comparable power but maintain battery life, yes?

[a64pilot]

He had it backwards guys and caught his mistake, for high RPM of course that takes a smaller prop, one disadvantage is of course reduced thrust and efficiency, advantage is simplicity of direct drive and lower weight and less drag when sailing.

Your just going to have to play with it is my guess, at least smaller props are cheaper

[Pauls]

Read "The Propeller Handbook" by Dave Gerr. This is engineering, get the data and do it right. And to answer your question, no, a huge prop with very fine pitch will be quite inefficient. Larger props require lower RPMs.

[manitu]

Previous poster got it right!
do the calculations, it isn't rocket science..

And, while at it, get or make the torque graph for the motor.

1:what is the max torque from the motor/controller combo? (Calculated from controller and motor specs)
2:What is teoretical max rpm at your minimum voltage ? (motor spec - rpm/v)
3:What is max amp the motor can handle continually? (motor spec - amps)

Ok. Max torque is at 0 rpm. from there the torque falls along a linear graph, all the way to max rpm. At max rpm you have zero torque.

you now have the peak output graph.

Now , you could be tempted to run the biggest prop ever , at as few rpm as possible, but you allso have to look at efficiency.
And at low rpm's the motor still has a really low resistance+impedance , and could/would overheat.

As amps are directly linked to torque, you should know by now how much torque the motor can produce continually. It can only make ths torque up to a certain rpm , as you can see in the peak output graph.

But the safest and most effective speed to run the motor at is just above this rpm , as you cant overheat the motor above this rpm (at a given voltage)

then comes the prop and gearing calculations....


.manitu

[Colin A]

Calder had an article in Proboat a few months back about prop size. With electric you really want a larger slower turning prop for best efficiency. As mentioned Gerr has an excellent book on the subject. In general for displacement boats it tends to be more efficient to run a larger prop at lower RPM. Of course it's hard to say with out the details.