Prop Selection for Electric Motor

[omaolchonaire]

Hello, I have owned my boat three years and often browsed the forums here, but this is my first post.

I have a Rawson 30, which is currently on the hard for paint and major mechanical. I removed the old Isuzu engine and hydraulic drive.

I am installing 8 x 100 W solar panels for my 48v system, with a separate 2 x 80 Watt into my 12v home power bank which I can switch to series wiring in a pinch, with a 48v 2kw wind generator up on a mizzen mast and some other means of charging. Most likely I will use LiFePo4 cells to make a 100 AH 48v battery bank, but if I can find these GM L-ion battery packs that are 24v 100 AH again I may use those. Either way I plan to double that to 200 AH ASAP.

My preference is to use the Curtis AC-34 motor but may use one of their DC motors for cost savings. Either way, the maximum RPM is 10,000. I've been told about 1000 RPM is ideal for a sailboat, max 1200-1500 from a good source, is that correct?

I am using a drop down belt-driven gear reduction. Should I be looking at gear reduction in the 6:1-8:1 range ?

Also, the current prop needs replacement, it is a 12". I am thinking a 5 blade may be the best idea to maximise prop drag, and increase charging capability. Thoughts ? Should I reduce prop size in that case ?

[roverhi]

Isn't the real advantage of electric motors instant high torque from near O rpm?? Thought they didn't need a reduction gear or transmission??

[StuM]

Yes, you want to aim for a prop speed of around 1000 RPM.

Why would you want to maximize prop drag on a sailboat?
(Presumably you are looking at re-generation - but on a boat like a Rawson 30, you are not going to be getting sufficient boat speed for any appreciable re-gen.)

[MartinMV]

It is not important, what RPM your motor does, but what the shaft RPM will be after reduction.

Higher RPM = smaller propeller
smaller RPM = bigger propeller

I just read "the nature of boats" and there was a chapter about propeller and calculation.

But without knowing, what RPM your shaft will do, that won't help.

I would take a motor that does between 600 and 1000 RPM in the first place. That would spare you a gearbox.

Also I would look at the IP or Nema savety classes. Water and high current do not mix well.

Your motor including cable plugs and controller should be watertight and have an encasing that won't rust. I doubt that the AC 34 motor is a good idea for a boat.

Otherwise you will risk your live sooner or later.

[grjfield]

I switched over to electric about 3 years ago and based on sage advice, stayed with about the same prop specs (12"). However because the electric does generate instant torque, I questioned why I couldnt use a larger prop, thinking I would get better spec at low revs. For now I am quite happy with the arrangement as I can almost hit hull speed at about 6.5 knots (lowers range significantly though)
My reduction is about 2:1.
I expect the science of electric boats will mature and the prop calcs may be upgraded for this relatively new product.

I'm curious if anyone has a line on lower cost Lithium batteries yet. For safety I'm leaning toward Lithium Iron Phosphate.

[Stumble]

Originally Posted by grjfield

I switched over to electric about 3 years ago and based on sage advice, stayed with about the same prop specs (12"). However because the electric does generate instant torque, I questioned why I couldnt use a larger prop, thinking I would get better spec at low revs. For now I am quite happy with the arrangement as I can almost hit hull speed at about 6.5 knots (lowers range significantly though)
My reduction is about 2:1.
I expect the science of electric boats will mature and the prop calcs may be upgraded for this relatively new product.

I'm curious if anyone has a line on lower cost Lithium batteries yet. For safety I'm leaning toward Lithium Iron Phosphate.

A larger prop will be more efficent, but there is a practical maximum. Most boats with a shaft drive already use the largest prop their drive system can handle, make it to large and it starts causing cavitation on the hull. So you could use deeper running gear, but that adds a lot of drag, reducing efficiency. I am not saying that shaft placement on sailboats is optimal, but it isn't that far off in actual terms.

So certainly conversions are generally wise to stick with the same size prop (assuming it was the right size to begin with). And match the electric motors rpm to the old shaft rpm.

Adding a reduction gear is just a massive mistake. It adds a lot of inefficiency to the system, makes the system more complicated, and doesn't really get you anything. Just buy a motor with the right rpm to begin with.

[grjfield]

Perhaps shaft drives have their own issues. My saildrive has enough clearance to handle a 17" blade, most certainly a 16".
The 'matching rpm and prop argument' may be the right way to go...its certainly the safest.
However, I cant help think I am missing an opportunity to get higher speeds at lower amp draws by not utilizing the higher torque available. And as noted, the major constraint for electric systems is range. Being able to gain speed at lower amp draw (via a larger prop) is possibly a great way to boost range.

[Stumble]

Originally Posted by grjfield

Perhaps shaft drives have their own issues. My saildrive has enough clearance to handle a 17" blade, most certainly a 16".
The 'matching rpm and prop argument' may be the right way to go...its certainly the safest.
However, I cant help think I am missing an opportunity to get higher speeds at lower amp draws by not utilizing the higher torque available. And as noted, the major constraint for electric systems is range. Being able to gain speed at lower amp draw (via a larger prop) is possibly a great way to boost range.

If you can swing a larger prop then you should add one. But that would be true if you had a diesel or electric.

Are accounting for 20% tip clearance berween the prop and the hull? That's would be a pretty big prop on a 40' boat.

[StuM]

Originally Posted by grjfield

Perhaps shaft drives have their own issues. My saildrive has enough clearance to handle a 17" blade, most certainly a 16".
The 'matching rpm and prop argument' may be the right way to go...its certainly the safest.
However, I cant help think I am missing an opportunity to get higher speeds at lower amp draws by not utilizing the higher torque available. And as noted, the major constraint for electric systems is range. Being able to gain speed at lower amp draw (via a larger prop) is possibly a great way to boost range.

Torque is not power.

It takes a certain number of Watts( or Horsepower) to drive a boat at a given speed in given conditions. Doesn't matter whether you are turning a bigger prop at lower speed or a smaller prop at higher speed - you still need the same amount of Watts/Amps.

[shyguy68]

My best suggestion for this is to browse youtube for Sailing Uma, and look at their videos as Dan took a motor from a forklift and used it on their sailboat and the only glitch they've had since is the batteries quit (can't remember why)

[keith4001r]

Originally Posted by shyguy68

My best suggestion for this is to browse youtube for Sailing Uma, and look at their videos as Dan took a motor from a forklift and used it on their sailboat and the only glitch they've had since is the batteries quit (can't remember why)

They were maintaining the batts improperly.

[omaolchonaire]

Originally Posted by roverhi

Isn't the real advantage of electric motors instant high torque from near O rpm?? Thought they didn't need a reduction gear or transmission??

The maximum RPM on the electric motors I'm looking at are max 10,000 RPM, and operate most efficiently near peak RPM.

[omaolchonaire]

Originally Posted by StuM

Yes, you want to aim for a prop speed of around 1000 RPM.

Why would you want to maximize prop drag on a sailboat?
(Presumably you are looking at re-generation - but on a boat like a Rawson 30, you are not going to be getting sufficient boat speed for any appreciable re-gen.)

I thought it would be a good idea to maximise prop drag because the prop drag is what causes the force for the charging. But yeah, the Rawson's max hull speed is 6.29 knots and the systems start charging at 6 knots.

[omaolchonaire]

Originally Posted by grjfield

I switched over to electric about 3 years ago and based on sage advice, stayed with about the same prop specs (12"). However because the electric does generate instant torque, I questioned why I couldnt use a larger prop, thinking I would get better spec at low revs. For now I am quite happy with the arrangement as I can almost hit hull speed at about 6.5 knots (lowers range significantly though)
My reduction is about 2:1.
I expect the science of electric boats will mature and the prop calcs may be upgraded for this relatively new product.

I'm curious if anyone has a line on lower cost Lithium batteries yet. For safety I'm leaning toward Lithium Iron Phosphate.

The LiFePo4 can be had online for about $126 US for a 3.2v 100 AH cell. I forget the name of the retailer but can look it up if that is what you're after. The other product I'm looking at is the 24v 100 AH L-ion battery pack for the Chevy Volt, which I saw new on Ebay for $649 US each a few weeks ago, but haven't been able to find since.

[omaolchonaire]

Originally Posted by Stumble

A larger prop will be more efficent, but there is a practical maximum. Most boats with a shaft drive already use the largest prop their drive system can handle, make it to large and it starts causing cavitation on the hull. So you could use deeper running gear, but that adds a lot of drag, reducing efficiency. I am not saying that shaft placement on sailboats is optimal, but it isn't that far off in actual terms.

So certainly conversions are generally wise to stick with the same size prop (assuming it was the right size to begin with). And match the electric motors rpm to the old shaft rpm.

Adding a reduction gear is just a massive mistake. It adds a lot of inefficiency to the system, makes the system more complicated, and doesn't really get you anything. Just buy a motor with the right rpm to begin with.

The motor would be in the bilge then, and the motors are all 10,000 RPM max. The boat had a hydraulic drive, and the lower pump sat in the bilge water. Pretty sure I would lose more efficiency than with a gear reduction if I put the hydraulic drive back in.