Prop for Electric Motor

[Fore and Aft]

I like the sound of that 5 foot prop outboard. On windless days you could swing it upside down and use it like a fan on your sails.
Cheers

[Icarus]

Originally Posted by coldfish

Hi, I had my boat hauled out yesterday and will be having an electric motor installed this week. It used to have an old 4 cylinder diesel volvo in there that got about 70hp. (pretty big for the old 32' boat, but in the 60's, that's what they did)

Now, I'm converting it to electric and bought a kit:
https://www.thunderstruck-ev.com/18-...lboat-kit.html

and a gear reduction:
https://www.thunderstruck-ev.com/cus...reduction.html

The motor is the equivalent of about a 70hp, but unlike the diesel, it has all the torque it gets from 0 rpm and goes up to 6000rpm, so I understand that perhaps I need to change things up a bit.

The old diesel couldn't really handle a quick gear change. You kind of had to let it work it's way up, but once it got "a grip", it could push the boat 7-8k with no problem, but as the RPM increased, you could almost feel it slipping.

It's got a little 2blade prop on it and I'm thinking that, with the new motor, I probably should move up to a larger diameter and go with 3 blades.

Here's the question: How in the world do you decide??? There are tons of props on ebay (I've really gotta keep it under 1k, my budget is tighter by the minute!) and I don't want to screw up.

Is there some way to calculate this in math I can understand?

(current prop: https://ds.morrisdev.com/mo/sharing/ZtUjOoujl)

The prop size depends on the optimal output RPM, around 2200' and average hull speed which is dependent on the WL length.
A 32ft yacht certainly does not need a 70hp motor, more like 25 to 30hp and a 2:1 reduction box. Maybe a a 16" × 12 or so?
Doesn't matter what type of energy source is spinning the prop. Diesel or electricity, the parameters should be pretty much the same for either.

[EWOL Props]

Originally Posted by coldfish

I was told about this, but the guy in the marina next to me also has electric and he said that it was a negligible amount of charge. No where near what he got off a little solar panel, yet the larger prop slowed him down. He changed to a folding prop.

That all depends on the prop type, a feathering prop has the same efficiency in forward and reverse (and hydro-generation). A fixed prop have a reduced efficiency in reverse (and hydro-generation), while a folding has a very poor efficiency in reverse (and hydro-generation).

[wingssail]

Originally Posted by coldfish

I was told about this, but the guy in the marina next to me also has electric and he said that it was a negligible amount of charge. No where near what he got off a little solar panel, yet the larger prop slowed him down. He changed to a folding prop.

It is easy to understand this if you think about it a little.

When you are motoring, say 6 kts using 10 HP, your propellor is throwing a column of water back at 15-20 knots due to propellor slip.

If you could pull that propellor at 15-20 knots while sailing you might generate 10 HP or 7500watts. But you can't sail that fast. Sailing at 5 knots, with slippage, you could generate 1-2hp, max, so 1000 watts, except that the slippage of that prop is much much more at such slow speeds and you get a very small amount of watts out of the regen. We've seen videos (from sailing Uma) showing less than 5 amps (60 watts).

If you had a large two blade propellor, as 60", like Sean advocates, your slippage would be less and you would generate more watts.

[Bullshooter]

I think this idea of a 60" prop can be ruled out. If you look at this diagram of a Cheoy Lee 32':

https://sailboatdata.com/sailboat/offshore-32-richards

there is nowhere near the space available for a prop that big, unless a major re-build of the boat were involved.

Plus the OP already said "I actually did that calculator and came out with a 24 inch prop recommendation. Unfortunately, that's not realistic. Not only can't I find one, but it wouldn't fit in the space I have between the keel and rudder. I've got a 17" now and could probably fit an 18 or 19 before it started just making it worse."

The other concept of also buying an outboard with a 60" prop, in addition to the inboard propulsion is pretty far-fetched. Who makes an outboard that can accomodate that?
Is anyone expecting the OP to carry this outboard with a 5 foot diameter prop on the transom of his boat? He just started the thread to get some input on what prop would match his thunderstruck. If this 60" prop proposed is feasible, please explain what would be involved to make this work.

[Adelie]

Both EEOL & WingSail are right but also wrong.

Props generally are more efficient in one direct than the other. This is because the prop is optimized for forward thrust, so front face has curvature in the flow direction and the back face is flat compared to the flow. There are 2 consequences of this:
A. Reduced thrust in reverse, the curved and flat sides are on the wrong sides for this reversed spin.
B. Mediocre output when using the prop for hydro-generation. While the prop is still spinning the same direction the pressure differential between sides is reversed because the goal is to pull energy out of the water instead of to put it into the water. To optimize for hydro-gen the flat and curved faces should be reversed.

With feathering props each blade flips so curved and flat sides are on the correct face.

OceanVolt has created a compromise propeller for its latest EP-hydro-gen saildrive. It uses symmetrical props curved front and back which are Controllable Pitch Props (CPP). With this system pitch can be set for the specific use of the moment: propulsion, reverse, hydro-gen or feathered for minimum drag sailing. With this system the gets more than twice as much hydro-gen at a very modest penalty in propulsive efficiency. That said it’s a $30-40k system small cheap boats are unlikely to adopt.

In the long run folks will be picking up CPP systems for inboards and driving them with EP. Currently there are a bunch of CPP suppliers in Europe, mostly Scandinavia. Most don’t have any offerings small enough for 26-45’ recreational sailboats but a couple do.

There is a corollary in aviation for the performance of asymmetrical foils, aircraft wings. Curved on top, flat in bottom. You can fly an aircraft upside down but it takes a lot more power to do so and a much higher angle of attack. There are a few aerobatic planes that use symmetrical wings, curves too and bottom.

[Adelie]

Originally Posted by Bullshooter

I think this idea of a 60" prop can be ruled out. If you look at this diagram of a Cheoy Lee 32':

https://sailboatdata.com/sailboat/offshore-32-richards

there is nowhere near the space available for a prop that big, unless a major re-build of the boat were involved.

Plus the OP already said "I actually did that calculator and came out with a 24 inch prop recommendation. Unfortunately, that's not realistic. Not only can't I find one, but it wouldn't fit in the space I have between the keel and rudder. I've got a 17" now and could probably fit an 18 or 19 before it started just making it worse."

The other concept of also buying an outboard with a 60" prop, in addition to the inboard propulsion is pretty far-fetched. Who makes an outboard that can accomodate that?
Is anyone expecting the OP to carry this outboard with a 5 foot diameter prop on the transom of his boat? He just started the thread to get some input on what prop would match his thunderstruck. If this 60" prop proposed is feasible, please explain what would be involved to make this work.

You have the wrong boat but same issue with no room for 60” prop on the inboard shaft.
https://sailboatdata.com/sailboat/alden-32-motor-sailer

My proposal was for an outboard. All of Sean’s experiments have been with outboards though I don’t recall him indicating this directly in his discussions.

[Bullshooter]

He has already bought an electric inboard and is having it installed. Can you explain what you are proposing? That he purchase an outboard in addition, with a 60-inch prop? And are there manufacturers that make an outboard that is capable of utilizing a prop of that size?

[RaymondR]

"There is a corollary in aviation for the performance of asymmetrical foils, aircraft wings. Curved on top, flat in bottom. You can fly an aircraft upside down but it takes a lot more power to do so and a much higher angle of attack. There are a few aerobatic planes that use symmetrical wings, curves too and bottom."

One of the most successful fighter aircraft of WW2 was the Mustang which had a laminar flow wing profile whereby the wing is thinner, has a less rounded entry and is fairly symmetrical in it's upper and lower surfaces. The Mustang allowed the US Army Airforce to extend fighter cover for their daylight bombers over most of Germany. It was as fast or faster then most other fighters of the time and more importantly could be fitted for very extended range.

[Adelie]

Originally Posted by Bullshooter

He has already bought an electric inboard and is having it installed. Can you explain what you are proposing? That he purchase an outboard in addition, with a 60-inch prop? And are there manufacturers that make an outboard that is capable of utilizing a prop of that size?

The OP has bought a motor for the inboard prop and wants to know what prop to use. The existing prop is probably a good match already. Given that the OP has gone to EP with the attendant decrease in motoring range and power it would be a good idea to improve sailing ability. To that end a folding or feathering prop would be a good idea. Since the prop is in an aperture a folding prop almost certainly won't work, so a feathering prop would be the answer. Another aid to sailing would be a drifter, a cruising chute or a Code Zero in order of increasing costs.

Sean expanded the topic to deal with what is most efficient. He brought up 60" props. It seems apparent that would not work for pretty much any inboard. I suggested an outboard and indicated that there is nobody currently manufacturing such but it would be a good idea for drastically increasing range under EP (though not long-distance by any means). I am actually researching where to source parts and how they would need to be modified and mounted. I doubt I will start a business for this but it's an interesting endeavor.

[Adelie]

Originally Posted by RaymondR

"There is a corollary in aviation for the performance of asymmetrical foils, aircraft wings. Curved on top, flat in bottom. You can fly an aircraft upside down but it takes a lot more power to do so and a much higher angle of attack. There are a few aerobatic planes that use symmetrical wings, curves too and bottom."

One of the most successful fighter aircraft of WW2 was the Mustang which had a laminar flow wing profile whereby the wing is thinner, has a less rounded entry and is fairly symmetrical in it's upper and lower surfaces. The Mustang allowed the US Army Airforce to extend fighter cover for their daylight bombers over most of Germany. It was as fast or faster then most other fighters of the time and more importantly could be fitted for very extended range.

I simplified my description for a general audience. Yes, the P-51 had a wing section that was curved top and bottom. And there are a whole lot of planes currently flying that are similar with top and bottom being curved. But they are not symmetrical, they may be close to symmetrical but they are not completely there.

The only aircraft that I am aware of that use truly symmetrical foils for the wings are racing and aerobatics aircraft such as the Extra EA-300.

https://en.wikipedia.org/wiki/Extra_EA-300

My position stands that a symmetrical prop is currently the best compromise shape for use in a hydro-gen EP system.

[RaymondR]

They tend to illustrate it as being fairly symetrical.

I tend to agree with you mostly and I suggested in Post 45 that a variable pitch prop might be a good application in an electric drive. If I was trying to design a system I'd look at one coupled with a slow RPM high torque multi pole motor with electronic commutation as an ideal solution.

[Adelie]

Originally Posted by RaymondR

They tend to illustrate it as being fairly symetrical.

I tend to agree with you mostly and I suggested in Post 45 that a variable pitch prop might be a good application in an electric drive. If I was trying to design a system I'd look at one coupled with a slow RPM high torque multi pole motor with electronic commutation as an ideal solution.

The foils I could find for it shows it as being even closer to symmetrical than what you have in that graphic, but even the ones I found were still asymmetric. That said my understanding is that a lot of laminar flow foils can be pretty touchy. A lot of the early ones didn't take well to icing, a little ice and lift went completely to crap. I have no idea if the current crop of laminar flow foils are more resilient.

Whatever, the issue is that if you want to maximize forward thrust efficiency you use a normal asymmetric foil shape the greater the diameter and the lower the rpms the better.

If you want the primary prop to also do duty for hydro-gen then the compromise foil shape is symmetrical, preferably with pitch control. Right now only Ocean Volt is doing this and it costs an arm and a leg.

[billcrowley]

One thing I did not see mentioned in previous replies is prop drag when sailing with the engine or motor turned off. Unless you are opting for a folding or feathering prop of some kind (which I don’t think is in your budget), this may be a factor worth considering in your decision to choose a 3-blade prop over a 2-blade prop. With a 2-blade prop, it may be possible to “park” it in a vertical position, so it is mostly shielded from causing drag by the keel and rudder. With a 3-blade prop, at least 2 of the blades will always be “parked” in a position “unprotected” by the keel and rudder, and thus adding drag for your sails to overcome.

[Adelie]

Originally Posted by billcrowley

One thing I did not see mentioned in previous replies is prop drag when sailing with the engine or motor turned off. Unless you are opting for a folding or feathering prop of some kind (which I don’t think is in your budget), this may be a factor worth considering in your decision to choose a 3-blade prop over a 2-blade prop. With a 2-blade prop, it may be possible to “park” it in a vertical position, so it is mostly shielded from causing drag by the keel and rudder. With a 3-blade prop, at least 2 of the blades will always be “parked” in a position “unprotected” by the keel and rudder, and thus adding drag for your sails to overcome.

It was mentioned earlier, my post #55 https://www.cruisersforum.com/forums...ml#post3673548

And probably another one earlier I think.

Regardless, you are corrected, if anybody is going to convert to EP then maximizing sailing ability follows from that except for daysailing from your normal marina or mooring.