| Velocity ratio is exactly what is states.

Thanks for the correction/clarification Rick.

So it seems...
| for the 18m proa in calm conditions at 6.5kts, It swings two, 3-bladed 540mm diameter props.

....at 7.8kts.

Which is a very different scenario than a typical sailing prop pushing a typical boat. Which may be why my guess is off.

Obviously you have extracted a significant amount of energy efficient performance from those electric drives. 54cm diameter props seem untypical. Did you make them?

1 a) At what RPM is the 18m props turning in the pics you attached?
1 b) What percent of rated max is that RPM?

2) Do you have a pic of those props? Edge on too?

| With regard to dragging the prop backwards, you are not thinking about a proa

Do you mean I am not thinking about your proa? Or maybe you are not thinking about the proa, as I have repeatedly said, I am thinking about? You know, the tipped tender on the HP?
Or did I misunderstand you?

| An efficient prop operates at very low angle of attack.

3) As the topic is dragging props, can you model the dragging of the 18m proas props?

4) Can you say at what speed that prop would need to be dragged to match the speed they are at in the pics (7.8kts)?

| So even if your blades have a very fine trailing edge they still perform quite well in reverse rotation.
| Most props do not have a particularly fine trailing edge so work quite well in either direction under load.

I was unable to understand the essential element of this. Regarding trailing edge sharpness and dullness;

5 a) Can you clarify the importance of it in this context?
5 b) What does either case (sharp or fine) help or hinder?

| As far as I can detect the 18m proa motors in either direction with similar efficiency..

That's quite interesting.

6 a) Did you expect them to be different? Please explain.
6 b) Do your prop blades have an airfoil shape? Will you share said shape?
6 c) Can you explain why the performance is not different?

| It is desirable to have the drives at the aft end if motoring in a decent sea because they get buried deeper when at the forward end of the boat; as occurs with any boat.

Good point.
7) But the pivoting tender of the HP seems to keep the prop buried right where you want it in seas, right?

| So, as a turbine, the blades could be working in the normal forward direction or the reverse direction.
| Once spinning the turbine will have very low velocity ratio because you normally charge at a much lower current rate than you draw under load.

I was kinda confused by this part. Forgive me.

I am assuming dragging a prop would give a velocity ratio ( negative velocity ratio?) less than would be expected from the prop being powered. Because, I assume, the prop is not an accidentally efficient windmill.

| Typically LiFePO4 cells have a maximum sustained discharge at 0.5C and recommended maximum charge of 0.2C; meaning 5 hours to charge from flat.

Right. But given a pack of a size for some amount of duration, it would be very hard to pull 0.5C, right? Even pulling 0.2C might be hard. Large pack size makes the charging amperage not be that problematic?

| I can assure you that you would not want to try to stop a 540mm diameter, 3-blade turbine from spinning when doing 20kts through water. It would slow insignificantly when charging at allowable charge rate if the controller allowed the high voltage required.

Well, they would be slowed by the velocity ratio, plus the inefficiency of being dragged vs being powered, I assume.

But if you wanted to depower it, you could raise the tender, or sail slower.

Which raises an interesting point.

8) What if you raised the prop on the tender until only part of the disk was submerged? Just enough to get the prop speed you want and add minimal drag?

Yes, this assumes calm seas, but is it an interesting option?