Electric propulsion thrust measurement?

[KTP]

So I got my $25 digital fish scale from Amazon today (400 pound max) and hooked it up to our 34 foot Pacific Seacraft sailboat (displacement 13,500 pounds) at the dock to see if I could get some sort of thrust measurements at various power levels into our electric propulsion drive.

The drive is a 10kW brushless motor being driven by a Sevcon gen 4 controller (a Thunderstruck kit)

Having the fish scale tied to the dock and the boat with all other lines slack we measured the following thrust at certain power levels:

100Watt 9 lbs

200W 21 lbs

500W 48 lbs

750W 63 lbs

1000W 82 lbs

1500W 111 lbs

2000W 136 lbs

3000W 180 lbs


So how does this compare with a measurement published by a manufacturer such as Torqeedo on their Cruise 2.0 2000 watt drive (they say 115 pounds of thrust) or their Cruise 4.0 4000 watt motor (they say 189 pounds thrust). They then have a disclaimer that their figures are low related how trolling motors are measured. I am not sure exactly what would be involved in measuring static thrust?

[cj88]

It would be interesting to find some people with Torqueedo's and trolling motors and do the same test with your scale and see how the results compare to the published specs.

[Dave_S]

Do you have speeds for those readings as well.

It would be interesting to see how many lbs thrust is needed to give a speed.

I.e. 1,000W is 2kn

[KTP]

We plan on going on tomorrow and will do some speed tests at those power levels now that my wife has written some software to read the Sevcon canbus and display/log all of the data.

From earlier this year we have a few data points:

"Anyway, we motored out of the marina at 50V @ 10 amps (500 watts) which pushed us along at about 2.1 kts. In the open water I saw us move at 1.1 kts at 3 amps (150 watts), 1.8kts at 8 amps (400 watts). 2.5kts at 16 amps (800 watts), 3.2 kts at 30 amps (1500 watts), and 4.5 kts at 80 amps (4000 watts)."


So at 500 watts which was a dockside static thrust of 48 lbs we were able to move the boat at 2.1 kts. At 1500 watts (111 pounds dockside thrust) we moved at 3.2 kts and for the 4.5 kt figure at 4000 watts we don't have a dockside thrust figure but the thrust curve looks linear so we could use the slope and say it was probably around 224 pounds of dockside thrust.

What I was really wanting to test with the fish scale setup was how our prop was performing with our electric motor and 3:1 gear reduction. If it were super easy to change out props I would want to try larger props, different pitches and find something that was super efficient for our motor. I don't think it is super easy or cheap to swap props in and out though.

Anyway it looks like our prop is maybe pretty efficient. What I don't know or understand is how a prop performs when the boat is in motion vs how it performs when at the dock and the boat is being held in place. Is it even a useful test at all what we did?

[44'cruisingcat]

A big diameter, low pitch prop spinning slowly might give excellent static thrust, while not being particularly useful for propelling a boat at anything but very low speed.

[KTP]

True I guess but I wonder if it would then be really really good at regen while sailing.

I remember reading somewhere that the ideal prop diameter for max efficiency is something like 1/3 the length of your boat lol.

[RaymondR]

I vaguely recall that the power curve for DC motors is a hump with the peak at about 50% of the free running revs of the motor. If this is the case you would need to prop your motor to run at about half the motor speed for maximum power.

Measuring motor RPM is fairly simple to accomplish and you can do it with the vessel underway.

[KTP]

Oh, the Sevcon canbus outputs motor rpm also, I just didn't mention it. I did record the rpm for some of the wattage/thrust measurements.

motor rpm 1283, prop rpm 427, power to motor 1000 watts, thrust 82 lbs

motor rpm 1489, prop rpm 496, power to motor 1500 watts, thrust 112 lbs

motor rpm 1650, prop rpm 550, power to motor 2000 watts, thrust 136 lbs

motor rpm 1904, prop rpm 634, power to motor 3000 watts, thrust 180 lbs

[Cpt Pat]

Originally Posted by KTP

(...) So at 500 watts which was a dockside static thrust of 48 lbs we were able to move the boat at 2.1 kts. At 1500 watts (111 pounds dockside thrust) we moved at 3.2 kts and for the 4.5 kt figure at 4000 watts we don't have a dockside thrust figure but the thrust curve looks linear so we could use the slope and say it was probably around 224 pounds of dockside thrust.

The propeller won't perform well at a dead stop. It's pitch should be optimized for a water velocity near hull speed. (This is why, when using an outboard engine that has a propeller optimized for around 10 knots, one should change that propeller for one optimized for about half the water speed.)

The propeller angle-of-attack and thrust versus induced drag will be different (worse) at a dead stop versus in motion through the water. You should find that you get more actual thrust when in motion for a given amount of power consumed.

Your method is very practical, as long as you don't use those numbers to calculate in-motion motor output power coupled to the water, which would yield artificially lower results.

[a64pilot]

Originally Posted by 44'cruisingcat

A big diameter, low pitch prop spinning slowly might give excellent static thrust, while not being particularly useful for propelling a boat at anything but very low speed.

Yes, but that is what your going to get with an electric sailboat anyway.
The biggest problem is that the best, most efficient prop for electric propulsion, will have huge drag when your under sail.
Sailboat props are of course a compromise between being under sail and under power, even folding and feathering ones are.

Look at a Scuba scooter or a torquedo to see a well designed electric propulsion prop, and imagine how much drag one would have under sail.

You know way back in the day when screw propulsion of sailing ships first became common, I think many ships could actually retract the whole drive system into the boat.

[cj88]

Originally Posted by a64pilot

You know way back in the day when screw propulsion of sailing ships first became common, I think many ships could actually retract the whole drive system into the boat.

Some are doing that today with their electric propulsion. Of course, if you do that then there's no regen possible.

[a64pilot]

Originally Posted by cj88

Some are doing that today with their electric propulsion. Of course, if you do that then there's no regen possible.

Yeah, I don’t see regen as all that viable myself. I’ve never thought, gee if I just slowed down a few kts, I could be charging my batteries.
Plus a driving prop, being driven isn’t very efficient, two different blade designs.
Others will argue that it doesn’t slow you down much, but just simple physics says if you pull significant enough power from the sails by adding hull drag to charge the batteries, your going to slow down quite a lot.
Just no free lunch. To pull just say 2000W is quite a lot of speed, a lot.

[KTP]

Originally Posted by a64pilot

Yeah, I don’t see regen as all that viable myself. I’ve never thought, gee if I just slowed down a few kts, I could be charging my batteries.
Plus a driving prop, being driven isn’t very efficient, two different blade designs.
Others will argue that it doesn’t slow you down much, but just simple physics says if you pull significant enough power from the sails by adding hull drag to charge the batteries, your going to slow down quite a lot.
Just no free lunch. To pull just say 2000W is quite a lot of speed, a lot.

Yes and no.

If you are sailing at close to hull speed in pretty good winds, then the sails are putting the equivalent of 15,000 to 20,000 watts of power into moving you. I mean this makes sense right? If you are doing 7kts on a beam reach in a 34 foot sailboat under sail only, it would take that much electric or diesel power to move the boat at that speed.

So knock 1,000 watts off that with propeller drag on regen and you are still getting 14,000 to 19,000 watts of power moving the boat. Ok for various efficiency and other reasons the drag would not be 1:1 with power generation but even so I can easily see how you could go 6.5kts while generating 1,000 watts of power instead of going 7kts. If the wind is enough to push that much water in front of your bow then it is going to do so even if your tiny prop is siphoning some off.

One other thing you can do to reduce prop drag and whatever performance hit that is on an electric boat in light winds that you really can't do on a diesel boat is actually run the electric system at a few amps, enough to spin the prop so it is not detracting from the boat speed. You really don't want to be running a diesel at idle for hours like that.

[a64pilot]

If I were to agree with your numbers it’s sounds logical, but I don’t agree with your numbers.
Don’t take it the wrong way, but I,think they are optimistic, and how often are we bombing along at hull speed and don’t mind slowing down?
Not as often cruising as I’d like.
Of course you can bump a Diesel up to 1000 or 1500 RPM and motor-sail quite well. It really helps if you have a prop that will increase pitch to load the engine. I find myself doing that to achieve hull speed or close to it and charge batteries, than I find myself at hull speed under sail alone.

I think if I had to estimate, I find myself at hull speed under sail when cruising less than 10% of the time, and that’s optimistic.

But then I don’t think electric drive is viable for a cruising boat, maybe if you had a spare million or so to throw around you could make it work.

It will happen, maybe, just isn’t there yet.