I think, what you are calling velocity ratio, I am more familiar calling advance ratio? Which is, IIRC, how big a bite if the fluid stream the prop takes. I continue assuming this is correct, but please correct if not.

While the advance ratio may be 1.2, that does not really address the prop wash velocity, which is the question of the moment. My thinking being, the prop wash velocity being some large multiple of the fluid speed the hull sees. Or are you saying the prop wash velocity will only be 20%? more than free stream? Do you know the prop wash velocity of the 18m proa?

Regarding your spectacular event; given a diesel at full throttle, with some gear ratio to spin the generator at max output (I assume 3600 rpm?), and spray in salt water? Amazing anything survived. Would not one fully seal any sort of generator/motor?

But I am not sure how that should compare to dragging a prop backwards through the water at 20kts. Its not like you would expect it to be super efficient and get to a huge speed, right?

On July 30, 2019 2:44:40 AM UTC, "Rick Willoughby rickwill@bigpond.net.au [harryproa]" <harryproa@yahoogroups.com.au> wrote:
| The velocity ratio on an efficient prop is relatively low. In my
| pedal boats it is of the order of 1.04. On the pedalled drives for
| the R2AK I aim to have less than 1.1 bur not always possible. With
| the 18m proa it is 1.2 in light conditions. The Krautner drive are
| higher because they are less efficient but designed for maximum
| bollard pull where the velocity ratio is infinite of course. It has
| been a while since I ran numbers on the Krautner for calm weather
| motoring but maybe 1.3 to 1.4 depending on the boat.
|
| I would not like to have the electric motor in the water above 12kts
| on the 18m proa for fear of tearing the motor apart or blowing it up.
| There is a real risk of flashover if you allow the motor/generator to
| run up to high rpm. The controllers are also limited in what
| overvoltage they can withstand. As a free running turbine, a
| propeller will have a velocity ratio very close to 1.
|
| Generatlly with battery electric drives, you are aiming for maximum
| range. That means you want an effiucient propulsion system. The
| velocity ratio is a direct factor in the efficiency. If your
| velocity ratio is 2 then you are immediately throwing away half your
| power moving water rather than moving the boat, without even
| considering the blade losses and the mechanical losses in the drive
| train.
|
| The first diesel on the 18m proa had a single generator in a series
| circuit with the two drive units. That meant the single generator
| had to run at a little more than twice the voltage of the two motors.
| The generator gearing was such that the peak revs of the diesel
| resulted in a little over the 90V rating of the generator. We had a
| reasonably spectacular event, aided by a little moisture from salt
| water, where the brushes flashed over and the short circuit torque
| stopped the diesel dead, The generator was recoverable by replacing
| the brush housing and brushes.
|
| Rick
|
|
|
| > On 30 Jul 2019, at 7:05 am, '.' eruttan@yahoo.com [harryproa]
| <harryproa@yahoogroups.com.au> wrote:
| >
| > Rick, I would like to revisit this point. I question the idea of it
| even being possible to over speed an electric thruster by towing it.
| >
| > While the hull may see some low knots when under power, certainly
| the electric prop will see many times the speed the hull does in that
| condition, in order to create the thrust needed to move the boat.
| Certainly it will see more than double the speed of the hull. I would
| guess 4 to 5 times is more probable, but I am often wrong.
| >
| > Does anyone know a good number?
| >
| > So, I am guessing, dragging a thruster at 20kts seems not
| problematic at all, as it, probably, will be no where near over
| speed.
| >
| > But, the potential charging amps seem quite attractive, as the mass
| available to extract energy is quite high. And it literally costs
| almost nothing in weight and complexity.
| >
| > Having said this, there is a problem of the tender thruster
| tolerating the forces of dragging the thruster at 20kts. But that's
| another topic.