Rick,
Thank you for your analysis. That is very generous.

You are right that each situation needs its own analysis ... and its own thread, I guess.

You pointed out already a number of issues. How to tackle them depends on the general mission for the boat. The intended goal would be to combine quiet operation as long as possible preferably keeping with a minimum pace (10 knots proposed) despite lack of wind. Wind drive above 15 knots could be sacrificed to fill up the batteries.

First of all, I forgot to mention that I was surprised with the noise your setup still made. Of course, there was no comparison with the outboard in the video, but other e-sailors claim little to no noise above the regular wave action.

The prop mounted on the rudder may introduce unwanted characteristics but I would assume that these can be kept bearable up to 15 knots. Going upwind a driven prop may even enhance the action of the rudder. I realize this is all in the conditional sense.

Luc

--- In harryproa@yahoogroups.com.au, Rick Willoughby <rickwill@...> wrote:
>
> Luc
> Each situation can be different so needs its own analysis. However
> if you aimed to do 10kts under motor the system needs to be designed
> to do that in its own right. You would treat any output from the
> sail as assistance. This approach is required because the prop and
> motor need to be matched so maximum rpm and target boat speed are
> matched.
>
> In the case of the 18m proa the system using two Mars motors will be
> capable of 8kts in calm conditions with a clean hull. That requires
> 7.7kw. If the target speed was 10kts then the power demand goes up
> to 14kw. So the last 2kts take almost as much power as the first
> 8kts. The possibility of doing this depends on how you derive your
> energy. For day sailing you might carry enough fuel to do it and
> have a large charger capable of producing the extra 7kW. 7kW from
> solar panels requires very large array. 7kW from a battery bank for
> more than a few minutes requires very large battery. Sails are more
> effective at moving the boat in wind than a wind turbine so wind
> turbine would not be beneficial.
>
> Using a propeller as a turbine is not ideal because the blades are
> optimised for thrust. Running the prop backwards as a turbine might
> be a little more efficient, which would be possible with bi-
> directional rudders. The controller we have enables full 4-quadrant
> operation meaning it can drive or brake regeneratively in either
> direction so it is not difficult to use the unit as a generator. If
> you want to sail at 15kts with the prop/turbine spinning then the
> system has to be designed for this condition or at least able to
> tolerate the mechanical stresses of severe overspeed and overvoltage
> condition. It probably means you are going to have to design a
> system capable of 40kW or so.
>
> The immediate objective with the electric drives on the 18m proa is
> to get the boat controllable under power so it can be easily docked.
> We feel two units separated across the beam is the best way to do
> this. Two directional units separated longitudinally would also work
> for docking but there is about 10% extra power required for same
> speed due to the thrust being offset from the drag.
>
> I also believe a prop mounted on a rudder will introduce unwanted
> characteristics particularly at high speed under sail. It would need
> to have retractable fairing to avoid this and more complexity. I
> know of a small sailing dinghy that did it well but sailing speed is
> around 6kts. I do not know of anything bigger but they might be out
> there.
>
> Rick
> On 23/05/2012, at 5:04 AM, LucD wrote:
>
> > Rick,
> >
> > Rob has drawn the 60' Seabbatical with an e-prop on each beam
> > mounted rudder. The e-prop can travel along the edge of the rudder
> > to be immersed or retracted. The rudder itself can also be
> > retracted (together with the e-prop). This is very clever.
> >
> > To simplify, what about fixing the e-prop in its optimally immersed
> > depth but modulate the control as to power the e-prop to keep the
> > speed at, let's say, 10 knots when not enough sail power, to
> > regenerate power and slow down the boat some when sail driven above
> > 15 knots, and power the prop just enough to neutralize drag in
> > between those speeds? Electronic complexity for mechanical simplicity.
> >
> > Am I overlooking something?
> >
> > Thank you in advance.
> >
> > Luc