Hi John,
I've got a very aggressive propeller, pics are available in the photo section of this group under "Eric's Serenity". The vendor that sold me the prop and the yard where I installed the prop both commented "that's not a sailboat prop, it will have too much drag". They were right, if stopped, it really drags under sail (I always spin it at less than 50W to reduce sailing drag). But this prop is optimized for drive thrust, not regen. Since few will sacrifice drive efficiency for regen efficiency, a dedicated hydro-collection device (like a towed generator) should be more efficient.
"Why go 6kts?" Energy output from mechanical collection increases exponentially with speed. This is true for wind generators and hydro-generators. At slower speeds less energy is available. Since most of our boats can hit at least 6kts, that seems to be our best opportunity for energy collection.
Unlike EVs with regen from tires on roads (very high friction coefficients) where almost all speed can be collected as energy, fluids like water or air will flow around collection vanes/blades at slow speeds if the resistance is too high. So heavy load from a generator will typically stall a prop (aggressive props more so) at slow speeds, increasing drag and generating nothing.
I'm sure that you have observed while sailing, at low wind speeds, any extra drag (bad sail trim, kelp, wind or hydro generation, etc.) slows the boat down. For example, my ketch will average about 5kts in about 10kt of breeze. At 15kts of wind, I'm sailing at 6kts or better, adding drag will still slow down the boat. As the wind increases more the boat can't really go faster because it is a heavy displacement boat, at those wind speeds, extra drag is overcome without a loss of speed. So when sailing in Beaufort 6 or higher, there is no significant loss of speed from any hydro-generation activities. So the top speed of your boat is based on the hull, LWL for displacement boats, if there is enough wind speed to overcome any additional drag from other sources.
After all of this, the best regen that I've seen from my boat was around 70W at 6kts in about 15kts of wind. I haven't been out in a gale since my conversion. I know that a full keel with the prop in an aperture costs me some, but even Mark with an exposed prop is reporting about 100W at 6kts.
But I encourage anyone here to try some of their ideas on their own boat, that's what I did. You never know, you might be the one that will make a significant breakthrough.
Fair winds,
Eric
Marina del Rey, CA
--- In electricboats@yahoogroups.com, John Francis <surv69@...> wrote:
>
> Why go 6 knots?
>
> If a person had a very aggressive propeller . . . one that would "really"
> drag on a sailboat under sail, could that extra torque be used to actually
> get a higher speed(and higher wattage) from the generator?
>
> Also, would it affect the function of hull-speed, or just, pain and simple
> slow the boat down under any given set of circumstances . . . as such that
> a blow or sail setting that "could" gain more than hull speed, which is
> evidently a function of the boats specs and wave length, actually allow the
> boat to gain that full hull speed, but now with higher winds and again
> increase the speed for the generator?
>
> In essense, just requiring more oomph to get to hull speed and coincidently
> getting more wattage at the costs of a knot or so in more normal conditions.
>
>
> John Francis
>
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