Hi Jim,
My sampling includes boats with 3 and 4 blade props that have swept areas exceeding 85-90%. My boat with a 4 blade 13x15.5 prop is one of those boats.
The commercial hydro-generator for boats people have been working on this for ages and still seem to be limited to less than 100W at 5-6kts. But maybe you'll figure out something that they haven't been able to. There is a $10k dedicated hydrogenerator on the market that uses a hydraulic variable pitch prop to optimize power to drag ratios while generating power. The system does the adjustments on its own, but the price has limited the user base to global racers for now.
On the reduction conversation, if you think about it, the prop isn't ever going to push more power into the system than the motor does, so I don't see where the issue come up. But if the vendor is concerned, I would heed their advice. I know that it is not an issue for the Propulsion Marine systems.
Fair winds,
Eric
Marina del Rey
--- In electricboats@yahoogroups.com, "jim_ranger_26" <jim_manley@...> wrote:
>
> Hi Eric,
>
> I had gotten the impression that the manufacturers/vendors weren't too keen on their reduction-gear/belt systems being driven from the prop, and that they were inefficient (particularly belt-driven systems) as speed increased because heat was generated at an exponentially-increasing rate. I'm guessing that your numbers are based on a two-bladed prop typically found on our vintage of boats that's not even really great at pushing, much less not optimized for being spun as it's being pulled through the water. Obviously, as the diameter, number, pitch, and hydrodynamic efficiency (in "pull" mode, including within a duct/shroud) of blades on a prop are increased, the amount of power it can generate will increase.
>
> Since the constraint (other than money ;) from that list, in our situation, is prop diameter, that would seem to be the more limiting factor, not just speed. All of the fixed-location water current electric power generation systems (e.g., river gravity-flow and tidal current systems) are designed to extract power at very slow flow speeds, and each turbine can contribute upwards of hundreds of kilowatts of power. However, they have prop diameters on the order of at least 10 feet, which is clearly at least 1,000% greater than we can handle (not to mention that budgetary gap again).
>
> So, the question is what is the best efficiency that can be expected from the most effective prop designed specifically for converting water flow into power? If your numbers are for a typical two-bladed, low-pitch, moderate-flow-area, 1970s-era, low-end-market sailboat prop, then just increasing the number of blades will result in a linear increase in power generated, and optimizing the blade pitch, area, efficiency, and number would improve power output even more. Now, I'm enough of a student of thermodynamics, hydrodynamics, and especially Murphy, to know not to expect to be able to extract anywhere near the kilowatts that most systems discussed here can put into the water. However, I'll bet a lot more than 100 watts can be generated at around five knots, and exponentially more as speed increases (although my boat is going to max out around 6 ~ 7 knots on its best days, with the hull completely clean, etc.).
>
> I was going to pursue the prop quest in the final phase of the development of my system, since a prop like I'm talking about could easily cost more than the entire rest of the system (fortunately, I possess the skills and I have access to facilities where I can do fabrication myself). First, I need to just be able to get in and out from the slip, so, I need to identify the prime mover that will later be optimal for regen, when the prop end of the equation has been solved. It may be that none of the available motor components are optimal for regen, and I would have to develop that part, too, but, that would have to wait until I'm retired to have time for the required tinkering effort. There just isn't enough of a market to keep myself fed to justify a full-time pursuit of foisting yet-another motor option Out There.
>
> Thanks for your usual level of analytical detail, and All the Best,
> Jim
>
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