T-Pain Is On A Boat: Re: [Electric Boats] Re: Direct Drive Motors and Regeneration

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.

Not true.
Timing belt transmission are about 98% efficient, far better than mechanical gearboxes, and creates a little noise (this is a drawback) and almost no heat.
They are routinely used upto 10.000 rpm and available for 100 kW and up in power.
When used to transfer several kw in much harder environments than boats (cnc stuff, industrial saws etc.) a timing belt drive is slightly warm, unlike a normal gearbox. No appreciable amounts of energy are wasted as heat- again unlike a normal marine transmission.

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.

Absolutely. Area and speed are the main problems, afaik.
? I believe the prop area is a area-squared function ? just like in wind power and for similar reasons.
And this is the main reason small propellers will likely never be efficient in re-gen.

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).

Absolutely.
A 10 ft prop would have 100 times the power generating ability compared to a 1 ft prop.

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,

It wont be linear ... just like more blades does not make a prop more efficient in a boat (it does make it smoother).

and optimizing the blade pitch, area, efficiency, and number would improve power output even more.

Abolutely. Get everythign right, and 5-15% better might be achievable. So instead of 100W you could get 110 W.

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,

No-one has managed it.
At least hundreds have tried, for 20-30 years. You might do better, but ..

and exponentially more as speed increases

Wont be exponential.

(although my boat is going to max out around 6 ~ 7 knots on its best days, with the hull completely clean, etc.).

Because the speed is still very slow, in abs. terms.

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).

Me too. Grin.

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

--- In electricboats@yahoogroups.com, "Eric" <ewdysar@...> wrote:
>
> Hi Jim,
>
> I'm not sure why you believe that a reduction setup (gears or belts) has any effect on regen capabilities. The problem with regen in boats our size the the limited speed potential (when was the last time you sailed faster than 6.5kts?) and the inefficiencies of propellers to collect energy at these speeds.
>
> Just like all of the wind generators or towed hydro-generators, which are optimized for energy collection (not propulsion), slow speeds just won't develop any significant output.
>
> All of the regen capable electric boat conversions that I have heard of, with either direct drive or reduction drives, generate about the same amount of juice through regen, somewhat less than 100W at 5-6kts. The only regen that I have heard of on a sailing vessel that sounds successful to me occurs around 8tks or faster. These boats are basically multihulls or fast monohulls in the 50' range.
>
> All of the drive systems (direct drive or not) from the regular vendors here can develop about the same amount of power via regen.
>
> The real question is how long does really good regen take to replace the power used while motoring out. Here's my answer, followed by a copy of my post that explains how I came up with the numbers:
>
> "Ignoring house loads, the ratio of drive distance (at 4kts) to regen distance (at 5kts) is about 1 to 40. Motor one mile at 4kts (15 minutes) and you need to sail 40 miles at 5kts (8 hours) to get the charge back. If you use a radio, autopilot or lights, the ratio gets worse."
>
> I've been using these estimates for years now, and I have never had anyone dispute them. I really wish that someone could prove me wrong.
>
> ----- Start post -------
> Message # 16453 - Nov 29, 2010 - [Electric Boats] Re: more regen conversation
>
> Larry,
>
> As the most visible "regen naysayer" here, the link that you provided completely supports my previous premise.
>
> According to the Whoosh site "... And since we average 5-6 kts, that's over 100 amp/hrs/day – for us quite possibly 150 amp/hrs/day – that is available for running the radar at night, making water with offshore water quality, feeding the SSB radio, and keeping the iPod charged..." So what does this mean? They get 100-150Ah at 12V in 24 hours, that's 1200-1800Wh in 24 hours or 50-75W at 5-6kts. They do say that the harvested water power is sufficicient for house loads, a statement that I wholeheartedly agree with.
>
> My statement is (and always has been) that regen at speeds around 5kts is not a viable source of charging for a traction battery bank. The speed is important because most displacement auxilliary sailboats between 27-32 feet (optimal for a 5kW electric drive) have a hard time hitting sustained runs above 6 kts. For this size boat, average sailing speeds of about 5.5kts is more realistic.
>
> So let's say that you use 4kWh of energy out of your 10kWh battery bank, that's about 10-12nm at 4kts in most of our boats. How far would you have to sail using the system described by Whoosh to bring your batteries back to full charge, assuming that you are using no electrical power in the mean time? Generating 75W at 6kts, that's 12.5Wh/nm. So 4kWh will take 320nm of sailing at 6kts and will take over 53 hours. Slow down to 5kts under sail and the recharge distance is up to 400nm and will take 80 hours. If you have a 12V house load of
> only 2A, the recharge times and distances double.
>
> Ignoring house loads, the ratio of drive distance (at 4kts) to regen distance (at 5kts) is about 1 to 40. Motor one mile at 4kts (15 minutes) and you need to sail 40 miles at 5kts (8 hours) to get the charge back. If you use a radio, autopilot or lights, the ratio gets worse.
>
> So regen works, but is not as effective at recharging your traction batteries as some drive vendors would like you to believe.
>
> Fair winds,
> Eric
> Marina del Rey
>
> --- In electricboats@yahoogroups.com, "Larry Doyle" <ldoyle@> wrote:
> >
> > Here's to those NaySayers that say regen doesn't work. Here's some boats that are doing it using towed gens as well as the free wheeling the prop/dc motor in regen mode.
> >
> > www.svsarah.com/Whoosh/WhooshPacificPrepTowGenerator.html
> >
> ----- End post ------
>
> As far as reliability or wear is concerned, my Browning reduction gear box is 97% efficient and has a MTBF of around 100,000 hours of use. So I don't think that there is any reason to be worried about that.
>
> Fair winds,
> Eric
> Marina del Rey, CA
>
> --- In electricboats@yahoogroups.com, "jim_ranger_26" <jim_manley@> wrote:
> >
> > Hi everyone,
> >
> > So, I've been continuing my research and it looks like the final decision point is the motor, which will be the most expensive single component in my system. The gist I've gotten is that, in order to support regeneration, a direct-drive motor has to be used for efficiency, as well as wear-and-tear on belts, pulleys, etc., associated with power/torque transfer/conversion components used in indirect-drive systems.
> >
> > It's not clear to me whom here is using a direct drive motor and/or regeneration, and I would greatly appreciate any thoughts and opinions on the current (double entendre fully intended) motors available in the 2 ~ 5 KW range. I'm repowering a Ranger Yachts 29 foot sloop that weighs about 5,000 pounds, and will only need to motor for a few minutes in/out of the slip, with most capacity held in reserve for no-wind cruising and emergencies.
> >
> > Thanks,
> > Jim
> >
>