Eric no afraid I have nothing at those speeds. It was the goodness of fit with known formulas that made me wonder.
Yours,
Andrew Gilchrist
fastelectrics.com
Australia
0419 429 201
From: electricboats@yahoogroups.com [mailto:electricboats@yahoogroups.com] On Behalf Of Eric
Sent: Monday, 22 April 2013 12:23 AM
To: electricboats@yahoogroups.com
Subject: [Electric Boats] Re: projected performance
Hi Andrew.
I have no personal experience motoring any of my boats at 2kts. And other than the performance testing that I have described a few times, I don't really spend any time motoring at 3kts. I find that 3kts feels barely more than drifting to me, I can still swim a few hundred yards at better than 2kts.
I typically row my inflatable dinghy at more than 3kts, when motoring the same with my Torqeedo, I rarely go slower than I can row. My cruising speed in my kayak powered by me is generally above 4kts.
But given that my 3kt motoring draw in the 5ton ketch is 500W, I wouldn't be surprised if 250W delivered 2kts and 125W delivered 1kt. With the numbers this small, how far off could they be? And realistically, I believe that external influences like wind, waves and current, even in calm conditions would make accurate data collection at these speeds problematic.
If you have collected any slow speed data with your electric boat(s), feel free to post it.
Eric
Marina del Rey, CA
--- In electricboats@yahoogroups.com, "Andrew Gilchrist" <andrew@...> wrote:
>
> Eric
>
> You may have said this.
> given the reduction in power you mention on 5 to 4 knots is bang on for the
> rule of cube for the relationship between speed and power
> have you got real word for 3 and 2 knots?
> regards,
>
> Andrew Gilchrist
>
>
> From: electricboats@yahoogroups.com [mailto:electricboats@yahoogroups.com]
> On Behalf Of Eric
> Sent: Sunday, 21 April 2013 1:54 PM
> To: electricboats@yahoogroups.com
> Subject: [Electric Boats] Re: projected performance
>
>
> Hi Richard.
>
> The power recommendation of 1kW/ton would indicate a 750W drive for your
> boat with 1500lb of displacement. That should provide speed up to 90-95% of
> hull speed based on your waterline length. You didn't mention the length of
> your proposed project, but if we picked 16' LWL, it gives a theoretical hull
> speed around 5.3kts or an electric speed a little less than 5kts. We also
> often observe that halving your power drops your speed about 1kt, so now
> your proposed full throttle of 350W should deliver about 3.5kts maybe a
> smidge more.
>
> You proposed battery pack is 5.2kW (110Ah * 48V) and if flooded batteries,
> should have a usable capacity of 3.1kW to a 60% depth of discharge. That
> 3.1kW should last 8.4 hours with a 350W load (allowing for the Peukert's
> effect) giving you a range at 3.5kts of 29nm.
>
> You can see in my analysis that I had to guess on the size of your boat and
> from that guess, estimate the watts to knots. I would bet that your
> performance, if properly propped should be pretty close, maybe even a little
> better. If the power-speed estimate is close, the range estimate for your
> battery capacity should be very accurate.
>
> In any case, if you can accept the 3.5kt limit to your top speed, then your
> plan as stated seems very doable.
>
> I hope that this helps,
> Eric
> Marina del Rey, CA
>
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