Well, if I can comment without sounding too much like Cliff on Cheers, ya gotta do the math. Using Keith's and Arby's amps vs speed data and the boat specs from the internet and Gerr's formula for boat speed and assuming 90% motor + controller efficiency:
AJA ARBY
LWL feet 26.1 20.5
Disp pounds 9700 7000
Speed mph 4.1 4.0 at 1200 watts input (24v @ 50a)
Speed mph 5.1 5.0 at 2400 watts input (48v @ 50a)
Keith's reported speed 5.0 mph at 2400 watts; prediction 5.1 mph
Arby's reported speed 5.1 mph at 1200 watts; prediction 4.0 mph.
Now 5 mph is 25% more than 4 so that's quite a difference but I have plenty of first hand experience taking amps to speed data and know it is difficult to get repeatable data as wind has a huge effect on speed and even doing two way trials does not perfectly compensate for wind as a little gust just when you are taking the data can have a significant effect. GPS data at these very slow speeds can vary a bit too. Check the speed readout over time with the boat tied to the dock. It is very hard to believe Arby's motor/controller is 25% more efficient than the Mars/Sevcon setup Keith has. It is easy to believe data accuracy accounts for most of the difference.
Also, the boats are quite different even though they sound somewhat alike. Aja's much longer waterline more than makes up for her 38% greater displacement. A dirty bottom could make a significant difference. Prop's have a big effect too.
If Arby's product works as promised he will have made the lives of sailboat converters much easier. As those who have actually done it know, there is a good bit of engineering (and often some expensive trial-and-error) required to make a functioning boat "propulsion system" out of a box of electrical parts. Having the motor/controller/
Denny Wolfe
www.wolfEboats.
PS: It's great to see actual data instead of the usual "powered into a head sea and foul current with authority" BS.
----- Original Message -----
From: aweekdaysailor
To: electricboats@
Sent: Sunday, July 19, 2009 11:14 PM
Subject: [Electric Boats] Re: Direct drive brushless DC motors now available
I did re-run some numbers today...best to see it in graphic form so I
posted the graph to the files section (Aja v Arby)
I did my runs inside the yacht harbor, just after high slack and with
almost no wind. I could not figure out how to normalize my amp values to
his, so the X-axis uses my values.
Either...my rig is doing very poorly or his is doing very well indeed...
(note that my numbers were taken in knots and converted up to mpH using
speed * 1.151) . If someone with a comparable vessel (to either mine or
his) using the Thunderstruck kit can post their numbers, that would be
helpful. Pay close attention to the value on the graph - I am running
at 48V nominal, he is running at 24V nominal. I did expect a 10-15%
difference based on the faster hull, but...this was a tad more...
One thing we should all do is start using watts - so that voltage drops
@ speed is compensated for. I did not take readings in watts today, but
will update in the future.
-Keith
--- In electricboats@
>
> Test results for the initial run of an Advanced Marine 24 volt BLDC
motor in an Ericson 27 (6800lb displacement)
> Conditions: Light wind, slack tide, batteries charged to 13.2v.
> We motored 5 minutes to the San Rafael Channel, and proceeded to make
two runs from San Rafael to the Brothers and back.
> Heading, Battery Current, and Speed (GPS)
> 100 degrees,10 amps, 2.6 mph
> 100, 20, 3.5
> 100, 30, 4.1
> 100, 40, 4.5
> 100, 50, 4.7
> 100, 60, 4.9
> 100, 65, 5.0
> 280, 60, 5.2
> 280, 50, 5.1
> 280, 40, 4.8
> 280, 30, 4.5
> 280, 20, 3.9
> 280, 10, 2.8
>
> Notes:
> 1) The motor is bolted hard to the original motor stringers. While
this makes the installation strong, it transmits far more motor sound to
the fiberglass than anticipated. The motor must be re-mounted using
heavy rubber isolation blocks, positioned to provide axial and torsional
displacement around the shaft center. In addition, a flex disk will
further isolate the shaft from motor output torque ripple. To ensure
grounding, a small wiper will contact the prop shaft.
>
> 2) The current/speed relationship was quite satisfactory.
>
> 3) The current limit, originally designed for 105 amps, was reduced to
80 amps peak to ensure controller integrity. The three blade 11 x 11
prop loads the motor/controller to just a little more than the 80 amp
limit can handle. The controller then operates in current limit at full
throttle until it the internal protection reduces the throttle to
limp-home. Once cool, the controller returns to full power, and cycles
again. With the current limit increased to 90 amps, this problem should
be abated.
>
> 4) The aluminum motor cooling fan resonates additional sound. A nylon
fan is now being produced.
>
> 5) The case temperature never exceeded 45C. The internal thermal
protection is set at 85C on the controller.
>
> 6) Rough water trials will be conducted when the new motor mounts are
constructed.
>
> Arby
>
>
>
>
> ____________
> From: kmannell kevmann@...
> To: electricboats@
> Sent: Saturday, July 18, 2009 5:09:00 AM
> Subject: [Electric Boats] Re: Direct drive brushless DC motors now
available
>
>
>
> Well, we dropped her in the water at 5:20pm 7/16/09. Lots of work was
done on the boat including the removal of an Atomic 4 ICE. Please keep
in mind that I have never worked on a boat before and am not a sailor.
We scraped her down to the gel and painted the topside and resealed the
bottom coat, weeks in the San Rafael dockyard with lot's of great people
encompassing a wealth of knowledge. We also had some newbie issues with
me overseeing the removal of the A4, I missed a bolt and when we pulled
it, bent the shaft just a tad. The A4 had a small prop designed for the
higher RPM it produced that we replaced with a three bladed 11x11 with a
shaft that was 12" longer so we could place the battery pack behind the
motor, from the Prop Shop in Richmond.
>
> Long story short, we just did the sea trials and recorded data Arby
will be posting soon. I will be doing power testing, as far as I can go,
to get a good drain on the 4 70lb, 100ah, AGM batteries in two banks of
24v and will let you know times and distance from a full charge. I am
working on our web site and will post the full story there, as soon as
possible with lots of pictures and some video. We will keep you
informed on changes we make to improve performance or efficiency and
follow with data on them.
>
> Kevin Mannell
> Advanced Marine electric Propulsion (.com soon)
> AMeP
>
> --- In electricboats@ yahoogroups. com, postal6@ wrote:
> >
> > Just finished talking with Kevin, who is working with Arby on an
Ericson
> > 27. They have changed the prop to a three blade and hopefully will
put
> > the boat in the water tomorrow, July 15, 2009. This is the 24v
system.
> > Don
> >
>
>
>
>
> [Non-text portions of this message have been removed]
>
[Non-text portions of this message have been removed]
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