Mike,
Not sure about the ballast issue but Idoubt you'll have any speed problems by reducing the motor to a 2.0. Power requirements go up exponentially with increase in speed so you'll probaby only lose 1/2 to 1 mph. Try the 4.0 at 50% trottle and see what you get, porbaby not that much less than at 100%.
To give you a benchmark on "how low you can go" consider this. I'm pushing a 50' long, 20 ton concrete boat 5 mph in calm conditions with an 8 kW input.(That's less than 10 hp) The speed vs power input for me goes something like this. (48V)
20 amps = 2 mph
40 amps = 3 mph
80 amps = 4 mph
160 amps = 5 mph
Things get a lot slower in heavier seas but surprisingly wind doesn't hurt nearly as bad as you would think. It's was a slow ride and we only averaged about 3 knots but I recently completed a transit of the entire intracoastal waterway of Florida powered exclusively by solar energy. Although the boat can go 5 mph on electric, since my solar array only gives me about 80 amps on a clear day we seldom exceeded 4 mph, unless the tide and wind where giving us a lift. I gotta sip off the batteries real hard to get that last mph so we always tried to save the battery for emergencies, like going thru a bridge when the current can pick up considerable and you want toget thru in a hurry. Although the system worked surprisingly well on partly cloudy days, there are times when it's just so overcast that I will have to switch over to diesel if I HAVE to be somewhere. Not to many night transits on electricity either.
Wind and currents come into play much more when you're underpowered like we are but on a good day with favorable wind and current we often made 5-6 mph. Unfortunately, on bad days, more like 2 mph but I can honestly say we always maintained control of the vessel and the sea never had it's way with us. Although sometimes our travels became frustratingly slow and when faced with these situations we often just dropped anchor until the tides and winds became more favorable.
I wouldn't recommend underpowering a boat like this unless you have a backup for safety reasons and the Arc is a parallel hybrid with a 38 hp diesel backing up the electric. For the record too, the sweat spot on the diesel is only 5 mph with a max speed of about 7.5 mph but since the darn thing is over 50 years old, I very seldom run it and when I do, I almost never push it past 5 mph.
Capt. Carter
www.shipofimagination.com
On Wednesday, April 29, 2015 5:50 PM, "dellariom@yahoo.com [electricboats]" <electricboats@yahoogroups.com> wrote:
[Attachment(s) from dellariom@yahoo.com [electricboats] included below]
Hi all
Hi all
I am new and just purchased a (one off) electric boat someone else built by converting a Venture 17 sailboat to an electric launch. He added a Torqueedo Cruise 4 with 16, 6v Trojan 105's (450AH) . The 1000lb battery weight is concentrated in the center of the boat to replace the ballast of the centerboard. He did some creative fiberglass work to move the Cutty Cabin to the aft end which houses a small day head and some resting bunks and converting the forward area into a lounge like the Duffy design. I love the concept of an all day electric boat that makes provision for both battery and bladder needs.
However, I wonder if it is overpowered and wonder if when it comes time to replace the batteries if I could replace the Cruise 4 (8hp-48v system) with a Cruise 2 (5hp-36v system) and reduce the batteries to 6(-620 lbs) or 12(-248 lbs). This would reduce the weight and hopefully increase efficiency? I am however concerned about power reduction and effect on ballast that may be needed to maintain stability. I am obviously not an engineer and hope someone can give me some insight on how to calculate this conversion.
Current Boat specifications are:
-Estimated weight 500lbs (without batteries and extra little things taken off boat). Original displacement 900lbs
-Length 17ft , Length at Water Line 15ft, Beam 6.5ft, Draft 18in,
-Max Engine size rating 10hp,
-Current speed is 6.5 mph max with an efficient cruise at 5mph with a Torqueedo calculated range of 80 to 100miles.
Any suggestion on how to calculate how much power is needed?
Thanks
Mike
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Posted by: Carter Quillen <twowheelinguy@yahoo.com>
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