Hi John,
Welcome!
The principal reason is efficiency. Much testing data exists to support the statement "a slow-turning, large diameter prop is more efficient than the same speed achieved with a faster-turning, smaller diameter prop at the speeds reached by most sailboats". Since electrically propelled boats generally store their energy in batteries which are mostly large and heavy, driveline efficiency has become somewhat of an obsession for most converts. If you pay attention to the efficiencies and take advantage where you can, for the same battery bank, you can more than double your range over a system that uses existing transmissions and prop.
When designing an electric propulsion conversion for an existing boat, what you generally want to start by determining the largest diameter propellor you can fit — clearances between shaft and hull, aperture size, perhaps moving the prop back an inch or two from the strut to gain diameter from the hull rising towards the stern. Next, you will determine your bocat's electric cruise speed. Many people mention "hull speed" which is 1.34(sqrt-LWL). For my boat, that is a waterline length of 25' so that makes 1.34*5 or 6.7kts. Hull speed is nothing more than the speed at which the power:speed curve goes from mostly horizontal to mostly vertical. The speed/power relationship is exponential. Each knot of speed costs about double the power. This is more or less proven with data many of us have supplied. So by slowing down, even just a half a knot, you see your range stretch out by miles. In my boat, at full throttle, I can get out to the race course and back (if I'm sure to sail most of the way back). At 5kts, I can get to my favourite anchorage and sail back (about 7nm). At 4kts, I can go to the far end of the lake (12nm). At 3kts, I can go there and back. So, while my hull speed may by 6.7kts, my practical cruising speed is more around 5nm. When you are range-limited, efficiency is EVERYTHING.
My boat's system is designed to hit optimum cruise speed around 800rpm and the system voltage and gear ratio mean than max rpm of the prop tops out just before 1000 rpm.
This is due to the rpm constant of my motor being 50rpm/volt and my system being a 48V nominal system, and the reducer is 2.55:1.
In other words, 50vdc * 50rpm/v = 2500rpm(motor)/2.55 = 980rpm(prop), let's call it 1000rpm.
I just read over some of my initial sea-trial data from 2014. I'm actually very pleased with the numbers.
I see 4.7kts at about 650rpm in flat water with a light wind and a very clean hull. That works out to 8.8" forward motion with each revolution of the prop.
My prop has a 14" pitch so my propellor efficiency at 4.7kts is 63%. That's actually pretty good. My prop is a 3-blade fixed 12dx14p, at about 65% blade-area-ratio. It's a beast compared to my 12x8 3-blade sailor prop from my diesel days.
I can reach 6kts with my setup at full throttle which is about 1000rpm. That gives 7.45" forward with each revolution, or about 53%. Also not bad, considering the rapidly approaching speed limit.
At some point, I intend to do prop efficiency measurements at all speeds from 2kts to full throttle, 0.5kt increments, but I just don't have the time in my limited sailing season here in Montreal. I need a full data logger for my controller's CANbus and my instruments' NMEA data so I can just collect it as a matter of course. This is coming soon…
Your boat probably isn't that different from mine, a 1978 Beneteau First 30, displacing 7500lbs (more like 8000…) with a deep fin (5'8"), so the numbers you could expect would be similar.
Those of us who have converted absolutely love to talk about our boats and are for the most part open about any mistakes we may have made along the way, so don't hesitate to ask, and also don't hesitate to search through the archives. There is a lot of valuable information in there.
Cheers,
/Jason
> On Feb 18, 2016, at 16:18, johnrhines@gmail.com [electricboats] <electricboats@yahoogroups.com> wrote:
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> I'm new and I'm sure this has been answered before, but here goes. I have an Ericson 29 with an Atomic 4. It has a small prop that spins pretty fast anyway. All of the motors say that they have full torque for the entire spectrum. So why not just program the controller to max out at 1300 rpm? If you have full torque, why not?
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> Sorry for the newby question,
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> John
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Posted by: Jason Taylor <jt.yahoo@jtaylor.ca>
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