James noted:
"The absolute measure of efficiency is pounds of thrust per kilowatt. Funny thing is, this is a curve not a constant."
Thrust per kilowatt is Force / Power.
And the reason it's a curve: Power ~ Force * Speed
Therefore: F/P ~ Force / (Force * Speed) = 1/Speed
In other words, this "Efficiency" measure is inversely proportional to Speed and points to the high power cost of speed.
Now, since this F/P term really only gives 1/Speed, it doesn't seem like an efficiency parameter.
But I get it---we're not interested in true efficiency in terms of a ratio of Power Out vs Power In.
Most of us are interested in maximizing Performance in terms of:
· Distance (n-mi) per Energy (kw-hour)
· Speed (knot) per Power (kw)
o Which is proportional to Sqrt(Force) per Power
So yes, Force vs Power is something we want to maximize---but it's not exactly an efficiency measure.
-MT
From: electricboats@yahoogroups.com [mailto:electricboats@yahoogroups.com]
Sent: Saturday, February 27, 2016 2:51 PM
To: electricboats@yahoogroups.com
Subject: Re: [Electric Boats] Re: Efficiency
The absolute measure of efficiency is pounds of thrust per kilowatt. Funny thing is, this is a curve not a constant.
James Lambden
Electroprop
6 Harbor Way #226
Santa Barbara, CA
93109
On Feb 27, 2016, at 12:25 PM, king_of_neworleans <no_reply@yahoogroups.com> wrote:
I'm no engineer but I agree. with commonly available motors, higher voltage and lower current, higher motor speeds and a reduction gear turning a larger prop, are going to be noticeably more efficient in spite of modest mechanical losses in the gearing. Going 24v rather than 48 does not allow use of half as big a bank. To get the same work done with half the voltage, requires twice the current and equal capacity. So comparing a 48v system with 4 12v batts in series with a 24v system with 12v batteries, they must be twice as big or be in parallel/series. Still need the same total v/ah to get the same work done, more or less. Cost of the reduction gear installation is almost trivial. You can homebrew a belt drive system with parts from McMaster Carr for probably $300. Salvaged junk, even cheaper. Small reduction gearboxes are cheap enough. Mine cost about $450 and is sealed, no maintenance and no additional parts needed and actually simplified installation because I did not have to fabricate anything for mounting a thrust bearing. That is not to say that direct drive doesnt work or doesnt have its place, becasue a motor can certainly be built that will work better in a direct drive system than what most of us are now using. I am thinking axial flow BLDC with about 4x the typical poles, large-ish diameter rotor and stator, and integral thrust bearing. Would this be significantly more efficient? Maybe, but maybe not. If there is one thing I have learned in putting my system together, it is that there is no free lunch. And all the great ideas have already been tried.
Posted by: "Myles Twete" <matwete@comcast.net>
| Reply via web post | • | Reply to sender | • | Reply to group | • | Start a New Topic | • | Messages in this topic (9) |
No comments:
Post a Comment