James
Very interesting information .....looks like electric boat conversions are finally going mainstream. But I would be careful about that rule of thumb unless they are just talking about sailboats. My 26' powerboat is 2-1/2 tons and I can tell you that 2,500 watts (1,000 watts/ton) is no where near adequate power. With my electric conversion I used a 5KW motor and 3:1 reduction gear and the boat performed just OK. Now I am now upgrading to have more speed and better performance. My powerboat and other conversions like it will need 10KW motors with belt reduction or special windings if you want to go with direct drive; and you will want to have water or forced air cooling. Rules of thumb may be OK for sailboats to get to and from the dock; but I suggest a pull test of your powerboat to determine how much power is required to cruise at a given desired speed; then you will have actual data to determine what size electric propulsion motors to install. My best guess is that if you take the rule of thumb you just mentioned ie 1,000 watts/ton x 4 = 4,000 watts/ton you are now in the ball park to determine the KW for your electric powerboat. Keep in mind that power requirements to push a boat climb substantially beyond hull speed.
Having said that there are some other important things I want to point out: The first is that every electric boat will need power from some source so make sure you install the right motor. Installing adequate and efficient power is key and keep in mind that motors run at their highest efficiency at 75% of rated speed so at the end of the day you want to size the motor(s) to give you what you want. This is much more complicated than it sounds when you consider gear reducers, torque curves, rated voltage and current, belt drives, direct drives vs parallel or planetary gears. Shaft speed should be kept below cavitation levels for electric boat drive installations so high propeller efficiency can be optimized and maintained. Also keep in mind that there will be some noise so you can design your electric drive system for the ideal rpm for highest efficiency and reduced noise as much as is practical.
Now This is where things get very exciting. We all know that internal combustion engines are loud, require maintenance, are expensive and extremely inefficient. Efficiency levels for IC motors are all over the map as I have seen 30%, 40%, 65% efficiency and who knows; with all the variables and maybe hundreds of heat producing moving parts that are required to operate an IC motor. The stark contrast is that electric motors have 3 moving parts; the shaft and the end bearings and motors are often over 90% efficient. The propulsion of an electric boat can be in excess of 90% efficient if you use the correct motor with or without a gear or belt reducer. Also, Planetary gears and some parallel gears as well as cleated belts are generally very efficient. You can direct drive a propeller shaft eliminating all gear reduction if the motor is specially wound to match the required propeller thrust rpm's for your boat. The drawback I see to having a motor specially wound for high torque and low speed to direct drive the propeller shaft is higher noise levels but the effciency may never be better. Also, you can sound dampen electric motors as long as they are kept running cool.
JCR
On Sun, Feb 14, 2010 at 5:19 AM, James Sizemore
<james@deny.org> wrote:
I was happy to see our happy little forum listed in the just released West Marine 2010 catalog. More so then that you can now buy multiple Mastervolt electric motors from your local West Marine! They are now selling the Aquapella, and Aquapella Hybrid drives. I was a little bummed that they do not yet sell the SilentProp 10.000. Which is the Mastervolt I'm most interested in.
One of the most interesting thing about the article was the section on "Trust vs Horsepower" They did a pretty good job of explaining the difference in power ratings between gas engine HP and electric ratings in watts. They listed a "rule of thumb" I have never heard before but sounded plausible.
The rule goes you should have about 1000 Watts of electric motor for every ton of displacement of your boat.
So using this rule my Chris Craft with a displacement of 26,600 pounds = around 14 short tons would need at least 14000 watt motor. Which is a little less then I plan on putting in. I plan on putting in 2 10000 watt SilentProps. Which makes my feel better about my own plans.
I'm guessing we may get a flood on newbies in the next few months, should be fun.
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