T-Pain Is On A Boat: Re: [Electric Boats] Thrust

Monday, June 3, 2013

Re: [Electric Boats] Thrust

Some comments inline.. and I dont mean them in any negative way.

I understand about the thrust-per-RPM rating being more a prop issue than anything else.

But, the electric motor can't be forever in a stressful state, building up enormous amounts of heat to maintain a given speed.

Actually, many many electric motors can do just that. They deliver rated power, at rated current, indefinitely until they break some very large nr of hours into them.

Natural coooled fully enclosed motors for industrial applications for example. My 3-phase motor is from about 1950, and has probably run 20.000 - 40.000 hours, or 10-20 years 24x7.
Its a bridgeport milling machine motor, original, from circa 1940-1950, for a Bridgeport M-head.
I run it with a VFD, and it runs very well from 12Hz to 100 Hz, at least, or 1/4 to 200% of its rated speed. It is nearly silent.
It is also about 18 kg in mass, and 3/4 Hp. This was before the definition of "continuous run" was around.

Ex: Stepper motors. They run hot, and typically last thousands to tens of thousands of hours like that. It is very rare for a stepper to fail from old age.
Ex: Servo motors.
Ex: IC motors rated continuous run.

Same goes for an I.C. engine. No one wants to run an I.C. engine forever at 3500RPM to maintain a speed of near hull-speed.

Many people do - like professional mariners, trawlers, shipping cargo and transport craft of all sort, and harbor tugs come to mind.

I'm personally willing to accept 3knots(plus or minus), as my base speed under power. Since a displacement hull begins wasting energy at a very much ever increasing rate about something like 3-4 knots regardless of the size(or oversized), motor, I have little interest in thise speeds, except, maybe, for a short... very short period of time.

Agreed 100%.

I have always been known(even when gas was $0.25 per gallon), as a very frugal user of energy. It has always literally hurt me in a very personal way to waste energy.

So, for me, I'm interested in my minimum thrust needs, along with a "little", more, and I'm interested in not taxing the limits of my electric propulsion.

Maybe my needs are too nebulous?

I think your needs are very well defined, and better than most boat "yachty" users.