T-Pain Is On A Boat: [Electric Boats] Re: Prop shaft torque

Tuesday, January 22, 2013

[Electric Boats] Re: Prop shaft torque

Hi Andrew,

You can also use a sophisticated controller to run the motor in constant RPM mode. Luckily, this pretty accessible to us, because they are not as rare as Carel believes. The ubiquitous Sevcon Gen 4 has a constant RPM option in its configuration, though I haven't seen it used for boats. I guess that in reality, constant RPM won't result in constant speed, due to varying loads (waves, wind, etc.) that affect a boat's speed. It has more use in land-based applications.

So for us, the constant power control setting delivers more consistent speed in the real marine world. This is part of the default configuration that Propulsion Marine and Sevcon jointly developed when the Gen 4 was first released. I believe that Sevcon uses that basic config for all Gen 4s sold for marine use. Since most,if not all, of the PMAC based marine drive systems come with Gen 4s now, if you're running a PMAC system (Electic Yachts, Thunderstruck, etc), your system is probably set for constant power control. With good instrumentation, like the Sevcon Clearview display, you can watch the amps and rpm constantly fluctuate for a given throttle setting. Of course, these fluctuations are small and happen many times a second, but it works well, you can't feel the changes under way or during static pulls.

Fair winds,
Eric
Marina del Rey, CA

--- In electricboats@yahoogroups.com, wrote:
>
> The so called load testing you are doing is also called bollard pull or thrust test - you can use a scale withthe boat pushing onto it or inline with a rope tied to the boat and the wharf to quantify it.
>
> This situation is as close as the motor/power system/hull will get to a stall ie the maximum load which will be seen or generated (there are some exceptions which are short term spikes or a fouled propellor)
>
> The only way to keep rpm constant and vary amp draw or load is to use a cvt gear box or a feathering prop to vary the load on the motor, unlik ICE motors electrics make power relative to the load level, ICE's make power relative to rpm .