Boat Forum / Building / November 2006

I am toying with the idea of driving a bow thruster with a 3 phase AC
motor and wonder if any one else has any experience in this field.

I have a genset that is 1 and 3 phase and a sail drive leg already
fitted with a 3 blade 12" kaplan prop - not exactly sure of pitch but
it is BT prop.
Vessel is 22m steel and displacement 65 tons. A friend in similar sized
vessel has a 5.5 kw motor, so I was planning on the same and either a 4
or 6 pole - so 1500 or 1000 rpm. I think that a 5.5 kw AC motor will
have the same end effect as a 10/12HP engine.

A reversing contactor is the easy part but my queries are:

whether to have a braked motor or not and

whether to feed the 3 phase through an AC drive??

I understand that an AC drive can incorporate a braking function, but I
think using a drive may just overcomplicate it all.

I guess that when the motor is switched off, it will stop pretty
quickly with the prop, so a braked motor may not be necessary.

The alternative is to use a DC 24v motor at 200-300A which will turn at
1440 rpm, so much the same power. Powered from battery and with
dedicated battery charger. DC motor is quite expensive option.

AC option will require me to start genset each time, DC option requires
battery and charger. Plenty of other vessels have a small diesel engine
driving BT, so starting genset is not an issue and I do not use the BT
all the time - did not use it for 2 weeks bringing vessel back from
Netherlands to UK along canals.