Marvin, remember that as the screw on the boats
turns faster the torque required goes up with it. Also
as the hull move through the water the faster the
torque reruired goes up propertional with speed. Thats
why if you were to look at the data suplied by Larry
and a few other on this forum they show that at 2
knots they draw very little current but at 4 or 5 knots
they draw four time or more many amps. The range of
the boat will be limited by the ability of the
batteries to deliver current. Batteries have diferent
rating or rates of discharge depending on how fast you
are taking current from it. A battery rated at a C10
(10 hour arting) rating of 200 can produce 20 amps
for 10 hours that same battery can also have a C2 (2
hour rating) of 160 that mean it would give you 80
amps for 2 hours. Idealy you want to have the most
effecient motor and controler coupled with the best prop
(dia and pitch) provide the least amount of drain on
your batteries at normal cruising speed of the boat.
This will give you the most range.<br> Getting back to
your original statement you want to control the speed
of the motor by changing the voltage to the field.
While this is done on some application on industrial
machine only where the application required high torque a
low speed with little or no torque at higher speed.
Just the oposite of what you need for the boat. If you
tried this in a boat you will draw a lot more current
from the batteries and drasticaly reducethe range of
your system since the motor will draw excessive
current in order to accomidate the higher torue need at
the higher speed of the boat.<br> Trust me on this
one the best system is to control the armature
voltage of the motor and leave the field alone. If you
insist on using a wud field motor then make sure that
you energise it before you energise the armature.<br>
If you want a very cheap speed control I would be
glad to provide a schematic of a 4 speed control using
4 batteries and 5 relays to give you reversing and
four seperate speeds. Otherwise you would do well to
spend the $300-$400 it takes to buy a well designed PWM
controller.<br><br>Regards,<br><br>Don<br><br>Regards<br><br>Don
