Steve
I can't help you with how to do it, but I feel your solution has to be
electronic, not relays. You are talking about switching up to 14 amps DC
perhaps 4 times per minute - that's about 40,000 cycles in a week of
continuous use. That's a number of duty cycles that would have one looking
carefully at specifications if it were an AC load. I don't think automotive
components will cut the mustard. No-one expects to start their car 10 times
per day, 365 days per year for 10 years!
So the solution has to be solid state; question is how you get it designed
and then prove it's reliability .....
Hope this helps.
David

"Doug Dotson" wrote in message
...
Steve,

I would use neither. MOSFETs would be my choice. Check out
International Rectifier's website.

Doug

"Steve" wrote in message
...
I finally located a stronger 12 volt linear drive unit for the tiller
pilot
on my 13 ton Ingrid 38. It has all the power and length of stroke that I
feel I need (when the going gets tough).

This unit has a 14" stroke (stop to stop) 1000# dynamic, 1500# static
and
overload clutch rated at 1200-1600# w/auto thermal overload.

However; the drive motors max current draw is 14 amps.. Now that sounds
like
a lot but under normal operating conditions it shouldn't take more than
5-6
amps and then only in pulses since the tiller load is normally fairly
light.
(from my observations of the AP performance recently, the duty cycle is
about 1sec on/15sec off. ) Under heavy conditions, I'm sure this will be
different and I have not data since the tiller load is too great for the
OEM
linear drive.

I will be using this AP set up mostly while motoring so power
consumption
will not be a primary factor.

I will still be using the very reliable AH3000 tiller pilot control head
but
since this unit is limit to only a couple amps output to the old linear
drive, I need to build a power controller circuit (not sure that the
right
term), to deliver the full 14 amps to the new linear drive motor.

Both the old and the new linear drive was controlled by the switching of
polarity to the drive motor. I would use this same output from the
control
head to switch the output polarity of the power controller..

First I would like to know which would be the most effiecent and
reliable,
Transistors or Relays in this controller.?? I realize I could build it
from
regular, off the shelf automotive relays and if they fail, I can just
plug
in a spare, but then if I mount my transistors in sockets, I could do
the
same thing.

BTW. Either controller will be protected, below deck in a sealed box.

Can anyone offer a curcuit diagram for either transistor or the rely
power
controller. I'm not an elex. whiz but I can build just about anything
that
you guyz can sketch up for me. Or a good reference that might have such
a
circuit.

Thanks,

Steve
s/v Good Intentions