Your power requirement is pretty close to what I got for Rutu. Now you
will find that there are so many alternatives that your brain will start
fogging over.

You have a starting point but the cylinder is going to be pretty big and
heavy and the tiller arm will have to be 18 to 19" long. You have
plenty of room to work with the pressure. With 700 lb. on a 12" throw
you can get by with a much smaller rod diameter. Look for a 1.5" to 2"
cylinder with a 5/8 to 3/4" rod and 12 to 14" of throw. It will be a
lot lighter and you can use a shorter tiller. The displacement will be
less and the pressure will go up about the same so the required HP will
be about the same.

Also, small 12VDC motors develop HP with RPM more than torque and the
lower you gear it the less efficient it gets. Try to stay under 3:1.

For example a 2x12x.625 cylinder will take about 34 cu. in. for a full
throw or about 136 CI/minute for a 15 second hard over. Now say we
choose a 3600 RPM PM motor (Surplus Center #10-1779 would be close but a
little over powered) and start with a belt ratio of 2:1 so the pump is
running 1800 RPM. 136/1800 is .075 so we need a pump with about that
displacement. The Surplus Center #9-5567 is a little small so the belt
ratio would have to be more like 1.8:1

Admitedly that is not an ideal setup but it will be very hard to get the
perfect match with surplus parts. The thing is, when you add up the
cost of all the parts including the mounts, fittings, hoses etc, you
will come out spending $500-600 and many hours of thinking and labor and
still not be quite sure that it will work the way you want. That makes
the $1,200 price of a complete professionally designed unit seem a lot
more attractive. You would have to be a compulsive DIYer like me to
rationalize it. :-)

OTOH, if you go through the exercise of designing one you have a lot
better understanding of how they work and know more about how to select
the correct one for your boat.

Steve wrote:

Sorry, another typo..

"Steve" wrote in message
...

Seems I should be using about a 2"X20"X1.5" cylinder, 318psi @ .186gpm.



Should read .... 2"X20"X 1 1/8" cylinder, .......

Steve
s/v Good Intentions



--
Glenn Ashmore

I'm building a 45' cutter in strip/composite. Watch my progress (or lack
there of) at: http://www.rutuonline.com
Shameless Commercial Division: http://www.spade-anchor-us.com