Hydraulic steering displacement vessel.

What would the limits of manual hydraulic steering be in a slow speed displacement vessel? (before adding an electric pump would be necessary)

I don't have my head quite wrapped around this subject, I've only dealt with small vessel manual hydraulic outboards, and not much since I never encountered any problems with it.

Vessel in question would be a 65ft dual helm (pilothouse) sailboat of approx 50 tons displacement. Would a Capilano 1275V helm pump (one aft and one in pilot house) be sufficient with the appropriate cylinder and a large wheel (say 48inch) for a vessel this size?
-if it wasn't and steerage was tiresome is it simple enough to just add an electric pump?
-how often do these pumps run? or do they need to run continuously when power steering is needed? (power usage is a concern) and sailboats can have frequent small course adjustments.
-im assuming a system with a hydraulic pump could be used in manual mode with pump turned off, and maybe only switched on when conditions warranted?

-main rudder would be locked at appropriate trim and sailed via windvane self steering on long passages so a hydraulic autopilot would not be required.

Appreciate any info anyone may have relevant to the subject!

I would suggest you call the folks at SeaStar, the Capilano product line's manufacturer, and ask them. Several years ago, I converted my power boat from Morse mechanical steering to SeaStar hydraulic and needed some advice. I called their technical support and received excellent, helpful information. 604-248-3858
http://seastarsolutions.com/support-2/technical-support/hydraulic-steering/

calling tech support is a prudent step...but .they will need a lot of info as they are concerned about the required torque to move the rudder (for the helm) and the stroke required (for the cylinder). look here:

http://seastarbom.com/training/RudderTorqueDatasheet.pdf

I will definitely do that before purchasing anything, was just curios if anyone had any real world experience with manual hydraulic steering and larger diameter wheels on larger boats, I'm keen on the simplicity and lack of power usage.
Thank you both very much for your replies and the links!