Full INTERNAL hydraulic where you replace the steering cable and factory power assist with a hydraulic helm and a gimbal mounted balanced ram.
Partial hydraulic where you add external ram(s) to the outdrive and tie into the stock steering "Brazil" valve at the end of the steering cable inside the boat
Full EXTERNAL hydraulic system where you completely replace the internal cable and Merc. assist ram and install external rams(s).
It's considered mandatory to go to either the partial or full external system whenever your boat is at or past the 70 mph mark. Aside from the precision it adds (and you really don't understand just how bad your cable steering is until you drive a full hydraulic boat) it provides a safety factor by preventing gimbal housing failures at speed.
There are 3 things in the stock system that can and have failed in boats running speeds above 70 mph (or at any speed for that matter), steering cable, tiller arm and gimbal housing. ANY one of these failing at speed can be fatal to you and your passengers. Adding the external ram system removes virtually all of the stress from the gimbal housing and tiller arm. It also locks the drive in place should your steering cable break.
The problem is magnified as the boat gets heavier and is prone to flying. Imagine the force of your 28' boat leaving and entering the water with 400+ HP slamming the prop into the water with a 3/4" cast housing being the only thing that's providing steering and holding the drive in place. Now consider that same force working on the 2 pins that support the gimbal housing onto the transom plate. That's a lot of force to be trusting to the cast gimbal housing with it's 2 location points on the top and bottom.
The cost breakdown is as follows (MSRP of Zeiger systems which I think are about the best going at this point in time)
Partial single ram external ~ $1795
Partial dual ram external ~ $2800
Full single ram external ~ $2850
Full dual ram external ~ $3690
You're living on borrowed time, take the cash you were going to spend on a stereo and put it into something that WILL save your life!!!!!