I don’t have a Grand Banks but I put a Side Power stern thruster on our boat 2 years ago. Pretty easy job. Took two of us about 2 days to install it. I did a lot of planning before we did the install. Measured a lot before I cut the 8” hole in the transom. The trickiest part was that the transom has a curve to it and it needs to be flat to mate up with the tunnel. I cut the hole and instead of trying to sand the transom flat, I used wax paper on the tunnel and spread thickened epoxy around the cutout. Then I bolted the tunnel on with just the top and bottom bolt. It pulled the tunnel up snug to the transom and the epoxy filled the gap around the sides to make the transom flat. Worked well. Then just caulk with 5200 and bolt it in. After installing the tunnel it was simple to install the thruster motor and wire it up. I uploaded a few photos that I have. Some uploaded upside down.
WESMAR has a perfect & easy to DIY install rear Stern-Thruster that works well on a Grand Banks & other similar simi-displacement trawlers.
Bolts right on back of the stern & has a tube with dual counter-rotation props in it that give you the side thrust as water is pushed out of the bolt on external tube for side to side thrust as selected by Captain.
Motor & electrical are all inside the boat & the prop & gear box is on outside.
One 4 inch or so size hole in the stern hull wall as far below the water line as you can & preferably in the rear middle, so maybe about a foot or so down.
Mine is a electric - 24 volt 13 HP, & is what I have - but for more power they have a 48-Volt -26 hp. or so size also available.
Mine has been very reliable & works well on 42 footer.
I had 2 Dickson Hydralic stern thrusters and they worked really well with a lot of power too.
I liked that only hydraulic hoses needed to go through the hull and the holes are ABOVE the water line.
The other cool thing is they come with a swivel mount so you can point them backwards and actually use them as “get home” power.
Does your Grand Banks have twin engines? The twin-engined boats I've observed don't need stern thrusters, let alone bow thrusters. On the other hand, a single-engined boat is much easier to handle (while pampering the transmission) with a bow thruster.