What size Jack plate

[charles p]

My boat is a 19.8 foot Carolina Skiff with a 90 four stroke Yamaha.  I need to run in some very shallow water to get to my boat lift.  I see hydraulic Jack Plates in sizes starting from a 4" set back to over 12".  What benefit does one get from moving the motor backward on the transom?  If shallow operation is the objective, is 4-5" the best length?  The shorter plates weigh less and all lift the motor vertically (what I need).

[eye shot]

charles p
Doesn't your outboard have power tilt? If so you should be able to run the prop about half out of the water and still have the water pump still work.

[Savage]

Jack plates allow the prop to run in cleaner water (Less disturbed from passing over the hull) and give the motor more leverage on the boat to plane it easier. Kinda like a cheater bar on a wrench. A power jack plate also allows motor height adjustment while under way to trim for max performance both  out of the hole and top end ranges. Be careful as you can get the motor too high to feed the water pump. You can also install a lower pickup unit and a water pressure gauge to make sure that doesn't happen. Seems like a costly solution, but it will allow you to run in shallower water at speed. With a motor in the 90hp range there won't be a large performance increase.
Savage
Edit: A 4" setback should be fine with your rig.