How much a boat drafts is strictly and totally dependant on two factors: The weight of the boat and how much surface area is touching the water. Nothing else figures into it, period. Larger, heavier boats can draft shallow that a smaller lighter boat if they have more surface area. Case in point, my current 21' RFL floats shallower than my old 18' RFL, and the 25' RFL floats shallower than my 21'.
How shallow a boat will get up is dependant on MANY more factors - but primarily lies with hull design. Flatter hulls will generally get up more shallow and speed/accelleration is major factor. Case in point, the reason my TRP gets my boat up so fast has NOTHING to do with "prop lift", which is really a fiction. The TRP, with it's twin props, has amazing accelleration - within a few feet it pushes the boat so fast that the hull begins to plane. However, as I said - MANY factors go into this.
To get ANY boat up, you have to push it. To push it the prop needs to be in the water. Can't get around that. Tunnels give ZERO advantage when the boat is at rest. Even with my RFL, I still have to lower the jack/prop enough so that it is sitting the water.
Here is where props come into play. Some props "bite" the water better than others. Some props require to be fully submerged to get any bite at all, some can be 3/4 submerged, and some - like the twin props on my TRP - can only be 1/2 submerged to get enough bite to start pushing the boat. Lots of factors go into the prop that control "bite": pitch, cup, size, blade design, etc.
Naturally, the more prop in the water - whatever the design - the more it will bite. This is where tunnels come into play.
The theory/operation of tunnel design - and there LOTS of different type of tunnels - is that the tunnel creates a stream of water that is thrown toward the motor. The more efficient the tunnel, the more water is thrown back. In most cases, this stream of water can/is directed in such a way that the stream of water is ABOVE the bottom of the hull.
When this happens the prop can be raised so that it can get the "bite" from the artificially created stream. In extreme cases, like with the RFL, a sufficient stream of water is throw above the surface of the water so that the prop is totally above the surface, and in many cases above the bottom of the boat.
But - to get this advantage - the boat has to be moving, and moving fast enough to create a large enough stream for the prop to take advantage of. Again, this is where accelleration comes into play and the reason why tunnel boat SHOULD be powered a little stronger than non-tunnel boat. You need to get that quick push to start the tunnel action.
There are dozen of boats, hull designs, tunnel design, motors, and prop configuration out there. You can take the SAME boat and put different motors on it and DRAMATICALLY different hole shot performance. You could also have two identical boat/motor setups and change out the props and get DRAMATICALLY different hole shots.
Most tunnel experts choose props that give more PUSH (lower pitch) so that they can get the accelleration and better hole shot. They sacrifice top-end speed (higher pitch) to get the hole shot. You can, and I have experimented with, put a high pitch prop on a tunnel boat and get them to go SUPER fast, but hole shot sux and it could take 100 yards to get the boat on plane. But MOST traditional design flat-bottom tunnel hulls, get really, really squirrelly and hard to control when they go over 50mph.
It's a beautiful thing when there is a perfect matching of hull, tunnel, HP, and prop. There are some great marriages that have been found over the years. For me, my Majek 21' RFL, with a 150 HP TRP is just one of the perfect combinations. Shallow draft, unbeatable hole shot, and a top end about 48mph. But I still get beat to death in a heavy chop.