Rob:
The older diesel 27 and newer outboard 27 are different hulls. Different strengths and tradeoffs. Its not just a case of what powerplant is "best". Its "best for WHAT"? Underwater, they are very different.
The traditional wisdom is a diesel has a longer operating life by a factor of hundreds of percent. But outboards are much easier to replace when the time comes. That old wisdom seems to be breaking down as outboards have become increasingly reliable and durable. Its still probably somewhat true, but that gap has closed some.
The real difference in overly-simple terms is the outboard is a go-fast engine. So in the redesign of the 27 the new hull is more of a go-fast hull with a cabin top design like an old go-slow trawler. They will behave differently because they are designed to optimize performance for different missions.
The vocabulary of hulls is misleading because words convey more hard certainty than is true in the real world. Do a search and you will read: a) displacement, b) semi-displacement, and c) planing hulls. The reality is that once you cross the Rubicon from displacement to semi-displacement, it is more of a smooth curve in the transition between semi-displacement and planing.
The true semi-displacement trawlers have a deep fore-foot V shape at the bow, that transitions back to flat at the stern transom. The chines are either hard angled or have some small amount of curve. A planing Deep V hull remains very much of a V shape at the stern transom. There are planing hulls that go flatter more forward and carried aft. Like I said, its not an easy thing to describe. But the over-riding concept is a planing hull is designed to provide a lot of lift in the shape to get on top of the water, and doing that requires speed, that speed requires horsepower, and that horsepower converts to more fuel burn.
The diesel Rangers all have some amount of V at the stern. And a keel like traditional trawlers. Look at the new R27 and you will see a couple of things. Its a deeper V at the stern. Forward in the shoulders there is something that looks like 4 inch discs have been cut into the hull maybe an eighth inch deep. Those discs are there to create air pockets to reduce water friction at planing speeds. That is speedboat planing hull design.
Again, words fail fast in describing this, but it seems to me there is a lot about the Ranger diesels that share more heritage with pilot boats than semi-displacement trawlers. Pilot boats are designed to speed fast in rough ocean conditions to incoming freighters and deliver pilots to them for the run to the destination port. They have some V in the stern for lift, have a keel, and have a reasonably deep forefoot. Its about stability at speed.
In calm water at displacement speeds a flatter hull will be more stable in feel because it has buoyancy at the edges vs. the centerline. Ditto at the dock or at anchor. When things get rougher the buoyancy at the edges of the flatter hull sections at the edges are lifted by waves more than the V whose buoyancy is more centered. The wider the beam for a given length the more this is true, and narrower the beam the less that is true. Levers with longer or shorter length. Best piloting strategies and techniques in rough weather can differ by hull design. I've seen some videos of fast planing cruisers running fast in rough water cutting across the wave tops and handling it very well. A semi-displacement trawler isn't going to do that so its strategies need to be organized around cutting through and over, or following the back of a wave at the wave's speed. The entire notion of what is "safe" is itself a tough concept. A fast pilot boat is plenty safe, and a trawler is plenty safe, but safe differently when handled differently.
No matter what, speed means more fuel burn per mile. You trade your time for your money.
Generally speaking, that tradeoff isn't a smooth straight curve, either. Fuel burn measured either in GPH or MPG plotted as a curve is flat at low speeds where a little more speed has a slight incremental cost in burn. But the curve does curve upward to points were one extra knot in speed has a big incremental cost in burn. In planing hulls there is that middle ground that is death to efficiency, where its pushing a wall of water but not yet up on plane. Push up onto plane and efficiency improves, the curve dips back down and then at high speeds goes vertical again. Its an S shaped curve. In true semi-displacement its a curve up until it goes vertical and no amount of extra power is going to produce more speed. Some of those designs are designed to flatten that curve into more of a straight line, but with some other tradeoffs I'll ignore.
See my comments at the end of the thread I started about "Speed Habits - Poll". The "Best Boat" depends a lot on how YOU plan to use it. If you want low fuel burn and go great distances at slower speeds, the older diesel R27 should be a great boat for that. If you want to get there fast and are willing to open the wallet for the gas bill to do it, the outboard R27 should be a great boat for that. Accommodations and interior will be pretty similar even if having some slight differences.
You have to choose.
I am trying to, myself. You can have anything you want, just not everything you want. Life can be a bitch that way. 🙂
Good luck.
