Tad, I believe, is Tad Roberts who is a naval architect. He doesn’t say that. I’ve read mostly about sailboats. Unlike power displacement boats they function at various heel angles and speeds so represent a different problem. However, it’s how much turbulence a hull leaves as it passes through the water. How much suction. The wake you see at the surface doesn’t disclose what’s going on below in its entirety. An hull exit with smooth, attached flow until it leaves creates less drag. So it’s what’s in the water not above that’s important. However, the sea is rarely flat. Boats slow dramatically when they hobbyhorse. Boats become unmanageable if the stern kicks around . The Willard 40 is my idea of how to best design a canoe hull for a small boat. It has a minimum of overhang so maximizes hull speed for LOA. It has fairly empty ends with a good gyradius. However in order to achieve these favorable characteristics it gives up load carrying ability. Compare these numbers
W40
LOA 40
LWL 36
Displacement 33000
HP 115
N40
LOA 39’9”
LWL35’5”
Displacement 55000
HP 105
So in actual use from what I can gather the transom stern is moving much more weight with similar HP and nmpg while paying no penalty in sea kindliness nor safety. I believe that’s efficiency.
Now look under water. Most transom sterns are associated with a smooth hull and no trailing edge except the rudder. The trailing edge is brought to the surface.you see this in the wake. To allow a balanced hull varying degrees of rocker are seen. Canoe sterns achieve the same balance by mimicking the lines of the forward half of the hull to some degree which often leads to a trailing edge in the hull. So the classic redningkoite has flow on the sides of the hull. Whereas the transom on the sides in the front and bottom at the back. Both work. But the second configuration allows greater displacement without a penalty in LOA. I apologize for my lack of knowledge to explain this with correct terminology.
The issue of reserve buoyancy is key. It allows a mannerly boat. It prevents hobbyhorsing. It allows additional displacement. It allows a good metacenter and gyradius. The Willard design does this brilliantly but at the expense of displacement and useful ability to carry weight when compared to the N40. The N40 isn’t a particularly efficient hull as concessions seem to have been made to maximize function as a cruiser. But if you define efficiency as energy needed to move a given weight in a boat of a given size a given distance and be safe you see why they aren’t in current production.
Willards are out of production for reasons unrelated to hull efficiency. The W40 traces its roots to a 1950s Wm Garden 34 design that was stretched to 36 feet - the W40. Designs from the Holy Trinity of west coast Naval architects - Monk, Defever, and Garden - were indeed wrought from experience vs computer modeling. Their recreational trawler style yachts incorporated these experiences. I believe it is a mistake to discount their designs as well intended but obsolete. They are proven hull forms with a deep and storied histories of truly knowledgeable owners who plied difficult waters with not much beyond seamanship skills carried between their ears.
I like the N40 a lot. But it's a huge leap to venerate it and use sales market share as evidence. Their buyers are typically long on desire and short on experience and a bit of disposable income. Nothing wrong with that but that alone doesn't make the boat anything more than well marketed. PAE has done a great job of developing a lifestyle brand in boats. I applaud them, an frankly, I liked doing business with them. I found their principals decent people who genuinely cared about their product and their customers. And they build a helluva product.
But at the end of the day, the N40 was successful where the W40 was not for two reasons. First, the same reaction your wife had when she saw the W40 - too small. Sitting at a trawler fest dock, simply no comparison. Second, the W40 was built in the good old USA. The N40 production was moved to China and base price was lowered to $399k which was around the same as the W40 at the time.
I have 1000s of offshore miles on both the N40 and the W40. They are both good boats, the N40 is better engineered and better built. But it's not more efficient, it carries a lot more stuff, which is important to people, especially people entering trawlers later in life when they are trying to shoe-horn a household full of stuff into a boat. Again, not a fault, just an observation. The N40 has a household refrigerator. The N40 had a Norcold. 'Nuff said on why the N40 outsold. The W40 50:1.
All I can tell you is I took a W40 from Long Beach to La Paz, roughly 1000 nms. She burned something like 1.3 gph and averaged 7.4 kts. I've taken a couple N40s from Dana Point CA to PNW, roughly similar distance, and averaged about the same speed but fuel burn was 2.5 gph. To compensate, the N40 carries almost 1000 gallons of diesel.
If this is about efficiency - and this thread is, that's the answer. Double ender or some variant is really efficient, which is more or less what Tad Robert said in his post.
The only reason no one builds round ended sterns is no one wants to buy them, not because they aren't a solid design. Technology has lowered the bar for new entrants to cruising. They either don't know or don't care about design attributes beyond what rose-tinted reviews tell them. They walk on a boat and want a capacious interior. That's it. Crowd-sourced design doesn't care much about the Naval architecture.
In the end, the N40 is an amazing boat. I'd own one in a heartbeat. But that doesn't make it efficient. Just makes it popular.
Peter