I have 8,5m x 2,85m displaysment boat with 75hp perkins engine, moves 6,2knots, weight 5.5tons, and its a nightmare in rough weather. So its not always a best and most stable. sidewawes are most horrible. tested it in baltic sea 16m/s and 21 tops... never again!
I don't have a boat or anything but displacement hulls are my favorite boards to surf. It's cool seeing how the design aspects of hulls as they apply to sailboats still apply to surfboards in the same fashion
Why do most displacement boats have a rocker (=bent keel line, with the midship sections sitting deeper in the water)? Doesn't that contradict the reasoning behind displacement hulls a bit? Is it only for maneuverability (=turning characteristics)? I see 2 problems with this: (1) having a rocker also implies that between bow and midship there is an inclined surface that pushes against the water and creates some lift. And that's a BIG(!) surface. If e.g. we take a look a hydrofoils, it's quite astonishing how little "wing" area is needed to generate substantial lift and we pay with a lot of drag to accomplish this. As I see it, we don't want that in a displacement hull - at least not if we optimize for efficiency (rather than top speed) (2) with a rocker the hull bottom is also rising between midship and stern. But this also implies that the aft section is less submerged --> we have less length below the waterline, hence less hull speed (because bow and stern wave are produced rather by the submerged _volume_ of the various sections than length alone); so why not a pointy stern and a keel that's straight over the entire length? The typical sailing vessel stern design also raises the question: why a flat transom (other than e.g. a double-ender kayak)? To me this only makes sense with planning hulls, so why not stick to 100% displacment characteristics only? In case it's about concerns related to pitch stability, we could always make the stern _above_ the waterline more boyant, in order to reduce hobbyhorsing tendency in rough seas, whilst preserving efficiency in calm waters. To sum it up: isn't a kayak type hull shape with no rocker a lot more efficient?
I'm a newbie also wondering about these questions. From what I've learned rocker is for maneuverability for tacking when sailing or for rivers. The waterline of displacement boats with a transom actually are still "double ended". At least I think so. It's possible it's a compromise, at low speeds the waterline is double ended and that helps with efficiency (water rushing back in providing pressure pushing the boat forward) while at faster "semi-displacement" speeds the stern sinks relatively lower in the water and the water gets cleanly separated from the transom.
Why do most displacement boats have a rocker (=bent keel line)? Because the water will get an impulse from the moving boat. The boat moves through the water, but relatively speaking the water flows past the boat. Viewed like this, the water is given time to accelerate and decelerate (before and after the "rocker"). This is more efficient than going in and out with a bang. (1) That's completely different. Put a hydrofoil on a displacement boat with hull speeds of 10 to 14 km/h (hull speeds of 6 to 10 meters boats). It won't do anything. Plenty of people have tested this, e.g. because they thought their boat was too deep in the water in the stern. (2) If the stern is submerged, there is no difference in the length of the water line, which is the only parameter that accounts for hull speed. Why a flat transom? 1. It is a better use of space. 2. Easier to design and cheaper to build. 3. Easy to mount things on it or put portholes in it (put a nice aft cabin in a sport boat in the first place). Concerning pitch stability: Where is the waterline in rough waters? It is constantly moving. I highly doubt that a flat transom is actually helpful when the waves are coming in from the stern. If it was bows, would be flat as well. But I might be wrong. I find that having the waves coming from the quarters can make yawing a problem and I think that in that situation not having a flat transom would be an advantage.
Depending on the exact design of the hull. The stern sinks into the water with more throttle and the boat might get flipped. The point is, every hull will provide "lift" at some point. A displacement hull will deliver a lot of lift not straight up but obliquely upwards and the slightest assymetrical force (a small wave, wind, a small rudder movement) will completely throw the forces out of balance. Most boats will roll or flip, before the stern sinks probably. I drove an overpowered charter boat once. Going even only 3 km/h over hull speed put the stern down by almost a meter, meaning it came down from just under a meter to water level within 3 km/h speed difference. Most hulls can probably go 4 to 5 km/h above hull speed and just inefficient, but still stable. At some point above hull speed a displacement hull will dig too deep into the water though. More power then would most likely only make the wave bigger that it pushes, if it can still drive stable and is sealed of against taking water in.
Random thoughts. All the displacement hulls, And or all boats in the oceans, Cause the rising seas?. not global warming...... well they both help each other lol. And another random thought iv had for a while. All the materials we move around the globe, IE cars, steel ,concrete ,ect ect. Wouldn't that throw the center of balance off? like a unbalanced car tyre, Slowly throwing our planet out of its normal orbit?. just spit balling btw lol
