FF wrote;
"Overbuilt displacement boat is fine"
Not fine at all. Overbuilt anything is bad. A waste and limit's the abilities of of whatever is overbuilt. Just like the all chain anchor rode. A waste and it limits the abilities of the rode and the boat.
Naval architects specify how a boat will be built and of what materials and the dimensions of those materials. Unless he makes a mistake (often happens) the boat should be at it's best as designed. If the boat is very well designed any change in the design will make the boat less able to achieve her design objectives.
So Fred to overbuild is not fine. But what is a boat built to in the first place? It's built to a list of objectives that may or may not be specified. Some may be implied or assumed. If you increase the thickness of the hull, add another engine and what goes with it, build a cabin on the sundeck or any other overbuilding and the boat will not perform as conceived, specified and designed. Design is an "organized solution to a problem" and if you take your eye off the ball (the "problem") you won't achieve your goals .... solution of the problem.
I'm not saying lighter is better at all times but it usually is if the original design to solve the problem is not compromised. Use plastic cleats? NO. Carbon fiber mast? NO. Remove a FB? Hmmmmmmmmm
So it's quite likely underbuilt may be preferable to overbuilt. Overbuilt may, however be safer most of the time. Changing a design is saying you know more about boat design than the NA that designed the boat. Sticking one's neck out to be sure but frequently changes are just a matter of taste. I don't want that big bed in the middle .. I want V berths. Or lets take that speaker out at the bottom and add another drawer. But you may say "I love this boat but want twin engines". Now that one you shouldn't do "off hand" but consult a NA before buying new engines ect.
But basically "overbuilt" is like putting a lead diving belt around your waist and going about your daily doings. The day wouldn't be the same. It would not be as good as usual. Overbuilt is not fine.