Diagonal fairing for high prismatic coeficient

Generally, the diagonals should have slight curvature at the bow and stern, and maximum curvature at the midsection. However, for a high prismatic coefficient, this is very difficult to achieve while preserving the design's displacement. If I will take it to the extreme, the only solution is to have low curvature at the entrance, exit, and amidship, with high curvature in between. Is it an acceptable solution?

 

Could you show us a drawing of the hull and the diagonal section you are talking about?

 

The only way I find to add a sufficient displacement, without increasing the diagonals curvature at the fore or aft, is by increasing the area of sections 4 and 6. This reduces the diagonals curvature at the turn of the bilge to be lower than the curvature at the fore and aft.

 

What is this for? That shape is a poor use of displacement, with several questionable changes in shape; so of course the diagonals are going to be funky. Are you trying to match some specific parameters? Because really, the hull shown is very inefficient from a stability and wetted surface perspective.

 

It is a full displacement motor yacht, with a specified D/L ratio, operation speed, length, and beam. What advice do you have?

 

Is the s-shaped hull a requirement or is it what you think a motor yacht hull should be shaped liked? You might want to make use of a round hull or a chine hull. Look for comparable designs to yours and you'll notice that there is probably no other motor yacht with a s-shaped hull. This kind of hull shape is more applicable for example for (traditional) sailboats.

 

DTMB Series 63.

https://apps.dtic.mil/sti/pdfs/AD0412788.pdf

 

Why are you set on the diagonals having that type of curvature? The shape will depend on where the origin point and angle from the vertical you decide on. In general, diagonals are used to make sure the lines are fair.

 

In order to fulfill the rule of thumb, draft = max( 1/2 beam, 1/7 length). A round hull will require a deep keel with a larger wetted area.

 

Where was that pulled from?

 

Several books. Maybe it should be applied to sailboats only?

 

Hand waves do not suffice for proper engineering. If it can't be read, it ain't been said.

 

The drawings show a very curious boat, very far of any motor boat (which are very different of sail boats) I've seen in my life. Full displacement boats are pretty simple round or hard chine. Basically a box with a bow and a stern, the final shape depending if it's rather heavy, pretty jeavy or extremely heavy and the wanted speed. To make a choice of shape and power of the engines you must have a first SOR.
Rather than struggling in trying to design a boat without enough knowledge (a person with good basic knowledge and having observed some real life boats would have never showed such drawings...) it's better to buy plans from a professional NA if you plan to build the boat. An amateur designed boat has none value.
It's a profession, you need some work at the university, with a good practice of rather basic math. In fact it's at least 4 years of studies, after that you're just a rookie with a diploma smelling the fresh ink and you'll need a few years of practice to acquire some experience. I've known a few NA in fishing boats without formal training, but they all were very able builders with many years of experience starting from well proven traditional shapes.