Re: Whats the reason for the huge differences in displacements in similar sized boats
Here is one of my long design explanation. I wrote this for a diffent purpose so I apologize in advance that it is not precisely on topic, but it might shed some light on the original question.
Like most forms of design, yacht design is an iterative process. It actually works backward of the way that most folks might think it does. It starts with defining the purpose of the design in terms of how the boat is primarily intended to be used, (distance cruiser, coastal cruiser, live aboard, performance cruiser, trainer, racer, etc.), where it is likely to be used, how many crew are likely to use the boat at any given time, how and where it will be constructed (production vs. one off, sophisticated vs. simple construction oriented yard), quality vs. cost trade offs, owner preferences (both aesthetic and functional), and designer experience, judgment and preferences.
With a good designer, this matrix of factors will drive the rest of the rest of the design. For example, if this was a distance cruiser, with an anticipated crew of four, it would need to carry a lot more gear, and be more robust, than coastal cruiser for four, so it might be heavier and longer for that same crew number. If it was intended for the Pacific, it might be deeper than if intended for an East Coast of the US. If it is deeper it might have a smaller ballast ratio and be lighter. If it were a coastal cruiser, it might need to adapt to more frequent wind speed changes over a narrower range of wind speed and performance might be more important, than a distance offshore cruiser. It might therefore need more sail area, and therefore need a higher ballast ratio, and simpler easier to depower rig.
But when the designer and owner are all done with filling in that matrix, the designer will ideally come away with a carrying capacity that the boat will need. There is a relationship been the required carrying capacity and the overall weight (displacement) of the boat that varies with the intent of the boat. On distance cruisers, the structural safety margin needs to be greater, so the weight of the hull and structure needs to be greater relative to carrying capacity. This added weight means that the boat needs a larger engine to over come the drag of this added weight and with larger engines comes the need larger fuel tanks to have an equal range. And with all of the combined added structural, engine and fuel weight, the sail plan needs to be larger to achieve a reasonable performance. The added sail area means that the boat needs stability to over come the drag of this combined added weight, and this means adding more weight in the form of more ballast. The added structural stresses of this added ballast and added rigging loads, requires that the structure of the boat needs to be beefed up which adds still more weight. And collectively that weight starts the next iteration of the design weight cycle over again.
By comparison, on a race boat, the carrying capacity requirements can be and should be a smaller percentage as compared to the overall weight of the boat. A mix of high- tech construction, and slimmer safety margins, allow the weight of the hull and structure to be proportionately less as well. This allows a larger ballast ratio, which allows a larger sail plan relative to the overall weight of the boat.
Once the design weight of the boat is understood, then the intended accommodations need to be considered. To some extent, but not always, the accommodation plan is a major driver in the length of the boat. You would think that accommodations plan would directly drive beam width as well. But at least in my experience, beam is mostly driven indirectly by the length of the boat, and its intended purpose, while length is directly driven by accommodations. Of course there are some trade-offs between the two, and so you make tweak the beam a few inches here, or there to help an especially tight accommodations plan.
In and of itself, weight does nothing good for a boat. Weight does not make a boat stronger, it does not make it more stable, weight does not give a boat a more comfortable motion, and it does not make a boat more seakindly. Weight simply increases the stresses on the parts of a boat, makes it physically harder to sail, more expensive to build and maintain, and makes it slower.
But once designer has set the overall weight that is needed, and roughed out a length that is adequate to house the accommodations, the next step should be (within reason) to maximize the over all and waterline length. All other things being equal, if you compare two boats of equal weight, one being longer and the other being shorter, the longer boat will offer better motion comfort, be more seaworthy, be easier to handle, have an ability to carry more supplies, and be faster. In most cases, if the boats are of equal weight they will have a similar cost to buy, and maintain. But often personal prejudices or some unique aspect of the intended usage, will mitigate against maximizing the over all and waterline length.
When you talk about similar purpose boats, of similar lengths, but widely divergent displacements, what you are often seeing is the way that the owner/ builder and/or designer, understood the decision matrix, considered the ability of the builder to precisely control construction weights within a budget, and the preferences and prejudices of all involved.
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Curmudgeon at Large- and rhinestone in the rough, sailing my Farr 11.6 on the Chesapeake Bay